Weather456's Tropical Weather Blog

Tropical Update

By: Weather456, 12:45 PM GMT on October 31, 2009

An area of low pressure along a frontal zone is being monitored for possible signs of subtropical development. This morning visible images already showed cyclonic turning taking shape along the frontal boundary, which drapes across the entire Western Atlantic. QuikSCAT also confirmed the area rich is cyclonic shear along the cold front. The feature has already secluded from the mid-latitudes, as there are no fronts to the north, rather a surface trough. This feature is expected to linger in the vicinity of the Central Subtropical Atlantic and then turn back west through the next day or two at which point, it will separate from any associated fronts as they move east. At this point, the feature stands a chance of becoming a subtropical storm over 26C waters and relatively low shear. The feature may also threaten Bermuda with high wind, wave and rain action.

I presented an overview of how subtropical cyclones typical form in my last blog, those north of 30N, and in reference to this same system.

An area if disturbed weather is located in the Bay of Campeche associated with the tail end of a cold front. No development expected.

Figure 1. Visible image loop of the area of non-tropical low pressure located in the subtropical Central Atlantic.


The 1502 Hurricane of the Dominican Republic: Columbus’ Hurricane

By: Weather456, 9:32 AM GMT on October 29, 2009

Everyone knew Christopher Columbus, we all knew him as an adventurer and discoverer. In some literatures, he is described as the discoverer of the New World and in others, simply the discoverer of the native Indians, which were already living there. However, not many persons knew Columbus made his first voyage during the peak of hurricane season which led him to the discovery of the Trade Winds, the most persistent and consistent winds of the globe. Moreover, Columbus was an amateur meteorologist, the one that predicted the violent storm that struck the Dominican Republic in 1502.

Columbus' first voyage departed Spain in early August, stopping in the Canary Islands to resupply in early September. Assuming modern day tropical climatology valid for this voyage, this would have put him on the downside of Atlantic hurricane season as he made his five-week trip across the ocean. By mid to late September, Cape Verde hurricanes become less frequent and storms that do develop tend to generate in the western Caribbean, western Atlantic or Gulf of Mexico.

Obviously, Columbus was familiar with some of the global wind patterns, heading for the subtropics to take advantage of the easterly trade winds on his voyage westward. During his return trip to Europe, Columbus made use of the prevailing westerlies in the mid-latitudes.

Now on his fourth voyage to the New World in 1502, Columbus had some falling-out with bureaucrats appointed by Spain to govern Hispaniola and extract gold, lumber and other commodities from the native land. Among these rulers was newly appointed, Don Nicolas de Ovando, the governor of Hispaniola, with whom Columbus had forbidden contact by his Spanish sovereigns. However, as Columbus entered the outer roadstead of Santa Domingo (a town in the Dominican Republic), he recognize the ominous signs of an approaching violent storm such as an oily swell emanating from the southeast and a developing veil of cirrostratus overhead.

Concerned for the safety of his men and ships, he sent a message to Governor Ovando urging him to allow his fleet to seek refuge in the harbour and to postpone any additional fleets departing from the country. Refusing both the request and advice, Ovando read Columbus’ note aloud to his minions, who roared with laughter at the forecast by the amateur meteorologist from Genoa, Italy.

The laughter was short-lived, the ships left port, and as they entered the straits between Hispaniola and Puerto Rico, the hurricane struck sinking 29 ships on the spot leaving one other brutally damaged.

Now Columbus was very smart and he positioned his fleet in the harbour on the southern side of the island as he was anticipating heavy winds from the north. Even with the protection of the mountainous terrain from the windward side, the fleet still struggled. In the admiral's words, “The storm was terrible and on that night the ships were parted from me. Each one of them was reduced to an extremity, expecting northing save death; each one of them was certain the others were lost.” Columbus’ ship was nearly swept out to sea but the fleet eventually survived with minimal damage.

I have done some further reading and it was found that Columbus returned to the island 18 months later, only to discover it had been largely destroyed by the hurricane. This could only mean that there weren't any re-colonisation of the area for over a year and I have hypothesise that a category 3-4 hurricane could of only caused such extensive damage at the time. The descriptions of the other events including this one were horrific and they include:

“ if it wanted to split heaven and earth apart from one another”

“The strong and frightful wind threw entire houses and capitals including people from the capital; tore them apart in the air...”

“...Europeans had never encountered such ferocity so they were tossed around by the wind. For the most parts Indians had crawled away and hidden themselves in holes to escape such disaster.”

Other reports indicated that the storm struck around 1 July and was sensed from Martinique so this system was most likely a Cape Verde Hurricane, but so early? Columbus' ships sheltered at the mouth of the Rio Jaina, the first Spanish treasure fleet sailed into the hurricane. Columbus' ships survived with only minor damage, while twenty-nine of the thirty ships in the governor's fleet were lost to the 1 July storm. In addition to the ships, 500 lives (including that of the former governor, Francisco de Bobadilla) and an immense cargo of gold were surrendered to the sea.

Columbus was the first governor of Hispaniola and was stripped of his position by Bobadilla due to treatment over the native people. Now you would of think Columbus was treated with a little more respect after this incident but following the death of Francisco de Bobadilla, the former governor of Hispaniola at the time, Columbus was accused by Ovando of magically invoking the storm out of vengeance. You could say Columbus was accused of wishcasting, lol.

I do not think he ever got the recognition he deserved as this tale was omitted from several historical literatures and was never taught in schools. The only part that was mentioned was the second governor of Hispaniola died in a storm at sea but no mention that the storm was forecasted by the first governor and ignored by the third.

Figure 1. An artist depiction of a violent storms "hurricanes" back in the sixteen century. This was drawn based on the accounts made by Columbus and his crew.

Sources and Further Reading

Divine Wind: The History and Science of Hurricanes by Kerry A. Emanuel, 2005

History of the New World, Girolamo Benzoni, 1565

Tropical Update

The Tropical Atlantic remains relatively quiet this morning with water vapour imagery showing a very hostile environment below 20N. This is due to a jet split with one axis along 15N and the other near 35N. The southern jet is interacting with the ITCZ to produce a swath of disorganize showers and thunderstorms stretching from Venezuela to the Cape Verde Islands, below 20N. In the middle of the split, low vertical shear and synoptic-scale sinking motion dominates and this may form the focal point of a subtropical system later this weekend into next week.

All the global models are predicting that a frontal system just offshore the NE United States will head southeast across the subtropical Atlantic over next 2-3 days. Most frontal systems are accompanied by an associated high-pressure system at the rear. Meanwhile, the climatological Azores High remains in place over the Eastern Atlantic. As the front advances east, the transitory high and the Azores High start to bridge (connect) causing the southern portion of the front to seclude and become trapped. A frontal low eventually forms along the front and a low-pressure area forms. The low-pressure area is cut-off from any cold air supply, meanwhile, the clockwise flow around the high ejects tropical heat and moisture into the system, causing it to lose its frontal boundary. The feature now stands a chance to become Subtropical Storm Ida. A combination of low shear, moisture and warm sea surface temperatures may allow development, but the system only has about 3 days to become organized before a cold front sweeps down and absorbs it. I will give a high chance of a low-pressure system forming over the Subtropical Atlantic over the next 7 days but a low chance of it becoming subtropical for now. The chances it affecting Bermuda, whether tropical or not is medium.

Elsewhere, I am having a difficult time finding any model that is forecasting development in the Western Caribbean next week but because the area is climatologically favourable, I will see how that goes. The tail end of a frontal boundary may supply some energy for a developing Eastern Pacific storm later this weekend and into next week.

Figure 2. The UKMET 00Z model run, just one of the models predicting development of a possible subtropical system later this weekend into next week.


Tropical Cyclones: Physics, Energetics and Mechanics

By: Weather456, 11:24 AM GMT on October 25, 2009

Today I will present an overview of the formation, development, maturity, decay of tropical cyclones using the power of physics, energetics and mechanics, and answer some of those not so obvious questions.

Why do we need warm sea surface temperatures?

A pre-existing disturbance is always needed to initiate development, whether it is a tropical wave, tropical low, surface trough, upper low, or sometimes a frontal boundary and extratropical cyclone. We often here that cyclones need warm sea surface temperatures but why? I mean water is water regardless of the temperature. The main reason why we need warm sea surface temperature lies in the fact that warm water is more energetic than colder waters. Tropical cyclones need warm moist air and the only source of such air is from the ocean. Temperature is simply the degree to which molecules move and evaporation can only occur if water molecules can escape the sea surface and enter the air and in order for that to occur; you need the temperature to be high.

How does a tropical cyclone cools the sea surface?

A tropical cyclone's winds causes upwelling of colder sub-surface waters but there is another process by which tropical cyclones remove heat from the ocean – evaporation. Evaporation is the process by which water molecules change from a liquid state (water) to a gaseous state (water vapour). Now the molecules in the liquid state are bonded closer than in the gaseous form and this pretty obvious if you ever boiled water. Now remember when I said warmer temperatures cause molecules to move faster, well to get those molecules spread out from the liquid form to the gaseous form you need energy (heat of evaporation). The heat comes from the surrounding ocean, which is referred to as latent heat.

