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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Still 2 hours until 11 am. TD 10 may have the name taken back by then... wait until NHC confirmed it.
Member Since: August 1, 2011 Posts: 28 Comments: 8075
3800. Msdrown
Quoting LargoFl:
notice where the Nam has it on sat morning..on sunday its even closer..water temps there near 90
Quoting LargoFl:
notice where the Nam has it on sat morning..on sunday its even closer..water temps there near 90
Quoting LargoFl:
notice where the Nam has it on sat morning..on sunday its even closer..water temps there near 90



Largo, I've noticed you have had alot of rain this past 3weeks and now Isiac coming. Doesn't this increase the possibilities of new Sink Holes????
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Quoting AussieStorm:

I disagree. The NHC is the only Authority that can name ATL and EPAC Hurricanes. No one else.

Well, isn't the ATCF (Automated Tropical Cyclone Forecasting) owned by the NHC?
Let's just wait. Ten will be renamed Joyce sooner or later.
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Quoting StormTracker2K:


That maybe a troll and not the real handle.


That is indeed a troll.
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3796. K8eCane

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE WILMINGTON NC
745 AM EDT THU AUG 23 2012

.SYNOPSIS...
UPPER LEVEL DISTURBANCES AND A STALLED FRONT OFFSHORE WILL KEEP THE
WEATHER UNSETTLED THROUGH FRIDAY. SOME SLIGHTLY DRIER AIR WILL WORK
INTO THE AREA OVER THE WEEKEND. EARLY NEXT WEEK...ALL EYES WILL BE
FOCUSED ON TROPICAL CYCLONE ISAAC TO OUR SOUTH AND ITS EVENTUAL
TRACK. THE RISK FOR WET WEATHER WILL RETURN DURING TUESDAY. REFER TO
THE LATEST BULLETINS ISSUED BY THE NATIONAL HURRICANE CENTER.

&&
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3795. LargoFl
Quoting ncstorm:


the 06z run is crossing SFL

..yes right now we cant trust the models..back and forth they go
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Name-spoof trolling = Worst. Hobby. Ever.
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3793. ncstorm
Quoting GTcooliebai:
The HWRF keeps flip-flopping like a fish, can't even get it off the hook. Largo posted the 6z run and it had Isaac going up the East Coast.


the 06z run is crossing SFL

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3792. LargoFl
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Quoting chevycanes:

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.
Oh the NHC is well ahead of the game and I'm not surprised by that, since they are the experts.
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3790. Grothar
Quoting GTcooliebai:
The HWRF keeps flip-flopping like a fish, can't even get it off the hook. Largo posted the 6z run and it had Isaac going up the East Coast.


I don't remember seeing the HWRF on the east coast.

Other than the Euro and the CMC almost all models have been pretty consistent in the general direction for a few days. They have been a lot closer than some other storms.
Member Since: July 17, 2009 Posts: 71 Comments: 27211
Quoting Tazmanian:



The atcf names things be for the nhc dos


So there for TD 10 is. No longer a TD it is now a TS

I disagree. The NHC is the only Authority that can name ATL and EPAC Hurricanes. No one else.
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Quoting gprxomstr:
Gee nrtiwInvragn forecasting abilities have really took a spiral downhill. That dude used to be good.


That maybe a troll and not the real handle.
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3787. VR46L
Quoting Tazmanian:



Not TD 10 it's now the J storm


Hmm I checked tropical atlantic site , NHC and Navy all calling it 10 still. where is it named officially Joyce?
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Quoting CypressJim08:
When can we expect the next HH mission?

Aren't they still there?
They're investigating the 700 mb level.
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3785. LargoFl
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okay, well I've got people from my job waiting for me to actually do something.
...SPECIAL FEATURES... (from 8 a.m. NHC Discussion)

THE CENTER OF TROPICAL STORM ISAAC AT 23/1200 UTC IS NEAR 15.4N 64.8W...MOVING WESTWARD 11 KNOTS. THIS POSITION IS ABOUT 225 NM/360 KM ESE OF SAN JUAN PUERTO RICO...OR ABOUT 315 MI...505 KM ESE OF ISLA SAONA DOMINICAN REPUBLIC.

