Isaac reorganizing as it blows through the Lesser Antilles

By: Dr. Jeff Masters , 8:34 PM GMT on August 22, 2012

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Tropical Storm Isaac continues to maintain top winds of just 45 mph as its center prepares to move through the Lesser Antilles islands late this afternoon and early this evening. The entire Lesser Antilles chain of islands is receiving heavy rains from Isaac, with Martinique picking up 1.46" of rain as of 2 pm EDT, and St. Lucia receiving 1.49". However, Isaac is not yet generating much in the way of tropical storm-force winds, and none of the islands had received winds in excess of 30 mph as of 4 pm EDT. During their storm penetration to obtain their 2 pm EDT center fix, an Air Force Reserve aircraft measured top surface winds of just 40 mph, and a central pressure of 1004 mb. Top winds at their 5000 foot flight altitude were 49 mph. Isaac is undergoing significant changes to its structure. The plane found the center had become a broad, elongated oval that extended 40 miles from NW to SE. The old center, fixed at 2 pm near 16.1°N, and closer to the dry air to Isaac's north, is being challenged for dominance by a new center that is attempting to form near a burst of heavy thunderstorms at 15.5°N. The Hurricane Hunters' latest fix at 3:50 pm EDT put the center near 15.9°N, a southwards shift of about 17 miles. The resulting battle between centers is giving Isaac a rather odd spiral rectangular shape, as seen on visible satellite loops. The Hurricane Hunters did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and Isaac will not be able to begin strengthening until it resolves the battle between it two competing centers, and casts out the dry air infiltrating it. This is going to take at least a day, since Isaac is a very large storm, and it takes more time to spin up a big chunk of the atmosphere. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister. There has been a modest increase in spiral banding since this morning.


Figure 1. Afternoon satellite image of Isaac, showing its odd spiral rectangle shape.

Latest model runs for Isaac
The latest set of 12Z (8 am EDT) model runs are very similar to the previous set of runs, which I discussed in detail in this morning's post. The models show a westward track to a point on the south coast of Hispaniola. Isaac's center shift to the south may require some modest adjustments to the south and west for the models. This would result in the storm spending a few hours less time over Hispaniola, and more time over or just south of Cuba. This would slightly decrease the risk to the Dominican Republic, the east coast of Florida, and the Bahamas, but increase the risk to the west coast of Florida. The ECMWF--our best performing model over the past two years--continues to be an outlier among the models. It predicts that Isaac will track just south of Cuba, cross the western tip of Cuba on Monday, then head north towards an eventual landfall in Louisiana. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity, and at this point, we don't know it its more likely that Isaac will go up the east coast of Florida, the west coast, or straight up the peninsula over land. At this point, I'd put the odds at:

40% chance of a track through the Gulf of Mexico, west of Florida
25% chance of a landfall in South Florida, and a track mostly over the Florida Peninsula
35% chance of a track along the east coast of Florida

Which model should you trust?
Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 2, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)


Figure 2. Skill of computer model forecasts of Atlantic named storms 2011, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence=a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; NOGAPS=Navy Operational Global Prediction System model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that lends together several of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) Image credit: National Hurricane Center 2011 verification report.

I'll have a new post in the morning.

Jeff Masters

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Quoting hunkerdown:

But as MH09 said, that should occur by Friday night, or at least know as it will be happening at that time...so I do agree with him. Once we see the shape he's in and what is the result from land interaction, we will also know the strength/position of the trough and condition of the ridge which will give a pretty solid track/forecast.
Oh man, look who it is. I haven't seen you around in way too long.
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What happens when the lower levels and the upper levels of a hurricane stop dating? They become decoupled...

I really wanted to say that, but now I have a serious question to make my comment relevant. Isaac has had strong convection all day now for the first time. Will d-max amp up storms as much as it has the past nights for Isaac, or will it play much of a difference now that its convection is more established?
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1299. Levi32
Quoting luigi18:

Levi what do you think is going to happen Expectation Vs Reality for us in PR and USVI ?


You will probably get some of the northern side of the storm but realistically it shouldn't be that bad for you guys. Certainly nothing like Irene's passage last year.
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1298. DDR
Quoting pottery:

Hi.
Looking at the Rainbow loop.
That area SW of us is concerning me.

Hi pottery
The radar is most disturbing,heavy rain is currently falling in south-south east and north-eastern areas.
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Some people at the library discussing TS/Hurricane Isaac.
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1296. LargoFl
Quoting Levi32:
18z GFS ensembles, basically unchanged:

..so florida it is
Member Since: August 6, 2011 Posts: 4 Comments: 42267
If you put the stove on, maybe you can get rid of some water vapor....


