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 islander101010:
only.see.cat.1.by.fl.so.far
islander... No offense, Just a question... Why the period after each word?
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Quoting bigwes6844:
That shift is getting more west

gfs is only 33 hrs in and there is no shift to the south or the west.

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Quoting Levi32:


You yourself just pointed out the lack of outflow. Watch the WV/visible and see the cirrus trajectories cut off by the upper low to the northeast. The shear has been present all through the last 48 hours and has been gradually lightening, now almost gone, but the cirrus flow is still confined. The CIMSS maps are not 100% accurate either. Just watch the cloud motions. It's not hard.

And again, I did not attribute the struggling NE quad fully to either shear or the flow pattern, rather a combination of both over the last couple of days. Isaac will begin recovering from those factors as it gets farther west and north.


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

3 strikes and your out. Thanks for playing.
Member Since: October 28, 2006 Posts: 57 Comments: 30562
Quoting UKHWatcher:


Funny, I was under the impression it was a tropical blog with an open worldwide membership???

Isaac is looking poor right now, don't be fooled... He's in transition and will organise well by this time tomorrow..


I would imagine a storm having a hard time organizing, when the storm is approximately the size of Pluto's moon, Charon.
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Quoting bigwes6844:
That shift is getting more west

sure looks that way
wow what a storm
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Quoting cyclonekid:
(click to enlarge)





I love your images you post. Keep them coming.
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344. 892mb
We just had a wicked storm roll through Grand Isle - tornado warning scared all the beach goers to safety. Sky is still black and winds are high, but all safe for now.
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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,

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Quoting Grothar:
I don't see any dry air to the west.


Central carribean maybe?
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for those just coming home..........
Member Since: August 6, 2011 Posts: 4 Comments: 39116
Quoting Drakoen:


Wrong. Try again.



You yourself just pointed out the lack of outflow. Watch the WV/visible and see the cirrus trajectories cut off by the upper low to the northeast. The shear has been present all through the last 48 hours and has been gradually lightening, now almost gone, but the cirrus flow is still constrained. The CIMSS maps are not 100% accurate either. Just watch the cloud motions. It's not hard.

And again, I did not attribute the struggling NE quad fully to either shear or the flow pattern, rather a combination of both over the last couple of days. Isaac will begin recovering from those factors as it gets farther west and north.
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Quoting WPBHurricane05:
Euro vs. GFS
Drakoen vs. Levi

Not sure which battle is more entertaining. :o
RELEASE THE CRACKON!!!
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only.see.cat.1.by.fl.so.far
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Quoting VR46L:
Hmmm seems rather big right now ...



But how will he cope with the dry air ahead



Looks like 2 TC :)
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The shear map hasn't updated since 1800UTC ie 2 p.m. EST.
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335. SLU
Winds in Barbados at 5pm were WNW - West. This certainly doesn't support the center near Guadeloupe.

Meanwhile the air pressure in Guadeloupe just hit 1005mb.
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Quoting LargoFl:
That shift is getting more west
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18z init

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Quoting LargoFl:
just went to publix and everyone in the line in front of me had the cases of bottled water etc..NOW is the time to get your supplies folks..and dont forget to gas up the car
I bought 5 cases of water yesterday.... If you have a Winn Dixie card it is only $2.99 a case.
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JMO
Enjoy all the comments and knowledge gained here. Too bad some people get too aggressive towards other poster's opinions at times.
As far as Isaac's future path and strength, I would have to say: once he passes the islands (Hispaniola/Cuba)
I believe "taking one for the team" would be a South Florida hit going into the peninsula as a Cat 1.
I know there would be some damage and flooding, but it would be much better than if he goes into the gulf (westerly track), or stays east and rides up the East coast possibly influencing the Florida east coast to New England. The result of either of those tracks would obviously give Isaac much more time over very warm water and thus have time to strengthen into a much more dangerous storm, causing much more damage. Stay safe all, and just be prepared. Rich
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Quoting hydrus:
I believe what is happening here is what the Dr himself was concerned with...It is eating or absorbing the huge thunderstorm complex that was hindering its development located to the S.E. All the while trying to form a dominant center. A lot for a large system to do in a rapid fashion.

how often in history has this happened.....what "looks" like two hurricanes in one? if so, what happened? is it really possible that one splits off and goes one direction and the other goes in another direction, as two separate hurricanes?
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(click to enlarge)



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Quoting VR46L:


LOL .. I was shocked when I first started lurking about 6 years ago .. could not get over calling each other poof . I still cant ..and its not even violating the infamous community standards huh..

Anyways back on topic Isaac in Rainbow


I took a bit of a double take when I saw that also, but they are just saying your going on the iggy list and your 'poof' as in gone
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Quoting Seflhurricane:
max mayfield on local10 in miami just said a hurricane watch or tropical storm watch appears very likely sometime friday afternoon or evening and that he is very concerned about the florida keys


He said that the Dry air is not a problem in terms of Intensification. Which is surprising maybe he is right maybe the only reason why dry air is messing up the NE quad is because of the elongation and lack of vertical stacking.
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Tropical cyclones are often well-insulated at the core from dry air even if it is in close proximity, as long as convective processes and constant mixing of a strong inflow of moist air are persistent.
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Quoting Levi32:


More like northeasterly shear, which allows the higher levels of the atmosphere to dry out naturally because of lack of convection, but not necessarily because of the intrusion of low-mid level dry air, which is normally blatantly obvious on TPW and relative humidity imagery.

