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 xcool:


HWRF

HEY It's Been a Long Time


Hey xcool! :)
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Quoting LargoFl:
way too soon to answer that


That's what I was thinking...

I'm wondering if the mountains on those islands are going to tear it up... I'm hoping anyway...
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Photobucket


It's amazing how large the circulation field of Isaac is. Due to how large it is a good portion of the circulation field covers the dry air regions of the subtropics. This dry air impinging on the inner circulation field of the storm is masking the true center of the storm. Also due to the southwest displacement of the storm center in relation to the overall circulation field it can be deceiving.

Photobucket

A closer inspection, with streamlines, shows the elongation of the center. Also, the apparent southwest movement of the center, which appears to me as a center of vorticity being expelled from the storm as it has become too elongated. I fully expect the dominant center to take over on the north side of the elongated center, closer to the center of the overall circulation of the storm.
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AL, 09, 2012082300, , BEST, 0, 158N, 622W, 40, 1004, TS
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1197. Relix
Center shifted again? You know what.. watch this stall, become a Cat 5, cross PR from S to N, then move W, go N to S over Hispaniola, then cross Jamaica and why not, Caymans so Wunderkid is happy, then cross Cuba, hit keywest, visit Miami then go over to Tampa and then run parallel in arc over the Gulf Coast, hitting from FL to NOLA and then finally getting inside Houston as a Cat 6.

Why not?
Member Since: August 3, 2006 Posts: 0 Comments: 2722
Quoting AussieStorm:


Which COC is the one to consider?
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1195. HrDelta
Quoting vince1:

Some of it is pseudoscience masquerading as facts (and some of it fails to acknowledge the even wilder swings in the climate record), since some of the loudest alarmists don't seem to realize the solution/end game for "anthropogenic global warming," which is to stop using oil overnight and massively reduce the size of the global population (Billy Gates admitted as much earlier in the week). This would ultimately require nothing short of enactment by a world government body, the likes of which Orwell envisioned in a certain "1984."


Why don't you go to a creationist or conspiracy blog instead? While there is a natural cycle underneath this, there is an excess of Greenhouse gases that does not appear to be from the normal cycle. And we do have ways of handling it that are not the drivel you come up with.

I have much less patience with your side than most of the others of similar thinking to me here. I lost relatives in the 7/22 Attack in Norway last year. And the community I am part has been attacked heavily in the past month.

Now, back to other things. Does it lock like a center is winning with Issac. For it to really strengthen, it needs one center. It sure as heck doesn't right now.
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1194. hydrus
Quoting xcool:


HWRF

HEY It's Been a Long Time
Ya think?..:)
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1193. Levi32
Quoting KoritheMan:


Well you certainly shouldn't shift your track at the whim of every single model shift. However, Isaac is struggling to organize, which makes me think the Euro solution is a little more believable. Perhaps not to Louisiana, but the Florida panhandle or points thereabouts? Sure.


The significance of that statement though is that it implies that the perception of many people is that Isaac is struggling more than expected to organize, thereby imparting a potential need to adjust the track. Relative to the initial NHC intensity forecasts which were overdone, this is true, but then none of the global models have shown any kind of real deepening until Isaac gets west of the lesser Antilles. In that sense, the storm is still right on schedule, taking its time, likely starting to strengthen a bit more just before running into Hispaniola.
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Quoting Thrawst:
Someone PLEASE tell me what's going on here right now with Isaac... too many mixed signals, and the fact that I am beginning senior year of high school isn't helping :P


just keep calm and buy your salchichas
Member Since: August 22, 2006 Posts: 2 Comments: 1983
http://www.ssd.noaa.gov/goes/east/tatl/flash-wv.htm l

Isaac is having a tough time organizing. Dry air to the north and an upper low to the west. Isaac has only 24-36 hours to intensify before interaction with DR, Haiti and Cuba.

If anyone recalls Hurricane Georges which was under perfect upper level conditions, and went over DR and Haiti and was still a very intense cyclone; making landfall in the Florida Keys.

South Florida still needs to watch Isaac as it emerges off the Cuban coast and is in a very warm water environment. The question is... will the upper level environment be conducive for delvelopment?
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Quoting AussieStorm:

Mate, once I start physically feeling 100% I'll start to feel better mentally. In fact, I told my wife, once it's gone, We are going to the Philippines and I'm going to trek up Mt Pinatubo and Mt Arayat, both are very close to our place. That will be my reward for getting through this period in my life.


