Isaac reorganizing as it blows through the Lesser Antilles

By: Dr. Jeff Masters , 8:34 PM GMT on August 22, 2012

Share this Blog
56
+

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

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 201 - 151

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 83 | 84 | 85 | 86 | 87 | 88 | 89 | 90Blog Index

IMO... The Islands are looking as a strong TS and the Sothern tip of Fl.... TS or Cat 1 hurricane at best. I am mot trying to down play this at all, because I know what damage flooding and wind can do. Although if Isaac winds up in the GOM for very long, as the EURO portrays.... it would be even worse on the Gulf Coast.
Member Since: June 13, 2009 Posts: 0 Comments: 6747
Quoting catastropheadjuster:
Just a question you all. Do you think the folks in Mobile,Al should be worried about this, or is it most likely going to do what the GFS shows? I'm probably like alot of other folks just don't have the money to go out and buy a bunch of stuff, seeing know that all my kids and there wife and grandchildren all live with us again. It's hard these days to make it, and a lot of you all know what I am talking about, I have a extra 8 mouths to feed not counting hubby and I. I have water and batteries.

Sheri
Quoting catastropheadjuster:
Just a question you all. Do you think the folks in Mobile,Al should be worried about this, or is it most likely going to do what the GFS shows? I'm probably like alot of other folks just don't have the money to go out and buy a bunch of stuff, seeing know that all my kids and there wife and grandchildren all live with us again. It's hard these days to make it, and a lot of you all know what I am talking about, I have a extra 8 mouths to feed not counting hubby and I. I have water and batteries.

Sheri


YES - Right now I'm biased to the Euro model. A storm in Mobile will close BOTH tunnels and the only way is north and west.....I'm a Frederick AND Camille survivor!
Member Since: Posts: Comments:
Quoting lottotexas:
models show td10 becoming Joyce and turning into the ridge before isaac

I see joyce on the line W of 50W (not sure what line that is) and ISAAC over E Cuba...
Member Since: Posts: Comments:
Isaac is still a very big storm. It could end up being kind of like Irene in the sense that its pressure may come way down but the winds never really catch up.

Member Since: Posts: Comments:
We could maybe,,,see that right Lobe fade and a new Overall become dominate tomorrow as it races west at 22...or vice versa as well.

: )


Dvorak Loop



Member Since: Posts: Comments:
Quoting Maineweatherguy20023:

I third that with
POOF!


I fourth the POOF.

Member Since: Posts: Comments:
Quoting Patrap:
WV Loop dee Loop



Looking at WV there is an ULL thats causing dry air for the system's NE quadrant this should persist for the next 24 hours. As the ULL is racing SSW. Also there is a TUTT cell over Cuba thats enhancing NW outflow. The atlantic ULL should pass to the E and SE of Issac tomorrow so the dry air should be focus between Issac and TD10 with the ULL wrapping that dry air into TD10. This should give Issac's NE quadrant a good break from dry air intrusion.
Member Since: Posts: Comments:
Quoting catastropheadjuster:
Just a question you all. Do you think the folks in Mobile,Al should be worried about this, or is it most likely going to do what the GFS shows? I'm probably like alot of other folks just don't have the money to go out and buy a bunch of stuff, seeing know that all my kids and there wife and grandchildren all live with us again. It's hard these days to make it, and a lot of you all know what I am talking about, I have a extra 8 mouths to feed not counting hubby and I. I have water and batteries.

Sheri

I'd start checking through your normal hurricane plan, making sure all is in order. Also, with that many mouths to feed (especially with the children), you may want to have some plans in mind to move some of your family away from the coast, just in case.
Member Since: August 3, 2008 Posts: 16 Comments: 5880
Quoting Patrap:


Seems to be on the side of the western models.
Member Since: Posts: Comments:
Quoting Kowaliga:
COC back over Guadeloupe?!?....I give up!


Need a new microwave.
Member Since: Posts: Comments:
Quoting catastropheadjuster:
Just a question you all. Do you think the folks in Mobile,Al should be worried about this, or is it most likely going to do what the GFS shows? I'm probably like alot of other folks just don't have the money to go out and buy a bunch of stuff, seeing know that all my kids and there wife and grandchildren all live with us again. It's hard these days to make it, and a lot of you all know what I am talking about, I have a extra 8 mouths to feed not counting hubby and I. I have water and batteries.

Sheri


From a fellow Mobilian.....of course keep an eye on it.
Member Since: Posts: Comments:
Quoting Patrap:
Where's the 2:00 P.M. ?
Member Since: Posts: Comments:
Quoting MississippiWx:
A true center would feel influenced to develop between the two big blobs of convection.


I think there is one there... not sure...
Member Since: Posts: Comments:
Quoting MississippiWx:
A true center would feel influenced to develop between the two big blobs of convection.



Er, dat is the COC in between and as I mentioned earlier, the "Bi-lobing" has balanced the Stack some, and the mean is rotating around that center.

Member Since: Posts: Comments:
186. yoboi
Quoting Patrap:


Your Local EMG Mgmt Team should have info for Special Needs Care in Evacs and other.



it's also good to pre-register with them because the first evacuated are special needs before general population....
Member Since: August 25, 2010 Posts: 7 Comments: 2334
Quoting mfcmom:
Word up guys I am pretty much a lurker, but live in Panama City, please keep us posted as to probabilities, I need an extra day or two to prep, have seven adopted kids five are handicapped. I am really depending on you all and thank you so much.

