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: 851 - 801

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 Grothar:
the FIM8

Now this is interesting






Mountains of Cuba take it apart?
Member Since: Posts: Comments:
18z GFS ensemble mean 144hr:

Member Since: Posts: Comments:
Link
Geeze - looking at this water vapor - it ure does look like there are two separate circulations out there in Isaac!!!!
Member Since: Posts: Comments:

CARQ Storm Position
Date: Aug. 22, 2012 18:00 Z (Wednesday)
Coordinates: 15.9N 60.4W
Wind Speed: 40 kts (46 mph, 74.1 km/h, 20.6 m/s)
MSLP: 1004 mb (29.65 inHg, 1004 hPa)
Storm Direction: W (280°)
Storm Speed: 19 kts (21.9 mph, 35.2 km/h)
System Depth: Medium
Isobar details: The last closed isobar has a pressure of 1010 mb. (29.83 inHg, 1010 hPa) The radius of the last closed isobar is 225 nm. (258.9 miles, 416.7 km)
Radius of Max Winds: 70 nm (80.6 miles, 129.6 km)
This position comes from the model file in the "aid_public" directory of the Automated Tropical Cyclone Forecasting (ATCF) System. CARQ stands for "Combined Automated Response to Query".
Member Since: Posts: Comments:
16.1 center coming back from the W.U. dead this evening.

Member Since: August 19, 2005 Posts: 5 Comments: 5451
Quoting Floodman:


Some of my friends who get to spend more time here than I do told me folks were asking after me, figured I'd dispel the rumours of my passing...LOL

What up?


Who do you think you are, a stranger coming on here and telling us what to do and making fun of us???
(How you doing, Flood??) :)
Member Since: Posts: Comments:
Quoting Levi32:
Interesting. NOAA-42 climbs from 750mb to 700mb and the wind shifts from southerly to easterly. Implies the mid-level center is still south of the surface center.



Tilted a little huh?
Member Since: Posts: Comments:
Quoting NoNamePub:


Hail Hail the gangs all here!

ALOHA BRAHDAH
hey no name Are you on Big Pine? I'm in Key West and I also speak Hawaiian;)
Member Since: Posts: Comments:
Hi Flood --- good to see you. Must have heard they were talking about you.
Member Since: Posts: Comments:
Quoting Levi32:


In general? Or mine specifically lol.


In general... I let ya'll explain them so I can understand them.
Member Since: Posts: Comments:
Several points for those lurking:

Storms often have competing circulations -- think the smarter ones on here told me once that is part of the formation process, leading to why the models prior to a storm becoming better defined are not as accurate. The beginning point changes.

Everyone has an opinion on the track, and they have good ideas behind them. But always listen to the pros for the real deal, they generally don't call it until they are sure. Its important to listen to the storm information for you area.

If you live anywhere in which this crazy mess of models has it possibly visiting, go over your hurricane plan. One thing is usually true: if you are 100% prepared, it won't hit you!
Member Since: Posts: Comments:
Quoting Stormchaser121:
"we are not off the hook by any means, we will have to watch Isaac for many days ahead" KFDM 6 Chief Meteorologist Greg Bostwick.


Lol. Saw that did ya? I can't keep up with the blog. :)
Member Since: Posts: Comments:
twc.is.on.fire.cantori.confused
Member Since: Posts: Comments:
Quoting serialteg:


i salute you miss
lots of work today and what remains
i still have to clear my house do prep work

tired :/

whelp, you live in a beautiful place
small price to pay is facing brutal winds, seas, and rains periodically :-)
Member Since: July 11, 2006 Posts: 14 Comments: 11358
Quoting Floodman:


I wouldnt make ANY CONUS bets this early; Hispaniola and Cuba look like foregone conclusins at this point but anything beyond say Saturday would be a bit premature


wow, someone with sense

thanks lol
Member Since: Posts: Comments:
the FIM8

Now this is interesting




Member Since: Posts: Comments:
Quoting nrtiwlnvragn:
Tropical Atlantic has the 18Z models


Link


DSHP looks legit
Member Since: Posts: Comments:
Quoting waterskiman:

Well the big question is will Biden be able to find his way out of the city


lmao
Member Since: Posts: Comments:
Quoting washingaway:


Who said that?
Carl Arredondo
Member Since: Posts: Comments:
Quoting HoustonTxGal:



18z GFS

Link
thanks for posting, and.... the words in my head sound like 'oh, shoot!!!!"
Member Since: Posts: Comments:
Quoting Floodman:


Some of my friends who get to spend more time here than I do told me folks were asking after me, figured I'd dispel the rumours of my passing...LOL

What up?


Hail Hail the gangs all here!

ALOHA BRAHDAH
Member Since: Posts: Comments:
Quoting captainmark:
New Orleans news met sticking with NHC forcast.
Says La not at risk. I would not make that bet.


I wouldnt make ANY CONUS bets this early; Hispaniola and Cuba look like foregone conclusins at this point but anything beyond say Saturday would be a bit premature
Member Since: Posts: Comments:
Quoting Floodman:


Some of my friends who get to spend more time here than I do told me folks were asking after me, figured I'd dispel the rumours of my passing...LOL

What up?