In summary, heat needs to be added to a group of water molecules to get it change from a liquid state to a gaseous state. The heat needed is called latent heat and comes from the surrounding ocean, which causes it to cool.

Latent heat versus sensible heat

Latent heat is the heat absorbed or given off by a substance while it is changing its physical state. Sensible heat can be sensed, or measured, with a thermometer, and the addition or removal of sensible heat will always cause a change in the temperature of the substance. In other words, sensible heat is the heat that affects the temperature of things; latent heat is the heat that affects the physical state of things. The summer time warming of the sea surface is related to sensible heat, while the heat removed from the ocean needed for evaporation is latent heat.

Figure 1. A schematic diagram showing the simply flow of energy from the sun to the ocean to the air aloft, illustrating the process of evaporation and condensation and the types of energy that are associated.

Why is moist air lighter than drier air at the same temperature?

Water vapour is a relatively light gas when compared to oxygen and nitrogen, the two other abundant gases in the atmosphere. If we take a parcel of moist air, the pressure of the vapour molecules will be greater than the heavier gases that share the parcel so in effect it will be lighter. Oppositely, a dry air parcel will have lesser vapour molecules and more of the other heavier gases so in effect will be heavier.

Another reason why moist air is lighter, is because of its temperature. Moist air is warmer because the tendency for condensation is greater and condensation releases heat. Dry air is cooler because the tendency for evaporation is greater and evaporation absorbs heat.

This is the reason why moist air (above 70%) is often a requirement for tropical cyclogenesis and why dry air interferes with tropical cyclone development.

How does dry air affect tropical cyclone development?

As mentioned before, dry air is heavier and cooler than moist air. If the air is dry in the mid levels of a tropical cyclone (between 500 mb-700 mb) then evaporation of raindrops or condensed water will occur. This causes the mid-levels to cool and become even denser. Dense air will sink which retards both lower pressure and rising air needed for convection. When it hits the surface it spreads out and the leading edge is what we called arc clouds or outflow boundaries.

Figure 2. Visible image of Tropical Storm Chris in August of 2006 showing a series of arc clouds emanating from within the central overcast, indicating that the tropical cyclone struggled with some level mid-level dry air. Sometimes you cannot use water vapor imagery to detect this dry since water vapor mainly shows the upper level moisture fields. For this you might use model soundings or total precipitable water loops.

What causes a tropical cyclone's warm-core?

The simple cause of a tropical cyclone's warm-core is heat of condensation. When condensation occurs, it releases heat because you are trying to bond the molecules from a gaseous form to a liquid form. The greatest condensation occurs near the center of the cyclone relative to the surroundings so there is where the greatest heat will be found, thus it fulfills the definition of a warm core.

What causes the pressure heights to fall in a tropical cyclone?

The main cause of the pressure falls associated with tropical cyclone or any low-pressure system is rising air. As air rise, it exerts less force on the earth surface and you have lower pressure. There is another process by which rising air cause pressure heights to fall – opposing the force of gravity. As air rises, it opposes the downward force of gravity. In order to compensate for this imbalance, pressure heights must be lowered. One person told me if this did not occurred, the troposphere would rise into the stratosphere but I have always oppose this by stating the tropopause, which is a seal, would not allow this.

Figure 3. A systematic diagram showing the formation of a tropical cyclone's warm-core and the associated pressure height falls.

What are the different types of energy associated with tropical cyclones?

You first have heat and potential energy stored in the ocean. Tropical cyclones obtain this heat energy through evaporation, which rises and condenses. Water vapour retains the heat and carries it from the sea surface to the air aloft. Now the heat ends back into the atmosphere through condensation. The air aloft warms, and becomes more buoyant and rises even further. The pressure heights fall in response and lower pressure is created. The pressure gradient increase and air begins to move and now we have kinetic energy. The moving air picks up more moisture from the sea surface and rises again to begin the cycle over again. As long as conditions remain favourable, this cycle intensifies with each round – positive feedback loop.

Now the moving air exerts a force upon the water surface to create and cause it to move in waves, this is now called mechanical energy. Mechanical energy is also responsible for physical damage and storm surges. The wind exerts a force on objects which causes them to move.

The heat and kinetic energy released by tropical cyclones is equivalent to 200 times the world-wide electrical generating capacity - an incredible amount of energy produced!

I hope after reading this blog you are left with a little bit more knowledge of how tropical cyclones work, and rather than just looking at one on satellite imagery, you can appreciate the energy, thermodynamics and physics that go in to making one.

Other News

I will also issue my October Summary next Sunday and Winter Outlook for the Western Atlantic sometime in early November.

Erika was really a forecasting challenge for me and probably one of my personal best forecasts in predicting her to go left of the forecast track but here is Erika’s model verification and I LMFAO last night when I saw how poor the models did with her.

Figure 4. Model tracks from 00 UTC 2 September and Erika's best track [actual (black)]. You can clearly see the large forecast error made by the models but this is no surprise because as an invest Erika never did follow the forecast models.

We often hear radar imagery is confined to land-based areas but wouldn't it be cool if numerical model-based radar imagery was available - which has no spatial constraints. The animation below is of Hurricane Fred from the COAMPS model of Fred that shows 1-hr increments radar based reflectivity from the 120-hr initial forecast – September 8 2009 at 12Z. This model had 95% accuracy in the forecasting of Fred's track and intensity but this accuracy was the same for most forecasters and other models.

Figure 5. COAMPS radar based reflectivity of Fred +120 hrs from September 8 2009 at 12Z. You can clearly see the pre-banding features that wrapped all the way around, the formation of the eye and the maturity and then the demise due to vertical shear.

Now, the season is winding down with only 5 weeks left and I do not expect much in November since this is an El Nino year and for various other factors. Now, I will still be here updating my blog on topics just like this one but for many others, they will be leaving, never to be seen again until next hurricane season, so for those that won't be here - Seasons Greeting and a blessed New Year.


Tropical Update

By: Weather456, 9:56 AM GMT on October 23, 2009

The area of disturbed weather associated with a broad area of low pressure in the Southwest Caribbean region continues to show little signs of developing at the moment. Shower activity has diminished along the feature and this is due to a combination of moderate shear and dry air associated with a lingering upper trough (which I don't think was supposed do this) north of the area. This is expected to continue over the next day or two until the upper trough splits and ridging takes over. At which point conditions may become a bit more favourable for development so despite the National Hurricane Centre’s take on things I will continue to monitor the feature.

Expect this feature to continue to drift north over the Western Caribbean through the weekend and into next week bringing an increase in moisture to Jamaica, the Caymans and Cuba. Most of the models show the system moving into the Gulf of Mexico through early next week where they cannot agree on its faith. Some have it absorbed into a frontal boundary while the GFS/CMC develops it into a tropical storm in the Southeastern Gulf of Mexico. At this point, I feel the disturbance will remain weak in the Caribbean but uncertain as to what it might do when it enters the Gulf of Mexico.

There is another area of disturbed weather associated with a low-pressure area over the Bahamas interacting with mid-upper level cyclonic flow over Cuba. Upper winds are extremely unfavourable for development but the feature might deepen some before becoming absorbed into a frontal boundary as it heads west.

Figure 1. Water vapor depiction of the Western Caribbean basin showing an overview of the features discussed and their relationship.


Little Change to 94L; Rick dumping heavy rains; Neki becomes a hurricane

By: Weather456, 11:12 AM GMT on October 21, 2009

There has been little change to the area of disturbed weather in the far southwest Caribbean. Satellite imagery along with surface observations place the center of this broad disturbed near the Costa Rican coast despite the low analysed on the 06Z Caribbean analysis maps. Last night’s QuikSCAT pass also places the low either near the coast or over Costa Rica itself. However, the latest satellite imagery proposed the low might have drifted more along the coast. This should delimit development in the meantime but the low is expected to slowly drift back over the SW Caribbean. Models are pretty much not helpful at the moment since the 00Z ECMWF has drop the system (the model which previously developed it) and the 00Z GFS develops the feature (the model that previously had a poor handle on the system). It could be the disturbance is too complex or complicated to be fully handled by the models, so we will have to derive any patterns from upcoming runs over the next few days. Upper shear environment and sea surface temperatures are still forecast to support development, but land interaction as previously mentioned will slow development. There is also a possibility that low stays too long over Central America to develop so currently it is a 50/50 chance. If it does develop, the feature should drift slowly north, maybe a bit east thereof over the next 5 days. Regardless of development, moisture is expected to spread across Central America and the Western Caribbean region through this weekend into next week. The GFS brings the system to the SE Gulf of Mexico in 1 week, but it’s way too early to put any validity or faith into it.

Figure 1. Satellite imagery of Tropical Invest 94L.

