Isaac lashing the Keys; an eyewall is building

By: Dr. Jeff Masters , 3:48 PM GMT on August 26, 2012

Share this Blog
44
+

Tropical Storm Isaac is steadily organizing as it lashes the Florida Keys with heavy rain and tropical storm-force winds. Sustained winds of 44 mph and 41 mph have been observed at Molasses Reef and Sombrero Key, respectively, this morning. Radar out of Key West shows an increase in spiral banding, and the beginnings of an eyewall. An Air Force Hurricane Hunter aircraft completed its first pass through the center of Isaac near 11:30 am EDT, and did not find the pressure had fallen, or that the peak winds had increased. Infrared and visible satellite loops show that Isaac is a large and increasingly well-organized storm. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is quite good and increasing to the north, but is lacking elsewhere. Moderate wind shear and dry air to the south are interfering with heavy thunderstorm development on Isaac's south side. Heavy rains from Isaac are lingering over Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least four people.


Figure 1. Morning reflectivity image from the Radar out of Key West radar shows the northwest section of an eyewall beginning to form to the southeast of the city.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly, and we can no longer be confident we know where Isaac will make landfall on the Gulf Coast. One camp of models, the UKMET and ECMWF, predict that a trough of low pressure moving across the Southeast U.S. will be strong enough to turn Isaac north to a landfall in the Florida Panhandle. The other set of models, the GFDL, GFS, and HWRF, predict the trough will bypass Isaac, and a ridge of high pressure will build in and force Isaac to a landfall over Louisiana. The official NHC forecast averages out these two extremes, calling for a landfall midway between the two solutions. Odds are, one of the two model solutions will turn out to be the correct one, and the NHC will be forced to make a substantial adjustment in their forecast track to the east or the west. Isaac has the potential to drop torrential rains capable of causing serious flooding and drought relief over the South. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over Southeast Louisiana, where it predicts Isaac will make landfall. The ECMWF model, however, these heavy rains will fall more over the Florida Panhandle, Alabama, and Georgia.


Figure 2. A hurricane forecaster's dilemma: which set of models is correct? The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs have diverged significantly. Our two top models--the GFS and ECMWF--have 72-hour forecasts that are about 350 miles apart. The ECMWF forecast is not shown here, but lies just to the west of the UKMET forecast (white line.)

Intensity forecast for Isaac
Isaac survived passage over Hispaniola and Cuba relatively intact. It's large size aided this. Isaac is over very warm waters of 31°C (88°F) with high total heat content in the Florida Straits, but is encountering moderate wind shear of 10 - 15 knots due to upper-level winds out of the southwest. This shear is predicted to relax to the light range tonight as an upper-level anticyclone becomes established over the storm. This should allow for more substantial intensification after Isaac passes the Florida Keys. However, the total heat content of the ocean decreases for Isaac Monday morning as it encounters a relatively cool ocean eddy in the Southeast Gulf of Mexico. If Isaac takes a more westerly track, passing due south of the Central Louisiana coast, the storm will encounter a modest warm eddy, which would aid intensification. The intensify forecasts from the various models are very divergent. The latest 06Z (2 am EDT) run of the GFDL model keeps Isaac as a strong tropical storm until landfall in Louisiana. Isaac will undergo rapid intensification into a Category 3 hurricane as it hits New Orleans, says the latest 06Z (2 am EDT) run of the HWRF model. The ECMWF model has Isaac as a strong Category 2 storm with a central pressure near 950 mb as it hits near the Alabama/Florida border.

Comparing Isaac with Ike of 2008
The current situation with Isaac is similar in some ways to that of Hurricane Ike of 2008. Ike spent considerable time over Cuba, weakening from a Category 4 to a Category 1 storm. The storm couldn't put its energy into building a strong inner core, but it was able to build up its outer rainbands that were over very warm waters. This resulted in a major expansion of its wind field, with tropical storm-force winds extending out 275 miles from the center at one point. Ike was able to intensify into a Category 2 storm on its path towards Texas, and had an unusually low pressure for a Cat 2 storm with 100 mph winds--944 mb. That's a central pressure more typical of a Category 3 storm, but Ike could only manage Category 2 winds, since it had such a large chunk of the atmosphere to keep spinning. With Isaac's TS winds already extending out to 205 miles, maybe we'll see another Ike-type situation as it intensifies--the storm will have an unusually low pressure in order to keep a huge wind field spinning, but never make it above Category 2, since it will take so long to spin up such a large wind field.

