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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4001. JasonRE
Quoting AussieStorm:

Euro is here.



Thank you AussieStorm, I appreciate the link.
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I did a tropical update again this morning....

My Isaac track is defintely west of NHC and model consensus. I think he'll do something more like Dennis '05 in terms of track (but be weaker than Dennis in terms of intensity)....

I also include in-depth stuff on TD 10 (and the rest of the Atlantic tropics as usual)....
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Quoting WolfStorm16:
Right now TD 10 looks better than Isaac in my opinion. It is 10 times more symmetrical and it has a much tighter circulation.



Per ATCF, we have TS Joyce now.
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Quoting MahFL:
Seems to be moving a positive WNW now.


MahFL
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Quoting stormpetrol:
Time: 13:28:30Z
Coordinates: 15.6667N 65.0W
Acft. Static Air Press: 697.1 mb (~ 20.59 inHg)
Acft. Geopotential Hgt: 3,151 meters (~ 10,338 feet)
Extrap. Sfc. Press: 1002.1 mb (~ 29.59 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 114° at 16 knots (From the ESE at ~ 18.4 mph)
Air Temp: 11.2°C (~ 52.2°F)
Dew Pt: 10.2°C (~ 50.4°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 61 knots (~ 70.1 mph)
SFMR Rain Rate: 21 mm/hr (~ 0.83 in/hr)
(*) Denotes suspect data

Doubt this accurate!

It's probably rain-contaminated.
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Quoting stormpetrol:
Time: 13:28:30Z
Coordinates: 15.6667N 65.0W
Acft. Static Air Press: 697.1 mb (~ 20.59 inHg)
Acft. Geopotential Hgt: 3,151 meters (~ 10,338 feet)
Extrap. Sfc. Press: 1002.1 mb (~ 29.59 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 114° at 16 knots (From the ESE at ~ 18.4 mph)
Air Temp: 11.2°C (~ 52.2°F)
Dew Pt: 10.2°C (~ 50.4°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 61 knots (~ 70.1 mph)
SFMR Rain Rate: 21 mm/hr (~ 0.83 in/hr)
(*) Denotes suspect data

Doubt this accurate!

Yeah... that looks VERY off, and very suspect.
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3994. Chiggy
Quoting luigi18:

i noticed

NOPE. Look at the PR radar link I posted...
Link
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3993. Drakoen
Quoting emcf30:
The problem currently see with the GFS track at the moment, no matter how well it had performed with Debby, is putting the Vort max, or center approaching the Southern Coast of Puerto Rico. If you look at the current relocation of what could or could not be the center way to the South, this seems highly unlikely.



It is possible they are picking up on another relocation of the center in 12 hours or so or a moment to the North ( which is unlikely ), which would support this.

If the center stays to the South for the next 6 to 12 hours, it would throw the entire GFS run off. It could maybe re-correct it's self if it were to intensify quickly to feel the effect of any possible weakness sooner.



You mean its placing the 850mb center south of the PR coast. They are not necessarily coupled.
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Quoting Thing342:
Say hello to TS Joyce.

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,


Ten named storms before September. If you'd said this before the season started, you would have been branded a troll or a wishcaster.

Whilst I do think activity will shut down mid-october, I can't see us not getting to at least 15 named storms in total, which is a very active season.
Member Since: August 10, 2010 Posts: 2 Comments: 1971
3991. MahFL
Seems to be moving a positive WNW now.
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Quoting Grothar:


True. Even if the center of circulation moves to the West coast, the right side of the storm is usually the "dirty side" Southeast Florida could see the worst weather with this. Also, if it slows down, which is also expected, all of South Florida, the Keys and perhaps even the Bahamas could see TS force for many hours.


Hi Grother,
Just peeking in. How exciting , eh? Too bad no matter what the storm or its outcome, it's always a big letdown. Hurricanes are like drugs: A big thrill until they're gone, then reality sets in and one realizes the excitment was nowhere near worth the trouble (especially if you get hit and lose things you care about - such as people, your home, power, water...)
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Time: 13:28:30Z
Coordinates: 15.6667N 65.0W
Acft. Static Air Press: 697.1 mb (~ 20.59 inHg)
Acft. Geopotential Hgt: 3,151 meters (~ 10,338 feet)
Extrap. Sfc. Press: 1002.1 mb (~ 29.59 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 114° at 16 knots (From the ESE at ~ 18.4 mph)
Air Temp: 11.2°C (~ 52.2°F)
Dew Pt: 10.2°C (~ 50.4°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 61 knots (~ 70.1 mph)
SFMR Rain Rate: 21 mm/hr (~ 0.83 in/hr)
(*) Denotes suspect data

