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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4051. hamla
Quoting CURIOUSWEATHERGRL:
PATRAP- where are ya?
What's your thoughts on the track??!
ppat started his vaca yesterday evening in cancun
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4050. yoboi
Quoting CURIOUSWEATHERGRL:
PATRAP- where are ya?
What's your thoughts on the track??!


i think he is on vacation in tampa at the rnc....he should be on later...
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4049. LargoFl
Quoting emguy:
Greetings everyone...Just popping in for a quick minute...This was a map I posted sometime just before 4AM this morning. Just sharing for those who have not seen it.

As of very early this morning, the red "cone" was my general thinking of what we are going to see based on what we have going on with Isaac. I feel more confident about this now the the morning models are showing continued support and ever slightly increasing consensus.

I look forward to the afternoon when we get to see the next run of the EURO. I suspect the EURO, for the fourth straight run will shift to the east a little farther again.

Have a good day. Happy posting.

..this is not looking good at all
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4048. GetReal
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4047. Drakoen
.3 degrees north and .2 degrees west. Poleward movement may be ensuing.
Member Since: October 28, 2006 Posts: 57 Comments: 30557
4046. Chiggy
Quoting Relix:
Oh, radar showing WNW movement. Center of 1001mb at 15.7 at 65W. Extrapolated this is a NW movement, but it was probably a relocation or another random center.

PR radar shows WEST...
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Wind Fields for Isaac
Last Updated On 8/23/2012 12:00:00 PM GMT
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4043. Relix
Quoting Chiggy:

Center suspect, peak Flt level winds 40.2mph which translated to 34mph..., If you go by that then it needs to be downgraded just on the mph

Huh? Center don't need to have the stronger winds, usually the stronger winds are a few miles to the NE or north of the center or anywhere for that matter.
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Quoting Seflhurricane:
which recon mission is currently in Isaac 5,6,7 i would suspect 7
5 and 6
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4041. MahFL
Quoting Chiggy:

NOPE. PR radar link..
Link


The part on the bottom right is the center and its going WNW.
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Quoting Chiggy:

Center suspect, peak Flt level winds 40.2mph which translated to 34mph..., If you go by that then it needs to be downgraded just on the mph

3
Quoting AussieStorm:

Sorry but the NHC still have it as TD10, they are the only authority that names Storms.


Didn't you agree on the fact that the ATCF is owned by the NHC?
It is a TS, per 12UTC ATCF. The name is supposed to be confirmed at 15UTC.
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Quoting AussieStorm:

And that's why society is breaking down. It should be taught no matter where in the world you live.
People should be surprised how useful Geography is... I won Geography Bee in Middle School and the winning question was... which state is north of Colorado? Other kid said Utah, while I answered Wyoming... It was the easiest thing I've ever won -__-
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Me either

Quoting pcola57:


It is what it is but Don't like where that goes.. :(
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4037. Relix
Oh, radar showing WNW movement. Center of 1001mb at 15.7 at 65W. Extrapolated this is a NW movement, but it was probably a relocation or another random center.
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which recon mission is currently in Isaac 5,6,7 i would suspect 7
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Quoting KEEPEROFTHEGATE:
whats your problem lack of brain cells

They are in his left toe.
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Quoting Bobbyweather:
1001.3 mb
(~ 29.57 inHg)

61 knots
(~ 70.1 mph)
21 mm/hr
(~ 0.83 in/hr)
51.4 knots (~ 59.1 mph)
Tropical Storm

Hmm...



Still 1000 mb steering level. Same steering as before
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4033. GetReal
Check out this loop of Isaac, and zoom in if necessary, and decide for yourself if Isaac is still moving west. (270)

Link
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Quoting Bobbyweather:


TS Ten...
Not Yet Joyce.

Sorry but the NHC still have it as TD10, they are the only authority that names Storms.

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4031. KEEPEROFTHEGATE (Mod)
Quoting gprxomstr:

NOLA HERE WE COME!!!!!!!!!!
whats your problem lack of brain cells
Member Since: July 15, 2006 Posts: 173 Comments: 54268
look on sattlite Isaac looks like it is a whole degree maybe even degree and half S of the forecasted position

Quoting snotly:
Now the GFS run did show Isaac going SW then NW.


hmm maybe GFS is sooo stuck of it previous forecast path and it is not taking Isaac's W and S movements too well so that why GFS shows that

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


nope sorry dude don't see it it looks like it may have even taken a Southward jog
Member Since: June 13, 2009 Posts: 2 Comments: 12138
4029. Chiggy
Quoting Drakoen:
Pressure in Issac appears to be steadily decrasing now down to 1001mb. minimum foundnear 15.7N

Center suspect, peak Flt level winds 40.2mph which translated to 34mph..., If you go by that then it needs to be downgraded just on the mph
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4028. LargoFl
Quoting SFLWeatherman:
From me
yes this is quite possible
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Quoting ecupirate:
Just what we need more rain....

I'm telling you guys, we may see a sharp hook NE in the forecast eventually.
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Quoting Drakoen:
Pressure in Issac appears to be steadily decrasing now down to 1001mb. minimum foundnear 15.7N
Here we go now this should clear things up on whether the center has been where it always has been or if a new one is forming where the MLC is down by 14.7 N.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
4025. luigi18
Quoting Chiggy:

NOPE. Look at the PR radar link I posted...
Link

Thanks lets wait until 11 am and check again!
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Quoting CURIOUSWEATHERGRL:
PATRAP- where are ya?
What's your thoughts on the track??!
Believe he's on vacation
Member Since: February 14, 2004 Posts: 2 Comments: 664
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
This system will not even get close the the Cayman Island!!!!!.
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Hello, here on the Gulf Coast of Louisiana, I am watching Issac closely, the more west he goes, the more nervous I get. Really don't want Issac here. Grounds are already saturated, bayous and marsh are very high. It would not take much to put this town under several feet of water. Definitively not wish casting this way. Put
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Quoting BahaHurican:
I'm supposing we'll see this upgrade at 11 a.m.....


