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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Product: Air Force Tropical RECCO Message (URNT11 KNHC)
Transmitted: 23rd day of the month at 13:56Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 300)
Storm Number: 09
Storm Name: Isaac (flight in the North Atlantic basin)
Mission Number: 7
Observation Number: 02

Mandatory Data...

Observation Time: Thursday, 13:52Z
Radar Capability: Yes
Aircraft Altitude: Below 10,000 meters
Coordinates: 26.5N 82.1W
Location: 18 miles (28 km) to the WSW (239°) from Fort Myers, FL, USA.
Turbulence: None
Conditions Along Flight Route: In the clear
Pressure Altitude: 7,320 meters
Flight Level Wind: From 210° at 17 knots (From the SSW at ~ 19.5 mph)
- The above is a spot wind.
- Winds were obtained using doppler radar or inertial systems.
Flight Level Temperature: -20°C
Flight Level Dew Point: -24°C
Weather (within 30 nautical miles): Scattered clouds (trace to 4/8 cloud coverage)
400 mb Surface Altitude: 7,580 geopotential meters
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Quoting StormTracker2K:


It looks as if he has finally gotten his self together this morning. Now the ramp up begins. I would put money down that this becomes a hurricane at 11pm.




Seems unlikely to me; or did I miss some wind readings from HH. Just don't see how it can become a hurricane until the decoupling issue is sorted out. Which it may be trying to do, but has not done yet.
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Quoting SykKid:
isaac a pathetic storm

Once it is hits Fl as a cat 2...u may want to take those words back!
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So the wrap up begins.

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Quoting NEwxguy:
over 4000 comments on just a TS.Pretty amazing.


A TS projected to become a hurricane, projected to hit the US, that might be the first major to hit since 2005. Plus, there's nothing else to do...
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4196. wpb
anyone locate center lat/long?
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Quoting 69Viking:


I don't believe it. Look at the satellites with the lat/lan checked. If the center is moving NW the rest of him is moving West or slightly South of West and the center will separate from all that convection South of 15N. Ever fram that goes by more of that mass of convection goes below 15N. Something's out of whack with this storm.


Even the automated center finder on RAMMB site is moving it SW frame by frame, and that's the most north-casting tool I've ever seen.


From PR radar, there's just no way any NW turn has occurred unless the LLC has been totally ejected.

PR Long Range

If this is a northward jog, I'm a baloney sandwich.


When you compare lat and long of the alleged NHC LLC, and use islands on the radar as lat/long reference points, you find the coordinates of the alleged NHC LLC is outside/North of the main 2 feeder bands on radar which are all moving west.

Based on their coordinates, the LLC would be north of the moderate and heavy rains shown on the radar SSE of the station, especially if they are alleging it turned more northwards....yet it's plain as day on radar that the movement of the spiral banding does not support that at all. If anything, it's consolidating more to the southwest.

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Outflow seems to be improving.
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If it can manage to consolidate a little, I think we'll see a slightly more defined center develop between 15 and 15.3. Between the two blobs.
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Quoting Neapolitan:
The NHC updates at regularly scheduled times, of course; they're not likely to interrupt their regularly scheduled program with a special tropical weather statement for a storm 1,000 miles from anywhere and going nowhere soon. But their own software on which they rely says it's Joyce, so Joyce it is--even if that naming hasn't yet been publicly announced. And that'll happen in 30 minutes.

What's the point of arguing?
I've said it so many times, but people are just denying it.
The ATCF did not rename Ten into Joyce. It's just Tropical Storm Ten until the advisory is issued at 15UTC.
Sorry, but it's the truth.
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Quoting FLWeatherFreak91:


It looks as if he has finally gotten his self together this morning. Now the ramp up begins. I would put money down that this becomes a hurricane at 11pm.

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Quoting LargoFl:
it was posted this morning a ways back several times
Yeah, I got it...thanks.
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Quoting weatherb0y:
Whoa... where did you come from buddy? Great analysis.


Me?? lol

I've been here forever...

