I live near Tomball, Texas (30 miles NW of Houston), and will write about whatever comes to mind. You've been warned.
By: jeffs713, 12:48 AM GMT on July 10, 2009
To start off, I want to be very clear in my intent for the second half of this post (that focuses on the Texas Coast) - I am not trying to predict anything... just educating, and telling people what to expect. Now that I have that out of the way, let's start by all getting on the same page, and learning some hurricane basics.
Major factors influencing storm formation:
Yes, I am getting just as tired of hearing about it as you are. Simply put, El Nino is an atypical warming trend in the eastern and Central Pacific Ocean. When El Nino is running full steam, it impacts the tropics by inhibiting storm formation in the way of a trend towards greater wind shear in the central Atlantic. This isn't to say that storms can't form... they will just have a harder time.
Most recent temperature anomaly map of the Pacific Ocean. Note the warm area (reds and pinks) across the central and eastern equatorial Pacific Ocean.
SAL (Saharan Air Layer):
SAL is basically the dust blown off the Saharan Desert, into the Atlantic Ocean. It inhibits storm formation by keeping the middle layer of the atmosphere drier and warmer than usual, holding down convection.
Current SAL map, the orange areas are areas of high dust concentration and dry air at the middle levels
SSTs (Sea Surface Temperatures):
Warm ocean water is the fuel that tropical cyclones live on. Cut off the warm water, the storm fades. Generally, waters at least 26C are needed to provide the fuel for storm formation. The warmer, the better (for the storm).
Current SST map, anything above 26C is sufficient for tropical storm formation
MJO (Madden-Julian Oscillation):
The MJO is basically a "pulse" that moves through the atmosphere from west to east that helps (or inhibits) convection. If you view the MJO like a wave, it has a crest, which is the "upward motion" trend that tends to help convection, and therefore storm formation. The MJO also has a trough, which is the "downward motion" trend that tends to inhibit convection. The upward pulse of the MJO isn't required for storm formation, but it sure helps.
Current forecast for the MJO. Orange areas are downward motion, green areas are upward motion.
Major factors influencing storm track:
The NAO (North Atlantic Oscillation):
The NAO is the basic position and strength of the two biggest players for the Atlantic hurricane season... The Azores-Bermuda High (A/B high) and the Icelandic Low. When the NAO is in a positive phase, the A/B high tends to be stronger, and located more east in the Atlantic, causing Cape Verde storms (storms that form near the Cape Verde Islands) to curve north before reaching a major landmass. When the NAO is in a negative phase, the A/B high is somewhat weaker, but also less concentrated, sprawling over most of the central and western Atlantic... forcing storms more westerly, before recurving north.
Upper Level steering:
This one is more important for storms "closer to home", in that where nearby high and low pressure systems are located will have a great influence upon where a storm ends up going. Remember that as a general rule, storms will avoid high pressure, and be drawn around low pressure. As an example, if there is a big dome of high pressure sitting over Florida, storms will generally either skirt to the south, or recurve north around the high, dependent upon where the storm first starts "feeling" the high pressure.
Current steering map for a strong TS/Weak hurricane. Note the high pressure center over the southern plains and the Atlantic Ocean.
Timing: This one is all about the timing of troughs that move from west to east across North America. If a hurricane is timed correctly, it can catch a ride on a trough and recurve north and east fairly quickly. If the timing is just a little off, it won't "feel" the trough, and will go westward (or WNW) on its merry way.
Major factors influencing storm strength:
TCHP (Tropical Cyclone Heat Potential):
TCHP is basically a measurement that combines the "amount" of heat in a given section of water. The measurement combines the SST, the depth of the 26C Isotherm (how deep the 26C+ temp water goes), and also how quickly the temperature drops off as you go deeper. As a general rule, the deeper and warmer the water, the more fuel a storm can have. Anything above a score of 80, and you start to set the stage for "Rapid Intensification", where a storm can jump several categories in as little as a day.
