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 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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Quoting Drakoen:
"WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER DATA
CONTINUE TO SHOW DRY AIR TO THE NORTHEAST OF ISAAC...AND A LACK OF
CONVECTION IN THIS AREA SUGGESTS SOME OF THIS AIR IS ENTRAINING
INTO THE CYCLONE AND DISRUPTING THE CONVECTIVE STRUCTURE. "

Jack Beven, PhD



Thank you!



He is filibustering the argument. You can post it till you are blue in the face.
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.
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850 mb vort at 69 hrs.

Member Since: August 6, 2007 Posts: 0 Comments: 693
BIG drop in precip...

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Quoting CCkid00:

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?
Probably, but under a extremely rare set of circumstances. I always wondered if there have ever been two tropical cyclones simultaneously in the gulf..It has never been recorded, but almost certainly has happened.
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Quoting 954FtLCane:


Looks huge on this view!


Roughly 1,100 miles from Eastern Edge to Western Edge, and about 600 miles from Northern Edge to Southern Edge.
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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.
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Quoting washingtonian115:
Men and Testosterone....Lordy lord lord.Sometimes you love it(older women know what I mean) and other times you hate it..


errr....

Link :)

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...
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Quoting HrDelta:


You are probably right.

Issac is gigantic. Only storm that I remember being larger have to be Ike, and Alex. Alex's out flow was completely stupid in size.
yes the storm is huge alright
Member Since: August 6, 2011 Posts: 4 Comments: 33375
Quoting washingtonian115:
Men and Testosterone....Lordy lord lord.Sometimes you love it(older women know what I mean) and other times you hate it..
I refer to them as Silverbacks
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BULLETIN - IMMEDIATE BROADCAST REQUESTED
SPECIAL MARINE WARNING
NATIONAL WEATHER SERVICE MELBOURNE FL
525 PM EDT WED AUG 22 2012

THE NATIONAL WEATHER SERVICE IN MELBOURNE HAS ISSUED A

* SPECIAL MARINE WARNING FOR THE COASTAL WATERS...

FROM MELBOURNE BEACH TO SAINT LUCIE INLET OUT TO 60 NM.

* UNTIL 730 PM EDT

* AT 521 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
A LINE OF THUNDERSTORMS...PRODUCING STRONG WINDS OVER 34 KNOTS
FROM 30 MILES EAST OF MELBOURNE BEACH TO WINTER BEACH...MOVING
NORTHEAST AT 15 KNOTS.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

THESE THUNDERSTORMS WILL LIKELY PRODUCE WINDS OVER 34 KNOTS AND
LOCALLY ROUGH SEAS. SMALL CRAFT...ESPECIALLY THOSE UNDER SAIL...
SHOULD MOVE TO A SAFE HARBOR IF POSSIBLE. MAKE SURE YOU AND YOUR
CREW ARE WEARING APPROVED FLOTATION DEVICES DURING THESE HAZARDOUS
CONDITIONS.

FREQUENT TO EXCESSIVE LIGHTNING IS OCCURRING WITH THESE STORMS. IF
CAUGHT ON THE OPEN WATER STAY BELOW DECK IF POSSIBLE...AND KEEP AWAY
FROM UNGROUNDED METAL OBJECTS.

WATERSPOUTS ARE ALSO POSSIBLE WITH THESE STORMS. A WATERSPOUT IS A
TORNADO OVER WATER THAT CAN BE DANGEROUS AND EVEN DEADLY. SMALL
CRAFT CAN BE SWAMPED OR OVERTURNED BY A WATERSPOUT. STAY AWAY FROM
THEM AT ALL TIMES. IF YOU SIGHT A WATERSPOUT...CONTACT THE COAST
GUARD VIA MARINE RADIO OR LOCAL OFFICIALS AND ASK THEM TO RELAY YOUR
REPORT TO THE NATIONAL WEATHER SERVICE.

&&

LAT...LON 2740 8002 2749 8034 2766 8038 2780 8047
2803 8039 2820 8004 2826 7952 2775 7932
TIME...MOT...LOC 2123Z 279DEG 9KT 2807 7998 2779 8032
2771 8037

$$



CRISTALDI
Member Since: August 6, 2011 Posts: 4 Comments: 33375
Quoting LargoFl:
maybe on those 27 specially made buses they have ready for them to get out of dodge, who wants to bet they have a wet bar inside lol


You are probably right.

Issac is gigantic. Only storm that I remember being larger have to be Ike, and Alex. Alex's out flow was completely stupid in size.
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Quoting TomTaylor:
See my post on the last page. ECMWF dust analysis shows some dry air has wrapped around the circulation.

I'll repost the image, although keep in mind this image is from yesterday's 00z run. I agree with you that dry air at the low to mid levels now is not as big of a deal as it has been getting mixed out. Now the bigger problem is northwesterly flow aloft limiting outflow and drying the upper atmosphere.



Look at it though. It's entrainment in the NW quad and western semicircle....not the NE quad. It came from the north but is wrapping in from the west, which does not directly impact the NE quad in so significant away as we have observed.
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Can be an indication of the true position of the center of Isaac ....

I think it fits more with the presentation of Isaac satelitte.

CIMSS 850mb



http://img585.imageshack.us/img585/1858/539519455 75000233922141.jpg
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Quoting PalmBeachWeather:
islander... No offense, Just a question... Why the period after each word?


He needs to update his Android.
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Quoting MississippiWx:
Starting to notice more growth and northward movement of cirrus in the northern semicircle.



Looks huge on this view!
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Men and Testosterone....Lordy lord lord.Sometimes you love it(older women know what I mean) and other times you hate it..
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WEAK ridge...
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Quoting washingtonian115:
Isaac is a very large storm..
What is Luigi doing?
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Quoting Jedkins01:



Indeed, I believe that you, me, and others here have stated many times that the 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.