And thank you.
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Quoting Seflhurricane:
looks like the center is tighting up near 15.0 N and 65 W


yeah I think its somewhere there

Quoting SFLWeatherman:
12Z


now CLP5 doen't seem far fetched anymore

Quoting RTSplayer:


As do I.

Even if that pressure is accurate, it's definitely not the center fix, because there's just no way.

In order for that to be a center fix, the storm would need to be anti-cyclonic...lol...

Both center fix candidates are on PR radar, both the known mid-level rotation, and the NHC alleged LLC are on radar, and neither of them is anywhere near that far north or east.

Not to mention, the suspected LLC has actually moved closer to the mid level circulation anyway.


The storm is far more organized now than it ever has been, which is plain as day on RGB and funktop, not to mention Dvorak.



true plus remember thats at 10,000 Ft HH RECON is flying into the storm at 5,000Ft so HH will give us a better idea

Quoting 69Viking:


I don't believe it. Look at the satellites with the lat/lan checked. If the center is moving NW the rest of him is moving West or slightly South of West and the center will separate from all that convection South of 15N. Ever fram that goes by more of that mass of convection goes below 15N. Something's out of whack with this storm.

got that right
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4186. NEwxguy
over 4000 comments on just a TS.Pretty amazing.
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The NHC is on top of the movement of Isaac read the discussion from 8 am....if they are uncertain then
why rely on a novice looking at far less data than
they have ...
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4184. mati
The two typhoons in the Pacific seem to be on a collision course as Tembin does a loopDeLoop
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4183. Gearsts
Quoting 69Viking:


What shear? Visible below shows good outflow to the ENE and that wouldn't happen if the shear you claim is happening was there.

Anticyclone is west of the system.
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4181. msphar
In a few more days Nicaragua ought to get worried.
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Quoting AussieStorm:

Opinions are welcome but Calling out a place like that is what's called wish-casting or doom-casting. A lot of people here don't like that here, especially me.
Like I said I have been on here for years and am very aware of what wishcasting is. Goes on here constantly. I think their funny. There a trusted few on here that give unbiased forecast and most people know that.Heck he might end up being right! Until this thing gets ita act together all bets are off.
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4179. fire635
Good morning all... Ive been watching for days now... rarely comment but I feel compelled to remind people that you cant hang on to every single frame of movement on the satellite. You have to take a group of frames over a period of time to decide on changes in movement. This is, and has been, moving west... I believe a more west northwest movement is coming.
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Quoting MahFL:
Here most of the white is below 15, due to the ENE shear of about 13 mph. But the storm is moving WNW.



What shear? Visible below shows good outflow to the ENE and that wouldn't happen if the shear you claim is happening was there.

Member Since: August 25, 2006 Posts: 1 Comments: 3055
4177. Drakoen
Quoting weatherb0y:
Whoa... where did you come from buddy? Great analysis.


He's been on wunderground for a while. One of the best, top posters on this site.
Member Since: October 28, 2006 Posts: 57 Comments: 30563
Just remember...

"Dying is easy, comedy is hard."
Old Hollywood Saying


These fish were among those caught in a fence by flood waters brought by Hurricane Ike in West Orange, Texas, Tuesday, Sept. 16, 2008.
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Aren't these storms great. No matter how much we think we know where their going they remind us we are not in charge here.
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Quoting AussieStorm:

And until the NHC names TD10 TS Joyce, It's still TD10.
The NHC updates the public at regularly scheduled times, of course; they're not likely to interrupt their regularly scheduled program with a special tropical weather statement for a storm 1,000 miles from anywhere and going nowhere soon. But their own software on which they rely says it's Joyce, so Joyce it is--even if that naming hasn't yet been publicly announced. And that'll happen in 30 minutes.
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4173. Relix
Quoting 69Viking:


You can't see the center on radar, it's too far South to see it. The WNW movement you see is the movement of the feeder bands around the center you can't see to the South IMO. At least I haven't seen a radar where you can see the true center.


