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 StormTracker2K:


Euro shifted EAST is big time from yesterday's 12Z run.

0Z


12Z


continue the run
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Quoting Neapolitan:
Joyce?

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,
Yeah 10/3/0 we are really ahead of schedule who say this season would be a bust:)bye leaving for school.
Member Since: October 15, 2011 Posts: 0 Comments: 4467



Typhoon Bolaven
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3698. LargoFl
Quoting presslord:


Isn't that interesting?!
sure is alright
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worst case scenario would be if Isaac hugged a large portion of the west coast of Florida, somewhat like Donna of 1960. Unfortunately it is a possibility right now.
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Quoting weatherh98:


these are the model run landfalls for the euro



that is not the pan handle

it would help if you posted the latest runs like the person above did.

you can go on believing that the 0z euro didn't shift to the panhandle of FL if you'd like.
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Looks like the NHC may nudge the track a little to the east at 11am as the models that were way west have all shifted east now basically converging on a SE FL strike.

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Quoting Drakoen:
What we are seeing is the models converging. A track through the straits or southern FL then into the GOM heading northwards towards the panhandle.


And it will be interesting to see what they will do once the NOAA jet's data is ingested after this afternoon's flight.
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3693. LargoFl
Quoting Waltanater:
You have a link for that GFDL model?
it was posted this morning a ways back several times
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Quoting LargoFl:


Isn't that interesting?!
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Member Since: July 14, 2006 Posts: 0 Comments: 1163
Quoting AussieStorm:

Don't know how to post it but here is a link to the GFDL loop.

Here is what it says at 90hrs...

114.5kts = 131.76mph or Cat 4 Hurricane.
This model can't be right. It assumes an initial position that is too North. The actual storm is south of that starting point, no?
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Quoting chevycanes:

euro was LA at 12z yesterday.

0z run was panhandle on FL.

so yes, it was a big shift east.



Provide your link...I would love to see that 200 mile shift..
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Quoting superpete:

A 60 mile difference equates to a much wider track spread a few days from now down the road. As has been pointed out, its south of guidance by a good margin. Interesting to see if the trend continues the next 12 hours.

read the 5am discussion from the NHC. they talked about this.

THE 00Z
GFS...ECMWF...AND UKMET MODELS DID AN OUTSTANDING JOB PREDICTING
THE RECENT SOUTHWESTWARD JOG OR REFORMATION OF THE CENTER OF ISAAC.
THOSE SAME MODELS ARE ALSO FORECASTING ISAAC TO MAKE A SHARP JOG TO
THE NORTHWEST OVER THE NEXT 12-24 HOURS...AND THEN STEADY OFF ON A
GENERAL WEST-NORTHWESTWARD MOTION THROUGH 48 HOURS
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3686. LargoFl
Quoting ProgressivePulse:


The ones that were east are trending west, the ones that were west are trending east. They are trending on a consensus is what they are doing.


tomorrow we could see them coming together better..today everyone is guessing..until that turn happens..even the NHC is guessing
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3685. Drakoen
What we are seeing is the models converging. A track through the straits or southern FL then into the GOM heading northwards towards the panhandle.
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Quoting muddertracker:


Innacurate based on incorrect COC data? Is that even possible?


I was even possible? My thought that a NHC track nudge to the east? Of course it's possible.
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Quoting Gearsts:
Not even the nhc knows where the center is exactly because it barely has one. We don't know if is moving west or wnw until we get a center fix.

I had this weird dream that isaac opened into a wave then hit Tampa later as a cat 5...
My ACTUAL prediction: Becomes hurricane south of Hispaniola, then skirts just over the southern tip of Haiti and crossing E Cuba before moving through the Keys as an intensifying CAT1 before moving offshore of Tampa as a strong and still intensifying CAT 2. It will then become a CAT3 before hitting the panhandle/peninsula junction moving NE then it will move NNE and affect the outer banks before moving into the atlantic.
Member Since: August 1, 2012 Posts: 0 Comments: 269
Stephanie Abrams looks like a hot mess in that purple dress: a purple potatosack.
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Quoting chevycanes:

euro was LA at 12z yesterday.