Don't forget the shear is the other reason the NE quad is struggling. I'm not attributing it solely to the monsoon flow I talked about.


Wrong. Try again.

Member Since: October 28, 2006 Posts: 57 Comments: 30562
Wow, just checked in to see what the latest forecast is, and it's really changed since this morning. Are conditions really changing that much, so quickly? Or are the models spazzing?
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Euro vs. GFS
Drakoen vs. Levi

Not sure which battle is more entertaining. :o
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Quoting Hurricanes305:


These LLCs should consolidate into one MAIN LLCOC most likely will happen further SE then it will absorb those two burst of convection solidify its CDO also this SE reformation would align itself under the MLC as the old one was too far NW of the MLC so I expect a big slow down. The tilt and lack of being vertically stacked is one of the main reason there has been a lack of strengthening over the past 24 hours. Once this happens the NE quadrant will become filled with more moisture and possible RI could ensue.


We'll see, the Doc said it would probably take another 24 hours for it to re-organize. I think eventually it will happen as it pulls away from the dry air but the question is how far West does it get before it makes that turn to the NW? I think we'll have a better idea on that tomorrow but it's still going to be a tricky track to nail down more than a few days out.
Member Since: August 25, 2006 Posts: 1 Comments: 3055
Member Since: August 6, 2011 Posts: 4 Comments: 39116
Quoting Patrap:
hey Pat do u think those two COC's will convert into one and create a huge monster out of this system?
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Quoting waterskiman:
My best advice is to avoid the rush and get your gas and beer now. I did that today. I stock and keep up hurricane supplies all year round so when it gets crazy I have alot less money to shell out and can deal with the more pressing stuff like protect my property.
just went to publix and everyone in the line in front of me had the cases of bottled water etc..NOW is the time to get your supplies folks..and dont forget to gas up the car
Member Since: August 6, 2011 Posts: 4 Comments: 39116
315. SLU
Quoting Drakoen:


The ECMWF is a good model and should not be discounted, even though it is an outlier. It is possible that if the internal structure of Issac doesn't get its act together (multiple vorticies), that the system takes a more western track.


Will also be foolhardy to call its stronger system and westerly track as a "outlier"
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Quoting waterskiman:
My best advice is to avoid the rush and get your gas and beer now. I did that today. I stock and keep up hurricane supplies all year round so when it gets crazy I have alot less money to shell out and can deal with the more pressing stuff like protect my property.
Yes, The essentials....Gas and beer.LOL
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Quoting Jedkins01:
My thought right now is that Isaac will travel through the keys and make landfall in the eastern panhandle getting close to the west coast of Florida, but not quite over it, however I don't have high confidence in that forecast, a landfall further west or east of Florida has only a slightly lower chance in my opinion, but very close.
Could you imagine the flooding that track would result in??? You know how saturated the ground here is already.
Member Since: December 1, 2006 Posts: 2 Comments: 3623
The RNC participants will be waving at Isaac from a distance, while they consume massive amounts of alchohol and keep singles on hand at all times.
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low.pressure.in.between.the.globs.
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Quoting redwagon:

Oh, I'm out there alright, but I'm also in here :)

'We're all here because we're not all there'

TX has one month left before the tropical gates close. It is infuriating to see all this moisture at our gates and this stupid dry air dropping to keep it at bay. Just one little system to come up and fill up Centex lakes...


Lol. That fits this place sometimes. :)

But as you say, we've got about a month left. Maybe they'll be a big, rainy tropical storm that will park over the lakes. We can also get them from the Pacific to do that too. Speaking of which I don't know if there's anything in the EPAC. Been a bit distracted lately. :)
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Quoting Drakoen:


Indeed. It is obvious that dry air has been affecting the NE corner. All one has to do is turn on the WV imagery.
Water vapor also shows the upper level confluent flow north and northeast of Isaac. Upper confluence will produce more dry air and obviously limit outflow and thunderstorm production in the area.
Quoting Drakoen:


Right now I think the GFS and UKMET have the best tracks for this system. It may be a South Florida or even more so a Florida straits event. But it is still too early to say.
Agreed. Yesterday I was saying south Florida or just west of there. Still like that forecast.
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18Z GFS now initializing
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LinkIsaacWVLoop
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Quoting weatherman12345:
When does 18z gfs come out?

Now! 12 hours in:


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Quoting Drakoen:


Notice on that imagery how the outflow channels do not extend to the northeast as a result of the overwhelming amount of obvious, undeniable, explicit, clear-as-day, dry air to the northeast of the system.


More like northeasterly shear, which allows the higher levels of the atmosphere to dry out naturally because of lack of convection, but not necessarily because of the intrusion of low-mid level dry air, which is normally blatantly obvious on TPW and relative humidity imagery.

Outflow channels are not determined by dry air. Dry air is just usually present as a result of lack of outflow.

Don't forget the shear is the other reason the NE quad is struggling. I'm not attributing it solely to the monsoon flow I talked about.
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I guess that appointment didn't help... ;)
Member Since: October 28, 2006 Posts: 57 Comments: 30562

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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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