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:

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"WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER DATA
CONTINUE TO SHOW DRY AIR TO THE NORTHEAST OF ISAAC...AND A LACK OF
CONVECTION IN THIS AREA SUGGESTS SOME OF THIS AIR IS ENTRAINING
INTO THE CYCLONE AND DISRUPTING THE CONVECTIVE STRUCTURE. "

Jack Beven, PhD



Thank you!

Quoting TomTaylor:
See my post on the last page. ECMWF dust analysis shows some dry air has wrapped around the circulation.

I'll repost the image, although keep in mind this image is from yesterday's 00z run. I agree with you that dry air at the low to mid levels now is not as big of a deal as it has been getting mixed out. Now the bigger problem is northwesterly flow aloft limiting outflow and drying the upper atmosphere.



He is filibustering the argument. You can post it till you are blue in the face.
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Member Since: August 6, 2011 Posts: 4 Comments: 33375
Quoting Drakoen:


Wrong. Try again.



There is >20kts of shear over the in the NE quad where there should be more outflow and banding features while the CDO is under <10kts. Once the SE LLC start to dominate and align itself with the MLC more I expect to the anticyclone to expand east to go along with the COC.
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Quoting washingtonian115:
Isaac is a very large storm..


So, is Mario thrusting in your avatar??
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At 51 hours out, the 18Z GFS is very similar to the 12Z, if a tad to the north.
Member Since: August 10, 2010 Posts: 2 Comments: 1970
Quoting Levi32:
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.
See my post on the last page. ECMWF dust analysis shows some dry air has wrapped around the circulation.

I'll repost the image, although keep in mind this image is from yesterday's 00z run. I agree with you that dry air at the low to mid levels now is not as big of a deal as it has been getting mixed out. Now the bigger problem is northwesterly flow aloft limiting outflow and drying the upper atmosphere.

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


ENE peice looks good.. NNE looks dry
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Quoting MississippiWx:


So cold in here...
Love it.
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Afternoon everyone.

I see we are Dmin... Downcasting minutes of the blog... lol

Lame attempt at joke aside, I'm going to be much more interested with what Isaac does overnight. Every afternoon this storm looks terrible, but never quite as bad as the afternoon before. Overnight, it usually seems to 'party hearty' as they used to say, so that its best presentation is just at sun up my time. I will prolly stick around for the night shift tonight, because this is one of two chances it has to make a run toward hurricane status before it gets tangled up in Hispaniola. despite current presentation I wouldn't be at all surprised to see Isaac as a strong TS by this time tomorrow morning...
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Isaac is a very large storm..
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Taking a overall look at Isaac there is no comparison on the structure and organization from yesterday and this evening at this time. Isaac is ready to begin his strengthening mode soon. Outflow continues to look better and better
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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.


So cold in here...
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Quoting HrDelta:


If the West Coast Forecasts are correct, they will be getting front row seats to something special.
maybe on those 27 specially made buses they have ready for them to get out of dodge, who wants to bet they have a wet bar inside lol
Member Since: August 6, 2011 Posts: 4 Comments: 33375
Quoting Grothar:


The Weather Channel and Max Mayfield and our local news just called it an outlier and gave the explanation why.


Curious to hear why. I've been wondering why the EURO has been weird with this storm.
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NOAA's P-3 is located WSW of Montserrat all there getting is E winds sugestive that LLC may already be in the caribbean looking on sattlite this could be that blob in the Caribbean deoes look better than the other part of the system

I'm just say this could be I'm waiting for HH C-130 to fly into that area
Member Since: June 13, 2009 Posts: 2 Comments: 9592
Quoting Levi32:


No. I think we're forgetting that mid-high level moisture is obviously going to be low when there is an absence of thunderstorms. The atmosphere is naturally dry up there in the absence of convection to bring moisture up from the ocean.

If you go down to the low-mid levels where it really matters, it is fairly obvious that the strong monsoonal flow is shoving moisture up into the eastern part of the circulation and keeping the dry air at bay to the northeast. There is an abrupt change to dry air but it is nowhere near Isaac's core and is not being entrained. Entrainment is only occurring on the west and SW sides.




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

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Quoting scott39:
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.

LOL!!!!
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Quoting SLU:


Will also be foolhardy to call its stronger system and westerly track as a "outlier"


The Weather Channel and Max Mayfield and our local news just called it an outlier and gave the explanation why.
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Quoting scott39:
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.


If the West Coast Forecasts are correct, they will be getting front row seats to something special.
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Quoting Jedkins01:
It had physics upgrades, I think they are working quite well, the euro on the other hand has been suffering from seemingly what the GFS has in the past. Although the EURO has still done better than many of the others.
Good link here describing GFS enhancements current and planned: http://www.emc.ncep.noaa.gov/index.php?branch=GFS

Recent analytical and data input improvements include:
=-=-=-=-=-=-=-=-=-=
* Use GPS RO bending angle rather than refractivity
* Include compressibility factors for atmosphere
* Retune SBUV ob errors, fix bug at top
* Update radiance usage flags
* Prepare for monitoring NPP and Metop-B satellite data
* Add NPP ATMS satellite data
* Add GOES-13/15 radiance data
* Add SEVERI CSBT radiance product
* Include satellite monitoring statistics code in operations
* Add new satellite wind data and quality control
* Update to current version of analysis trunk for optimization and preparation for future updates
=-=-=-=-=-=-=-=-=-=

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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,


Isaac must have crashed it with the latest blob switch.
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It's like Frazier Ali all over again when these two heavyweights go at it. Love it. Really enjoyed the back and forth between you two during Debbie.
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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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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.