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: 2901 - 2851

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 AllStar17:
Any center is probably SOUTH of 15N.


I agree at this point .. as the possible center for this storm.. We will c.. Might have some more tricks up his sleeve.. grrrr
Member Since: August 15, 2008 Posts: 0 Comments: 2133
wow RECON is now at 14.8N 63.7W and still has not hit the LLCOC
Member Since: Posts: Comments:
Quoting Levi32:
0z GFS ensembles: still really no change. It has the SW dip with the center during the first 12 hours though which is interesting.


Link?
Member Since: Posts: Comments:
Just took a look at the 00z GFDL and HWRF models. Both have shifted to a SE Florida landfall. Take it for what it's worth, just a model run. Expect the 5AM track from the NHC to shift east.
Member Since: Posts: Comments:
Any center is probably SOUTH of 15N.
Member Since: Posts: Comments:
RNC yes, my it is late. First time I've drank in awhile too. :)
Member Since: Posts: Comments:
2894. Levi32
0z GFS ensembles: still really no change. It has the SW dip with the center during the first 12 hours though which is interesting.

Member Since: November 24, 2005 Posts: 635 Comments: 26652
Quoting wunderkidcayman:
boy if this was to go into the next model runs it could potentialy chage track Further S and W by quite a bit
and it should shift the forecast track S and W as well


Yawn - anything else to say buddy.
Member Since: Posts: Comments:
...Nope. No center on this batch of data.
Member Since: Posts: Comments:
Quoting KoritheMan:


Whatever will it show, I wonder?
Well right now it shows a humongous ridge in the Atlantic.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting GTcooliebai:
While you guys look for the center, Euro initialized:


That is a crapload of highs marked stretching form Bermuda back towards the Azores.
Member Since: Posts: Comments:
Member Since: August 15, 2008 Posts: 0 Comments: 2133
Still an hour or so from Isaac showing his cards.
Member Since: Posts: Comments:
2887. Grothar
Quoting AllStar17:
Center is probably near 15 N...or maybe even a little south of 15 N.


It would be closer to 14.8 or 9. Should show a little pressure drop, too.
Member Since: Posts: Comments:
Quoting trHUrrIXC5MMX:
I posted this up earlier...but its still my current idea for the track
the white shade inside the cone is my best bet...

add or take 5 mph from the wind intensities shown...



GFS the best model solution right now, your forecast is lying to the south in the next 24-48 hours,the 11 pm track of NHC is were i think isaac is going to go,specially if is coming to slow down....even if the COC is relocated a little bit to the south
Member Since: Posts: Comments:
Quoting GTcooliebai:
While you guys look for the center, Euro initialized:



Whatever will it show, I wonder?
Member Since: March 7, 2007 Posts: 582 Comments: 20768
Quoting Tribucanes:
NHC getting moved or postponed has gotta be pushing 10-20% at this point.


You mean the RNC?
Member Since: Posts: Comments:
While you guys look for the center, Euro initialized:

Member Since: August 31, 2009 Posts: 0 Comments: 5628
boy if this was to go into the next model runs it could potentialy chage track Further S and W by quite a bit
and it should shift the forecast track S and W as well
Member Since: Posts: Comments:
RNC getting moved or postponed has gotta be pushing 10-20% at this point.
Member Since: Posts: Comments:
Quoting RussianWinter:
Sigh, there are two centers around a common center. Stop and stare at the shortwave for a while and you will begin to see it. The center between those two is a bit north of 15n.


yeah i know, but its hard to stare at it for awhile if u r really tired and sometimes ur eyes r playing tricks with ya.. It should be a broad center once it becomes more healthier..
Member Since: August 15, 2008 Posts: 0 Comments: 2133
2879. Levi32
The thing is showers moving from the NNW are visible around 15.5-16.0N on PR radar, implying a center east of there, not necessarily the only center:

Member Since: November 24, 2005 Posts: 635 Comments: 26652
Quoting Levi32:
15.25N no center yet.


That is well east of most of the convection.