Yep, and until the update at 11am it's still TD10.
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4020. kwgirl
Good morning all. Here in the Florida Keys, specifically, I am in Key West, we are preparing for this storm. Even though my weather sense does not anticipate this storm, I am preparing just in case. I have been surprised one too many times. Mostly for the better, once for the worse. I keep looking at the forecasts and keep asking how they can predict the northerly turn when it appears to be going mostly west. If it continues west, that would be a good thing for Florida and a bad thing for anyone on the Gulf Coast, because eventually it will turn. I am pray casting that this storm continues west, eventually turns north to cross the yucatan and hopefully go into Mexico. That way the Keys are on the east side of the storm. Everyone have a good day and heed the directions of your emergency managers.
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4019. emguy
Greetings everyone...Just popping in for a quick minute...This was a map I posted sometime just before 4AM this morning. Just sharing for those who have not seen it.

As of very early this morning, the red "cone" was my general thinking of what we are going to see based on what we have going on with Isaac. I feel more confident about this now the the morning models are showing continued support and ever slightly increasing consensus.

I look forward to the afternoon when we get to see the next run of the EURO. I suspect the EURO, for the fourth straight run will shift to the east a little farther again.

Have a good day. Happy posting.

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Quoting AussieStorm:

Per NHC we still have TD10.


TS Ten...
Not Yet Joyce.
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4017. centex
Quoting GTcooliebai:
I'm going by what the NHC and Recon found and that center is moving more in line with their track.
But they said "The initial motion estimate is an uncertain 270/12 kt."
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Quoting wakd3Xn04:


Geography is not an FCAT testable subject. They don't teach it in Florida schools any more.

And that's why society is breaking down. It should be taught no matter where in the world you live.
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4015. Gearsts
Quoting Chiggy:

NOPE. PR radar link..
Link
PR radar shows wnw heading
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Quoting AussieStorm:

Per NHC we still have TD10.
I'm supposing we'll see this upgrade at 11 a.m.....

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4012. Drakoen
Pressure in Issac appears to be steadily decrasing now down to 1001mb. Minimum found near 15.7N
Member Since: October 28, 2006 Posts: 57 Comments: 30557
4011. Gearsts
ime: 13:27:00Z
Coordinates: 15.7833N 65.0W
Acft. Static Air Press: 695.0 mb (~ 20.52 inHg)
Acft. Geopotential Hgt: 3,175 meters (~ 10,417 feet)
Extrap. Sfc. Press: 1001.5 mb (~ 29.57 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 84° at 33 knots (From the E at ~ 37.9 mph)
Air Temp: 12.3°C (~ 54.1°F)
Dew Pt: 4.7°C (~ 40.5°F)
Peak (10s) Flt. Lvl. Wind: 35 knots (~ 40.2 mph)
SFMR Peak (10s) Sfc. Wind: 20 knots (~ 23.0 mph)
SFMR Rain Rate: 0 mm/hr (~ 0 in/hr)
(*) Denotes suspect data
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1001.3 mb
(~ 29.57 inHg)

61 knots
(~ 70.1 mph)
21 mm/hr
(~ 0.83 in/hr)
51.4 knots (~ 59.1 mph)
Tropical Storm

Hmm...
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4009. pcola57
Quoting AussieStorm:

Euro is here.


It is what it is but Don't like where that goes.. :(
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Quoting TropicsGirl:
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.


I'm in Lantana. I remember the NHC kept saying the storm was headed west when it was clearly headed southwest...
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PATRAP- where are ya?
What's your thoughts on the track??!
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For anyone interested in seeing the Fujiwara effect in action, watch the interaction of Bolaven and Tembin over the next few days. Bolaven is frankly HUGE and strengthening, meaning thank Tembin is likely to run a perfect oval across Taiwan. V rare to see a cycle make such a movement, and expect some gargantuan rainfall observations out of southern Taiwan (1000mm/400"+)
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4005. Chiggy
Quoting MahFL:
Seems to be moving a positive WNW now.

NOPE. PR radar link..
Link
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Quoting CybrTeddy:



Per ATCF, we have TS Joyce now.

Per NHC we still have TD10.
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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
I'm going by what the NHC and Recon found and that center is moving more in line with their track.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
It would appear a more well-defined center is beginning to emerge around 15.3°N 65.5°W. Overshooting thunderstorm tops can be seen moving westward along 15.8°N and a pivoting action seen in the CDO along the southern edge of the deep convection.

The slower forward motion has also allowed moisture to begin streaming up the eastern quadrant of the storm as well, which should aid in the development of the inner core of the storm as it starts to close off to the dry air to the north. Once that happens a much more consolidated, symmetrical storm should ensue and intensification should occur at a steady, but not rapid, pace due to its large size. I would be surprised if Isaac made it to hurricane strength before interacting with Hispaniola.
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4001. JasonRE
Quoting AussieStorm:

Euro is here.



Thank you AussieStorm, I appreciate the link.
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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.