That's it mate! That's exactly it...we'll want photos!
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Quoting Levi32:
Still no wind shift SW of the center....this is getting comical.

Hey Levi, do you think it is Isaac's swift foward speed that is prohibiting rapid development at this point? When or what area is Isaac expected to slow down in?
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Quoting hurricane23:
GFDL 18z..



I don't like that at all. Watching channel 10 here in Miami, based on the models/possibility, Miami is not going to have fun either way.
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Quoting JLPR2:

(Click to access the radar)

Anyone willing to take a guess where is Isaac's circulation? XD


lol gemme a dartboard, we can just pick our own center then
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Quoting Hurricanes305:


Here's a link that I showed earlier here this would take 24 hours to consolidate a new COC like DRr. M said in his entry as its happening NOW New LLCOC becoming better define at a rapid rate


ok, i'll do it

Member Since: August 22, 2006 Posts: 2 Comments: 1983
Interesting little fact; if the initial advisory were to verify, Isaac would be a hurricane at 2a.m tonight. At least the track forecast is holding up pretty well.

INIT 21/0900Z 15.2N 51.2W 30 KT 35 MPH
12H 21/1800Z 15.3N 53.8W 35 KT 40 MPH
24H 22/0600Z 15.7N 57.2W 40 KT 45 MPH
36H 22/1800Z 16.0N 60.5W 50 KT 60 MPH
48H 23/0600Z 16.3N 63.6W 65 KT 75 MPH

72H 24/0600Z 16.9N 69.3W 80 KT 90 MPH
96H 25/0600Z 18.0N 74.0W 95 KT 110 MPH
120H 26/0600Z 20.0N 77.3W 95 KT 110 MPH
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1184. Thrawst
Someone PLEASE tell me what's going on here right now with Isaac... too many mixed signals, and the fact that I am beginning senior year of high school isn't helping :P
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right HH RECON is heading home NOAA P-3 is still flying

no HH RECON is more accurate its flying lower better reading and no reading says Isaac is at 1004mb

Quoting TropicalAnalystwx13:

Which means the pressure readings will be more accurate.

999.1 millibars confirms that Isaac has begun intensifying.


as I said before I think that mess called Isaac will come togeather near 15N 63W and according to last center fix and sattelite that seems to be happening that part that is E of the Islands will come in toward that part which is in the E caribbean

now that part E of the islands are now moving in
yep everything is coming together I think though we may still get them WSW movements


I think Isaac will stay S of Hispaniola and track S of Cuba but what I ain't too sure about is how far S it will go
the farthest S I see it make is passing S of Jamaica
the farthest N I see it going is just off the Cuban S coast maybe skirting the coastline at times

eventually I see this making it way along Fl W coast/gulf coast

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Quoting CybrTeddy:
The pressure reading is not legitimate as the recon is at a 10,000 ft altitude. Realistically, the pressure is 1003-1004mb.


The readings are extrapolated surface pressures

This all the way down at
14.917N 62.217W

Extrapolated
Surface Pressure
1002.8 mb
(~ 29.61 inHg)
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Am I too late?
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Quoting ILikeIke:

Scary...

Meh...pitiful
Probably no Joyce until tomorrow.
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1179. hydrus
Quoting Floodman:


I'm good brother and I'm glad I got to see you...how have things been?
Crazy..Busy..trying to keep from falling behind...Which I am barely doin.. Isaac is ready to blossom into a dangerous storm..Naturally I am concerned what this Caribbean Bruiser/Cruiser is gonna do.
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1178. LargoFl
Quoting connie1976:
Curious what you all think.... How likely do you think South Florida is going to be hit? I went to open house at my son's school and the parents were talking about how it isn't going to hit... A lot of them were saying that it is going to be a really weak storm too...
way too soon to answer that
Member Since: August 6, 2011 Posts: 4 Comments: 38451
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Quoting CothranRoss:
If Isaac still had two centers, would it be theoretically possible for both centers to develop eyes so that Isaac had a set of eyes like us humans?
No, but it would be really cool if it could.
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Sorry, but I don't buy sub-1000 mb pressure readings with a storm as disorganized as Isaac. Likely contaminated.
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Quoting Levi32:
Still no wind shift SW of the center....this is getting comical.



