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Isaac remains disorganized

By: Dr. Jeff Masters, 8:29 PM GMT on August 23, 2012

Tropical Storm Isaac has strengthened slightly, data from the hurricane hunters show, but the storm remains disorganized and difficult to forecast. If you have to make decisions based upon what Isaac will do, I highly recommend that you wait until at least Friday morning to make a decision, if at all possible, as the forecasts then should be of significantly higher accuracy. Isaac continues to have a large area of light winds about 50 miles across near its center. This makes the storm subject to reformations of the center closer to areas of heavy thunderstorms that form, resulting in semi-random course changes. Until Isaac consolidates, the lack of a well-defined center will make forecasts of the storm's behavior less accurate than usual. An Air Force Reserve hurricane hunter aircraft is in Isaac this afternoon, and has found that surface tropical storm-force winds on the east side of the storm, south of Puerto Rico, have undergone a modest expansion. These winds were mostly in the 40 mph range, with a few areas of 45 mph winds. The surface pressure remained fairly high, at 1004 mb. Infrared and visible satellite loops show that Isaac has fairly symmetric circular cloud pattern, with developing spiral bands that are contracting towards the center, which suggests intensification. However, the storm has a very clumpy appearance, and is a long way from being a hurricane. Given the storm's continued reluctance to organize, Isaac is unlikely to reach hurricane strength before encountering Haiti and Cuba. An analysis of upper level winds from the University of Wisconsin CIMSS shows an upper-level outflow channel well-established to the north, and an intermittent outflow channel to the south. Radar imagery from Puerto Rico shows some weak low-level spiral bands that haven't changed much in intensity or organization this afternoon. NOAA buoy 42060 reported 1-minute mean winds of 35 mph and a wind gust of 40 early this afternoon. At St. Thomas in the Virgin Islands, wind gusts up to 45 mph were observed early this afternoon. Isaac's rains caused major flooding last night in Trinidad and Tobago, the southernmost islands of the Lesser Antilles chain, according to the Trinidad Express. Isaac's rains have not been heavy enough today to cause flooding problems on other islands.


Figure 1. True-color MODIS image of Isaac taken at 1:40 pm EDT August 23, 2012. Image credit: NASA.

Latest model runs for Isaac
The latest set of 12Z (8 am EDT) model runs have shifted to the west compared to the previous set of runs. The models continue to show a west-northwestward track to a point on the south coast of Hispaniola, then across eastern Cuba and into the Florida Straits between Florida and Cuba. A trough of low pressure is then expected to pull Isaac to the northwest and then north, towards the Florida Panhandle. The big news in this model cycle is that both of our top models--the GFS and ECMWF--predict that 5 - 6 days from now, the trough of low pressure pulling Isaac to the north may not be strong enough to finish the job. These models predict that the trough will lift out and a ridge of high pressure will build in, forcing Isaac more to the west. The GFS predicts this will occur after Isaac makes landfall in the Florida Panhandle, resulting in Isaac moving slowly to the west over land, from Georgia to Alabama. The ECMWF predicts the westward motion will happen while Isaac is in the northern Gulf of Mexico, resulting in an eventual landfall near the Louisiana/Texas border on Thursday. There are some huge issues to resolve to make an accurate long-range track forecast for Isaac. Where will its center consolidate? How will the interaction with the mountains of Hispaniola and Cuba will affect it? Where will Isaac pop off the coast of Cuba? Hopefully, the data being collected by the NOAA jet this afternoon will give us a more unified set of model forecasts early Friday morning. For now, pay attention to the cone of uncertainty. If you're in the cone, you might get hit.



Figure 2. Predicted 5-day rainfall total ending at 8 am EDT Tuesday August 28 from Tropical Storm Isaac. Graphics were generated from the 12Z (8 am EDT) August 23, 2012 run of the HWRF model (top) and GFDL model (bottom). The heaviest rainfall is expected to occur in a 50 - 80 wide wide swath along and to the right of where the center tracks. Amounts in excess of 8 inches (yellow colors) are predicted for portions of the Dominican Republic and Haiti from the HWRF model, but not the GFDL model. Given the current disorganization of Isaac, these rainfall amounts are probably at least 20% too high. Image credit: Morris Bender, NOAA/Geophysical Fluid Dynamics Laboratory.

