About Jeff Masters
Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 3:25 PM GMT on August 27, 2012
Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.
Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.
Isaac's rains
Isaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least nineteen, and two died in the Dominican Republic.
Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.
Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.
Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.
Intensity forecast for Isaac
Isaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.
Figure 4. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.
Storm surge forecast for Isaac
Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.
The affect of storm size and angle of approach on storm surge
A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.
Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.
Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.
New Orleans flood defense info
Army Corps of Engineers map of the new flood defenses
Army Corps of Engineers video showing the flood defenses
New York Times article, Vast Defenses Now Shielding New Orleans
Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.
Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Wednesday morning.
Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.
Jeff Masters
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather
1759. sporteguy03
3:42 AM GMT on August 29, 20121758. HimacaneBrees
11:08 AM GMT on August 28, 20121757. 2ifbyC
1:46 AM GMT on August 28, 2012You, sir, are my hero!
1756. BahaHurican
12:07 AM GMT on August 28, 20121755. WetBankGuy
11:52 PM GMT on August 27, 2012A number of people have expressed surprise or dismay at the number of New Orleanians who are not leaving. There are several reasons for this, but primarily it is tied to former Mayor's Ray Nagin's poorly executed phased return policy.
Under this policy after Gustav (hardly a disaster for New Orleans) people were denied the ability to return and secure their homes from any damage on the theory that we couldn't possibly get by without full restoration of Cox Digital and a chance for the beer to get cold at Rouse's Supermarket.
We know how to get buy without power and with a boil water order. We're not going to grab a downed power line. We're not children, and the older of us have experience of this back to Betsy. Even the lead editorialist of the Times-Picayune (writing in a personal column) announced he would not leave for a mandatory evacuation order unless he thought it product and the phased return regime was eliminated.
It didn't help that FEMA went on national television promising to cover people's hotel bills, etc. during the whole phased return thing and then reneged on that promise.
We all watched anxiously during Gustav as the lake battered at the east side seawall of the Industrial Canal as a heroic few men secured a loose barge. Everything held.
We're not all throwing hurricane parties. We all learned that lesson from the people on the coast who died during Camille, thank you very much. People are hunkering down, well supplied (if they can afford it at the end of the month) in houses secured as best they can. We are following the directives of our local officials to shelter in place.
We are the survivors. We are the people who came home. We are not easily giving up what we have reclaimed.
The Tragedy of St. Bernard
-- Mark Folse
ToulouseStreet.net
1754. TreasureCoastFl
11:20 PM GMT on August 27, 20121753. leftlink
11:00 PM GMT on August 27, 2012I have been saying it will take 975mb to get this to a cat 1 most likely...
1752. WetBankGuy
10:56 PM GMT on August 27, 2012Look at that beautiful dry air. I see it closing up on water vapor but keep it comin'.
1751. WetBankGuy
10:52 PM GMT on August 27, 2012FB poll: all NOLA peeps are staying. Updates from the nearest station as long as the UPS keeps the phone up, which should get me through the storm. Houses are secure as they can get.
Cloud cover approaching 100%. F3 or F4 out, barometer 29.7 and falling.
Gave my sterno stove away when I thought I was leaving but everybody is invited over for cold canned mac & cheese and warm beer at the Fortress of Squalitude. Hope Patrap is getting a really nice tan somewhere.
1750. Dakster
10:49 PM GMT on August 27, 2012-- New Orleans authorities expect Isaac to hit the city as a Category 1 hurricane early Wednesday, with tropical storm-force winds arriving shortly after midnight Monday, Mayor Mitch Landrieu said.
-- Landrieu did not issue an evacuation order for the city but urged people outside the city's protective levees to leave and told others, "If your plan is to go, now's the time to go."
1749. Dakster
10:48 PM GMT on August 27, 2012Did they finally release one? CNN said it wasn't MANDATORY.
1748. Dakster
10:47 PM GMT on August 27, 2012No mandatory evacuations that I have heard about either. Last statement I heard was that 900 people living outside the levees protection zone were asked to leave -- and if anyone else was thinking of leaving 'Now would be a good time.'
Again, look for official news releases from NOLA public officials and do not rely on this blog when making life or death decisions.
FWIW, I would be one of those people deciding to leave... and I do not live in NOLA and have never even been there. But seeing what happened with that 'K' storm I wouldn't chance it... (I am getting better at not mentioning the full name)
1747. Dakster
10:43 PM GMT on August 27, 2012I haven't heard about any major damages in Florida that would require that...
FWIW, FEMA usually reimburses local governments for expenditures on natural disasters when declared. And this was declared ahead of time by Governor Rick Scott...
