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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okay, here we go...
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NHC taking their sweet time with Isaac's Advisory.
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Quoting HrDelta:


Accuweather failed both ways in 2005. They made predictions that Katrina would go into Florida, while bashing the NHC.

FEMA used the NHC, but the Department of Homeland Security used Accuweather. As a result, at least 30 people died in the catastrophic week after Katrina.

Then they hyped up Rita hitting Houston, causing a hysteria induced evacuation. Killing somewhere between 90-117 people.

There are many companies I get ticked at for various reasons. The Super-Banks, Big Oil, and that company that sucked up Union Carbide (Bhopal). But no company on the planet makes me more angry than Accuweather. For the inaccuracy of reports, then lobbying to sabotage the NWS. For Katrina and Rita, I wish Accuweather could be sued.

I f---ing hate Accuweather. With the fury of a thousand suns.


I fail to see how Accuweather killed 30 people after Katrina (especially since the storm had already passed in your context). People die during bad storms. The best forecasting in the world won't save them. Even the Native Americans told the white people they were crazy for building their city in a hole.
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2047. GetReal





It will be west, with an occasional WSW wobble with Isaac. IMO, with the current and forecast steering layers the core of Isaac will pass well to the south of Hispaniola. Now Jamaica and Cuba down the road may have serious problems.
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting scott39:
Dude, If you already knew....then why did you ask the question? That feels like a setup now!


Because I wanted everyone to understand the bias. Not a set up at all. And for the record, I was out the eye of Hugo looking at stars when I was 17; so I understand what it's like to go through a major. The point is, right now the EMCWF is an outlier and should generally be treated as such until we have solid info to state otherwise. It's not going to be there tomorrow if it does verify at all.

Quoting Hawkeyewx:
Rapid scan satellite just started.


Awesome, thanks! Now we just need to get the whole system in to the field of view.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
2045. geepy86
Quoting HoustonTxGal:
Wow, what just happened? One minute we are all discussing and share info on Isaac and then next minute and half a dozen arguments have broken out.

that's how WU rolls
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Quoting Thrawst:
I think the next Recon flight will be VERY interesting...

totaly agree

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Night everyone. Impressive convection burst is still going. Lets see if Isaac can do something with it.
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2041. scott39
Quoting Tazmanian:


You better chill or things like that will get you banned
I know Taz thanks
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Quoting tornadolarkin:


Agreed. But Jim Cantore is an absolute weather nut, which is something I think TWC needs just to keep them decent. And Greg Forbes worked with Ted Fujita, so he knows his stuff.


Granted, he has the passion. I wish my weather passion was as strong as his. I don't like the way he stumbles over other meteorologists during high pressure times though.
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Quoting Tribucanes:
I like the wunderkidcayman quite a bit, and his forecasting bothers me none. That being said, ya'll complaining about his very consistent trend of wishcasting are valid and understandably annoying to some if not many. Isaac coming together nicely now, three to five hours from now Isaac should be one nice solid ball of convection with an easily distinguishable center. I'm not calling you a wishcaster wunderkidcayman, but I understand completely why some do.


I just can't respect opinions that are not scientific, when we're dealing with science! I don't care if you wishcast a storm to land or out to sea. If you make a claim, you better have some good reasons for it. That is how you garner respect, if you care about that sort of thing. Me, Drak, Levi, TA, and Mississippi didn't get the recognition we have now overnight. It was a gradual process, through the presentation of demonstrable evidence.
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Half of WU is probably down to their knuckles right now after pressing F5 keys and refresh buttons for the past 25 minutes without breaks.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
2037. jonelu

The waiting.....is the hardest part
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Quoting HurricaneDean07:

Not a good thing to see, at this point in time... But as of right now... Isaac is poised to strike land either way... It can't back itself of this situation now... Its stuck, and not getting out of the situation without doing some damage...
E.I. Bull in a China closet.


And he's going to be pushing a lot of water. He's a big boy.
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Wow, what just happened? One minute we are all discussing and share info on Isaac and then next minute and half a dozen arguments have broken out.
Member Since: September 18, 2007 Posts: 0 Comments: 1074
Quoting Relix:
What a nice outflow to the north.... does this indicate a future WNW movement? Maybe within 6 hours?
and so much for the ne quadrant being void of energy
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Quoting HrDelta:


Accuweather failed both ways in 2005. They made predictions that Katrina would go into Florida, while bashing the NHC.