****


A word of advice. Do not rely on the crowd for advice. Rely on the NHC.
Member Since: Posts: Comments:
Quoting Maineweatherguy20023:

Not Levi but TD 10 would be too late!
models show td10 becoming Joyce and turning into the ridge before isaac
Member Since: Posts: Comments:
A true center would feel influenced to develop between the two big blobs of convection.

Member Since: Posts: Comments:
Quoting Patrap:

RNC or NOT
Member Since: Posts: Comments:
If there is better organization and Isaac starts to intensify over night it would would shift the models back once again. To many variables to let your guard down. I think the windshield wipers are on now for the models.
Member Since: Posts: Comments:
Quoting clwstmchasr:


I second that POOF.

I third that with
POOF!
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Quoting hurricanehanna:
People need to remember that this is a large system. If it does become a hurricane, it will affect more than just the anticipated landfall area. Areas to the North and East of the storm are greatly impacted by the rain, winds and storm surges. Think "outside the cone" once they can pinpoint the landfall area.

LOL 'think outside the cone', I like that one,
Member Since: Posts: Comments:
Just a question you all. Do you think the folks in Mobile,Al should be worried about this, or is it most likely going to do what the GFS shows? I'm probably like alot of other folks just don't have the money to go out and buy a bunch of stuff, seeing know that all my kids and there wife and grandchildren all live with us again. It's hard these days to make it, and a lot of you all know what I am talking about, I have a extra 8 mouths to feed not counting hubby and I. I have water and batteries.

Sheri
Member Since: Posts: Comments:
Quoting jeffs713:

Translation = Issac is a hot mess, and is fighting with his wife over where to put the LLC.

LOL LOL LOL LOL Does enyone think isaac will be a wave??
Member Since: Posts: Comments:
Quoting bappit:
Quoting Levi32:
The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.

That is so bogus. I won't waste time pointing out the evidence. These statements are based on fantasy.



...Or science, but you know. To each his own, I guess.
Member Since: Posts: Comments:
Quoting Levi32:
The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.



So were to believe you're right while Dr. M and all he experts at the NHC are wrong? Normally I have no problem taking your word for it but too many experts are talking about the dry air in the NE corner of Isaac and you're saying they're all wrong. I'm no weather expert but even I can see the upper level low driving dry air into the NE corner of Isaac, pretty obvious on the water vapor loops.

Member Since: Posts: Comments:
ok right now I will leave Isaac as a big mess with multiple LLCOC moving W untill they can find a good LLCOC
Member Since: Posts: Comments:
Sorry if that sounded a lil rude u guys... im just not having the best day... u know... one of those i wish i slept in days??? however... i will never sleep in when an active storm haha
Member Since: Posts: Comments:
Quoting Kowaliga:
COC back over Guadeloupe?!?....I give up!


I see 2 COCs
Member Since: Posts: Comments:
Quoting bappit:
Quoting Levi32:
The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.

That is so bogus. I won't waste time pointing out the evidence. These statements are based on fantasy.



If you won't support a differing opinion then I can't really reconsider mine.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Quoting lottotexas:
Levi,do you think TD10 will play a part in Isaac's turn ? TD10 should help the trough erode the ridge.

Not Levi but TD 10 would be too late!
Member Since: Posts: Comments:
Quoting Levi32:
The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.

That is so bogus. I won't waste time pointing out the evidence. These statements are based on fantasy.

Member Since: May 18, 2006 Posts: 10 Comments: 5995
People need to remember that this is a large system. If it does become a hurricane, it will affect more than just the anticipated landfall area. Areas to the North and East of the storm are greatly impacted by the rain, winds and storm surges. Think "outside the cone" once they can pinpoint the landfall area.
Member Since: Posts: Comments:
Quoting mfcmom:
Word up guys I am pretty much a lurker, but live in Panama City, please keep us posted as to probabilities, I need an extra day or two to prep, have seven adopted kids five are handicapped. I am really depending on you all and thank you so much.


Your Local EMG Mgmt Team should have info for Special Needs Care in Evacs and other.

Member Since: Posts: Comments:
Quoting MAweatherboy1:
Sorry if it was posted but here's today's CPC map:


Wish that green would shifta lil west as the rest of the country needs it
Member Since: Posts: Comments:
Quoting Kowaliga:
COC back over Guadeloupe?!?....I give up!


yes definitely
Member Since: Posts: Comments:
Quoting Levi32:
The only place dry air has been wrapping in during Isaac's entire life has been the western side, and that hasn't affected him greatly. He has a very nice moisture bubble that has remained perfectly intact, with again the only intrusion from the west, and minor at that.

Levi,do you think TD10 will play a part in Isaac's turn ? TD10 should help the trough erode the ridge.
Member Since: Posts: Comments:
Quoting Levi32:
The sudden wind shift from northerly to southwesterly as the plane turned west of Martinique is a great illustration of the WSW to ENE elongation of the center.


Translation = Issac is a hot mess, and is fighting with his wife over where to put the LLC.
Member Since: August 3, 2008 Posts: 16 Comments: 5880
looks.like.a.pacific.system.
Member Since: Posts: Comments:
Quoting jeffs713:

He still looks kinda flat in the NE quad, tho.


Because his circulation is literally elongated.
Member Since: Posts: Comments:
Sorry if it was posted but here's today's CPC map:

Member Since: Posts: Comments:
COC back over Guadeloupe?!?....I give up!

Member Since: Posts: Comments:
The sudden wind shift from northerly to southwesterly as the plane turned west of Martinique is a great illustration of the WSW to ENE elongation of the center.

Member Since: Posts: Comments:

Viewing: 201 - 151

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 83 | 84 | 85 | 86 | 87 | 88 | 89 | 90Blog Index

Top of Page

About

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

Local Weather

Overcast
65 °F
Overcast