Tropical Storm Rick and Hurricane Neki

Tropical Storm Rick is moving towards the Mexican coast after sparing the Baja Peninsula with hurricane forced winds and direct impact. The storm is dumping heavy rains across Western Mexico and this is expected to continue over the next 24-36 hrs. Rick is expected to dump up to 12 inches of rain across the region closest to landfall and inland along the path.

Figure 2. Rainfall forecast for Tropical Storm Rick for the next 12-18 hrs.

Hurricane Neki has continued to strengthen over the Central Pacific just southwest of the Hawaiian Islands and pose to become a major hurricane in 3-4 days. This is the first hurricane in the Central Pacific since Ioke in 2006 and reinforces 2009 as an El Nino year.


Atlantic tropics liven up

By: Weather456, 10:46 AM GMT on October 20, 2009

A broad area of low pressure continues to linger across the Southwest Caribbean, producing somewhat organize convection across the area. Satellite imagery along with surface observations revealed clear cyclonic turning with an estimated 1007 mb low at 12.8N-82.0W, quasi-stationary. Much of the reliable global models continue to forecast this low to meander across the area for at least the next 3 days, slowly organizing as it does so. A tropical wave over the Eastern Caribbean may interact with the feature and aid in genesis. Upper winds are anticyclonic about 5-10 knots and sea surface temperatures are near 29C so conditions appear favourable for the continued slow development of the system and an invest could form today or on Wednesday.

The initial motion of the system will be influence by weak steering flow and a deep layer ridge parked over the Southeastern United States. This should delimit much motion northward early in the forecast cycle. The GFS, ECMWF and CMC are coming in good agreement that a depression will be located just northeast of Honduras in 5 days. Long-term speaking, 7 days, the models largely differ on the faith of the system with the CMC taking it over the Yucatan, the ECMWF over the tip of Cuba and the GFS in the middle. They differ because they handle the amplification of the upper trough differently, that is expected to dig across the region in 7 days and help pull the feature more towards the north and eventually northeast. I will go with the ECMWF/GFS solutions since the CMC is having trouble moving the feature when it goes over the Yucatan, which is a sign of inconsistency; and the fact that the path favoured by the ECMWF/GFS models is climatologically reasonable.

Such a path would affect Central America, Jamaica, the Caymans, Cuba and eventually Florida. Regardless of development, these areas should anticipate an increase in moisture activity later this weekend into next week.

Tropical Invest 93L

In my blog entry on Sunday, I alluded to the area of disturbed weather in the Eastern Atlantic...

“I am surprise we have not one but two areas of interest in the Atlantic this morning. First, the area of disturbed weather I kept mentioning in the eastern Atlantic has actually consolidated thunderstorm activity this morning. This feature is associated with a broad area of low pressure and tropical wave along the ITCZ. Further development of this feature is uncertain due to high vertical shear to its northwest. Regardless, I will continue to monitor this feature over the next few days.”

The feature was eventually tagged Tropical Invest 93L and satellite imagery showed a remarkable increase in organize relative to 72 hrs ago. Shortwave infrared imagery along with this morning QuikSCAT pass revealed a closed circulation associated with this feature but it remains expose and west of the convective mass due to strong upper winds. If the system somehow manages to get 25 knot winds, it is likely a tropical depression will form. However, upper winds are extremely unfavourable for development, running near 80 knots! I will continue to monitor this feature and the SW Caribbean disturbance.

Figure 1. Infrared imagery of the Atlantic tropical basin showing the area of disturbed weather in the Southwest Atlantic (left) and Tropical Invest 93L (right).

Pacific Update

Rick has weakened severely to a tropical storm due to dry air and strong wind shear so the impacts are less than anticipated. Meanwhile, weakening has occurred with Lupit and it is expected to make landfall on Thursday/Friday as a category 1 hurricane. In the center of the mix, Tropical Storm Neki has form southwest of Hawaii and the Central Pacific Hurricane Center described this as unusual, likely due to El Nino.


Tropical depression may form later this week

By: Weather456, 10:16 AM GMT on October 19, 2009

A broad area of low pressure located in the Southwest Caribbean is showing some signs that it wants to develop into the season’s eleventh depression. Satellite imagery continues to show gradual increase in shower activity and pressures remain low and slowly falling across the area. This area is tied to a passing tropical wave but the wave has since moved ashore Central America leaving an area of low pressure behind. Most of the global models are now hinting that the low-pressure area will meander over the Southwest Caribbean over the next 4 days at which point a tropical wave near 60W will enter the picture and likely kick off genesis. The models differ considerably with the time of genesis since the GFS has a much slower solution than the ECMWF. I will have to go with the latter model since I do not think it will take until next Monday for a low to still be south of 15N. I guess the models heard my plea as yesterday I was trying to understand why not one model has joined the ECMWF; and now as of 00Z today, the CMC, NOGAPS and GFS have all come onboard. The likely time frame of substantial development should be between 4 and 7 days, which is probably why the National Hurricane Center has not mentioned anything in their Tropical Weather Outlook yet. Upper level winds are forecasted to be light with high ocean heat content and deep layer moisture along the projected path so there is a possibility of rapid growth.

Now, the likely motion of any such system will be slow to occur, initially, as a ridge will build over the Southeastern United States through day 4, slowing any northward motion. Meanwhile, a deep layer trough will intrude just south of the high near Cuba/Florida to Honduras and thus the system might appear to move slightly east of due north. This pattern should abate as second frontal trough digs across the Eastern CONUS beyond 5 days, and the system should be attracted towards it, thus resuming a west of due north motion. This effectively places Jamaica, the Caymans, Cuba, the Yucatan Peninsula and the Southeastern Gulf in the line of fire. It is still too early tell whom but the greater chance lies with Jamaica and the Caymans.

Regardless of development, heavy thundershowers are now spreading across Central America and the NW Caribbean region today and this should continue over the next day or two, with moisture spreading further north in part by a stationary front over the Caymans and Jamaica later this week. I will continue to monitor the situation.

Figure 1. AVN enhanced infrared image of the Western Atlantic region showing the area of disturbed weather located in the Southwest Caribbean.

Hurricane Rick

Satellite presentations of the once spectacular Rick as degraded substantially over night with the overall appearance ragged and asymmetric due to dry air intrusion on the left side of the system and 20 knots of upper westerly winds. Rick has also slowed substantially as it begins to recurve. The system should continue to gradually weaken today and tomorrow as the elements continue to batter the storm but only enough to weaken the storm to a category 2 hurricane before crossing the Baja Peninsula. Second landfall will occur as it crosses the Gulf of California about 20 hrs after in a much weaker state. The storm’s left eyewall should pass over Cabo San Lucas, sparing the city from the core of heavy winds located in the right eyewall. However, moderate damage is still expected across the area. Rainfall amounts are expected to be about 2-6 inches with higher localise amounts. Waves of 20-30 ft will batter the coastlines and a surge of 2 m could inundate the coastline, however the bathymetry of the Baja and Mexican mainland region is not one that is subject to high storm surge.

Tropical Depression 03C and Typhoon Lupit

Tropical Depression 3 has formed in the Central Pacific and is expected to become a hurricane later this week well southwest of Hawaii. The next name on the list is Neki. Meanwhile, Typhoon Lupit has resume its westward motion and should be nearing the Philippines later this week with little change in strength. I will have more updates on Lupit as the week goes by.


Rick becomes 2nd strongest Eastern Pacific hurricane

By: Weather456, 1:04 PM GMT on October 18, 2009

Successive rounds of rapid intensification over the past 24 hrs resulted in Rick attaining category 5 status on the Saffir-Simpson Hurricane Scale, the highest ranking. Rick topped off with sustain winds near 180 mph and a minimum central pressure of 906 millibars, not to mention a Dvorak reading of 7.5. Rick is the first category 5 hurricane in the Eastern Pacific since Hurricane Kenna in 2002 and the second strongest Eastern Pacific Hurricane ever, behind Hurricane Linda in 1997. Ironically, those two years and records were El Nino years – 2002 and 1997. The last time we saw a category 5 in the National Hurricane Centre’s domain was Hurricane Felix 2007.

Hurricane Rick should fluctuate in intensity over the next day or two, as expected of cyclones of that intensity. The cyclone should weaken to minimal hurricane as it approaches the Baja Peninsula due to progressively cooler waters and increasing shear accompanied by the same upper trough that will help recurve it.

Figure 1. GOES-11 infrared/visible image of Hurricane Rick this morning just offshore Mainland Mexico as a category 5 hurricane. Image credit: NASA.

Typhoon Lupit

Typhoon Lupit has slowed over the warm waters of the Western Pacific just east of the northern Philippines and has intensified into the equivalent of a category 4 hurricane. The typhoon is still forecast to maintain this status as it begins to pick up speed through mid-week.

The Atlantic Tropics

I am surprise we have not one but two areas of interest in the Atlantic this morning. First, the area of disturbed weather I kept mentioning in the eastern Atlantic has actually consolidated thunderstorm activity this morning. This feature is associated with a broad area of low pressure and tropical wave along the ITCZ. Further development of this feature is uncertain due to high vertical shear to its northwest. Regardless, I will continue to monitor this feature over the next few days.