Hey Flood, good to see ya.
Member Since: Posts: Comments:
Quoting HoustonTxGal:


I have never been able to understand these graphics from the HH.


In general? Or mine specifically lol.
Member Since: Posts: Comments:
i think tropical storm emily is a good analouge for isaac right now stregnth wise. i remember there were 2 balls of deep convection but the pressure was high and winds were weak
Member Since: Posts: Comments:
Quoting HoustonTxGal:


Lord, I sure hope not.. I am in south TX

This storm has almost no chance of affecting texas.. Maybe 3%
Member Since: Posts: Comments:
Quoting Kowaliga:
Pondering its future?......looks almost stationary...



i believe that's the rotation, more N
Member Since: Posts: Comments:
Quoting hydrus:

Off topic but anyone notice the out of season NW US storm??
Member Since: August 1, 2012 Posts: 0 Comments: 269
Tropical Atlantic has the 18Z models


Link
Member Since: Posts: Comments:
Quoting gprxomstr:

Jerry!! What's going on man! Long time no hear.


Some of my friends who get to spend more time here than I do told me folks were asking after me, figured I'd dispel the rumours of my passing...LOL

What up?
Member Since: Posts: Comments:
Quoting Levi32:
Interesting. NOAA-42 climbs from 750mb to 700mb and the wind shifts from southerly to easterly. Implies the mid-level center is still south of the surface center.



I have never been able to understand these graphics from the HH.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Quoting Grothar:


Just ask me. I get them a half hour before :)



Pretty soon I'll be plotting my own from the ATCF data file as soon as it appears on their server. I might actually dabble with that this afternoon while we watch Isaac do the same 'ol same 'ol.
Member Since: Posts: Comments:
anyone think we will have Joyce at 11pm?
Member Since: October 15, 2011 Posts: 0 Comments: 4410
Quoting zoomiami:


That's so the rest of us know where you live!


bring me a gift! lol
Member Since: Posts: Comments:
If you think Isaac is a mess, have a look at Typh Tembin Forecast Track - looks like Taiwan may well get a double whammy!
Member Since: Posts: Comments:
Quoting captainmark:
New Orleans news met sticking with NHC forcast.
Says La not at risk. I would not make that bet.


Who said that?
Member Since: July 14, 2006 Posts: 0 Comments: 1163
Quoting waterskiman:

Well why didn't you post that in the first place instead of your political opinion. Tampa is a city with heaps of people wether there is a convention there or not


I typically start with quoting the headline and if you read the article you will see that there is "no political opinion" from me or anybody else. There are only documented facts and science.
Member Since: Posts: Comments:
Quoting Stormchaser121:
My thoughts on Isaac is that it will clip Cuba...and move northwestward towards LA...then the ridge builds in and forces it more west...maybe to western LA or TX. Thats my thoughts on it. Im getting this idea from the Euro model.


Did you catch the locals? Both said we'd have to watch it. Greg said the high over us could move northward slowing Isaac down in the gulf. And we're not out of the woods yet. No time to panic of course. Just keep an eye on it. And hope it stays weak wherever it goes. You know the drill. :)
Member Since: Posts: Comments:
Quoting CybrTeddy:
Definite closed circulation at ~15.5N.

Winds went from pure west shifting to the east as they passed through the circulation with a pressure of 1004mb now.
Winds are still incredibly lackluster. No reports of 40kt surface winds that I've seen?
Member Since: Posts: Comments:
Quoting serialteg:


i know where i live chick LOL


That's so the rest of us know where you live!
Member Since: Posts: Comments:
Interesting. NOAA-42 climbs from 750mb to 700mb and the wind shifts from southerly to easterly. Implies the mid-level center is still south of the surface center.

Member Since: Posts: Comments:
New Orleans news met sticking with NHC forcast.
Says La not at risk. I would not make that bet.
Member Since: Posts: Comments:
Pondering its future?......looks almost stationary...

Member Since: Posts: Comments:
Quoting Tazmanian:




i give you a cookie
Aww, thanks Taz. :'')
Member Since: Posts: Comments:
just curious, based on its apperance where do you think isaac's center is? (coordinates approximately)
I see something near 15.5 or 6 N
Member Since: Posts: Comments:
Quoting Grothar:


Just ask me. I get them a half hour before :)


...Or 24 hours late.
Member Since: Posts: Comments:
Quoting Grothar:


Just ask me. I get them a half hour before :)



Those are initialized back at 52W. Those seem pretty old...
Member Since: Posts: Comments:
Er, today's the ah, 22nd Gro.


Looks for his Tabasco to pack deep in the Suitcase.
Member Since: Posts: Comments:
Quoting Chicklit:

But I don't! We have Ponce Inlet here in New Smyrna Beach, but Isaac is a long way off from here. :-)


i salute you miss
lots of work today and what remains
i still have to clear my house do prep work

tired :/
Member Since: Posts: Comments:
Quoting Grothar:


Is that the 18z early models? If it is its more northerly.
Member Since: Posts: Comments:

Viewing: 851 - 801

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.