THE ESTIMATED MINIMUM CENTRAL PRESSURE IS 1003 MB. THE MAXIMUM SUSTAINED WIND SPEEDS ARE 40 KT WITH GUSTS TO 50 KT. ...

CONVECTIVE PRECIPITATION...NUMEROUS STRONG FROM 12N TO 17N BETWEEN 62W AND 69W AND FROM 14N TO 17N BETWEEN 60W AND 61W AND FROM 10N TO 16N BETWEEN 59W AND 61W INCLUDING IN TRINIDAD...SCATTERED MODERATE TO ISOLATED STRONG IN OUTER RAIN AREAS ARE REACHING THE EASTERN SECTIONS OF THE DOMINICAN REPUBLIC FROM 18N TO 21N BETWEEN 68W AND 71W.
Member Since: July 11, 2006 Posts: 14 Comments: 11424
Doesn't slowing of forward speed usually mean a change in direction?
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Quoting RTSplayer:


It does not matter anyway, because the 0z euro is wrong, since it went north right away, along with the other models. That was initialized with data that was both wrong and 12 hours old.

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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3780. Drakoen
The 12z TVCN is a little to the west of the NHC forecast maybe only a slight shift westward of the NHC track at 11 am.
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Quoting presslord:
Yet again...people are coming outta the woodwork to help Portlight respond to this....This is truly a remarkable community we're part of here...



How can we donate?
Member Since: August 25, 2010 Posts: 0 Comments: 78
Quoting Grothar:
The CMC shifted west and the EURO shifted East.



Miami straight up to Tampa seems like the track i'm starting to favor now. NHC may nudge the track east ever so slightly at 11am as there is a tight concensous across SE FL.

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3776. Grothar
Quoting LargoFl:
..well it looks like this for sure..so far..IS our storm huh GRO


Getting to look like it.
Member Since: July 17, 2009 Posts: 71 Comments: 27211
When can we expect the next HH mission?
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Quoting Grothar:
The CMC shifted west and the EURO shifted East.




Still a tremendous amount of variability in the Ensembles; both timing and intensity (hence track).

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Quoting 900MB:
Does everyone agree that we are on the extreme left side of the cone right now?
A. Yes
B. No
Depends on who "we" are.
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Quoting Grothar:
The CMC shifted west and the EURO shifted East.

The HWRF keeps flip-flopping like a fish, can't even get it off the hook. Largo posted the 6z run and it had Isaac going up the East Coast.
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3771. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 27211
3770. LargoFl
Quoting StormTracker2K:


GFS is back to that sceanrio except it goes right up the west coast of FL.

Even the GFS ensembles shifted east.
..yes if this is the same tomorrow we are in for it im afraid
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Quoting Tazmanian:



The atcf names things be for the nhc dos


So there for TD 10 is. No longer a TD it is now a TS

That's what I said : TS Ten.
NOT Joyce yet. It will be updated shortly, likely in 30 minutes or less.
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you are 100% correct. well said.
Quoting Jedkins01:
I must stress something HIGHLY important, a more eastern track up the west coast of Florida and or peninsula is based on Isaac gaining at least decent intensity, if Isaac fails to intensify for too long it will most likely go much further west, it's just a matter of physics.

With that said, some of the model shifting to west farther into the central gulf is only because of Isaac's southward relocation and its failure to intensify, if Isaac begins to intensify steadily from here on out however I expect model groups to further tighten into the eastern gulf.

Make sense?
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Quoting nrtiwInvragn:
Good news for those in south Florida. Models continue to shift west. Expect the NHC cone to be altered left soon. Keep an eye on this one Texas!


I think TX is safe from this one. Also pretty much all the models are tightly clustered around SE FL specifically the Miami area up to Tampa then up the FL Big Bend.

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3766. Grothar
The CMC shifted west and the EURO shifted East.