Closer to home, a tropical wave near 80W is interacting with a broad area of low pressure over the Southwest Caribbean Sea. Vorticity in the lower levels along with QuikSCAT and satellite imagery revealed broad cyclonic turning along the axis of the wave with noticeable increase in shower activity. The feature is already interacting with prefrontal flow as the thunderstorms stretch towards the northeast over Jamaica, SE Cuba and Hispaniola.

The ECMWF is hinting that this activity will serve as the focal point for the development of a tropical depression later this week. Such a feature will move slowly northward over the next 5 days due to high pressure behind the current cold front. The ECMWF is hinting under favourable upper winds and warm ocean heat content, the system will undergo explosive development as it heads towards the Yucatan Channel. Not many other models develop an area in the Western Caribbean so it remains to be seen whether development occurs or not. In addition, shear remains pretty high across the area so the degree of shear that lifts out will determine how much development occurs, if any. However, the model is being reasonable and consistent so folks in Jamaica, the Caymans, Central America, Cuba and the southern Gulf of Mexico should check up on the tropics this week.


Rick becomes a category 4 hurricane

By: Weather456, 12:36 PM GMT on October 17, 2009

Under an upper anticyclone, humid air mass, and extremely warm SSTs of 30C; Hurricane Rick rapidly intensified during the overnight hours and is now a solid category 4 hurricane of winds of 135 mph and further intensification is forecast, thus Rick could become a category 5 hurricane over the next 48 hrs. Rick is currently moving off towards the west-northwest under the influence of a deep layer ridge whose axis goes from Central Mexico into the Central CONUS. This motion should continue over the next 72 hrs until a deep layer trough digs across the Pacific Coast and help pull Rick more towards the north, and finally recurve into the California Baja Coast. Much of the intensity guidance is showing additional strengthening of Rick over the next day or two with a gradual decrease in strength thereafter due to progressively cooler waters and increasing vertical shear. The models are very divergent with the intensity of Rick as it nears the Baja coast, ranging from a category 3 to 1 hurricane. There is 70-80% chance the Baja coast will be affected by the hurricane but I am going with the lower percentile of maybe a weak category 2 or category 1 hurricane since SST forecasts are extremely reliable out 5 days.

Figure 1. GOES-12 infrared/visible image of Hurricane Rick this morning just offshore Mainland Mexico. Image credit: NASA.

Typhoon Lupit

On the other side of the Pacific, Typhoon Lupit continues to steadily approach the northern Philippines, the storm is now equivalent to a category 2 hurricane and is forecast to strengthen to a category 4 hurricane before striking the already battered islands of the Northern Philippines.

The Atlantic Tropics

The MJO has arrived in the Atlantic and with it an increase of convective activity especially along the Eastern Atlantic ITCZ. Ironically, upper level winds have become increasingly hostile in the Tropical Atlantic with a band of high upper westerlies extending from the Caribbean to Africa. The shear forecasts agree that this is not expected to let up anytime soon. Despite this, several global models are developing a weak feature in the Eastern Atlantic, likely from the current flare up near 25W. I will continue to monitor for signs of changes.

I will also be monitoring the Western Caribbean, which is a more climatologically favourable area than discussed above. The ECMWF continues to forecast the development of a depression over the SW Caribbean late next week that moves towards the north. Such a feature would recurve under the influence of advancing frontal systems.


About Weather456

By: Weather456, 9:56 AM GMT on October 16, 2009

Today I will give you a short biography of my young life and the processes of my analysis. My actual name is Cavin Rawlins and I was born in the capital city of Basseterre, Saint Kitts in July of 1988. I was brought up by my mother mostly with three other siblings. I attended primary school at an early age of 4, which is the equivalent of elementary school in the United States. I spent seven years here and this is the point of my life which I began to get interested in not only weather but also natural sciences (biology, geology, astronomy, meteorology and hydrology). I attended the Basseterre High School at the age of 11 where I narrowed down my interest to weather while doing Caribbean Geography. Upon graduating high school at age 16, I quickly decided to go college though I had no funds. I took a loan and signed up for a Bachelors Degree program in environmental sciences but with a major in tropical meteorology at the University of the West Indies (UWI). It took me 5 years since I had to get my Associates first. I finally graduated in June 2009.

Upon graduation, I now work with the Met Office here in Saint Kitts in local weather forecasting, after several internships over past summer breaks. My hobbies including maintaining an online blog dedicated mostly to tropical storm tracking, surfing and limin’ (slang for hanging out with friends). My dreams are to further my studies to obtain a Doctorate later in life; to visit the United States Hurricane Center and get married ;).

I currently live on the outer edges of the capital where it overlooks Basseterre and the Caribbean Sea and my work office is just a mile west near the airport. While I do much of the work at the office, I still have time to maintain this blog and this is where I get into the process of my analysis.

I have over 500 links that I go to each morning when updating my blog. Sometimes too much information can be confusing so I actually group them into categories and sub-categories as in figure 1. I use Opera Web Browser since it is a useful blend of Internet Explorer and Firefox. In addition, Opera browser is equipped with tools useful for making quick and accurate analysis. Each morning I open predetermine default web pages such as the National Hurricane Center, FSU cyclogenesis fields, etc, and then branch out from there. I have two display monitors and a very powerful computer, which are also very useful. The main idea behind my analysis is to observe, scrutinize and reason to come up with the best and most accurate forecast and I don’t want to brag but it has work pretty well during this and last hurricane seasons. I began tracking hurricanes in 1998 via the Weather Channel, and DIFAX images obtain from the Antigua Meteorological Office, I shifted to the computer and Internet in 2002.

In addition to my existing friends and I have made many friends here on WeatherUnderground because face it, in a country of 43, 000, it is hard to find persons that share the interests you have, so the online community is a medium of sharing those interests.

So that’s it, the story of a young dude living in the tropics that likes tracking hurricanes.

Figure 1. Screen-shot image of my desktop showing Opera Web Browser bookmark menu. In the background, notice several websites within square preview boxes which illustrate a speed-dial feature that can bring up a website with one-click. Also notice the other program open in the taskbar - Corel Paint Shop Pro, the program I use to draw my images.

Hurricane Omar one year later

The center of hurricane Omar pass just to the northeast of Saint Kitts during the early morning hours of October 16 2008 as a powerful category 4 bringing high winds, heavy rains and most notable of all effects, rough surge. Omar’s unusual easterly trajectory came almost 10 years after another devastating category 4 hurricane – Lenny, that moved in a similar direction and struck the very same region. Because a hurricane’s strongest winds are on the right side of motion, these type of storms have the capability of delivering a heavy blow on the Caribbean facing sides of the Leeward Islands, the side that is normally sheltered in storms coming from the Atlantic, like Hurricane Georges. On the island of Saint Kitts, we experience a surge that inundated the coastline, washing away several homes; however, the exact height of the surge is unknown. On the island of Nevis, the Four Seasons Hotel which was closed for several years after Lenny, was finally closed for good after Omar damaged the hotel and surrounding infrastructure along the coastline. Other effects of the hurricane included power outages mainly in the countryside, fallen trees and debris and minor flooding especially along the Marriott Golf course.

Figure 2. Omar wind analysis at 0430 UTC 16 October 2008 as it passed to the northeast of Saint Kitts. Omar was upgraded to a category 4 hurricane in post season analysis which I suspected since the winds experience in Saint Kitts could only be the result of an expansion of the wind field during passage. Image credit - NOAA AOML Research Division.

Omar was the first hurricane since Hurricane Lenny in 1999 to visit the islands and ironically, it took the same path. This is a blessing since we escape the destructive hurricane seasons of 2004 and 2005 at the expense of others. This decade has also been quiet considering the four major hurricanes that affected the nation in the 1990s. The previous decade saw Hurricane Hugo, which was basically one of the few in the 1980s so if history is creating a pattern, the next decade 2010s, is going to be an active one for the Leeward Islands.

Figure 3. The Basseterre Bay Road near the Ferry/Bus Terminal. This boat was moored about 15ft over water until Omar washed it ashore. Notice the black sand which is volcanic in origin. White sand is found further to the south on the Southeast Peninsula.

Figure 4. Waves battering the island around 6am 16 October or about 3 hrs after the peak of Omar.

Tropical Storm Rick

Tropical Storm Rick, the 17th named storm of the Eastern Pacific Basin continues to rapidly intensify just south of Southeastern Mexico and is expected to become a hurricane later today and major hurricane in 3 days. The storm is expected to move west-northwest to northwest away from the Mexican mainland but may begin to turn more towards the Baja Peninsula at the end of the forecast cyclone in about 5-7 days. This is the third storm to threaten the region and Rick places the eastern Pacific Basin about 2 storms above average, typical of El Nino years.