0z run was panhandle on FL.

so yes, it was a big shift east.


these are the model run landfalls for the euro



that is not the pan handle
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Quoting LargoFl:
man that GDFL model scares me..a direct hit on hmm maybe miami area, then up thru florida, coming out somewhere near tampa..........omg plse be wrong
You have a link for that GFDL model?
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Quoting LargoFl:
Post 3676: HWRF shifted considerably East.
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Quoting Chiggy:
Issac missing the next NHC forecast point by about >1deg to the south. May not sound much but it's actually huge this early in the game...

A 60 mile difference equates to a much wider track spread a few days from now down the road. As has been pointed out, its south of guidance by a good margin. Interesting to see if the trend continues the next 12 hours.
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It seems Isaac is slowing down to consolidate the COCs that have hampered development and direction.
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This was the most recent published RGB image.




Now you can't zoom in here, but what you can do is go to the animation, pause on the last frame and look, and then put it in motion.

Zoom in close enough to see the internal bands, but not so close that you can't see the overall outflow.

What they are calling the "center" is definitely not the center. It is a dry line between an inner and outer rain band.

If you look at various radars, it's plain to see that the wind is blowing in a direction that would be impossible if their fix was correct.

the center is 3.0 to 3.5 degrees due south of Caguas, PR on radar, almost at the boundary of it's range.

The wind direction would not allow anything else.
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Quoting Chicklit:

Really? I thought models were trending west.


The ones that were east are trending west, the ones that were west are trending east. They are trending on a consensus is what they are doing.


Member Since: August 19, 2005 Posts: 5 Comments: 5458
3673. LargoFl
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Quoting GeorgiaStormz:
If we think we have it bad with Isaac...Taiwan has got problems



Yep and forecasted landfall is to be VERY near Tai'tung which has a population of over 230,000 people. Also this storm is forecasted to do a loop de loop.





Catastrophic flooding and winds.
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Good day

I live on a small island one mile North of St John US Virgin Islands - Lovango Cay.
I have set up a Davis Weather station.
So far Tropical Storm Isaac's winds have been around 23 mph, high wind speed of 37 mph and only 0.38 inches of rain.
The barometer has started to rise, its now 30.45 inches at 4:30 thsi morning it was at its lowest 30.40
We are still erxperiencing some gusty winds and rain but conditions are really not too bad.
I will try and keep a post going later on today.

Be careful - be safe !

9:00 am AST 8-23-12

Just registered a 40 mph gust. Looks like Isaac's backside will be stronger than his front !
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3670. LargoFl
Quoting Kristina40:
The panhandle cannot handle anymore water. We still have standing water everywhere from the past two months of raining almost every day.
im afraid right now it looks bad for you folks up there,wether the storm hits florida or goes up the west coast..all the rains will go northward, same for me here..tons of rain coming yet again early next week
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Quoting GeorgiaStormz:
If we think we have it bad with Isaac...Taiwan has got problems


Yes, yes it does.
Typhoon Tembin
10-minute sustained winds: 75 kt
1-minute sustained winds: 105 kt
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I don't see much north movement.
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Quoting cat6band:


Euro moved east??? I don't think so...Pretty much ALL models have shifted westward..unless I'm missing something..


Euro shifted EAST is big time from yesterday's 12Z run.

0Z


12Z
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3666. Chiggy
12Z CIMSS maps 500mb vorticity.. still way south of where the NHC center is. Until this lines up I do not expect a lot of strengthening..
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On Sunday, Aug 26, the HPC has the high centered over Texas bridging with the Atlantic high (purple). The GFS (green) shows the weakness between the two highs. Also notice the large low pressure system off the west coast. The HPC is forecasting a faster speed than the GFS. This is the system that may eventually turn Isaac to the N/NE. The interaction of the two highs -- one in the Atlantic and the one over Texas --- will be critical in Isaac's future track. TD 10 (Joyce) will play a role in the positioning of the Atlantic high, whether it retreats east a lot or a bit, or it stays in about the same position nosing close to the Florida coastline.