Member Since: July 17, 2009 Posts: 71 Comments: 27211
3765. luigi18
Quoting stjohnviweather:
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 !

nice cay to live! good info i in PR !
Member Since: September 21, 2008 Posts: 0 Comments: 483
Quoting AussieStorm:


Taz, NHC has not named it yet. Until the NHC names it then it's still TD10.



The atcf names things be for the nhc dos


So there for TD 10 is. No longer a TD it is now a TS
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Quoting StormTracker2K:
Expect Issac to really gain some wind speeds today. My guess is we will have a 60mph Issac at 5pm then 75mph Issac at 11pm.

Thats pretty funny guy! Considering they could not even get an accurate center fix!  Calm down there buddy, its going to take some more time for Isaac to get his act together.
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3762. LargoFl
Quoting Grothar:
..well it looks like this for sure..so far..IS our storm huh GRO
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3761. GetReal


Isaac appears to be consolidating itself into a tighter core.
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3760. hubcity
Impact Weather Forcast:

Impact Weather has decreased the intensity forecast while Isaac is in the Caribbean. Isaac is forecasted to strike Haiti as a moderate tropical storm instead of a strong tropical storm. In addition, the track forecast has shifted a little west of the previous advisory. This brings the system farther into the Gulf of Mexico.
Isaac continues to move to the west. However, a turn to the west-northwest should begin later today due to an upper low in the northwest Caribbean. This should take the system toward the southwest Peninsula of Haiti tomorrow. Thereafter, the latest model data indicates a bit more ridging to the north of Isaac. The model tracks have shifted to the west in response to this. Long term, significant intensification is likely in the Florida Straits and the southeastern Gulf of Mexico. Isaac is forecast to become a hurricane with 90 mph winds. However, it is quite possible that it could become a category 2 hurricane.

Tropical Storm Isaac Advisory 9. August 23, 2012 from www.impactweather.com
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Quoting AstroHurricane001:


Err...

Have a look at this loop and you'll say the same thing....

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3758. LargoFl
Quoting LargoFl:
notice where the Nam has it on sat morning..on sunday its even closer..water temps there near 90
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3757. Grothar
Member Since: July 17, 2009 Posts: 71 Comments: 27211
Quoting GTcooliebai:
Here ya go weather98:

00z ECMWF 168 hrs.



It does not matter anyway, because the 0z euro is wrong, since it went north right away, along with the other models. That was initialized with data that was both wrong and 12 hours old.

We now know the CoC moved at least half a degree south over night, which put it back more in line with the previous run of the Euro anyway.

And in any case, even the three previous westward tracks were not and are not some massive absurd outlier, at least not at this point and not as big as some people try to claim.



and





And finally, look at the graphic the Dr. Masters posted.





With Debby being an exception, the Euro beats the GFS from 36 hours forwards, and beats the Official NHC forecast from about 60 to 72 hours forward most of the time.



It's still inside the cone of uncertainty for the end of the track, especially if you project another day out on the cone, so you can't just ignore the thing given it's history.


If it was doing something ridiculous like 100 to 180 degrees off from the other storms, ok, but it's not...
Member Since: January 25, 2012 Posts: 33 Comments: 1520
I don't want to start a panic. but Fl. did have a Gov. Kirk. I leave out the politics. Although he was a bit of a Claude if you as me.
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Quoting LargoFl:
...wow back on page 8 lol..well here is the GDFL..straight up florida..just like the GFS had a few days ago..GDFL hasnt budged from this track..notice the intensity


GFS is back to that scenario except it goes right up the west coast of FL.

Even the GFS ensembles shifted east.
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Quoting Tazmanian:



Not TD 10 it's now the J storm


Taz, NHC has not named it yet. Until the NHC names it then it's still TD10.
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Quoting newt3d:
Does not compute ... SHIPS is an intensity model, only.


The white line on the computer model traversing Cuba and headed for Central Gulf is the UKMET. Thanks!
Right. Ships does intensity only.
Member Since: July 11, 2006 Posts: 14 Comments: 11424

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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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