Typhoon Lupit

On the other side of the globe, Typhoon Lupit continues to steadily intensify and is now equivalent to a category 1 hurricane. Like Rick, this storm is expected to become a category 4 hurricane by the time the weekend is finish and if that was not enough, threaten the Northern Philippines, the region hardest hit by typhoons Ketsana and Parma.

Atlantic Tropics

The Atlantic tropics remain quiet this morning and I do not expect anything today. Some of the global models are developing a weak feature in the Eastern Atlantic ITCZ so I guess no one told them that Cape Verde Season was over.

I will have another update on Saturday.


California system; Patricia nears Baja; Volcano spews ash; Quiet in the Atlantic

By: Weather456, 10:21 AM GMT on October 13, 2009

California Storm System

A storm system over the Northern Pacific Ocean continues to deepen and is now boasting 977 millibars, which is deeper than the original forecast for this morning, and it could get deeper as it makes a U-turn over the Gulf of Alaska. The trailing frontal system that extends south of the low-pressure center is now approaching the California coast bringing showers, gusty winds and heavy surf. Long-range radar out of Eureka, in northern California has already revealed pre-frontal showers streaming onshore but rainfall totals so far have been minute. Wet weather will become more widespread later through the day and rainfall totals are expected to reach 2-4 inches, possibly 5-7 inches in the Sierra Nevada where upper level forcing combined with orographic lifting will be taking place. Some flooding is possible. Winds in excess of 30 knots will also make their way onshore with increasing wind speeds with height that could reach 70 mph or more, with widespread urban flooding and moderate wind damage–including downed trees and power lines– a possibility in Northern California.

Figure 1. Forecasted rainfall totals (inches) for the California region for the next 48 hrs.

Yesterday afternoon I commented on why the system was able to produce such a deep layer of moisture in the atmosphere similar to tropical system and it lies within the jet streak aloft. A jet streak is a region within a jet, in this case, the subtropical jet stream, with high velocity, consisting of an entrance region (convergence) and exit region (divergence). Divergence associated with jet streaks are capable of forcing air to lift which consequently, cools, condense and precipitate. Another reason is that there is humid tropical air invlove ahead of the system where southerly flow exists.

Figure 2. 00Z GFS +24 hrs 200 mb heights and winds for the Northern Pacific and Western coast of the United States illustrating the jet streak's entrance and exit region. The entrance region is where convergence occurs because air slows down as it enters the main area of the jet streak. On the other hand, divergence occurs in the exit region as air speeds up and spreads out as it leaves the main region of the jet streak. In the tropics we know that upper divergence is good for convective development. Now match this up with this morning's infrared imagery and you realize that the convergence region has little cloud activity, mostly characterized by cold air stratocumulus, while the divergence region has deep cloud cover.

Tropical Storm Patricia

Tropical Storm Patricia as of 5am EDT was located near 20.7N-109.3W or about 155 miles south of the southern tip of the Baja California peninsula. Tropical storm watches remain in effect, as Patricia nears the region but high pressure is expected to build north of the storm and turn Patricia back towards the west just before making it onshore. The exact timing of both features will determine of Patricia makes it ashore but regardless, she is expected to bring tropical storm conditions to Baja along the current forecast track.

Figure 3. Patricia's wind field for the next 21 hrs as it nears the coast of Baja showing tropical storm force winds reaching onshore regardless of landfall.

Smoke Plume from Montserrat

Soufriere Hills volcano on the island of Montserrat has been showing signs of recent high activity as the volcano has been spewing ash on several occasions between October 5-12. Scientists on the island have blamed the recent activity on the result of a new lava dome growing near the volcano's summit and there is some concern but the activity is not escalating at the moment. Any ash from the dome is currently being blown into the Caribbean Sea by the prevailing winds away from neighbouring islands as my own – Saint Kitts, which is a good thing. Ash is a substance that could cause respiratory problems and not to mention, “dirtying up the place.”

Figure 4. GOES-12 visible image of the Northern Leeward Islands on Monday 12 October showing the smoke plume coming from Montserrat which was obscured from view by patch of clouds between the two islands.

Atlantic Tropics

The Atlantic tropics remain quiet this morning but there is a region southwest of the Azores islands that is expected to meander in the vicinity and makes it to a non-frontal, cold core system, which is prime candidate for subtropical development. You can see the system on shortwave infrared imagery or visible imagery near 30N, east of 40W.


Significant fall system to impact West Coast

By: Weather456, 9:13 AM GMT on October 12, 2009

Figure 1. Infrared image of the storm this morning over the Northern Pacific Ocean.

A significant fall system is expected to impact the West Coast of the United States later this week bringing heavy rains, high winds, and rough surf. The system is currently located over the Northern Pacific as 988 mb gale system and is expected to deepen to a 970 mb storm over the next day or two. The storm southern portion of this storm system is expected to tap on some tropical moisture, creating a long fetch of moisture into California and Oregon, producing up to 5 inches of rain, with locally heavier amounts especially inland along higher elevations. The storm is also expected to drive 30-40 knot winds, which will kick up swells of 7-15 ft. Numerous high wind advisories have already been issued for counties along central California is anticipation of the gale force winds that precede these systems. Widespread urban flooding and moderate wind damage–including downed trees and power lines–is a possibility in Northern California and persons in the affected area could begin to see the effects as early as Monday night PDT.

This system is expected to turn northward and back over the Gulf of Alaska by Thursday/Friday but another trailing frontal system extending all the way to Hawaii will create a moisture fetch extending to the Washington/British Colombia area. The biggest issue here will be the rain and these locations could pick up 5-7 inches late this workweek into the weekend.

The first system could be explained as the remnants of Typhoon Melor but the second system only reinforces the change in seasons and possibly the beginning of an El Nino winter.

Figure 2. The Ocean Prediction Center 48hr forecast of the Central/Eastern North Pacific showing the storm system near the coast and the second trailing system north of Hawaii.

The Atlantic tropics remain quiet this morning and the system formerly known as Henri is not showing any signs of regeneration. Thus, I will have additional updates on the West Coast system later this week.


Understanding the Atlantic Hurricane Season: Tropical Jet Streams; Thermocline and El Nino

By: Weather456, 11:22 AM GMT on October 11, 2009

Tropical Jet Streams

Jet streams are fast flowing, narrow air currents found in the atmosphere normally created and maintained by pressure and temperature gradients. When most people think of jet streams, they often relate to the polar jet stream, which is the largest jet stream on earth, affecting the planet’s weather on synoptic and sub-global scales. However, the tropics are home to few jet streams both at the lower levels and aloft.

The Subtropical Jet Stream

These are climatological features of the tropical general circulation and have their greatest amplitude at 200-300 mb near 20-35N over the Atlantic and Pacific Oceans in the northern hemisphere winter, spring and early summer. This jet maybe formed by a southern branch in the polar jet stream separated/split either by the tropical tropospheric trough (TUTT) or the subtropical ridge. The jet may exceed 200 knots when it is fully matured in January.

African Easterly Jet

This is a summertime feature of the African continent monsoon system and plays a role in the development of African Easterly Waves. The jet develops between the temperature/humidity gradient between the hot and dry Saharan/Sahel region and the cool wet sub-Saharan coast/Gulf of Guinea. The jet is strongest near 600 mb so it is considered a mid-level jet and is normally found near 10N-20N from 30E to 40W. Instability in the jet causes the formation of tropical waves.

The Caribbean Low Level Jet

This jet has not been widely recognised in literature until the 2000s partly due to the launch of QuikSCAT. The jet is embedded within the easterly trades south of the subtropical high-pressure systems but is enhanced and channelled through the sea with the help of the Colombian Low. The jet plays a role in the weather of Central America, Jamaica, the Caymans and the remainder of the Western Caribbean region. In addition, the trade winds carry moisture and heat from the tropical North Atlantic into the Caribbean Sea where the flow intensifies forming the Caribbean Low-Level Jet (CLLJ) and creates the Atlantic warm pool. The CLLJ affects tropical storm formation by reducing the ability of systems to form west winds south of the low-pressure system (hence no closed low-level circulation). A strengthening of the CLLJ also increases the difference between it and the upper westerlies (wind shear). A weakening of this jet signals an increase potential for the development of tropical systems in the basin. The jet is stronger in El Nino years and weaker in neutral and La Nina years.

Figure 1. A systematic overview of the three main tropical jet streams over the Atlantic Ocean. There are other tropical jet streams of the Indian and Pacific Ocean but this entry focused on those in the Atlantic.

Understanding the Atlantic Hurricane Season: The SST Factor
Understanding the Atlantic Hurricane Season: Technical Terms
Tracking Tropical Cyclones Part 1: Center Fix
Tracking Tropical Waves
Understanding the Atlantic Hurricane Season: Vertical Wind Shear
Understanding the Atlantic Hurricane Season: The Saharan Air Layer

The Thermocline in El Nino

The thermocline is a thin but distinct layer in the ocean and/or atmosphere, in which temperature changes more rapidly with depth than it does in the layers above or below. The thermocline in the equatorial Pacific Ocean is normally found between the 15-20C isotherm; is a main indicator of the current ENSO mode and can be use to forecast the transition of the oscillation.

