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: 1451 - 1401

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

Quoting pottery:

Yeah, I am watching that too.
Still rumbling down south.... you must be seeing the flashes.
Try RAdar doppler, is working for San Juan, but the center is not showing yet..if there is one that is..
Member Since: Posts: Comments:
1450. KEEPEROFTHEGATE (Mod)
T.C.F.W.
10L/TD/XX/CX
MARK
13.01N/39.95W
Member Since: Posts: Comments:
1448. SLU
Quoting pottery:

Yeah, I am watching that too.
Still rumbling down south.... you must be seeing the flashes.


Batten down the hatches. Beware of the potent tail of Isaac.
Member Since: Posts: Comments:
Quoting HoustonTxGal:


Thats almost the size of Texas


Hurricane Katrina was 415 miles in diameter
Member Since: Posts: Comments:
1446. GetReal
Member Since: Posts: Comments:
1445. LargoFl
where he goes, no one knows
Member Since: Posts: Comments:
Quoting Tazmanian:




i hate the TD 10s they nevere be come named
I agree last year didn`t get name either nor 2007.
Member Since: Posts: Comments:
This convection keeps growing and getting cooler. U can see a little white pixel now.

Member Since: Posts: Comments:
1442. icmoore
Quoting pottery:

I'm going with the middle one.....


You are always so sensible :)
Member Since: Posts: Comments:
Quoting hydrus:
Why...Thats almost as big as jfv,s ego.

nar, that immeasurable. lol
He'll admit it also.
Member Since: Posts: Comments:
Quoting aislinnpaps:
I have figured out the solution of the two blobs of Issac. It's simple and keeps the majority of people happy. The northern blob splits off and goes to Florida, the southern one goes into the Gulf to keep everyone trying to figure out where...


nominee for best comment of the day. Humor is needed in long-hours of storm tracking to maintain sanity.
Member Since: Posts: Comments:
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:








Here the ridge is going to be broken (GFS 84 hour):

Levi will you change the number of name storms you predict for this season or you are going to stay with the number you forecast?
Member Since: Posts: Comments:
1438. Relix
Holy... just as I shed off any worries about Isaac it pulls that out of nowhere. Now the COC would be relocated to the NE. Amazing AMAZING convection.
Member Since: Posts: Comments:
1437. hydrus
Quoting JLPR2:
Using Google Earth's ruler, I got that Isaac is 509miles from N to S and 850miles in its widest area from E to W.
Why...Thats almost as big as jfv,s ego.
Member Since: Posts: Comments:
1436. trey33
Quoting luigi18:

yes i saw hin on TWC lol


Wish he would hurry up and come to Tampa. He's a great storm deflector.
Member Since: Posts: Comments:
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:





Here the ridge is going to be broken (GFS 84 hour):



Georges was also a major hurricane before he started interacting with the islands. Isaac should not be more than a Cat 1 before making landfall or land interaction with an island.
Member Since: Posts: Comments:
Quoting AtHomeInTX:
12z EURO ensemble mean. All over LA and jumps west at the end.





In a lot more agreement than yesterday.
I think we will find even more agreement with the next euro run. compared to the other major models
Member Since: Posts: Comments:
Quoting JLPR2:
Using Google Earth's ruler, I got that Isaac is 509miles from N to S and 850miles in its widest area from E to W.


Thats almost the size of Texas
Member Since: Posts: Comments:
1432. pottery
Quoting HuracanTaino:
North winds ,who said that... ?

I'm asking.
What direction are the winds?
Member Since: Posts: Comments:
Quoting mcluvincane:


Most definitely

yep

Quoting floridaboy14:
i think the center is now just south of the intense convection near 15.4N. where do you see it?


yeah I would say its about there

Member Since: June 13, 2009 Posts: 2 Comments: 12180
1430. pottery
Quoting aislinnpaps:
I have figured out the solution of the two blobs of Issac. It's simple and keeps the majority of people happy. The northern blob splits off and goes to Florida, the southern one goes into the Gulf to keep everyone trying to figure out where...

I'm going with the middle one.....
Member Since: Posts: Comments:
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:








Here the ridge is going to be broken (GFS 84 hour):



georges was a good one. last cat2 to really hit PR. last real good cane we've had.
Member Since: Posts: Comments:
1428. hydrus
Quoting Wunderwood:
Someone posted on another forum that Jim Cantore said that the northern part of the storm was taking over.
Complete opposite of whats been..And that definitely means Isaac is ready to what happens normally this time of year.
Member Since: Posts: Comments:
Wow 6 post a minute.
Member Since: Posts: Comments:
1426. JLPR2
Using Google Earth's ruler, I got that Isaac is 509miles from N to S and 850miles in its widest area from E to W.
Member Since: Posts: Comments:
Isaac is leaving behind that ULL from the NE that was holding it from moving NW and consolidating its NE CDO....

Looks like it will start moving WNW more or the expected NW turn into Hispaniola... and will develop...