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3664. A4Guy
Quoting RTSplayer:



The radar doesn't lie.

Just because you measure a lower pressure in a thunderstorm doesn't mean jack.

The circulation is nowhere near the official fix, and it's freaking obvious.


Gee, thanks. It's amazing all those PHDs at the NHC can't see that. I hope they figure it out soon so we can get an accurate track.

Sorry for the sarcasm...but I trust the NHC, not some blogger sitting on the couch in his pajamas.
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3663. bappit
NHC sounds like a broken record commenting about the dry air affecting Isaac.

UPPER-AIR DATA FROM ST. MAARTEN AT 00Z CONFIRMS THE DRY AIR IN THE MID-LEVELS OF THE ATMOSPHERE BETWEEN 600 AND 300 MB AS ALLUDED TO IN PREVIOUS DISCUSSIONS. THIS LAYER OF VERY DRY AIR HAS BEEN HINDERING CONVECTIVE DEVELOPMENT IN THE NORTHEASTERN QUADRANT FOR THE PAST 3-4 DAYS.
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3662. GetReal
The last run of the UKMET has also shifted way to the left; pushing Isaac through the Florida Straits on a NW track towards the N central GOM.

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Quoting cat6band:


Euro moved east??? I don't think so...Pretty much ALL models have shifted westward..unless I'm missing something..

euro was LA at 12z yesterday.

0z run was panhandle on FL.

so yes, it was a big shift east.
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Wow what a surprise a system moving across the Caribbean with a LLC and a MLC. The Doc suggestion of this staying disorganized all the way across the Caribbean may be possible. I like the one model that runs it out to west and then when it runs to the end of the high it stops, gets it's act together and comes north.
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Quoting Chicklit:

Really? I thought models were trending west.


they are we are dealing with the floridians.

its still moving west.
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Quoting SFLWeatherman:
DID you see the CMC today the CMC and GFDL is going for S FL

The CMC was off the Florida east coast to begin with, so is shifting west.
Member Since: July 11, 2006 Posts: 14 Comments: 11423
Quoting LargoFl:
.see the GDFL models isnt budging..right up thru florida..im preparing, regardless..luckily so far we got days to do that


Always be prepared, but you've been all over this since it formed. IMO I don't think we are going to know the full effects Issac will have, if any, on florida until it has crossed Cuba, if it does. The system is still rather disorganized so the odds of it bombing out are getting less and less if it wants to dance around the islands anyway.
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3656. sar2401
Quoting StormHype:


Since most all rigs are well west of the forecast path, will any of those be evacuated anyway if the forecast for an eastern GOM track (as they call for now) verifies?


Depends on both track and intensity. If the storm tracks a little more west and gets to a cat 2, the non-essential personnel will be evacuated. A cat 3 and production will be shut down. That's why the wholesale price of gas has stalled right now. The high priced met consultants don't know more than we do about where, and how strong, Isaac may be. The bets yesterday were that it would get close enough to affect production.
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Quoting Tazmanian:
hey look what i found on TD 10


AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS,


oh joy
ce
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If we think we have it bad with Isaac...Taiwan has got problems



This is a growing eyewall that is much improved from just a few hours ago.
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3653. 7544
morning all the big change today is issaac has slowed down to 12 mph which will give him a better chance to get his act together where he could not do yesterday at 20 mph

more models going east so we could see a new cone at 11 am imo rick scott is doing a news confrence at this hour making all fla awhere that issac could become hurricane tomorow and tells fla to get prepared stay tuned
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Quoting Neapolitan:
Joyce?

AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS, 34, NEQ, 40, 30, 0, 40, 1013, 225, 40, 0, 0, L, 0, , 0, 0, TEN, M,



yep

evere thing shows that TD 10 is now a TS



YAY the year that TD 10 did not fail
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Quoting Tazmanian:
hey look what i found on TD 10


AL, 10, 2012082312, , BEST, 0, 149N, 416W, 35, 1006, TS,
Looks like Joyce, which would put us at 10 named storms for the season, Taz.
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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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