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 tropicfreak:


The shear from Isaac is keeping it at bay.

But I suspect that we will see TS Joyce tomorrow at some point.
Amazing considering this is supposed to be an El Nino year and the GFS is still spitting out storms off of Africa.
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1400. pottery
Quoting HuracanTaino:
Here on the west of PR, we are getting son rain an its windy 25 mph and some higher gust so far..I wonder what this thing would do overnight...?

On the WEST side of PR ? North winds ???
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Isaac is bi-bloblular. (bi-blobular?)
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I am definitely having trouble performing my normal activities with Isaac out there.

If the heaviest area of convection becomes its official LLC, then it has some elbow room due to the moisture out in front.
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Quoting opal92nwf:Hey! your name applies for me too! (:




(:
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Had to lurk before getting some shut eye..someone asked in earlier post how large issac was..Cantore mentioned something about 230 miles from center..being lopsided prolly about 500 miles from end to end..have a good night all..thanks for the info all...
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1395. HrDelta
Quoting StormJunkie:


It is getting a little out of hand tonight huh?


So here is a challenge for all the experts out there...Prove that this is not an open wave at the moment, and has not been much of the afternoon.


I think it's Dolly, all over again.

Remember, Dolly at one point over the Gulf of Mexico was similar. I think it was around 50mph at that time too.
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Quoting hydrus:
Yes..If this storm continued west unimpeded by the mountainous terrain into that super heated water, then took the turn north into the gulf it would be almost a mathematical certainty that a cat-4, and possibly a cat-5 would present itself...and I am not doom-casting by any means.

I totally agree. If Isaac stays weaker and moves further west he could shoot the gap between Yucatan and Cuba which would also mean he's pass over the nuclear fuel and R.I and hold that strength possibly all the way to landfall since the OHC in the GOM is very high. This is just a scenario of what could happen, not a doom-cast.
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Quoting NativeSun:
Up the middle of Florida as a cat 2or 3. first hit Isalmodada in the Florida Keys any takers?
I'll see your Florida Keys and raise you a Tampa Bubble.
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Quoting tropicfreak:


The shear from Isaac is keeping it at bay.

But I suspect that we will see TS Joyce tomorrow at some point.
I have seen that td 10 and td 2 are the two numbers of depression that almost always fail to become a name storm.
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Quoting tropicfreak:


Under Dmin too.

Once it can co
nsolidate look for this to take off!
Looks like I'd consolidating to me.







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1390. pottery
Quoting Grothar:


Flow following finite-volume Icosahedral Model

Ah!
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The NOGAPS and CMC (the farthest outliers to the right) have even shifted west towards Florida. My guess right now is that this thing can strike anywhere from the east coast of Florida to Lousiana. Everything depends on if Isaac can get organized enough in the next couple of days to feel the weakness. In the short term, my prayers are with the people of Haiti and DR.
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Quoting pottery:

Cant access the local radar.
Hearing some thunder to the south.
Hope it does not get too heavy.
Seems like a lot more to come though, looking at the loops.

DMAX and all......
Here on the west of PR, we are getting son rain an its windy 25 mph and some higher gust so far..I wonder what this thing would do overnight...?
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1387. HadesGodWyvern (Mod)
Japan Meteorological Agency
Tropical Cyclone Advisory #41
TYPHOON TEMBIN (T1214)
9:00 AM JST August 23 2012
====================================

SUBJECT: Category Three Typhoon South Southwest Of Ishigaki Jima

At 0:00 AM UTC, Typhoon Tembin (955 hPa) located at 22.5N 123.8E has 10 minute sustained winds of 75 knots with gusts of 105 knots. The cyclone is reported as almost stationary.

Dvorak Intensity: T4.5

Storm Force Winds
================
55 NM from the center

Gale Force Winds
=================
120 NM from the center

Forecast and Intensity
========================

24 HRS: 22.7N 121.7E - 75 knots (CAT 3/Strong Typhoon) Yaeyama Islands
48 HRS: 22.3N 119.4E - 50 knots (CAT 2/Severe Tropical Storm) Taiwan Strait
72 HRS: 21.9N 119.0E - 45 knots (CAT 1/Tropical Storm) Taiwan Strait
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1386. LargoFl
Quoting NativeSun:
Up the middle of Florida as a cat 2or 3. first hit Isalmodada in the Florida Keys any takers?
some models do that
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Yeah, Pottery i think it has something to do with when you kick the bucket. As far as this storm is concerned I'm about to put two pennies in the bucket and chuck it.
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Quoting scooster67:
Those 2 blobs are coming together to form 1 central low. D-max is going to be interesting.