Storm surge forecast for Isaac
Isaac is a very large storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. The latest 3:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0 on a scale of 1 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 1 to 6. A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located about 650 miles west of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 50% chance of developing by Tuesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop late this week, and potentially take a more westward track towards the Lesser Antilles, arriving around September 2.

Angela Fritz will have a new post here by 6 pm EDT. For the next few days, I plan to do the morning blog post, and Angela will be doing the late afternoon post. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will be on either in the afternoon or evening on Sunday, Monday, and Tuesday.

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: 476 - 426

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 | 45Blog Index

476. CJ5
Using Key West radar w/storms on, you can clearly see the center rotation. Still open though. I think one problem he has had is his size. He has a huge broad circulation and I believe it is harder for a broad system to tighten up. He has alot going for him but struggles to tighten up convection.
Member Since: Posts: Comments:
475. Gorty
Quoting Levi32:
Isaac still has to overcome a large amount of dry air that has become inseparably integrated with its circulation. One can see the center is of banded nature with an "S" shape, not a CDO, indicative of dry air entrainment. The only way this can be overcome is for the storm to perform lots of convective mixing to re-saturate the air inside it. It will accomplish that eventually, but it is large, and it will take a little while yet. Rapid strengthening is not likely until it is out in the open water of the gulf.



But look, as Issac is moving away from Cuba, more and more banding features are seen. What do you think about that?
Member Since: Posts: Comments:
Even lower now 929 mb!!!





this is now getting out of hand, and has to be over-done...
Member Since: July 4, 2005 Posts: 204 Comments: 8806
You have to remember that the HWRF has been very consistent with a strong hurricane the past couple days.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Getting some pretty strong winds and rain here on Grand Bahama. Impressive for a storm that's so far away.
Member Since: Posts: Comments:
54 hours.

Member Since: Posts: Comments:
Quoting TomballTXPride:

Hope you are feeling better today. Man even for feeling under the weather like you said yesterday, you are still one of the funniest people on here! You keep the mood light and that is what is needed in times like these!!


When I'm good, I'm very good; when I'm bad, I'm even better.:)
Member Since: Posts: Comments:
Quoting tomas5tex:
5667. weatherganny 4:37 PM GMT on August 26, 2012 +0
Quoting tomas5tex:


Buna here also

Buna texas???
Action: Quote | Ignore User

Yes

I am in Newton.
Member Since: Posts: Comments:
Quoting HCW:


Why is it dreaded ? If I lived in Pensacola I would be happy that the Storm Shield was in my city cause you know that you are safe from Isaac when 4'9 Jim C is near

You people are just NOT NICE! Shame, shame!
Member Since: Posts: Comments:
Will this storm be like most with the worst weather being east of the center or is it possible for some strong activity to the west of center?
Member Since: Posts: Comments:
Quoting SykKid:
lmao HWRF bringing it up to cat 4/5 what a joke


don't laugh too hard

sometimes things change
in but a blink of an eye
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Speaking About Tornadoes
_________________________________________________ ______

195
WFUS51 KLWX 261651
TORLWX
MDC005-261715-
/O.NEW.KLWX.TO.W.0035.120826T1651Z-120826T1715Z/

BULLETIN - EAS ACTIVATION REQUESTED
TORNADO WARNING
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1251 PM EDT SUN AUG 26 2012

THE NATIONAL WEATHER SERVICE IN STERLING VIRGINIA HAS ISSUED A

* TORNADO WARNING FOR...
WESTERN BALTIMORE COUNTY IN NORTHERN MARYLAND...

* UNTIL 115 PM EDT

* AT 1251 PM EDT...A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A
TORNADO WAS OVER WESTERN BALTIMORE COUNTY...OR NEAR PIKESVILLE...
AND WAS MOVING NORTH AT 15 MPH.

* LOCATIONS IMPACTED INCLUDE...
GLYNDON...
BORING...
BUTLER...
UPPERCO...

PRECAUTIONARY/PREPAREDNESS ACTIONS...

TAKE COVER NOW. MOVE TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF A
STURDY BUILDING AND AVOID WINDOWS. IF OUTDOORS OR IN A MOBILE HOME OR
VEHICLE...MOVE TO THE CLOSEST SUBSTANTIAL SHELTER AND PROTECT
YOURSELF FROM FLYING DEBRIS.

THIS TORNADO MAY BE WRAPPED IN RAIN AND HARD TO SEE. TAKE COVER NOW.



LAT...LON 3952 7685 3957 7683 3956 7669 3941 7670
3940 7676
TIME...MOT...LOC 1652Z 163DEG 15KT 3943 7674



GMS
Member Since: Posts: Comments:
461. ITCZ
Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.