Normally, the thermocline is slanted downward from east to west with deeper +20C waters residing over the Western Pacific Warm Pool and shallower waters +20 over the Eastern Pacific where the cold Peru Current and upwelling exists. We normally see this pattern during neutral years.

Figure 2. Dept-longitude section of equatorial temperatures in degrees C. The thermocline is near the 20C isotherm as that is where the greatest change in temperatures occur. This is the thermocline during September 2008 when the ENSO was neutral. Notice that the thermocline is naturally deep in the Western Pacific and shallow in the Eastern Pacific.

During the transition from neutral to warm episodes (El Nino), easterly trades which transfer heat energy into the Western Pacific weakens and equatorial westerlies begin to develop, causing a reversal in the transfer of heat energy. A warm tongue eventually shifts east to the Central and Eastern Pacific and thermocline gets deeper in the east and shallower in the west, thus during the mature stage of El Nino, the entire thermocline is leveled horizontally. This process can take several months thus the transition to El Nino is more gradual.

Figure 3. Dept-longitude section of equatorial temperatures and anomalies in degrees C for the El Nino episode of October 2002. Notice that the thermocline is leveled horizontally throughout the Western Pacific Basin.

As the El Nino matures and the thermocline is the same dept everywhere, and it does not take much for easterlies to re-develop and upwelling to re-establish over the Eastern Pacific. The transfer of heat returns to normal and because the Eastern Pacific thermocline is normally shallow, it returns to normally very quickly. The thermocline deepens in the west and becomes shallow in the east and the transition from El Nino to neutral to La Nina begins. This process takes a shorter period and thus transition from El Nino is more rapid.

Figure 4. Dept-longitude section of equatorial temperatures and anomalies in degrees C for the La Nina episode of August 2007. Notice the thermocline is oriented similar to neutral years but the Eastern Pacific waters are shallower than normal and Western Pacific waters are deep than normal. In neutral years (not shown), the entire basin is near normally dept.

This is part 3 of a series of El Nino-orientated blogs. The previous entries discussed the Southern Oscillation Index and how the Earth’s Angular Momentum (Earth’s Rotation) and ENSO are linked. My next entry is the global tropical cyclone effects of El Nino.


Henri weakens to a depression; little change in 2nd AOI

By: Weather456, 10:43 AM GMT on October 08, 2009

Tropical Depression Henri is located near 19.9N-61.3W or about 165 miles northeast of the Northern Leeward Islands moving off towards the west-northwest near 15 mph with sustain winds near 35 mph and a central pressure of 1009 mb. Satellite imagery revealed the expose low-level center continued to decouple overnight as Henri was impacted by 25 knots of southwesterly shear. This morning satellite imagery continues to show signs that the system is very disorganize and hanging onto life support. There continues to be signs of another center within the heaviest thunderstorm activity further east and south but as long as this secondary center remains open, the NHC will downgrade this feature to a tropical wave. Models including SHIPS continue to forecast an improving upper environment beyond 2-3 days and the statistical intensity guidance has reflected this by showing Henri regaining tropical storm intensity. That being said, the feature will be monitored for any signs of development beyond then or regeneration if it does dissipate.

A trough swinging north of Henri continues to cause a weakness in the subtropical ridge, which is helping to pull Henri west-northwest and north of the Islands. This morning radar imagery showed the inflow bands of convergence generating some showers over the islands and this is expected over the next day or two. Beyond 3 days, the trough lifts out and deep layer ridging takes over, steering what is left of Henri more west and possible, southwest, impacting the Greater Antillean Islands of Puerto Rico, Hispaniola and Cuba. The system will be monitored as it has the potential to bring heavy rains to these islands. There is a possibility that Henri will move out to sea, but that possibility is growing thin since the trough is moving to his north.

There is another area of disturbed weather to Henri’s southeast which is about to move into the Southern Antilles bringing squally weather. Conditions remain favorable for some slow development over the next few days but there are no significant signs of development currently.

Figure 1. Infrared imagery of Tropical Depression Henri and the other area of disturbed weather to its south-southeast which is being monitored for signs of development.


Tropical Storm Henri

By: Weather456, 10:05 AM GMT on October 07, 2009

Tropical Storm Henri is located near 18.1N-56.1W or 460 miles. East of the Northern Leeward Islands, moving off towards the west near 13 mph. Maximum sustain winds have increased to 50 mph with a central pressure of 1005 mb. Satellite imagery revealed a deep pulse of convection developed near the center overnight and this allowed Henri to intensified some. However, over the past few frames the thunderstorm blob appears to be shrinking but I’m not sure of this is a downward pulse or a continuing trend. The storm is moving through an environment of 20 knots of vertical shear which increases to 25-30 knots within 12 hrs, so some fluctuations in strength is expected today. This shear is expected to impact Henri for the next 24-48 hrs and the system may weaken to a depression by Friday. Beyond then, models have been consistently forecasting that the upper environment will improve and thus the system will be monitored, especially if it does move southwest around the ridge and away from any advancing troughs that might case heavy shear. Dissipation is always a possibility but I’m not 100% buying the NHC scenario.

Henri is being steered by mid-level flow which basically supports his northwestward motion yesterday then turn towards the west this morning. Since Henri is not expected to be a significantly deep system we can exclude the deep layer steering and continue to maintain the level he is in. The storm will eventually resume his west-northwest to northwest motion trough 2-3 days as a trough lifts down and passes to the north. This should carry Henri north of the Leewards but might still bring a few showers to the islands. If he is not pulled out to sea, which remains a limited chance for now, a significant deep layer ridge develops over the SW Atlantic and forces Henri back west, or even southwest through 5 days; and affect areas like Puerto Rico, Hispaniola or Cuba. I do agree if Henri ever makes it into the Caribbean as intact entity it could be a real concern.

There is a feature southeast of Henri associated with a tropical wave that has increase in shower activity this morning. This system will be monitored as it blows through the Southern Antilles later this week brining showers and thunderstorms.

Figure 1. Infrared imagery of Tropical Storm Henri and the other area of interest in relation to the Leeward Islands. This image is almost identical to Tropical Storm Erika, back in early September.


91L heads northwest; Grace gone

By: Weather456, 10:34 AM GMT on October 06, 2009

Tropical Invest 91L is located near 16.5N-51.2W or 811 miles east-southeast of the Northern Leeward Islands moving off towards west-northwest to northwest near 15 knots. Estimated surface winds are near 35mph with a pressure of 1007 mb. Shortwave infrared imagery revealed the mid-low level circulation of 91L and it is rather unclear if this is at the surface – but it’s at the lower levels. The discovery of this feature revealed a disorganize system with the heavier convection within a curve band on the eastern, arcing along the northern sides of the expose center of circulation. This is probably due to south southwesterly shear the storm has encountered since its movement northward. Shear might cause some problems for this system over the next 48 hrs, as the band of upper westerlies is not budging as quickly as previously expected. Beyond 3 days, the models are consistent with keeping the upper environment favourable for development so the disturbance will continued to be monitored for any signs of strengthening.

The disturbance has really taken a shift towards the northwest over the past 24 hrs as a deep layer trough sits just offshore the Eastern CONUS. This is allowing a weakness near 60W between two upper ridge centres over Mexico and just west of the Cape Verde Islands. I expect this motion to continue over the next 2-3 days with deviations between west-northwest and northwest. This should place the system near the Northern Leeward Islands by Thursday/Friday and according to how close the system gets, breezy squally weather could affect the extreme northern islands. Models are taking the system north of the islands and while I do agree it is a possibility, this forecast was not based on numerical models since they have not initialized a new center yet – rather a blend of looking at the previous track and the forecasted steering flow. Beyond 4-5 days, a ridge builds over the Southwest Atlantic, forcing 91L back west and possibly southwest according how strong it is – this is expected to be a deep layer ridge so nothing is going to get through it, rather around it. I have to admit, I am anxious to see this system play out with the upcoming ridge.

Figure 1. Shortwave infrared imagery of 91L with the CIMSS shear analysis overlaid.

Grace has been absorbed into a frontal boundary just southwest of Ireland. It might have some impact on Ireland but as a non-tropical entity.


Tropical Invest 91L and Grace are born

By: Weather456, 10:21 AM GMT on October 05, 2009

Tropical Invest 91L

A tropical wave over the Central Atlantic is located along 45W south of 15N, moving off towards the west near 20-25 mph. Shortwave infrared imagery indicate broad cyclonic turning can be centred along 45W and this is the location I ‘m going with for 91L. The invest is generating clusters of deep convection that appear somewhat organized but covering a large area. The system continues to lie under broad anticyclonic flow and moving over 29C waters thus some organization is possible today and what I would look for is continued consolidation of thunderstorm activity. There is a band of upper shear that continues to linger just to its northwest and north and may cause some problems in the near term, however, the upper environment is forecast to improve in the long-term by most reliable models, despite them not compensating for a change in strength. Only the statistical intensity guidance bring 91L to a strong tropical storm in about 5 days which is entirely possible given the shear forecast.