Jeff Masters

Hurricane

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE TAMPA BAY RUSKIN FL
509 AM EDT FRI AUG 24 2012

FLZ039-042-043-048>052-055>057-060>062-065-242115 -
LEVY-CITRUS-SUMTER-HERNANDO-PASCO-PINELLAS-HILLSB OROUGH-POLK-
MANATEE-HARDEE-HIGHLANDS-SARASOTA-DESOTO-CHARLOTT E-LEE-
509 AM EDT FRI AUG 24 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR WEST CENTRAL AND SOUTHWEST
FLORIDA.

...POTENTIAL TROPICAL IMPACTS...
TROPICAL STORM ISAAC...CURRENTLY LOCATED SOUTH OF THE DOMINICAN
REPUBLIC EARLY THIS MORNING...IS FORECAST BY THE NATIONAL
HURRICANE CENTER TO MOVE ON A GENERAL WEST-NORTHWEST TRACK
THROUGH THE END OF THIS WEEK. ISAAC IS THEN EXPECTED TO TURN
NORTHWEST...MOVING IN THE GENERAL DIRECTION OF FLORIDA DURING THE
WEEKEND AND THERE IS A POTENTIAL FOR ISAAC TO IMPACT THE FLORIDA
PENINSULA EARLY NEXT WEEK
. THERE IS SOME UNCERTAINTY WITH REGARDS
TO THE EXACT TRACK AND INTENSITY OF ISAAC AND AS TO WHETHER OR NOT
ISAAC WILL HAVE DIRECT IMPACT ON WEST CENTRAL AND SOUTHWEST
FLORIDA....HOWEVER ALL INTERESTS ARE ENCOURAGED TO CONSULT THE
LATEST GUIDANCE AND INFORMATION FROM THE NATIONAL HURRICANE CENTER
CONCERNING THE POSSIBLE EFFECTS OF TROPICAL STORM ISAAC.

...THUNDERSTORM IMPACT...
MOISTURE BEGINS TO RETURN WITH SCATTERED SHOWERS AND STORMS FOR
THE WEEKEND...BUT BECOMING NUMEROUS ACROSS THE SOUTH. NUMEROUS
SHOWERS AND ISOLATED THUNDERSTORMS SPREAD ACROSS THE ENTIRE AREA
MONDAY. SHOWER AND STORMS BEGIN TO TAPER BACK TO SCATTERED
TUESDAY. A FEW OF THESE STORMS MAY BE STRONG WITH GUSTY
WINDS...BUT THE MAIN THREAT WILL CONTINUE TO BE LOCALLY HEAVY
RAINFALL AND OCCASIONAL LIGHTNING STRIKES.

...TORNADO IMPACT...
DEPENDING UPON THE TRACK ISAAC TAKES OUTER BANDS FROM THE STORMS
COULD GRAZE SOUTHWEST AND WEST-CENTRAL FLORIDA MONDAY AND RESULT
IN THE THREAT OF TORNADOES.

...WIND AND SEA IMPACT...
WINDS OVER THE ADJACENT GULF WATERS WILL BE ON THE INCREASE DURING
THE UPCOMING WEEKEND AND INTO EARLY NEXT WEEK DUE TO HIGH
PRESSURE TO THE NORTH AND TROPICAL CYCLONE ISAAC TO THE SOUTHEAST.
THE INCREASING WINDS WILL LEAD TO BUILDING SEAS WHICH WILL PRODUCE
VERY ROUGH BOATING CONDITIONS FOR SMALL CRAFT OPERATORS. ALL
MARINERS SHOULD CONTINUE TO MONITOR THE PROGRESS OF TROPICAL
CYCLONE ISAAC THROUGH THE REMAINDER OF THE WEEK AND INTO THE
UPCOMING WEEKEND.
:-)
Quoting mcluvincane:
NHC not to enthusiastic about a strong storm in the gulf.... might just be a rain maker after all




That's heresy around here, you know...but I think you may well be right...(forgot who I was for a moment there)
Real COC coordinates?