Means we don't have a closed circulation, it's literally an open wave
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Member Since: September 30, 2007 Posts: 9 Comments: 15935
MH9 Bobby's blog. :)
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Quoting Chicklit:


Looks like the east flank is jumping into the Caribbean finally. Now they can unite in bathwater...
Sweet. Maybe they will give each other loofah scrubs in the bath.
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Quoting CothranRoss:
If Isaac still had two centers, would it be theoretically possible for both centers to develop eyes so that Isaac had a set of eyes like us humans?


No.
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1169. SLU
ISAAC is an extremely decoupled storm. The mid-level center seems over 150 miles to the south of the LLC according to the radar and RECON data.

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Quoting KoritheMan:


No offense to the guy who said it, but people are way too quick to call rapid intensification. I don't quite think they understand the mechanisms that have to be in place for that.
Not even close to rapid intensification at this point in time, imo.
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1167. Levi32
Quoting floridaboy14:
just curious. what went wrong with your debby forecast? i remember you said it would drift north and then the texas ridge would catch her and shove her into south texas. what caused her to move northeast instead?


Too much amplification in the pattern. Forgot to account for the flow around a meridionally-oriented ridge (north-south) being unfavorable for blocking a tropical cyclone in the gulf.
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Quoting PensacolaDoug:
Quoting drg0dOwnCountry:


I typically start with quoting the headline and if you read the article you will see that there is "no political opinion" from me or anybody else. There are only documented facts and science.


You're kidding right? Thst whole thing is political and insulting.


Which part is it? Quote it.
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1165. xcool


HWRF

HEY It's Been a Long Time
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Quoting MiamiHurricanes09:
Damn, that is true. Do you believe that a center reformation would really dramatically impact the track though? I couldn't imagine that half a point of longitude/latitude would really change the situation drastically.



Probably not, but the intensity would. A weaker Isaac will have the tendency to wobble around the trough and move with the high.
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Quoting icmoore:


Aw I hope you are both back in the same location soon. Ack I am just someone having fun with a camera, get your mind out of the gutter Press, but thank you for asking!


Yep, we hope so too...

Just Someone with a camera? LOL...yep, and Picasso was just this guy with a paintbrush. You have a wonderful knack...

As fior Press getting his mind out the gutter? That would require some kind fo surgery...he's a filthy old reprobate and I love him like a brother...LOL
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Quoting CaicosRetiredSailor:

No problem with deep convection tonight:)
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If Isaac still had two centers, would it be theoretically possible for both centers to develop eyes so that Isaac had a set of eyes like us humans?
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Quoting JLPR2:


First we need a defined circulation.


Here's a link that I showed earlier here this would take 24 hours to consolidate a new COC like DRr. M said in his entry as its happening NOW New LLCOC becoming better define at a rapid rate
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Quoting lurkersince2008:
Is there a trough or something that would keep Isaac away from central Gom?

Hey! your name applies for me too! (:
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Quoting drg0dOwnCountry:


I typically start with quoting the headline and if you read the article you will see that there is "no political opinion" from me or anybody else. There are only documented facts and science.


You're kidding right? Thst whole thing is political and insulting.
Member Since: July 25, 2006 Posts: 0 Comments: 563
1157. JLPR2

(Click to access the radar)

Anyone willing to take a guess where is Isaac's circulation? XD
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Quoting CybrTeddy:
The pressure reading is not legitimate as the recon is at a 10,000 ft altitude. Realistically, the pressure is 1003-1004mb.

No, this plane is farther to the ground than the typical one, making the pressure readings more accurate.
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1155. Levi32
Still no wind shift SW of the center....this is getting comical.

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Well looks like the two convective blobs are sorta connecting.
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Quoting Levi32:


So far in my "career" it has served me well more times than it has bitten me in the rear.
just curious. what went wrong with your debby forecast? i remember you said it would drift north and then the texas ridge would catch her and shove her into south texas. what caused her to move northeast instead?
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1152. LBAR
So, Isaac appears to be coalescing towards the more northern blob of convection?
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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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