Wind shear is defined as the difference in wind speed and direction between the 200mb layer (approx 35,000 to 40,000 feet), and the 850mb layer (approx 5,000 feet). Generally, the 200mb layer will be moving faster, and if it is moving significantly faster than the 850mb layer (or in a different direction), it can literally tear a storm apart. Anything above 15-20kt of wind shear generally will inhibit a storm's strength.
Current wind shear map. Shear values are outlined in bold yellow, and the upper level air flow is in the light orange arrows.
Simply put, land can be a pain for tropical systems. Not only will land cut off the flow of fuel (warm ocean waters) for a storm, but if the land is mountainous enough, it can also disrupt the circulation. Many storms have been significantly weakened by land interaction, especially when traversing western Cuba, or Hispaniola.
Yep. Luck. We don't fully understand hurricanes yet, and they tend to be fickle. A storm can just as easily blow itself apart in the middle of the ocean, as it can jump from a weak tropical storm to a cat 3 hurricane in a day.
Now that we have the basics out of the way, lets get to the guesstimation part.
Overall, the NAO is trending towards a negative phase during the peak of the hurricane season, which will potentially push more storms towards the continental US and the Gulf of Mexico (GOM). Weather456 made an excellent blog posting (check it out here) regarding how the NAO impacts Cape Verde hurricane tracks, and also a way that the NAO phase can be predicted for the peak of hurricane season. For those of us in Texas, a negative NAO is BAD. 2008 had a negative NAO, and we got the gift of Hurricane Ike.
On the flip side, an El Nino event has officially been declared, which means that wind shear will tend to be higher across the central Atlantic Ocean, which would, at first look, hold back storm formation. That said, its only a tendency, and storms still can form (and intensify) in the face of an El Nino. Also... during El Nino seasons, more storms form "close to home", and provide less warning to people along the coast.
And in the middle, SSTs are trending more or less around average overall (combining the Atlantic, GOM, and Caribbean). There are some hot spots of course, but nothing glaringly unusual.
Factors to Watch for on the Texas Coast:
With the persistent high pressure system that has been causing drought all along the Texas coast, the GOM has been largely cloud-free, and combined with warmer temperatures caused by the high pressure... Gulf waters are mighty toasty. SSTs across most of the GOM have been running 1-2C above normal, with the western GOM heating up the most. What does this mean? Anything that gets into the GOM is likely to have jet fuel sitting below it. A storm can easily blow up to something very nasty very quickly with that kind of fuel potential.
Also, another big factor for the Texas coast will be the timing and positioning of troughs that cross the continent. If a storm doesn't catch a trough (like Dolly last year), they can maintain a very straight track. If a storm does catch a trough (like Ike last year), it can curve pretty sharply, impacting further north than expected.
And finally, with El Nino influencing things, and with how warm the GOM is, special attention needs to be paid towards storms that form "close to home". A storm that can pop up quickly (like Humberto in 2007), or ramp up quickly on a short track (like Alicia or Camille) are especially dangerous.
Here are some examples of close-forming storms that impacted Texas:
Hurricane Humberto, 2007
Hurricane Alicia, 1983
Hurricane Bret, 1999
What can I do?
PREPARE. If you haven't already gotten your hurricane supply kit together, why are you waiting? Tropical Storms and Hurricanes can strike at any time during the hurricane season, it doesn't make sense to wait until one is looming to get prepared.
Here is a link to the National Hurricane Center's hurricane supply kit page.
What if it is a weak/slow season?
It doesn't matter if it is a very weak season, like 1992 or 1983, or a very strong one (like 2001 or 2005). It only takes one storm.
What can I do to help if a storm strikes?
Donate your time (or money) to Portlight Strategies. Portlight was created to help those most in need after natural disasters, and help them get back on their feet. I personally support them with both my time and money, and I encourage you to help out as much as you are able.
As always, if you have any questions, comments, or just want to say "hi", feel free to comment below!
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The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.