Nicer beaches? :-)
Member Since: Posts: Comments:
Quoting Chicklit:
Wonder why the newsfolk are all in Pensacola rather than Mobile.
NE quad is worst
Member Since: August 1, 2012 Posts: 0 Comments: 244
Eastern Shore Alabama:

Gas stations about 75% fuel pumps still good with less than 10 min wait, Walmart full but all registers open so not too crazy, bread and hot dogs and hamburger meat gone but stocking shelves continually.

Marinas locking down today, moving trailers etc.

Weather absolutely beautiful today! Expecting about 4-5 ft surge on Tuesday afternoon.

Looks like eye wall forming right now, anticipate strengthening throughout afternoon, will be interesting to see final growth through landfall.

Looks more like NOLA now but Eastern Shore Alabama watching closely, especially as we will be on the dirty side.
Member Since: Posts: Comments:
Quoting flcanes:

who

Max Mayfield
Member Since: Posts: Comments:
Quoting Chiggy:
12Z HRWF - Overdoing it?

That's very funny.Definitely entertaining to say the least.If that happens will Santa give me everything I want for Christmas?.
Member Since: August 14, 2010 Posts: 10 Comments: 16409
Isaac still has to overcome a large amount of dry air that has become inseparably integrated into its circulation. One can see the center is of banded nature with an "S" shape, not a CDO, indicative of dry air entrainment. The only way this can be overcome is for the storm to perform lots of convective mixing to re-saturate the air inside it. It will accomplish that eventually, but it is large, and it will take a little while yet. Rapid strengthening is not likely until it is out in the open water of the gulf.

Member Since: November 24, 2005 Posts: 635 Comments: 26547
So what's the latest track the nhc is forecasting for Isaac once in the Gom?
Member Since: Posts: Comments:
5667. weatherganny 4:37 PM GMT on August 26, 2012 +0
Quoting tomas5tex:


Buna here also

Buna texas???
Action: Quote | Ignore User

Yes
Member Since: Posts: Comments:
About to say hi to $4 per gallon of gas in Central NC...
currently $3.73.

@wunderground

Some oil platforms in the path of #Isaac have begun evacuations.
Member Since: August 1, 2011 Posts: 28 Comments: 7899
Quoting bappit:

Could be the microphone they gave him.

who
Member Since: Posts: Comments:
Wonder why the newsfolk are all in Pensacola rather than Mobile.
Member Since: Posts: Comments:
Quoting Chiggy:
12Z HRWF - Overdoing it?

As someone said earlier, add about ~15 mb to the pressure. However, that still means Isaac would slam into LA as a low-end C4.
Member Since: Posts: Comments:
Quoting Chiggy:
12Z HRWF - Overdoing it?



Hopefully.
Member Since: Posts: Comments:
Just heard my first TV met say for Port Arthur/Beaumont to be alert.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
There's a reason the HWRF is the king of overdoing systems, lol... Its solution is possible, but very unlikely.
Member Since: Posts: Comments:
444. ITCZ
Quoting WhereIsTheStorm:


A cast iron skillet works great on the grill. It takes a little getting use to though. You might want to try it a couple of times before the power goes off so you don't waste your time and food. Once it gets hot you can remove it from the grill and it will stay hot for awhile without getting hotter which is why you removed it from the grill. Easy way to make eggs without melting the handle of a regular skillet.


Thanks! After years of camping in the Everglades, that much I've got down to a science :-)
Member Since: Posts: Comments:
Quoting LargoFl:
GFS at 18 hours


that is interesting

makes me think the GFS thinks the system COULD feel the weakness, but the weakness just ends up not being strong enough I guess

still a cloudy situation similar to Debby
Member Since: Posts: Comments:
Quoting Bluestorm5:
Adding 20 mb because HWRF is usually overdone still meant Category 3.



ok
Member Since: Posts: Comments:
Quoting MiamiHurricanes09:
Catastrophic hurricane headed towards Louisiana on the 12z HWRF in 48 hours.

what does it say for 18-24 hours
Member Since: August 6, 2011 Posts: 4 Comments: 36867
Large void of convection to the east of the center of circulation. Will be more impressed when that fills out..
Member Since: Posts: Comments:
Quoting flcanes:

ridge is stronger than many people think
So we are looking prolly well West of NOLA?
Member Since: Posts: Comments:
Haven't seen any tornado warnings in FL yet but the threat is still there.

MESOSCALE DISCUSSION 1815
NWS STORM PREDICTION CENTER NORMAN OK
1158 AM CDT SUN AUG 26 2012

AREAS AFFECTED...PORTIONS SRN FL AND KEYS

CONCERNING...TORNADO WATCH 589...