Did you zoom out the radar to its maximum point? You can definitely peak a rotating band of rains to the SE side. I really want a new center fix and hear what the NHC has to say.
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This is the Catholic Diocese of Charleston's Disaster Response Plan...it may be useful to some of you in coming daysLink
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The LLC is rotating around the massive mean circulation.IMO
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Sullivanweather is really knowledgeable, he usually concentrates on North East weather.
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Do we have the return of the Blob!?!
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Quoting Relix:


Well the system did develop a SW COC yesterday, could also do the same now. It could also be developing at 15N, 65W like Selfhurricane says. The radar shows the supposed real center moving at WNW, and the visible it seems its also lifting. The steering also supports this, and the NHC and the GFS. That's why there are multiple planes flying over Isaac, they need to keep feeding good information because this is one wild system with catastrophic probabilities.

I know some of you WANT the system in your backyard, but there are facts. I stand with the WNW movement, but the NHC has the final say


You can't see the center on radar, it's too far South to see it. The WNW movement you see is the movement of the feeder bands around the center you can't see to the South IMO. At least I haven't seen a radar where you can see the true center.
Member Since: August 25, 2006 Posts: 1 Comments: 3055
if you look at the water vapor loop region wide the trof is begining to dig down into the Se USA and should create a weakness very soon as the models are indications the big question is going to be how far west is the high going to build and whats isaacs location when that occurs
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Quoting StormTracker2K:


It's almost not even worth clarifying that as some just don't get it. I mean one can look at at this and tell this is for sure Joyce.


It is, just not until the upcoming advisory.
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4163. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 39262
4162. Relix
Is the MLC (which somehow seems like a separate entity) rotating around the NE center? Kinda seems like it.

And I vote for B. Moving WNW-NW in my eyes
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Quoting sullivanweather:


It isn't so much that the center keeps reforming...

It's more that there hasn't been a true inner core of the system, but rather a board tropical low pressure system. These center reformations are simply individual vorticies spinning around the overall broad circulation, which would be much easier to discern in a system with less convection. These little vorticies would eventually take over and become the dominant center is a smaller, more symmetric storm. But due to the large, broad, off-centered nature of Isaac these little areas of vorticity get spun off. As they spin off they sometimes drag the broader center in their direction but never far enough to change a longer term vector, which is why the storm 'center' (area of lowest pressure) has remained north of 15°N, despite all these apparent 'jumps' of the center. It's simply a representation of how the storm is currently structured.
Whoa... where did you come from buddy? Great analysis.
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4160. MahFL
Here most of the white is below 15, due to the ENE shear of about 13 mph. But the storm is moving WNW.

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Quoting Seflhurricane:
reed the models are currently argree with the NHC track thus far except 2 outliners
He probably thinks it will hit him from the west coast now. lol
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4158. JLPR2
Well, some breeze and not much to report besides that. XD Was expecting to be raining by now, but Isaac seems to have other plans.


Also, one can easily get an idea of where the LLC is, MLC, no idea. This storm still has some work to do before it can become a hurricane.
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Quoting StormTracker2K:


It's almost not even worth clarifying that as some just don't get it. I mean one can look at at this and tell this is for sure Joyce.

Yes should be Joyse and way better organized than his big brother Isaac...
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Quoting Bobbyweather:
1007mb (29.74 inHg) Sea Level (Surface) 26.0°C (78.8°F) 22.9°C (73.2°F) 205° (from the SSW) 40 knots (46 mph)

And exactly who are you referring to?
I think I'm not making up stuff.
Directed to trolls...
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4155. LargoFl
Quoting TriMOPER:


Great, I can rest easy now...LOL
LOL i was thinking the same thing..wonder if he will help board up my house
Member Since: August 6, 2011 Posts: 4 Comments: 39262




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1007mb (29.74 inHg) Sea Level (Surface) 26.0°C (78.8°F) 22.9°C (73.2°F) 205° (from the SSW) 40 knots (46 mph)
Quoting Bluestorm5:
I'm getting tired of morons making up stuff on this blog. It's disgusting... please get a job at McDonald's or something if you really want attention. They will need you.

And exactly who are you referring to?
I think I'm not making up stuff.
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Quoting LargoFl:
Gov. Scott says Florida 'will be prepared' for Isaac


Great, I can rest easy now...LOL
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Member Since: December 1, 2006 Posts: 2 Comments: 3625

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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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