Doubt this accurate!
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Right now TD 10 looks better than Isaac in my opinion. It is 10 times more symmetrical and it has a much tighter circulation.
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Taiwan is going to get absolutely crushed.
That storm looks like it's strengthening as it approaches landfall, and the amounts of rain that are going to get wrung out on the mountainous terrain of the island are going to be phenomenal. :-/
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Quoting wunderkidcayman:

yep even now I think it already off of offic track and models




I don't know how in the word you guys see any Nward movement if anything it had Sward movement


Floridians Get antsy.
Everyone does.

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Quoting AussieStorm:

Well, if you can't tell that is not Isaac then you don't know your geography and you need to go to the Optometrists.


Geography is not an FCAT testable subject. They don't teach it in Florida schools any more.
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Quoting RescueAFR:
Good morning everyone from Fort Lauderdale.. Thanks for the great energy in providing your thoughts / weather knowledge on Issac.. Granted mom nature has a plan.. It will be interesting to see what lays ahead and how close you all figure that out..

Be well, G'day..
Steve
USAF Retired
Thanks for serving our country
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Quoting Thing342:
Say hello to TS Joyce.

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,


10-3-0... Impressive.
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Quoting Thing342:
Say hello to TS Joyce.

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,

Don't say Joyce until Advisory 5 is issued.
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Looks to me in the water vapor loop its edging more Southwest
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3979. luigi18
Quoting samiam1234:
is some one else seeing a northward jog in the last two frames.. is this thing finally making a turn towards the north west as predicted

i noticed
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Quoting JasonRE:


Where could I see the EURO model? Sorry, trying to keep up with everything.......thanks!

Euro is here.
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Isaac is still having trouble getting together on the forecasting note. The MLC and LLC are not vertically stacked but will probably be so soon. Isaac has really hit the brakes and has slowed down to 12-13 mph. The main reason that Isaac is so disorganized is because of the notorious trade winds in the eastern Caribbean, causing Isaac to accelerate. If Isaac continues to move at this reduced speed it will begin intensifying late today. Doubting a hurricane before it hits Hispaniola, which might also lead to less weakening once it enters the Bahamas. Not the best of situations for there to be a strengthening hurricane over the Bahamas with such warm SSTs.. Especially as Isaac is so large. The euro remains adamant that this could also get into the Gulf and strengthen. Has shifted to more inline with the NHC track..
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Quoting jeffs713:

Beyond that, just looking at the banding, the COC isn't where the crosshairs are...

Based on banding alone, I'd suggest the COC is somewhere SSW of the "center". (like 14.5/65.5)




Was looking At that two.

Hope some of the south floridians don't get upset at this
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Quoting SFLWeatherman:
From me


Could you please post a point of the approximate location of your house, so we can see if there is any bias?
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Good morning everyone from Fort Lauderdale.. Thanks for the great energy in providing your thoughts / weather knowledge on Issac.. Granted mom nature has a plan.. It will be interesting to see what lays ahead and how close you all figure that out..

Be well, G'day..
Steve
USAF Retired
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Quoting RitaEvac:
Bulk of storm nearly due south of Puerto Rico, deep in the Caribbean



Exactly, Rita. NHC's forecast track is initializing from the still disjointed, and mostly exposed llc to the neast of the main mid-level circ that is doing all the heavy lifting and is the unofficial real center of the whole system rotating.

And, while fwd speed has slowed, its motion is still west, and I fully expect that trend to continue till further west in the Caribbean. It will miss Hispaniola outright, imho. Tracks will adjust left as the weaker, non-heightened, non-maturing storm is unable to really push up against the 1012/1016mb of high pressure still above it.

Until it gets vertical, until it gets a llc either generating moisture or the mlc fully works to the sfc, this will still be a weak system tracking further west. The main circ is tracking considerably southwest of the official forecast points.
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3972. yoboi
Quoting drs2008:
Im ,now , offically concerned about the North central gulf.



yep not looking good...
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Quoting WarEagle8:
OK. Great. :( This comes about 1 mile south of my house in Dunedin, FL.
I'm not too far away from you in Largo. I went to Dunedin High School.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
3969. emcf30
The problem currently see with the GFS track at the moment, no matter how well it had performed with Debby, is putting the Vort max, or center approaching the Southern Coast of Puerto Rico. If you look at the current relocation of what could or could not be the center it is way to the South, as compared the latest GFS run. This seems highly unlikely.