VALID 261658Z - 261900Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 589 CONTINUES.

SUMMARY...POTENTIAL FOR ISOLATED TORNADOES WILL PERSIST INTO THE
AFTERNOON OVER S FL...BUT GREATEST THREAT IS EXPECTED TO EVOLVE OVER
THE SRN TIP OF THE PENINSULA THIS AFTERNOON.

DISCUSSION...AT 11 AM EDT THIS MORNING THE CENTER OF TS ISAAC WAS
LOCATED 180 KM SE OF KEY WEST MOVING WNW AT 15 KT. SEE LATEST
DISCUSSIONS FROM NHC FOR MORE INFORMATION. LATEST VISIBLE IMAGERY
SHOWS WIDESPREAD CLOUDS AND RAIN ASSOCIATED WITH THE OUTER
CONVECTIVE BANDS HAVE OVERSPREAD THE SRN HALF OF THE FL
PENINSULA...EFFECTIVELY LIMITING DESTABILIZATION.
MOREOVER...CONVECTIVE MODES HAVE BEEN PREDOMINANTLY LINEAR. THESE
FACTORS HAVE SO FAR SERVED TO LIMIT A MORE ROBUST TORNADO THREAT.
HOWEVER...AN AREA OF CLEARING EAST OF THE CENTER SHOULD EXPAND NWD
INTO THE SRN TIP OF FL THIS AFTERNOON WHERE LOW LEVEL HODOGRAPHS ARE
MAXIMIZED WITH 200-400 M2/S2 STORM RELATIVE HELICITY IN THE 0-1 KM
LAYER. ADDITIONAL CONVECTION MAY DEVELOP ALONG CONVERGENCE BANDS
WITHIN THIS ZONE OF BOUNDARY LAYER DESTABILIZATION THIS
AFTERNOON...LIKELY INCREASING THE THREAT FOR EMBEDDED SUPERCELLS AND
ISOLATED TORNADOES.

..DIAL.. 08/26/2012
Member Since: Posts: Comments:
Quoting Tazmanian:



thats likey a strong cat 4 or low cat 5 with a 934mb
Adding 20 mb because HWRF is usually overdone still meant Category 3.
Member Since: August 1, 2011 Posts: 28 Comments: 7899
12Z HRWF - Overdoing it?

Member Since: Posts: Comments:
GFS at 18 hours
Member Since: August 6, 2011 Posts: 4 Comments: 36867
Catastrophic hurricane headed towards Louisiana on the 12z HWRF in 48 hours.

Member Since: Posts: Comments:
Quoting seflagamma:

Huh??? rofl.. I don't have you on ignore! It takes a lot of stupid statements for me to put you on ignore
but there have been about 18 bloggers in the past 2 days that earned that designation.
I respect different opinions.. just not the stupid posts or trollish posts...
Well stated gamms. I just cannot bring myself to put anyone on ignore, personally, but that is probably why my hair is going gray and I hit the keys on my computer way too hard...
Member Since: Posts: Comments:
HAHA, this is funny.. located in Houston. Our local forecast about two hours ago was sunny and mid nineties for Wed/Thursday of this week - and just went to 90F and some thunderstorms! Of course I cannot tell if this is due to reaction of the latest 12Z GFS or not...
Member Since: Posts: Comments:
Quoting cutlerbay32:

While I am counting my blessings, the rain and howling wind we have been experiencing is anything but a "dud". This weather is exactly what I expected because I do lurk and learn here. Be careful in your generalizations. It is difficult to hear that what I am experiencing now is nothing because I just gotta tell ya it ain't.

If you think this is howling winds then you are either young or new to the area. I'm way south of you in the Redlands and have been here for 30 years , this is what a light TS feels like down here! I guess is just a matter of perspective!
Member Since: Posts: Comments:
429. ITCZ
Link


school closures, Tampa
Member Since: Posts: Comments:
Quoting msgambler:
Did the track models shift more West? They were around Mobile Bay or Pascagoula, MS when I left the hotel a couple hours ago and now everyone is talking about the other side of NOLA. What gives?

ridge is stronger than many people think
Member Since: Posts: Comments:
Quoting newportrinative:


LOL! I do have to turn the volume up signifantly to hear him and then down again after he's done!

Could be the microphone they gave him.
Member Since: May 18, 2006 Posts: 10 Comments: 5951

Viewing: 476 - 426

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 | 45Blog 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
71 °F
Mostly Cloudy