Member Since: Posts: Comments:
Quoting pottery:

On the WEST side of PR ? North winds ???
North winds ,who said that... ?
Member Since: Posts: Comments:
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:





Here the ridge is going to be broken (GFS 84 hour):

if it shows a major in the gulf the next 2 runs, we shouldnt discount it yet
Member Since: Posts: Comments:
Quoting GTcooliebai:
Amazing considering this is supposed to be an El Nino year and the GFS is still spitting out storms off of Africa.
I know.Bust season for sure...
Member Since: Posts: Comments:
Quoting Levi32:
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:





Here the ridge is going to be broken (GFS 84 hour):

I'm with the GFS too, the Euro is too slow imo and Isaac is still moving at a fast pace. The pattern should start to become more progressive as we enter into El Nino.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
You guys and gals going to fast! Slow down! Im on page 24
Member Since: September 18, 2010 Posts: 0 Comments: 5234
Quoting TropicalAnalystwx13:
I have doubts about Tropical Depression Ten becoming anything more than a minimal tropical storm. Wind shear from the outflow of Tropical Storm Isaac and from an Upper Level Low is causing the system to be disorganized and the center is very elongated from north to south. With the system embedded within a dry environment, I wouldn't be surprised if it dissipates completely. The NHC is far too bullish with the intensity forecast.




TD 10 better get name


i hate TD 10s evere year this about do not get named
Member Since: Posts: Comments:
I have figured out the solution of the two blobs of Issac. It's simple and keeps the majority of people happy. The northern blob splits off and goes to Florida, the southern one goes into the Gulf to keep everyone trying to figure out where...
Member Since: Posts: Comments:
Quoting sunlinepr:


But the last flare-up makes it look like relocating N....

Seems its back at 16.2 and 62, west fo Guadaloupe...
Member Since: Posts: Comments:
this is driving me nuts...Gordon was in water barely warm enough for tropical development, basically in the SAL, and maintained its border line cat 2/3 strength....GET IT T/G isaac! im tired of being on this blog 24/7 waiting for you to do so! blob over the LLC again, but thats happend 3 nights in a row lets see if it stays this time???
Member Since: Posts: Comments:
Quoting Charliesgirl:
yeah, I heard that. shakes head...


Anyone send the local met a email and tell him to not so easily disregard the best weather model in the world 5-7 days away from landfall?
Member Since: September 18, 2010 Posts: 0 Comments: 5234
1414. icmoore
Quoting NativeSun:
Up the middle of Florida as a cat 2or 3. first hit Isalmodada in the Florida Keys any takers?


Rubbing salt into Floridan's worries by chance?
Member Since: Posts: Comments:
1413. Levi32
I still think the Euro is more likely to be bogus. The last time I can remember a storm running parallel along Hispaniola and Cuba, then continuing the shortcut track into Louisiana, was Georges in 1998, and it took this strong of a ridge to keep him from recurving into the Florida area:








Here the ridge is going to be broken (GFS 84 hour):

Member Since: Posts: Comments:
I have doubts about Tropical Depression Ten becoming anything more than a minimal tropical storm. Wind shear from the outflow of Tropical Storm Isaac and from an Upper Level Low is causing the system to be disorganized and the center is very elongated from north to south. With the system embedded within a dry environment, I wouldn't be surprised if it dissipates completely. The NHC is far too bullish with the intensity forecast.

Member Since: Posts: Comments:
1411. luigi18
Quoting Wunderwood:
Someone posted on another forum that Jim Cantore said that the northern part of the storm was taking over.

yes i saw hin on TWC lol
Member Since: Posts: Comments:
Quoting RitaEvac:
This is much farther south of Puerto Rico than was anticipated seems like




looks to have made a jog south of west if you ask me... just watch the 15 degree mark
Member Since: August 6, 2011 Posts: 0 Comments: 226
1409. klew136
Quoting Bradenton:
I'll see your Florida Keys and raise you a Tampa Bubble.

Please no wish casting, this is scary

Member Since: Posts: Comments:
Would be interesting if the center was located at 15.3N 63W.
Member Since: Posts: Comments:
The models have been trending west the last few runs as Isaac has remained weak and dis- organized. They may start trending back to the east the next few runs if Isaac strengthens faster than expected.

Deeper storm more likely to be drawn to weakness in ridge and all that...

The angle that this storm will take coming north makes forecasting the effects on Florida maddening. Even 36 hours out, the margin of error, if it is say, 30- 50 miles either way (radius--from the center point) could make the difference at any one location on the Fla peninsula between "Maybe" strong hurricane force winds and minimal TS winds.

Anyhow...that was a long winded post for a long time lurker..
Member Since: Posts: Comments:
Someone posted on another forum that Jim Cantore said that the northern part of the storm was taking over.
Member Since: Posts: Comments:
Quoting tropicfreak:


The shear from Isaac is keeping it at bay.

But I suspect that we will see TS Joyce tomorrow at some point.




i hate the TD 10s they nevere be come named
Member Since: Posts: Comments:
The current NHC forecast does not take Issac to a hurricane until he is around 73w due South of Haiti. If he is able to reach hurricane strength before that longitude, I would think that the NHC track would shift a tad back towards Florida as opposed to into the extreme Eastern Gulf.......Have to see how he looks in the am.
Member Since: Posts: Comments:
1403. ackee
I wonder where the NHC will postion Issac centre at 11PM ? it does seem like the centre is further south to me . I expect the model will shift further west, also the longer Isaac battle with two centre the weaker it will reamin for now
Member Since: Posts: Comments:
1402. LargoFl
Member Since: Posts: Comments:
Quoting tropicfreak:


The shear from Isaac is keeping it at bay.

But I suspect that we will see TS Joyce tomorrow at some point.
Amazing considering this is supposed to be an El Nino year and the GFS is still spitting out storms off of Africa.
Member Since: August 31, 2009 Posts: 0 Comments: 5628

Viewing: 1451 - 1401

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
54 °F
Overcast