1746. WatchingThisOne
10:35 PM GMT on August 27, 20121745. WatchingThisOne
10:34 PM GMT on August 27, 2012The dry air thing has been going on for some time now. Isaac has been fighting it since he was an invest just west of Africa. I don't recall him ever being in an environment where dry air was not an issue. So that doesn't seem like a variable.
1744. Roxanne
10:06 PM GMT on August 27, 2012He sent FEMA days ago to Fl. I saw it on the local news here in Orlando
1743. watchdog40
9:49 PM GMT on August 27, 2012What about Florida I wonder?
1742. redwagon
9:47 PM GMT on August 27, 2012My take is Isaac is going to cash out and head back to the FL panhandle.
1740. Carnoustie
9:29 PM GMT on August 27, 2012she had the lowest pressure as a TS after being a hurricane.
1738. Elena85Vet
9:26 PM GMT on August 27, 2012Exactly
Isaac has made some progress, but that dry air could just as easily set him a step back.
1737. oracle28
9:24 PM GMT on August 27, 2012Funny, I thought the opposite was true, for example... although Katrina hit as a 3, it had the "built-up" water of a 5, since it had been a 5 previously.
1736. catastropheadjuster
9:21 PM GMT on August 27, 2012Taco r u for real. what the TWC has people here already or something?
sheri
1735. Adamcu81
9:18 PM GMT on August 27, 20121734. hydrus
9:17 PM GMT on August 27, 20121732. ScottLincoln
9:16 PM GMT on August 27, 20121730. HarryMc
9:15 PM GMT on August 27, 2012It has taken all day but the core has almost closed. Looking a lot more like a near-hurricane now.
1728. acl8610
9:15 PM GMT on August 27, 2012Yes that's Daytona International Speedway... I was just there, it was crazy. I have never seen anything like that before, being from NE Ohio originally.
1727. WCSCTVCharleston
9:15 PM GMT on August 27, 2012This is jus.t one of those storms I think it will surprise in the end
1726. GTcooliebai
9:15 PM GMT on August 27, 20121725. hahaguy
9:15 PM GMT on August 27, 20121724. taco2me61
9:14 PM GMT on August 27, 2012Come on folks at least get the city Right :o)
Taco :o)
1723. NCHurricane2009
9:14 PM GMT on August 27, 2012Nah....what about Fay 2008 when she was near Lake Okechobee? Here tropical storm eye was a bit meaner than this IMO
1722. WunderAlertBot (Admin)
9:14 PM GMT on August 27, 20121721. Category5HitsFl
9:13 PM GMT on August 27, 2012I agree, especially since this storm could intensify into a mid level Cat 2 right as it nears the coast, which can catch people off guard.
Katrina created its most destruction with storm surge, and storm surge and flooding rains always tend to be the most devastating elements of tropical cyclones.
Due to the large nature of this storm and its general slow movement it has the capacity to be much more destructive in those elements than the actual strength of this storm would indicate.
1720. NCHurricane2009
9:13 PM GMT on August 27, 2012Yeah...I am suprised though that the SS scale has central pressure as a reference value...I'd have just tossed those out if I had contributed to it...because its the wind (not the pressure value) that causes the tangible damage the SS scale is based on.
I guess the pressure values on the SS scale represent the statistical 90th percentile of storms in that category...
1718. weatherh98
9:13 PM GMT on August 27, 2012The ants are lunatics
1717. Doppler22
9:13 PM GMT on August 27, 20121716. TreasureCoastFl
9:13 PM GMT on August 27, 20121715. jascott1967
9:12 PM GMT on August 27, 2012Storms have been slow to develope this season and although the gulf waters are warm there are no deep water eddys for it to pass at current. I see a low to moderate CAT 1 at landfall. The NHC has pretty much had this storm pegged from the beginning and that's what they've been calling for. See no reason to deviate.
1714. KEEPEROFTHEGATE (Mod)
9:12 PM GMT on August 27, 20121713. stoormfury
9:12 PM GMT on August 27, 20121712. STXHurricanes2012
9:12 PM GMT on August 27, 2012Unlikely...just increase in swells at the coast and dry air resulting in temps rising to 100 or more the next few days as the dry n/nw flow from isaac filters in.
1711. stormchaser19
9:12 PM GMT on August 27, 2012People of NOLA are lucky people that dry air is present in upper levels
1709. Bluestorm5
9:12 PM GMT on August 27, 2012BREAKING NOW: President Obama directs @FEMA to give LA, MS & AL governors the resources they need