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 stormpetrol:
It appears to me that even REcon is having a hard locating a true center, just sayin....


That's been the case all day. There are a couple of different circulation centres, which will probably end up meeting somewhere in the middle. Doesn't mean the circulation isn't closed.
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108 hrs. approaching the FL. Keys as a 995 mb. storm.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting washingtonian115:
Ever since Brian Norcross joined TWC they've dragged the Andrew story....
But I do love Brian... I remember watching him during Andrew....... He was calm and well informed...
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Most of em went west. Im seeing the track of Issac moving ever so slightly to the west. Will it do the same??
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Quoting Jedkins01:



Indeed, water vapor loop cannot necessarily determine moisture accurately, in comparison the PW analysis, it's quite futile. Even with PW analysis, you can still be fooled. This is thanks to the fact that you could have significant water mass through the column, but still have shallow layer of relatively drier air present that although my seems minor, it is all that's needed to disrupt a tropical cyclone's inner core. That is what is happening here with Isaac.


However, it doesn't mean that a dry layer from the north isn't being sucked into the core at some given (nth)mb level where the water vapor does show dry air. I don't have any access to the tools needed argue for against how much dry air entrainment is occurring and where, so I take a neutral stand here. But dry air entrainment appears to be occurring, that seems reasonable to conclude.


On the satellite imagery, Isaac continues to resemble a tomato slice that has been chopped in half and then set down on a plate. In this analogy, the two nodes of convection that are currently visible on the infrared could be thought of as a mutated seed core that has somehow been divided into two seed cores.

On top of the dry air entrainment, which is at least partially responsible for the above-mentioned appearance, at least part of the storm's circulation will have to contend with land masses in the Greater Antilles in a day or two. It is still possible that the COC could pass over Hispaniola and/or parts of Cuba as it progresses W or WNW.

Add all of this together, along with the wild card of unpredictable wind shear profiles five or more days down the road as well as still yet more land interaction with the Florida peninsula and you have the recipe for a possible underachieving tropical cyclone. I know it is way early to predict such a thing but it is certainly one possible scenario.

What will be interesting to watch is how the RNC and the Tampa City government react in the coming days. Imagine if they cancel the Republican convention a bit early due to gun shyness or a sense of "due diligence and precaution" only to find the deserted streets of downtown Tampa experiencing just a cooling moderate rain and a couple of overcast days during what would have been the convention? This is a perfect example of how trying to make the right decision in a circumstance like this is unavoidably a roll of the dice, after all is said and done.
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It appears to me that even REcon is having a hard locating a true center, just sayin....
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444. FOREX
Quoting PalmBeachWeather:
......I remember years ago you could tune into the TWC and get weather... I tuned into the channel to catch what they would say about Isaac. Storm Stories was on... I thought the weather on the 50's of the hour was the Tropical Update.... To me this seems more important... It reminds me of when MTV came on and you could watch music videos.Hardly any music anymore.


It's all about ratings. TWC, like many channels would rather have 15 million idiots watching reality shows, than 5 million watching the Tropical updates. Just my opinion.
Member Since: August 17, 2011 Posts: 0 Comments: 2335
Quoting Chicklit:


We finally have an updated shear map.
I can see an area of 40 kt shear over Hispaniola. If the does not let up, a track further west would ensue also..jmo
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Ever since Brian Norcross joined TWC they've dragged the Andrew story....
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440. 892mb
Quoting PalmBeachWeather:
......I remember years ago you could tune into the TWC and get weather... I tuned into the channel to catch what they would say about Isaac. Storm Stories was on... I thought the weather on the 50's of the hour was the Tropical Update.... To me this seems more important... It reminds me of when MTV came on and you could watch music videos.Hardly any music anymore.


Amen!! All you see are Ice Pilots, Iron Men and other nonsense. Thankfully places like this exist!!!
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438. TXCWC
As long as Isaac struggles with it's core - it would be unwise to simply discount the Euro more west solution - as mentioned below - NHC is not. I think DR. M is right in stating the following and may be a good gauge of which model - GFS or Euro eventually ends up being the more correct solution: "...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..."
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Quoting KEEPEROFTHEGATE:
Post 430. This one is coming with a lot of water.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
yeah im outta here also, see ya all later..nothing new for awhile
Member Since: August 6, 2011 Posts: 4 Comments: 40965
Quoting Maineweatherguy20023:

YES yes maybe no NO
I dont know seems reasonable...

You'll see like we did last night when Isaac hit some sort of wx brick wall and all the convection kept going W and the center was beautifully exposed for a brief moment..
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Quoting 69Viking:
OK, I'm out for a while! Might check back in on things later! I must say you guys have taken Dry Air analysis to a whole new level and are teaching some of us a alot, thanks!