Currently the system is moving westward under the influence of the subtropical ridge. This motion is expected over the next 48 hrs until an upper trough swings down and induces a more northerly motion. This should take the system in the general direction of the Northeastern Caribbean. I do not believe that the trough will pick up 91L since it does not dig deep enough to entirely break down the ridge, just influence a pull. Through 3-4 days, the trough lifts out and 91L resumes a westward motion at which point it could be located anywhere between 15N and 22N. Some of the models are showing that the ridge will re-build strong enough to force 91L back southwest (the latest track guidance). However, I don’t think it will be as dramatic as illustrated here. The exact timing of both features will determine where the storm will end up but the current steering speed of 91L does not change much as it heads towards the islands. Folks in the Central and Northern Antilles, Puerto Rico and Hispaniola, should check up on the progress of 91L.

Figure 1. Infrared image of disturbance 91L, located about 1200 miles east of the Windward Islands.

Tropical Storm Grace

In an unusual turn of events last evening, the NHC thoughts on Invest 90L went from code yellow, to code red to Grace in little under two hours. Tropical Storm Grace was named last night and has now strengthened into a strong tropical storms, just 5 mph shy of being a hurricane. However, Grace is easier said than found, literally.

Tropical Storm Grace is located near 43.0N-18.0W or 585 miles northeast of the Azores islands in the far Northeast Atlantic, moving off towards the northeast near 28 mph. Maximum sustain winds are near 70 mph with a central pressure of 989 mb. This morning satellite imagery showed Grace remains a well-organize, very small and tight system – one of the smallest systems I ever tracked. Tropical storm forced winds extends outwards only 50 miles and thus is not readily seen on QuikSCAT imagery. The system is racing off towards the northeast under the influence of an advancing upper trough seen on both European water vapour imagery and the Ocean Prediction 500 mb charts, and this motion should continue until the system is absorbed within 24-36 hrs. The system is moving over sub-20C waters and shear is about 15-20 knots, so little strengthening is likely.

This system has already defied forecasters and is the most interesting and controversial system of the year. This is the same system that I commented on in my blog on Saturday and still firmly believe that it should have been name a subtropical storm earlier. My reasoning lies with the transition as cyclones don’t jump from being extratropical to tropical one time. They have to go through a transition or move along the spectrum from one phase to another. Extratropical cyclones transition to tropical cyclones through becoming an hybrid (subtropical storm), which is one reason I do not believe the occluded front that the NHC claimed in their forecast discussion of Grace was there for as long as they analyzed it for. I have no problem in them waiting to name a fully tropical system as Grace but if you going to name subtropical systems (as of 2002), then Grace should have been named earlier.

Figure 2. Infrared image of the far northeast Atlantic with Tropical Storm Grace in far right corner of the image and the Azores to the southwest.


Tropical wave to reach the Islands later this week; Melor becomes a category 5

By: Weather456, 12:11 PM GMT on October 04, 2009

A tropical approaching 40W, 37W, south of 15N to be exact, has increase significantly in shower activity over the past 12 hrs and should be monitored for development since it has the potential. This morning infrared imagery showed the thunderstorm coverage has doubled and visible imagery along with a few surface observations indicates the wave has amplified enough, that a surface circulation is forming near 9N. QuikSCAT supports this. The environment around this wave appears to be becoming a bit more favourable with the expansion of shower activity a sign of anticyclonic outflow (though an anticyclone has not form as yet) and less dry air – this is also supported by water vapour imagery. The steering for this wave is so certain; you can bet money its going west. The wave is currently being steered by a mid-upper ridge and this motion is expected through 3 days. By day 3, a trough swings down and starts pulling it more towards the north. At this point, the feature may already be located near 55W. The models don’t show the trough picking up this wave, and as the trough lifts out the wave resumes it westward motion. The exact timing of both features will favour a track through the Caribbean Sea or north of Puerto Rico. Vertical wind shear remains highly unfavourable north of this feature but most of the reliable models are showing shear will relax enough to allow some development along whichever path this wave decides to take. Though, most of them keep it weak, not really sure why since they reduce shear, so should compensate by showing some strengthening. Regardless of development, an increase in moisture is expected to reach the islands in about 3-4 days. I will have more updates.

Figure 1. Infrared imagery of the Central Atlantic showing the wave as the area with the large expanse of shower activity.

Typhoon Melor

Typhoon Melor intensified into the Western Pacific’s second category 5 in just one month and they are sure making use of the available energy not used earlier in the season. The system intensified into a super typhoon just after passing the Mariana Islands as a moderate typhoon. The storm dumped up to 2 ½ inches of rain across the islands and tropical storm forced winds. It could have been a lot worst if the typhoon passed 20-30 miles further south, so they were very lucky. No major injuries or significant structural damage were reported on Saipan, according to the Tribune; so some good news there.

Figure 2. Multi-spectral image of Typhoon Melor with infrared (UL), Dvorák-enhanced infrared (UR), 85 GHz microwave imagery (LL) and 39GHz microwave imagery (LR). This typhoon has solid core of convection around a very circular eye. Dvorák numbers reached 7.0 or 160 mph/898 mb. This is probably the most intense cyclone this year, globally.

Figure 3. MODIS TERRA capture this image earlier today around 1:40AM UTC (9:40PM EDT) at a resolution of 250m, compared to operational geostationary satellites (GMS, GOES, MSTAT) furthest resolution of about 1KM. Image credit: NASA


Another Subtropical Cyclone

By: Weather456, 1:08 PM GMT on October 03, 2009

Did the NHC dropped the ball on another subtropical cyclone?

Tropical Invest 90L was designated on 1 October after a well-organize non-tropical low began to seclude from the westerlies and gather some tropical characteristics. Initially, the disturbance was very well organize south of the Azores but remained tied to an occluded front (which is disputed). Early on the 2 October, the disturbance lost all organized convection and was looking like it would not of made it and thus was deactivated as an invest. Later that day, the storm made a significant comeback, with increase convection near the center, which resulted in a warm-core system seen both on numerical modeling (cyclone phase diagrams) and on satellite observations (AMSU). Probably, some of the hardest evidence was the QuikSCAT pass from yesterday, which showed the strongest winds right at the low-level closed circulation (LLCC) typical of tropical systems. The system remained embedded within a broad upper level trough and over SSTs of 22C (1 degree below the threshold), so if it was classified, it would of been a subtropical entity. Dvorak subtropical numbers just before the system was deactivated was around ST 3.5 but was discontinued afterwards. I theorize that based on the satellite presentations during the late part of 2 October, that the disturbance would of probably have a ST number near 2.5-3.0 based on the shear pattern analysed at one point.

The Tropical Prediction Center continued to analyze this feature along an occluded front despite satellite signs that indicate the front was decaying or already decayed. I really see no reason why this feature should not have been classified at the time. This is the same area where Tropical Invest 92L developed on the first day of the hurricane season and as many of my readers know, I fully debated this Invest should also been a subtropical storm.

Currently, Tropical Invest ex90L is spinning just north-west of the western Azores islands with satellite observations, justifying a sheared sub-tropical depression.

Figure 1. UW-CIMSS Advanced Microwave Sounding Unit (AMSU) vertical temperature anomaly profile of Tropical Invest 90L taken aboard the NOAA-18 around 1442 UTC yesterday. The red line denotes to the center of 90L and notice the warm-core both at the surface and upper levels of 90L's center.

Figure 2. Infrared enhanced satellite image of Tropical Invest 90L around 8PM EDT last night, showing the organize deep convection around the center of circulation.

Meanwhile, the area of interest near Cuba yesterday, probably has become part of a frontal boundary but development was not anticipated.

Showers have increased along a tropical wave nearing 50W but upper winds continue to impact the feature and thus development is not expected today. However, the feature will be watched over the next 3 days as upper winds may relax some.

Typhoons Parma and Melor

Typhoon Parma slammed into the Northern Philippines Islands today, bringing heavy rains and high winds to the already battered region. The winds whipped the coastline and felled power lines in northernmost Cagayan Province. Debris littered the roads, making evacuations even more difficult.

Parma veered off to the north, avoiding a direct hit on heavily populated Manila. But the real menace in the Philippines capital was not wind. It was water, and there was no escape from it with Parma.

The storm could dump as much as 8 to 20 inches of rain in areas still water-logged from last week's Typhoon Ketsana. That storm resulted in the heaviest rainfall in 40 years and at one point, 80 percent of Manila was submerged.

Figure 3. AQUA-1 rain measurements and MSTAT visible image of Typhoon Parma as it made landfall along the Northern Philippines Saturday afternoon local time.

Meanwhile, Typhoon Melor passed to the north of Guam but long-range radar out of Guam was able to capture the eye of Melor as it passed, and while Guam was spared, the Mariana Islands were not so lucky.

Figure 4. Long-range radar out of Guam showing the passage of Melor, just north of the island.