Member Since: Posts: Comments:
2877. Ryuujin
Gotta Felling Mississippi and Gothar got you on this one Levi. May end up wrong, but just a gut feeling.
Member Since: Posts: Comments:
I see one south of 15n and one north of 15. I see to vortexes, srs.
Member Since: Posts: Comments:
Quoting AllStar17:
They should come across a center by the time the next batch of data comes in, I'd think.

and if not
Member Since: Posts: Comments:
Good low pressure energy this evening over Colombia.



Member Since: Posts: Comments:
It's always really difficult to find a center at night. Especially in disorganized systems like Isaac.
Member Since: March 7, 2007 Posts: 582 Comments: 20768
They should come across a center by the time the next batch of data comes in, I'd think.
Member Since: Posts: Comments:
Sigh, there are two centers around a common center. Stop and stare at the shortwave for a while and you will begin to see it. The center between those two is a bit north of 15n.
Member Since: Posts: Comments:
well RECON reached 15.25N 63.9833W still don't hit the LLCOC hmm seem I may be right its S of 15N
Member Since: Posts: Comments:
Quoting AllStar17:
Center is at least 75 miles SW of where the NHC had it at 11. Winds now just starting to shift a bit.


Where u think the actual dominate center is at.. Some reason im thinking it is SW like u said compared what the NHC has said.. Im leaning towards around 15n and near 65w in that general area.. I want to wait for a new image to be sure tho..
Member Since: August 15, 2008 Posts: 0 Comments: 2133
2868. Levi32
18z GFS ensembles had Isaac dipping down near or below 15N this evening, actually.

Member Since: November 24, 2005 Posts: 635 Comments: 26652
At 15.2N and still no center fix.
Member Since: Posts: Comments:
Center is probably near 15 N...or maybe even a little south of 15 N.
Member Since: Posts: Comments:
2865. Levi32
15.25N no center yet.

Member Since: November 24, 2005 Posts: 635 Comments: 26652
2864. Grothar
Quoting AllStar17:
Center is most definitely south of where the NHC has it.


So we weren't seeing things? This could change a lot of the models.
Member Since: Posts: Comments:
It just looks like Isaac did a wsw job for a few frames.. but like i said earlier, im probably seeing stuff and really tired..
Member Since: August 15, 2008 Posts: 0 Comments: 2133
Took awhile, but here's my blog if anyone wants to read it. Not sure who's still up to do that, though... lol
Member Since: March 7, 2007 Posts: 582 Comments: 20768
Center is at least 75 miles SW of where the NHC had it at 11. Winds now just starting to shift a bit from E to NE.
Member Since: Posts: Comments:
What's your thinking on Isaac's future and path Skyepony? Hope he's not going to be venturing in your neck of the woods.
Member Since: Posts: Comments:
Quoting redwagon:

Recon is still in it? Whoa. Hope they were far away from that beyond-extreme convection burst.

nope new flight

Quoting AllStar17:
Center is most definitely south of where the NHC has it.


I did say it was

Quoting MississippiWx:
Recon is at 15.7N and they still haven't reached the center. The center position at 11pm was at 15.8N.


as I said I think the LLCOC is further S and W of NHC location and last RECON fix
Member Since: Posts: Comments:
Quoting gordydunnot:
Joshsen18 whats the worst that can happen? You can't leave the hotel room.


A lot of other things you could do inside if it's a bit windy and rainy!
Especially when on your honeymoon.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Want to read pure terror; read Jeff's Hugo HH mission story. Literally he was seconds, not minutes, from death. Talk about an oops moment. 4 missions never made it back, Jeff's story puts you with those who never made it back. Truly a story of trust your intuition or else.
Member Since: Posts: Comments:
Quoting MississippiWx:
Recon is at 15.7N and they still haven't reached the center. The center position at 11pm was at 15.8N.


Maybe its my eyes or I am just really tired.. To me it looks this broad center might be near 15N and 65W by looking at these images and all.. But most likely it might be incorrect..
Member Since: August 15, 2008 Posts: 0 Comments: 2133
2854. Skyepony (Mod)
OSCAT missed too.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Quoting cirrocumulus:
Yes.


I knew I wasn't alone.
Member Since: Posts: Comments:
so far RECON says LLCOC is further SW than the 11pm advisory location
Member Since: Posts: Comments:

Viewing: 2901 - 2851

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

Mostly Cloudy
53 °F
Mostly Cloudy