I think you have supplanted some of the so-called experts with your simple post about the dry air. Have a good one.
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OK, I'm out for a while! Might check back in on things later! I must say you guys have taken Dry Air analysis to a whole new level and are teaching some of us a alot, thanks!
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top places that I think the LLC could be

14.8N 63W
15.5N 61.5W
15.0N 61.0W
Member Since: June 13, 2009 Posts: 2 Comments: 12442
Will be interesting to see which way the models will come in agreement with: weaker storm would take a Katrina like track with Euro model, while the other models have it stronger and going more East. So far this season the storms have been weaker, so i agree with Euro for the moment unless it reaches hurricane strength before entering the Gulf. If it is stronger than a flora bama or florida strike.
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Quoting SLU:
WOW ... so the EURO was right when it showed ISAAC as a large mess with multiple lows.



and the euro puts it on NOLA
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It looks like to me the center is at 15N 61W between the 2 main blobs of convection. It also appears that Isaac is sucking up that cluster of thunderstorms to the SE that Dr. M was talking about earlier. Just my 2 cents, back to lurking
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Quoting TomTaylor:
The shear is low. The problem to the northeast is that the winds our of the nw are limiting outflow creating upper level convergence.

By the way, what's with the attitude? This isn't Maury not Jerry Springer. We can debate like normal human beings.


You're right nobody is missing their baby daddy. This is a weather blog. I'm surprised this is even up for debate the quality of this blog has really gone down.
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Quoting redwagon:

Think the 'real' center is at 15N 61W.


We finally have an updated shear map.
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......I remember years ago you could tune into the TWC and get weather... I tuned into the channel to catch what they would say about Isaac. Storm Stories was on... I thought the weather on the 50's of the hour was the Tropical Update.... To me this seems more important... It reminds me of when MTV came on and you could watch music videos.Hardly any music anymore.
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Quoting HrDelta:


Curious to hear why. I've been wondering why the EURO has been weird with this storm.


I believe the EURO was using the data for a much weaker storm which would have responded less to the influence of the impulse which is expected to pull Isaac North. Also, there was another trough which may have been expected to be pull out earlier and force the high pressure more to the west, keeping Isaac on a further west track. Right now they are the only model I see having Isaac on the far west coast of Cuba. I would assume that the NHC is in the middle of the consensus of the Euro on the west and the others on the East. The Euro is a very reliable model. Also, the GFS is having a computer problem and they have not updated here. I am sure they are passing on the information though.
Member Since: July 17, 2009 Posts: 71 Comments: 26852
423. TXCWC
"...THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC..."

NHC is not discounting Euro - nor should they as long as it too is being pretty consistant right now.
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Quoting Levi32:
It forever amazes me the inability of this blog to consider views different from the consensus of experts, models, what have you, just because it's not the generally accepted explanation. Does that make such views automatically wrong? Do I have to answer that?

I love meteorological debate but you guys make it so difficult to have fun tracking the weather.


Amen Levi, a few here have one opinion, theirs, and it's always the right one. I personally think a more westerly track than you, but thats just me. Not going to sit here and argue that point to the point of being ridiculous. Sure not going to say your wrong because you say east I say west..for all I know, you could be right. Kind of obvious you know way more than me. I'm just a coastie thats likes weather..
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Member Since: August 6, 2011 Posts: 4 Comments: 40965
Quoting Jedkins01:


True, but mid level and upper dry air can effect a tropical cyclone thanks to shear. Weaker tropical cyclones also are much less efficient at "insulating" the inner cor then well developed ones. Given this tropical storm is also undergoing major shifts in inner core structure, it doesn't necessarily take deep layered drier air to disrupt the cyclone. If this had a well established and developed inner cor, it would be hard for me to disagree with you, but given Isaac's current state, it doesn't take much to inhibit organization.


You are right, but with dry air only truly present at 500mb and above, if there was any significant low-level convergence at all, one would expect thunderstorms to at least be able to pop up due to a moist lower 10000 feet of the atmosphere, and then perhaps collapse due to wind shear and the dry air above 500mb, leaving outflow boundaries and what not.

However, that has not been the case, rather the NE quad has been utterly empty of any updrafts. To me this implies that there is another mechanism besides the shear working against forced ascent of air in that part of the storm. Earlier I provided my theory as to why.
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Quoting masonsnana:
Wow amen to that..You tell them Levi.


Same here Levi... one of the reasons why I stopped posting as much. Enjoy your posts, even though you aren't always correct. But no one is.
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418. Vero1
Drokoen.. Each yr you poof me so keep throwing the the ball...there is really no "out".

Since Issac is has competing centers, enlongated, and rectangular, I must side with Levi as he has been avocating this for days.

Stay in school.

Quoting Drakoen:


Woooo 5-10 knots of deadly northeasterly shear. I'm surprised the coc isn't exposed.