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Quoting Bluestorm5:
Well, one half is saying Isaac is getting strong FAST while the other half is saying it's still struggling... *facedesk*


Well... I'll go in the middle and say it is beginning to get its act together.

Now how's that? ;)
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Quoting Bluestorm5:
Also, it's starting to wrap up with the new, but strong convection. Making people to freak out that it's RI, which is not the case. Slowly strengthening though...


Gradual to Rapid intensification into a hurricane cannot be ruled out I've seen it dozens dozens of times (recently Gordon). Its been VERY
slow in intensification since its develop it becoming the time where this finally start it's intensification process.
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1291. quante
Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.


Cat 2 west coast Florida.
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Showing E Gulf now

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Soon Isaac will have plenty of energy to feed on.



and this is what's still to come. A pass just south of Haiti could be really bad for down the road with so much OHC.

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Ah...

1000mb

Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting stormchaser19:


Isaac will be a strong storm in the next 24, specially GFS is slowing down the storm systematically,so i will not be surprised if isaac start to become stronger soon


Ernesto was projected to slow too. It never did. I have difficulty fathoming Isaac becoming too strong before reaching Hispaniola. Maybe a minimal hurricane, but it will have to slow down first.
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1286. JLPR2
Quoting MiamiHurricanes09:
I mean 700mb isn't that far up in the atmosphere. I'd imagine we'd have to adjust it upwards a few millibars, but 1000-1002mb MSLP seems very plausible with the newer convective updrafts.


Maybe Isaac will open to a sub 1000mb 50knots Tropical wave.

XD
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Quoting luigi18:

Levi what do you think is going to happen Expectation Vs Reality for us in PR and USVI ?
Sorry about Guanica earlier!
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1284. LargoFl
Quoting Grothar:
Look what just came out. You think maybe Florida will be out of the cone? By the way, expect a model shift at 11:00.


..geez GRO..maybe the GDFL was right afterall..by the way..on that map..where IS florida? LOL..geez, never saw anything like this before
Member Since: August 6, 2011 Posts: 4 Comments: 42267
Quoting Chicklit:




You might not be so lucky in 6-8 hours.


Getting that kinda scary look.
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Quoting stormchaser19:


Isaac will be a strong storm in the next 24 hours, specially GFS is slowing down the storm systematically,so i will not be surprised if isaac start to become stronger soon
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1281. HrDelta
Quoting HoustonTxGal:
Do we have an estimate on the actual size (across) Isaac is? I have not seen that.


No, but looking at the entire thing, East-West, it is 1770 km. (Mid-way in size between Pluto and Charon). North-South is 960 km, about the size of Ceres.
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Quoting MississippiWx:


There is a reason that the recon never found anything lower than 1004mb. Recon was in there not long ago and there is no way it has strengthened by 5mb since that time.
I mean 700mb isn't that far up in the atmosphere. I'd imagine we'd have to adjust it upwards a few millibars, but 1000-1002mb MSLP seems very plausible with the newer convective updrafts.
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Okay kids, I had to step into a bar to get internet access (Long sstory into which I will not delve into here) and the house band has butchered the last three songs they've done so I am escaping this place before it gets any worse...

I'll be in for time to tie to keep tabs on Isaac...great seeing all of you!
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Quoting Levi32:


Movement faster than 20mph certainly isn't helping, and neither are the accelerating trade winds to the west. Some of Isaac's struggles are very similar to Ernesto's when he was in this position.


Isaac will be a strong storm in the next 24, specially GFS is slowing down the storm systematically,so i will not be surprised if isaac start to become stronger soon
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1276. pottery
Quoting Floodman:




Hang in there, brother...this one will become, or it won't.

I feel so very reassured at this comment. Thanks.

heheheheheh
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1275. quante
18 z models tightening the spread. Florida in the bullseye. Ugh.
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Ack - my hometown!!



Forecasters now calling for Tembin to restrengthen after curving SW off Taiwan. There's a tongue of warm water to the right of Bolaven's projected track.
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1273. Levi32
18z GFS ensembles, basically unchanged:

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Quoting wunderkidcayman:

South



well I did say it will come together



Stop giving me a headache...
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Quoting Grothar:
the FIM8


Hei {{Gro}}, what is FIM8 short for?
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1270. Levi32
The FIM illustrates the great problems that the greater Antilles present for the forecast track. Hispaniola has a habit of pulling storms poleward quickly into its mountains, then spitting them out westward in a jerky fashion. Depending on the situation, this can put storms more poleward or more southward from the path that they would have taken without Hispaniola being in the way. Here the FIM shows the scenario where Hispaniola hands the storm off to Cuba and it gets stuck trying to get away from the mountains, resulting in a track farther west.