FEMA used the NHC, but the Department of Homeland Security used Accuweather. As a result, at least 30 people died in the catastrophic week after Katrina.

Then they hyped up Rita hitting Houston, causing a hysteria induced evacuation. Killing somewhere between 90-117 people.

There are many companies I get ticked at for various reasons. The Super-Banks, Big Oil, and that company that sucked up Union Carbide (Bhopal). But no company on the planet makes me more angry than Accuweather. For the inaccuracy of reports, then lobbying to sabotage the NWS. For Katrina and Rita, I wish Accuweather could be sued.

I f---ing hate Accuweather. With the fury of a thousand suns.


It goes deeper then that, for one the DHS should stay out of the emergency management aspect of "homeland security" leave that to FEMA. If the government is serious about protecting people in disasters then they will let FEMA be an independent agency again, with a director in a cabinet level position as it was pre 9/11. Bottom line is that the government should rely on the NHC as the law, and they can use other services as supporting material but nothing more.

Above all else though, people need to take responsibility for themselves. They also need to monitor weather forecasts and plan accordingly, FEMA can't hold everybody's hands. People need to look out for themselves, and let the government help them do it.
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Quoting chevycanes:
looks to me that Isaac has slowed down in the last few hours.
the blog ate my post but I attempted to post that earlier... 18 mph speed when I logged on earlier. Thought..."oh...now he's working on organizing."
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2030. Relix
What a nice outflow to the north.... does this indicate a future WNW movement? Maybe within 6 hours?
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Quoting TropicalAnalystwx13:

Looks are deceiving.


Can you provide any evidence that's it weakening?
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Quoting scott39:
SCREW YOU KORI...Iopen up on here and thats the CRAP I get from you. I WAS JOKING the other day!!!!!


You better chill or things like that will get you banned
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I like the wunderkidcayman quite a bit, and his forecasting bothers me none. That being said, ya'll complaining about his very consistent trend of wishcasting are valid and understandably annoying to some if not many. Isaac coming together nicely now, three to five hours from now Isaac should be one nice solid ball of convection with an easily distinguishable center. I'm not calling you a wishcaster wunderkidcayman, but I understand completely why some do.
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Quoting scott39:
SCREW YOU KORI...Iopen up on here and thats the CRAP I get from you. I WAS JOKING the other day!!!!!


Crap?

Whatever. I'm not getting into this. I know exactly how this will go. But I will say, if you didn't want people taking your comments at face value, you should have made an active endeavor to provide some clarity or insight. It's not my fault I interpreted it exactly how you said it.
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Quoting NYCyclone86:


What do you think about the ridge weakening to 5880M?
Nothing unexpected; as Wxlogic stated, the cyclone should begin to slow down somewhat with a gradual shift towards the WNW over the next 12-24 hours. At this point, it's looking as if Isaac will be crossing over the center portion of Hispañola, which luckily (for us) is also the most mountainous region.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting KoritheMan:


That's not a significant weakening. Moreover, the trough actually looks weaker based on water vapor imagery. Only slightly weaker, though. Not at all saying it's dissipating or something like that.

Looks are deceiving.
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Quoting KoritheMan:


I'm seriously considering it. And I generally have a lot of patience, and even attempt to defend some bloggers that get trounced on.
I generally don`t ignore people that is not JFV or Jason and I actually laugh everytime he says the cayman islands.
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lol
Quoting TampaFLUSA:
Guess where I think its going to hit... Don't look at my username... ;)
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2021. trey33
Quoting TampaFLUSA:
Guess where I think its going to hit... Don't look at my username... ;)


I know, I know!!! Alaska right? :)

What do I win???
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Quoting KoritheMan:


That's not a significant weakening. Moreover, the trough actually looks weaker based on water vapor imagery.