3 strikes and your out. Thanks for playing.
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Looking at the data from both HH you can clearly see how lopsided the cerculation is the center at 10000 feet is many miles S of the cerculation at 5000 feet
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Quoting MiamiHurricanes09:
Argument turning into a congressional hearing now? ;)

I mean, you could give textbook explanations as to why dry air isn't being infiltrated by the northeastern quadrant, but at the end of the day you don't need 18-sylabble words and all the jazz: just a WV loop.

It ain't happening now. The bogus assertion was that it has not affected TD9/Isaac at all.
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Quoting Levi32:
It forever amazes me the inability of this blog to consider views different from the consensus of experts, models, what have you, just because it's not the generally accepted explanation. Does that make such views automatically wrong? Do I have to answer that?

I love meteorological debate but you guys make it so difficult to have fun tracking the weather.
Levi, Stop drinking ice cold Sprite and start drinking ice cold Beer when your on here.... and tracking a storm and debates will be ALOT more fun.
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Quoting HrDelta:


Roughly 1,100 miles from Eastern Edge to Western Edge, and about 600 miles from Northern Edge to Southern Edge.

Yikes, I think florida is around 450 or so miles long, thats a couple three good days of pounding weather right there
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Quoting Drakoen:


Woooo 5-10 knots of deadly northeasterly shear. I'm surprised the coc isn't exposed.

3 strikes and your out. Thanks for playing.
The shear is low. The problem to the northeast is that the winds our of the nw are limiting outflow creating upper level convergence.

By the way, what's with the attitude? This isn't Maury nor Jerry Springer. We can debate like normal human beings.
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500 mb vort at 75 hrs.

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Quoting MississippiWx:
Starting to notice more growth and northward movement of cirrus in the northern semicircle.



Yup cirrus clouds forming to its NE. Once the LLCOC becomes dominant I Believe it would add energy to those cirrus cloud a new eastern COC focus more of the moisture from the south up the east side to the Northeastern side to meet with a will establish NW outflow. Again RI could be underway.
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Quoting Levi32:
It forever amazes me the inability of this blog to consider views different from the consensus of experts, models, what have you, just because it's not the generally accepted explanation. Does that make such views automatically wrong? Do I have to answer that?

I love meteorological debate but you guys make it so difficult to have fun tracking the weather.
Wow amen to that..You tell them Levi.
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Quoting Grothar:
Well, it doesn't seem we are going to get any GFS models soon. Seems there is still a computer glitch somewhere. Why do they always go down in the middle of a developing system,



Ok, so it's not just me. Probably why the FIM hasn't run or shown anyway. BTW they put the FIM9 on the Tropical Atlantic page as a spaghetti. Of course this may have been there a while and I just noticed it. :)
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SW ball of convection is the new COC right?
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Kermit didn't find a wind shift at all?
Member Since: August 17, 2005 Posts: 26 Comments: 16874
Quoting redwagon:

Think the 'real' center is at 15N 61W.

YES yes maybe no NO
I dont know seems reasonable...
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http://mag.ncep.noaa.gov/GemPakTier/MagGemPakImages /gfs/20120822/18/gfs_wnatl_075_precip_p24.gif
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Quoting Levi32:


Yes but it depends greatly on what level the dry air is at. With no thunderstorms, the tropical ocean atmosphere is automatically pretty dry above 600-500mb. Tropical cyclones get disrupted a great deal more when the dry air is deep-layer and affects the storm at the low-mid levels, which you can usually observe quite well by looking at the 700mb level.

Take a look at the 18z GFS initialization of 700mb relative humidity vs. 500mb relative humidity. See how at 700mb the core of the storm is actually pretty well insulated. It's not until you get up to 500mb that the moisture collapses on the north side because there is no convection to bring the moisture up from the lower levels. That upper-level dry air is a natural result of the other processes I mentioned that prevented convection in the NE quad. It's the egg, not the chicken, and is not the reason for the lack of convection.

700mb Rel Humidity:



500mb Rel Humidity:



True, but mid level and upper dry air can effect a tropical cyclone thanks to shear. Weaker tropical cyclones also are much less efficient at "insulating" the inner cor then well developed ones. Given this tropical storm is also undergoing major shifts in inner core structure, it doesn't necessarily take deep layered drier air to disrupt the cyclone. If this had a well established and developed inner cor, it would be hard for me to disagree with you, but given Isaac's current state, it doesn't take much to inhibit organization.


I agree with some of what you have said, nut still disagree somewhat on the reason for Isaac's struggle. But that doesn't matter, it's ok to disagree.
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Quoting Levi32:
It forever amazes me the inability of this blog to consider views different from the consensus of experts, models, what have you, just because it's not the generally accepted explanation. Does that make such views automatically wrong? Do I have to answer that?

I love meteorological debate but you guys make it so difficult to have fun tracking the weather.
Superbly stated.
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Quoting MississippiWx:
Starting to notice more growth and northward movement of cirrus in the northern semicircle.


Think the 'real' center is at 15N 61W.
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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.