Until we are past the mountains, they represent a large inherent uncertainty in tracks like this one.

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1269. Grothar
Look what just came out. You think maybe Florida will be out of the cone? By the way, expect a model shift at 11:00.


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Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.


D/E Florida and if it goes to the GOM F.
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1267. MahFL
That wv image has a spot that is off the chart !
Member Since: June 9, 2004 Posts: 0 Comments: 3850
Quoting AstroHurricane001:
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.



E
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Quoting KoritheMan:


No, because then you have to take into account land interaction, which could easily result in a center reformation, or at least erratic motion.

But as MH09 said, that should occur by Friday night, or at least know as it will be happening at that time...so I do agree with him. Once we see the shape he's in and what is the result from land interaction, we will also know the strength/position of the trough and condition of the ridge which will give a pretty solid track/forecast.
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1264. pottery
Quoting DDR:
Some feeder band activity starting to take shape over Trinidad.

Hi.
Looking at the Rainbow loop.
That area SW of us is concerning me.
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doesnt.take.long...boom
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Quoting louisianaboy444:


AHHH change of heart my dear?


Not yet.
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Update:

Isaac appears to be getting better organized and may be on the verge of beginning a slow intensification process.

The ULL to the NE of the system appears to be weakening and it has slowed it's SW motion. This is enabling Isaac to begin developing a NE quadrant for the first time. This ULL has been injecting dry air into Isaac slowing it's development. Concurrently the UUL to the west was Isaac appears to be weakening as well. All of this in addition to the system moving into waters of high heat content should allow some slow strengthening.

Isaac should maintain it's near due west movement over the next 48 hours with some slight decrease in forward speed over the next 48 hours. Forward motion is forecast now to be more west moving south of both Hispanola and Cuba slightly following the ECMWF solution and ending up in the central GOM which is a worst-case scenario for the potential of explosive development given the extremely high heat content of these waters.

Next update 7:00PM Thursday.
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1260. luigi18
Quoting Levi32:


Movement faster than 20mph certainly isn't helping, and neither are the accelerating trade winds to the west. Some of Isaac's struggles are very similar to Ernesto's when he was in this position.

Levi what do you think is going to happen Expectation Vs Reality for us in PR and USVI ?
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Quoting MississippiWx:


There is a reason that the recon never found anything lower than 1004mb. Recon was in there not long ago and there is no way it has strengthened by 5mb since that time.
Also, it's starting to wrap up with the new, but strong convection. Making people to freak out that it's RI, which is not the case. Slowly strengthening though...
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Quoting serialteg:


ok, i'll do it

Lol you got that from our site ;-), we still back the NHC, please don't get the wrong idea
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1257. pottery
Quoting serialteg:


ok, i'll do it


Even as far south as Trinidad, we had WSW winds (light) swinging to WNW at about 2:00pm today. Calm now.

There are reports of circulating winds all over the Island chain.
How come the Recon did not find any westerlies?
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1256. ackee
I think Ecmwf did show Isaac with two centre and bit weaker too guess we see
Member Since: July 15, 2008 Posts: 0 Comments: 1376
Quoting Levi32:


"Sprawling" actually helps. The larger it is, the larger of a depression it has in the 500mb height field, which can help it move north, but I can see where you're coming from. Another very interesting storm to track.


Hopefully neither of us are too far off in the end, lol.
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Quoting icmoore:


Sorry guys last personal note I know it's busy but I have to say thank you so much, blushing, very nice of you to say! :))
As far as you know who I agree but what kind of surgeon? :)


That's is indeed the questions...as for you blushing, that's one of the reasons you're as good as you are. No pretense, you do what seems good to you...
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Quoting AussieStorm:
...




This sort of says it all. Mess of a system. It looked better as a naked swirl.
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Quoting sunlinepr:
Are we Ready.... almost here...



wow what a blow up
Member Since: August 22, 2006 Posts: 2 Comments: 1988
OK, time for the most-hated quiz of all time:

How strong will Isaac be at peak on the SSHS?

A. TS
B. Cat 1
C. Cat 2
D. Cat 3
E. Cat 4
F. Cat 5

I'm going for C, cat 2 with 90 kt winds, 961 mb about 36.8 nm S of Clearwater, Florida.
Member Since: Posts: Comments:

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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