Not a good thing to see, at this point in time... But as of right now... Isaac is poised to strike land either way... It can't back itself of this situation now... Its stuck, and not getting out of the situation without doing some damage...
E.I. Bull in a China closet.
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Quoting scott39:
No I dont mind and yes I live in Mobile.... as you can tell Im a little anxious. My mind lights up when I see models trend westward and the EURO bombing in the GOM. I went through Major hurricane Fredric when I was 10 years old. That kind of trauma doesnt wear off. So if I phych myself up to a possible something... I will be ready.....if not then thank God I missed another one. Thats how I deal with it.
id be anxious also living there and what you experienced during Katrina.
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Quoting Bluestorm5:


Bull****...

no really I am not no Bull
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Quoting wunderkidcayman:


that was at diffrent height and fron NOAA's P-3 mission not the airforce Hurricane Hunter,s C-130 mission lowest pressure/circulation was 15.7N 62.2W



NOAA is reporting extrapolated surface pressures, P-3 is in the storm now.
Member Since: August 19, 2005 Posts: 5 Comments: 5452
Quoting aasmith26:


Norcross is excellent. Cantore is good but overdramatic... I have all the respect in the world for Dr. Greg Forbes.


Agreed. But Jim Cantore is an absolute weather nut, which is something I think TWC needs just to keep them decent. And Greg Forbes worked with Ted Fujita, so he knows his stuff.
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2014. GetReal
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting SFLWeatherman:
no shift


could shift east off cuba's coastline a bit
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2012. Thrawst
I think the next Recon flight will be VERY interesting...
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Guess where I think its going to hit... Don't look at my username... ;)
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2010. HrDelta
Quoting stormhank:
anyone feel a dennis track from 2005 could verify??or a fla west coast hugger up to the big bend?? thanks for any input..as possibly a cat. 1 or 2?


I think West Coast Hugger, maybe getting up to 115-120 mph.
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2009. scott39
Quoting StormJunkie:


He does, I was just wondering if that is what was influencing his insistence on the Euro.
Dude, If you already knew....then why did you ask the question? That feels like a setup now!
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Quoting HuracanTaino:
The lowest pressure is around 16N/62W...


that was at diffrent height and fron NOAA's P-3 mission not the airforce Hurricane Hunter,s C-130 mission lowest pressure/circulation was 15.7N 62.2W

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Quoting wunderkidcayman:


-9999999999999
I am not wish casting at all


Bull****...
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Quoting NYCyclone86:


What do you think about the ridge weakening to 5880M?


That's not a significant weakening. Moreover, the trough actually looks weaker based on water vapor imagery. Only slightly weaker, though. Not at all saying it's dissipating or something like that.
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Think your forecasting skills are better than anyone other blogger here ? For those of you in college, look up the WxChallenge at your school. I believe it is open to anyone, undergraduate, graduate, staff or faculty. Participants file forecasts 4 days a week for the next day's high and low temperatures, max sustained wind speed and precipitation in 1/100 of an inch. Sounds simple right ? Participants have access to all the usual forecasting tools and models.

It is a heck of a lot harder than it sounds. I did this for three years back when it was named something different and while our team did very well year to year, I wasn't anywhere near the top in any given year.
Member Since: May 19, 2009 Posts: 0 Comments: 252
If there is a landfall in Florida it will be within 50 miles of Fay's (thats a super safe statement 400 miles of Coastline, LOL).
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.
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2001. HrDelta
Quoting trey33:


Same station (different met) that said Charley was the "worst possible scenario" for Tampa. Everyone freaked out, evacuated, in panic and then it went south and they were the last station to report the turn. That met retired not long after. The met who said tonight's statement is better, but I think the station needs to have a training seminar on the open mouth, insert foot theory. And yes, Charley was supposed to be a direct hit, I'm not arguing with the forecast so much as the adjectives used to hype it up.


Accuweather failed both ways in 2005. They made predictions that Katrina would go into Florida, while bashing the NHC.

FEMA used the NHC, but the Department of Homeland Security used Accuweather. As a result, at least 30 people died in the catastrophic week after Katrina.

Then they hyped up Rita hitting Houston, causing a hysteria induced evacuation. Killing somewhere between 90-117 people.

There are many companies I get ticked at for various reasons. The Super-Banks, Big Oil, and that company that sucked up Union Carbide (Bhopal). But no company on the planet makes me more angry than Accuweather. For the inaccuracy of reports, then lobbying to sabotage the NWS. For Katrina and Rita, I wish Accuweather could be sued.

I f---ing hate Accuweather. With the fury of a thousand suns.
Member Since: Posts: Comments:

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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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