It is possible they are picking up on another relocation of the center in 12 hours or so or a moment to the North ( which is unlikely ), which would support this.

If the center stays to the South for the next 6 to 12 hours, it would throw the entire GFS run off. It could maybe re-correct it's self if it were to intensify quickly to feel the effect of any possible weakness sooner.

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Say hello to TS Joyce.

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,
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Quoting hulakai:
I's coc seems to be consolidating at the moment near or below 15N and he is still moving West. If so, and this continues, models and offl. will have to shift West. Jamaica should put up its antennae and get ready to put down its shutters.

yep even now I think it already off of offic track and models


Quoting GTcooliebai:
Looks like it is getting back on track with the NHC forecast track.


I don't know how in the word you guys see any Nward movement if anything it had Sward movement
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is some one else seeing a northward jog in the last two frames.. is this thing finally making a turn towards the north west as predicted
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3964. luigi18
Quoting Relix:
Hmm... ok. 2 yes, 2 no. I'll just keep watching haha

it seem to me is climbing too! at 290
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Quoting RTSplayer:
What's wrong with this image?





Give up?



It has nested white inside of double nested gray, far to the south-west of the official center fix, including it's own automated center fix.

Nested "weak" color inside of double nested stronger color usually signifies a CoC...

Beyond that, just looking at the banding, the COC isn't where the crosshairs are...

Based on banding alone, I'd suggest the COC is somewhere SSW of the "center". (like 14.5/65.5)

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Quoting gprxomstr:


Please put a name with the system so readers aren't confused and think that is Isaac. Thanks!

Well, if you can't tell that is not Isaac then you don't know your geography and you need to go to the Optometrists.
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Quoting mikatnight:


Not to mention that the NHC issued it's 1st Hurricane Warning on Katrina (in Florida) only 18 hours before landfall...
I didn't take Katrina seriously (as it was expected to make landfall 2.5 hours north of where I lived, even though we were in the hurricane warning area. I figured it would be a weak hurricane at best and that with the center being so far north we would only have a few heavy squalls and a lot of rain. Boy was I wrong!! The darn thing ended up going right over my house and did quite a bit of damage to our property and the roof (not to mention all of the wind driven rain that came in through the window and door openings. I won't play that game again.
Member Since: August 22, 2011 Posts: 0 Comments: 32
Quoting SFLWeatherman:
From me
OK. Great. :( This comes about 1 mile south of my house in Dunedin, FL.
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3959. snotly
Now the GFS run did show Isaac going SW then NW.
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Quoting RTSplayer:
What's wrong with this image?





Give up?



It has nested white inside of double nested gray, far to the south-west of the official center fix, including it's own automated center fix.

Nested "weak" color inside of double nested stronger color usually signifies a CoC...


That's what I think too, only time will tell.
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Gonna assume Dr. M will post a new entry after the NHC 11am. Looking forward to his opinion.
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Quoting RTSplayer:
What's wrong with this image?





Give up?



It has nested white inside of double nested gray, far to the south-west of the official center fix, including it's own automated center fix.

Nested "weak" color inside of double nested stronger color usually signifies a CoC...
It's not vertically aligned and the center is still off to the NE off that convective mass.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
3955. Relix
Hmm... ok. 2 yes, 2 no. I'll just keep watching haha
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Quoting Grothar:


Well, Storm, You and Largo and a few others know, this is the exact tract we have had from the beginning. You bloggers know I NEVER seriously write that. I'll tease with lines like "I saw it first" and "I nailed It", but you all know I am joking. However, My original track forecast has not changed.

I said it would strengthen in the Easter Caribbean, move south of the DR and move over southern Haiti, then move into the Windward Passage over Easter Cuba and move towards Florida. I have not changed on this.

Now Doctor Masters will come along and no one will ever see this LOL. Just joking Doc.


Gro, what's your personal opinion on strength as it reaches FL?
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From me
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Isaac is pretty huge...

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3951. drs2008
Quoting JasonRE:


Where could I see the EURO model? Sorry, trying to keep up with everything.......thanks!
Im ,now , offically concerned about the North central gulf.
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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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