4507. Grothar
The FIM8 and the FIM9

Quoting sunlinepr:
Real COC coordinates?


Sorry but I see a loop of Typhoon Bolaven. That's the problem posting anything from CIMISS.
I use sniping tool on win 7 and upload the image to imageshack.us .
4509. auburn
I am hearing that the ECMWF model hints that Isaac could stall out over the Tennessee Valley for several days after landfall.(I am just a parrot..repeating what I hear)
I'll continue going with the western edge of the cone until I see real evidence of the NHC fix being right.




The 1445 frame admittedly gives me pause because it looks like something shifted during that time, but I'm still not convinced it's as far as 16.3N.
Quoting auburn:
I am hearing that the ECMWF model hints that Isaac could stall out over the Tennessee Valley for several days after landfall.(I am just a parrot..repeating what I hear)

I don't know where you heard that from but this loop shows differently.
4512. auburn
Quoting AussieStorm:

I don't know where you heard that from but this loop shows differently.


Like I said..I am just a parrot...I depend on you guys,just thought it was worth a mention.
4513. kwgirl
Quoting Grothar:
Get ready


Can someone tell me what the F5 keys is for. I never use them, leaving key strokes behind with the DOS system.
4514. Grothar
4515. Grothar
The lastest


4516. kwgirl
Quoting interstatelover7165:
NHC Y U NO ALREADY UPDATE
They are checking with the Governor and the GOP. I think their cone has changed.
4517. Grothar
Quoting kwgirl:
Can someone tell me what the F5 keys is for. I never use them, leaving key strokes behind with the DOS system.


It is the button to refresh a page. Some bloggers (not mentioning who) hit the F8 button to switch sites and hit the F5 button every second so they can be the first to post the NHC information. Not that I have ever done that myself.:)
interesting to note that the gfs ensembles show paint a much more eatern picture over south florida...
Quoting kwgirl:
Can someone tell me what the F5 keys is for. I never use them, leaving key strokes behind with the DOS system.

F5 = Page Refresh.
Tropical Storm Isaac is having an impact on two more cruise ships sailing out of Florida.

Royal Caribbean says the 2,350-passenger Majesty of the Seas and 2,390-passenger Monarch of the Seas will not call this weekend at CocoCay, the line's private island in the Bahamas. The ships are setting sail today from Florida's Port Canaveral and Port of Miami, respectively.

The announcement brings to five the number of vessels affected by Isaac. Industry giant Carnival's 2,974-passenger Carnival Valor skipped a call Thursday in Curacao to stay ahead of the storm. The ship instead is today visiting Nassau in the Bahamas. Another Carnival ship, the 2,978-passenger Carnival Liberty, also is visiting Nassau today after skipping a call Thursday in Grand Turk.

Also on an altered course this week due to Isaac is Royal Caribbean's 5,400-passenger Allure of the Seas, which departed from Fort Lauderdale on Sunday. The world's largest cruise ship has sailed for the Western Caribbean instead of heading to the Eastern Caribbean as planned. It's visiting Falmouth, Jamaica and Cozumel, Mexico instead of St. Thomas and St. Maarten.

As of 11 a.m. ET, Isaac was 165 miles south-southwest of Santo Domingo, Dominican Republic and moving to the west-northwest at 14 miles per hour. The storm has maximum sustained winds of 60 miles per hour.
The new GFS track issued at 1pm now shows it hugging the coast all the way up.

The Ew will up date its track around 3:30 pm.
4523. Dakster
Quoting beachman42:
The new GFS track issued at 1pm now shows it hugging the coast all the way up.

The Ew will up date its track around 3:30 pm.


Who is the Ew?
Quoting weatherman12345:
interesting to note that the gfs ensembles show paint a much more eatern picture over south florida...


What site are you looking at?