The PERFECT storm.
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Cruising 20mph westbound....not even technically closed off.....major changes coming to forecast track. I expect starting now and next 24 hrs
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1382. LargoFl
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Up the middle of Florida as a cat 2or 3. first hit Isalmodada in the Florida Keys any takers?
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Quoting hydrus:
I mentioned earlier with a great deal of caution that its possible Haiti may not get the center, but it is still to soon to say.
Haiti will get the bad part of the storm, per nhc track. But the Haitians are in trouble during small tropical storms. So, I feel for them. Giant tent city.
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Quoting Tazmanian:
no TS yes from TD 10


AL, 10, 2012082300, , BEST, 0, 135N, 389W, 30, 1007, TD,



grrrrr


The shear from Isaac is keeping it at bay.

But I suspect that we will see TS Joyce tomorrow at some point.
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1377. LargoFl
Quoting HoustonTxGal:
LMAO..i want that sign
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Quoting RitaEvac:
This is much farther south of Puerto Rico than was anticipated seems like


Hard to even see PR under Isaac. Big storm.
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Quoting hurricanehunter27:
These cloud tops are crazy. Coldest of the year in ATL.

Those 2 blobs are coming together to form 1 central low. D-max is going to be interesting.
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Quoting Wunderwood:
The parochial prognostications on this forum are starting to rival Storm2K.


It is getting a little out of hand tonight huh?


So here is a challenge for all the experts out there...Prove that this is not an open wave at the moment, and has not been much of the afternoon.
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Quoting HoustonTxGal:


Lmao!! Like that one!
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12z EURO ensemble mean. All over LA and jumps west at the end.





In a lot more agreement than yesterday.
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Quoting Chiggy:
Look how far south the 500mb vorticity is from the LLC



Meanwhile look at the various vorticity levels for tropical depression 10. A very healthy and stacked system at its early formative stages.

On the competing LLC's in Isaac...go to the infrared loop and turn on the "MSLP" field at the top. The isobars clearly show that the areas of lowest pressure are on the NE side of the circulation, thus if one vorticity in the center of the storm is going to become dominant it is most likely to be the one to the north.
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1370. hydrus
Quoting RitaEvac:
This is much farther south of Puerto Rico than was anticipated seems like


I mentioned earlier with a great deal of caution that its possible Haiti may not get the center, but it is still to soon to say.
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1369. pottery
Quoting DDR:

We'll probably get the usual,a few gusty winds,couple inches of rain,if you ask me.I'm using the Barbados radar,our radar has been offline for over a year.

Yeah, I am watching that too.
Still rumbling down south.... you must be seeing the flashes.
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Quoting RitaEvac:
This is much farther south of Puerto Rico than was anticipated seems like




But the last flare-up makes it look like relocating N....

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1367. Grothar
Quoting LakeWorthFinn:

Hei {{Gro}}, what is FIM8 short for?


Flow following finite-volume Icosahedral Model
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boriquens.your.next
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1365. Chiggy
Quoting Tazmanian:



that was not the recon finding the 1000mb or be low that was the noaa plane flying at 10,00ft

Wish people read the data correctly and understand them before posting on here - nothing worse than misinterpreting recon data
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1364. tkdaime
They said on nbc that Isaac was the size of Texas and not fully together yet
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Quoting Chiggy:


I am saying passes just south of Jamaica... Anybody agree on this!?


Most definitely
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Quoting Chiggy:


I am saying passes just south of Jamaica... Anybody agree on this!?
If I lived in New Orleans I think I would pay more attention now..
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Quoting TropicalAnalystwx13:
Recon found many, many readings below 1000 millibars...why did the NHC go with 1004 millibars?


Because it wasn't at the surface for one. Secondly, the center was poorly organized when recon left. It hasn't organized that much in just an hour and half or so.
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1359. klew136
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

I think the Keys are going to get smacked either way it goes east or west, oh I can only say please let Cuba save us.

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Quoting 1900hurricane:
Looks like Taiwan might be in for a double-dose of fun...



I'm not surprised, there is a rather strong cold front moving south towards the China coast. Can be clearly seen on Vis and WV.

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Quoting MississippiWx:
Definitely some intense convection:



Under Dmin too.

Once it can consolidate look for this to take off!
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Quoting MAweatherboy1:
The 0z dynamic model suite was very similar to 18z...
18z:



Basically unchanged, very slight poleward shift in the longer range, but the consistency that we're seeing amongst the dynamical suite is rather remarkable.
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Quoting TropicalAnalystwx13:
Recon found many, many readings below 1000 millibars...why did the NHC go with 1004 millibars?



that was not the recon finding the 1000mb or be low that was the noaa plane flying at 10,00ft
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Quoting MississippiWx:
Definitely some intense convection:

i think the center is now just south of the intense convection near 15.4N. where do you see it?
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right next recon is same time as we had this ealy morning so plane lifts off at about 1am I think
Member Since: June 13, 2009 Posts: 2 Comments: 12716
1351. hydrus
Quoting AussieStorm:
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.

Yes..If this storm continued west unimpeded by the mountainous terrain into that super heated water, then took the turn north into the gulf, it would be almost a mathematical certainty that a cat-4, and possibly a cat-5 would present itself...and I am not doom-casting by any means.
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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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