Tropical Update

By: Weather456, 10:26 AM GMT on October 02, 2009

Mid-upper cyclonic flow (trough axis) covers the area from 85W-70W helping to generate scattered showers and thunderstorms across the area as it interacts with surface stationary front. Surface analysis indicate that a surface trough continues to extent southward from the stationary front but little signs of organization due to unfavourable upper winds. Little development expected over the next 24 hrs but the area will be watched as it continues eastward.

Meanwhile, a tropical wave approaching 40W continues has increased in shower activity this morning but water vapour imagery clearly shows a heavily sheared system with much of the convection east of the axis. Vertical wind shear is expected to remain unfavourable for the next 3 days but could life/ease through 5 days, at which point the wave and its neighbour (55W) will be located in the Caribbean, the areas to watch in October.

Figure 1. Water vapor depiction of the two features of interest.

Tropical Invest 90L was designated early yesterday just southwest of the Azores Islands. The system has since then moved towards the northeast, across the western-most islands is now meandering just north thereof. The system has become significantly less organize since yesterday, especially after looking like a subtropical storm on NASA MODIS images. The system has a subtropical T-number (ST) of 3.5, which would place it near 50 mph and moving over waters just on the borderline of the subtropical threshold of 23C. Upper levels remain near 5-10 knots, typical of these systems as they are embedded within the center of an upper low (shear resides on the periphery of upper lows). However, the system remains cold-core in the upper levels and tied to an occluded front, two features that go hand-in-hand. If the system manages to shed its front and gather more convection, it stands a chance, but the current track takes it east-northeast then northeast over the next 4 days, over increasingly cooler waters, at which point it maybe absorbed into a larger system. It will be monitored.

Figure 2. Terra 1km True Color image for 1/10/09.

I will have an update on these features and the Western Pacific typhoons Saturday morning.

If you missed, I issued my outlook for the remainder of hurricane season yesterday.


Outlook for the remainder of hurricane season

By: Weather456, 9:30 AM GMT on October 01, 2009

September Summary

Table 1. Tropical activity recorded for the month of September 2009.

Activity for the month of September, the peak of the hurricane season, was deplorable and well below the long-term and 1995-2008 averages. September normally sees three tropical cyclones per year but only two storms were named during the month and one tropical depression. In my September outlook back on 31 August, I expected a 70% chance of 4-5 named storms based on the activity for August and an upward pulse in the MJO. This forecast did not verified due to the unexpected change in the Madden-Julian Oscillation (MJO) forecast during the first week of September, but shear and other aspects of the forecast verified.

Figure 1. Tropical activity recorded for the month of September 2009. Notice that all of the disturbances, storms and hurricanes originated in the Eastern Atlantic as this was the only area where low shear resided.

Vertical wind shear for the month of September was above average across most parts except for the far Eastern tropical Atlantic and the subtropical regions of the North Atlantic. This is the region where Fred intensified into a category 3 hurricane and where all the disturbances originated, which was expected. What was also expected was little activity in the Caribbean Sea, which typically shuts down during El Nino years. The one and only storm to enter the Caribbean met its demise due shear from the Tropical Upper Tropospheric trough (TUTT). The jet stream was also positioned well south of the forecasted position and along with Erika; Fred, TD 8 and the two other invests met their demise through shear.

Another factor, which played into the low activity of the month, was dry air and troughs in the westerlies. Much of the dry air was enhanced by the downward motion of the MJO and these troughs help pulled storms northward towards the band of heavy shear. It is safe to say that these conditions all worked together to make this month the most inactive September since I began observing hurricanes in 1998.

Figure 2. 31-Day average Precipitable Water, inches (top) and vertical wind shear m/s (bottom). Notice that much of the Atlantic was covered within a dry air mass and because precipitable water mainly measures the water content from 500 mb to the surface, it really assesses the potential tropical systems had last month. In the shear diagram, notice that low shear resided over the Eastern Atlantic, where the systems originated but a band of shear near 20N, where most of the systems met their demise. Also, notice where Erika met her faith.

October and November Outlook

This is the period when the favourable conditions that build up towards September begin to decay and tropical cyclone formation tends to gradually decline; faster in some years than other, especially in El Nino years. On average 1-2 tropical cyclones develops in October while November has an average of about one storm per year. At this point in the hurricane season progression we are due for three additional tropical cyclones before the season ends usually in the Caribbean, but because this is an El Nino year and progression thus far have been well below average, I am forecasting a 70% chance of no more than 2 named storms for the remainder of the hurricane season. There have not been any recorded storms beyond mid-October for any classic El Nino year since 1994.

Figure 3. Tracks of all storms occurring in October (top) and November (bottom) between 1886 and 2006. Notice that October is illustrated here as a rather active month likely due to the second peak with a very sharp drop off in November.

Vertical Wind Shear

Models have differed on the vertical wind shear forecast for next 15-30 days over the climatological development areas of the Caribbean. The Climate Forecast System (CFS), shows above average shear for the region while the Global Forecasting System (GFS) shows low shear values through mid-October. I will have to go with the GFS due its excellent handle on long-term shear and the fact that lower shear is product of upward MJO pulses. This lower shear may allow some development over the extremely warm waters of the Western Caribbean. Beyond October 31, an increase in westerlies associated with cool season troughiness will likely shut down development basin wide. However, I would not discount a subtropical storm where the jet splits.

Figure 4. GFS vertical wind shear for the Tropical Atlantic, valid through 13 October 2009.

Sea Surface Temperatures

Sea surface temperatures have remain near average overall across the basin but extremely high ocean heat content continues to reside in the areas that are prime for development in October and November. This should make development quick and easy for any storm if upper winds allow. Heat content does not usually cool down until mid-December and are capable of producing Wilma-Omar-Paloma- and Lenny-type storms this late in the hurricane season.

Figure 5. Ocean heat content for the Tropical Atlantic through 15 September 2009. Notice that the main development areas of October and November have extremely high heat content and thus can support major storms under the right upper level winds. This is comparable to 2005 at this time.

Madden Julian Oscillation (MJO)

Forecasts for the MJO have recently become inconsistent, with the Empirical Wave Propagation (EWP) and the Global Forecast System (GFS) predicting a modest upward pulse during the next 2 weeks as oppose to the strong upward pulse previously forecasted. The GloSea model is predicting a 60-80% chance of continued below normal precipitation/above-average pressure for the Caribbean basin and a 60-80% chance of above normal rains/below-normal pressure for the Gulf Region. This could be the result of increase frontal activity for the Gulf region but it does not look like the MJO, despite the forecasted upward pulse will have much impact on the Caribbean in the months ahead.

Figure 6. 200-hPa velocity potential forecast by the Empirical Wave Propagation, which assumes the MJO is a perfect oscillation. The MJO is not a perfect oscillation but because the CFS and GFS show the same pattern, this model was used to smooth out regions of enhanced convection (green) and areas of suppressed convection (brown). The MJO upward pulse is forecasted to be over the Atlantic through the latter part of October. Such as short period cannot have much effect on cyclogenesis for the month. The MJO returns downward through early November.

El Nino

Tropical Pacific Ocean temperatures remain warmer than average in all of the key El Niño monitoring regions and continue to exceed thresholds considered typical of an El Niño event. These conditions are forecasted to persist until at least year's end by most leading climate models. El Nino years tend to end during the latter part of October and do not usually extend much into November due to an increase in upper westerlies over the Atlantic.

Figure 7. Sea surface temperature anomaly for the four equatorial NINO regions (1+2, 3, 3.4 and 4). Notice the warm anomalies well established across all four regions indicating the presence of a moderate El Nino.

Steering Flow

El Nino conditions coincide with increase frontal activity earlier than usual and that is basically the pattern we have been seeing all season. In addition, regardless of the NINO node, the seasonal transition to the cool season brings increase troughiness. Now this does not totally protect the United States and/or Eastern Caribbean from storms forming in the Caribbean Basin itself and examples include Wilma, Paloma, Omar and Lenny.


At this point in the hurricane season, we are due for three additional cyclones but due to the slow progression of this hurricane season and El Nino, the chances of such numbers are low at this point. A combination of low vertical wind shear, a modest up-surge in the MJO and very warm sea surface temperatures may combine to allow the genesis of 2 additional cyclones but history dictates that this season may not linger much pass mid-October. Areas across the Western, Northern and Eastern Caribbean including South Florida should still be alert as these are the areas vulnerable to late seasonal activity.

Recent Pacific/Indian disasters

Floods, super typhoons, earthquake and tsunamis have taken the lives of over 800 persons during the past week between the Indian and Pacific Oceans. All they need now is a volcanic eruption (God forbid) and this would be freak occurrence of natural disasters. I do encourage readers of my blog to donate to any aid effort for these guys half-way across the globe and our hearts continue to go out to the persons who lost everything, including loved ones.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Weather456's Tropical Weather Blog

About Weather456

With a Bachelors Degree in Environmental Sciences (2009), began tracking tropical storms in 2002 and is now a private forecaster.