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: 2176 - 2126

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

Quoting bappit:

So now that we have the knee jerk HUH? out of the way, what might account for the intensity forecast?

Nothing. There is no reason it's so low.
Member Since: July 6, 2010 Posts: 113 Comments: 31502
2175. ITCZ
Quoting stormchaser19:
Richard Pasch


He think Isaac will be a weak Cat.2



Love to see faces for names i read alot--thanks!!! :)
Member Since: September 5, 2008 Posts: 0 Comments: 47
2174. Walshy
HURRICANE WARNING ISSUED FOR NORTHERN GULF COAST
Member Since: Posts: Comments:
Man if we had enough common sense in Louisiana we would cancel all schools and close non essential businesses in the NEWLY posted Hurricane Warning Area. That is yet to happen. There will be enough traffic as it is with people evacuating. It will take a good 48 hours to evacuate everyone, even with contraflow. The message is better to be overprepared, but then they say, "but we will wait and see what happens."
Member Since: August 3, 2012 Posts: 0 Comments: 39
Quoting wolftribe2009:


Article posted late last year on MSNBC

http://www.msnbc.msn.com/id/45066867/ns/us_news-l ife/t/six-years-after-katrina-slow-progress-nation s-levees/#.UDqGSaPALbw


And there are serious engineering questions about the floodgate pumping systems put into place at the head of the drainage canals. The levees will likely hold for a Cat 1 but enough rain could cause substantial flooding because the city's historic capacity for pumping out rainwater will not be met (1 inch first house, including storage and a 1/2 in an hour after that).
Member Since: Posts: Comments:
Quoting goldenpixie1:


So they're sending "The Shield" to NO? Hope his protection continues to work for them.


I call him "The Angel of Death." In the real world, his arrival is not seen as a good thing...quite the opposite. I am not happy that he is coming here, not at all.
Member Since: Posts: Comments:
Richard Pasch


He think Isaac will be a weak Cat.2
Member Since: Posts: Comments:
2169. bappit
Quoting MiamiHurricanes09:
No one's bashing anyone. This is a blog where we share our opinions and that's exactly what we're doing.

So now that we have the knee jerk HUH? out of the way, what might account for the intensity forecast?
Member Since: Posts: Comments:

Member Since: Posts: Comments:
THIS IS OFF TOPIC..but I do want to apologise to BOBSLED27 ,who was not the poster that prompted my reactions...I sent you a note Bobsled..OK..I also say sorry to all here for this disruption...I am going to go lay down by my dish now.
Member Since: Posts: Comments:
2166. yoboi
Quoting nola70119:


What do surface maps tell you?


how far west it can go and a weakness to the east...
Member Since: Posts: Comments:
2165. ITCZ
Weather station at Key West, FL is reporting sustained winds of 63 mph. http://bit.ly/MUr9zT

.....this blog is kind of addictive isn't it?
Member Since: September 5, 2008 Posts: 0 Comments: 47
Quoting GeoffreyWPB:


That will change very quickly. 7-years ago today...

Pretty close to the same track and the NHC mentioned the warm water loop current. I wonder what other factors are the same as back then.
Member Since: Posts: Comments:
Quoting deb1:
Quite eerie, watching this. Some of the forecasts are showing Isaac hitting New Orleans 7 years to the day after Katrina.


They say History repeats itself, though 7 years sames a bit too soon history wise. WE had Paloma in 2008 hit Cayman Brac 76 years to the date after it was devastated by the 1932 Hurricane( strange, weird and makes one wonder)
Member Since: Posts: Comments:
I don't get it.. if gfs & gfdl are moving west why are they still saying N O ?
Member Since: Posts: Comments:
Quoting AtHomeInTX:


They're not buying the GFS.

I wonder why?
Member Since: Posts: Comments:
Quoting yonzabam:



That's a great pic. Looks like he's thrown off the confining shackles of Cuba and Florida and is now going to show the world what he's made of. He's a big boy and getting bigger, and I fear he's also going to be bad. Maybe only a cat 2 at landfall, but pushing an Ike sized wall of water in front of him. Major disaster looming.
Very good point. Geographic size usually means larger wind-field size which means greater surge potential. Unfortunately, the current category system only accounts for wind speed. Rainfall potential and surge potential should be considered for land-falling storms. After all, flooding is the leading cause of fatalities.
Member Since: Posts: Comments:
2159. WxLogic
Quoting TropicalAnalystwx13:

Up a millibar? It's down to 992 millibars.


Not sure why... didn't see a reading that would be even rounded down to 992MB... but then again I'm not in the HH nor NHC to get the tools they have. So it stayed at 992MB... I'll go with that.
Member Since: Posts: Comments:
2158. flsky
Band going thru Ponce Inlet, FL at the moment. Decent amount of rain and wind.
Member Since: Posts: Comments:
What guidance other than the HWRF is there for faster intensification than what Pasch has?
Member Since: Posts: Comments:
Quoting washingtonian115:
It seems the major news networks are more interested in the convention schedule then Isaac and his potential affects on the U.S...


That will change very quickly. 7-years ago today...

Member Since: Posts: Comments:

This shear map is from noon today. Hopefully there will be an update at 6 p.m. est
Member Since: Posts: Comments:
Could the political people go talk on a politics blog? Those of us in harm's way are not in the mood for that right now.
Member Since: Posts: Comments:
Quoting Stormchaser121:

we arent out of the woods yet!


Yeah, we'll see. At this point I hope the NHC is right.
Member Since: Posts: Comments:
2152. GetReal
Quoting RTSplayer:
Anyone else see some weirdness with the CoC on radar?

Something is changing, and I'm not talking about intensity.

Maybe it's stalling or changing directions, I'm not sure.



Just looking at that from a different perspective... You may find your answer here... Very interesting; I have never observed coud tops boiling before!!!

Link
Member Since: Posts: Comments:
Quoting MiamiHurricanes09:
Good question. Stewart should be the designated discussion writer. Even if it isn't his shift and he's at home, he should e-mail it to them.


I'm just amazed at how nonchalant Pasch seemed to be in the discussion. It seemed like he had no worry that it could be a bad storm.
Member Since: Posts: Comments:
Quoting 1900hurricane:
Differences in the big four: @72 hours:



@96 hours:

Well, considering how well the GFS did with Debby - the GOM is a fish tank compared to open-water analysis - and it wants to take Isaac West after landfall... I would drop the Euro if I was planning emergency operations for LA and TX.
Member Since: Posts: Comments:
Quoting kidd5433:
Jim Cantore is going to be in NOLA after SNF. As you were...


Bet he won't go to Biloxi again...
Member Since: Posts: Comments:
THE TROPICAL STORM WARNING FROM SUWANNEE RIVER TO ANCLOTE KEY IS
DISCONTINUED.

5 pm nhc report

This is good news. I felt earlier they would drop the warning down to a watch. But they went to nothing.
Odd, because they left the warning up for Pinellas, which of course is south and adjacent to Pasco.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
2145. airmet3
Quoting weathermanwannabe:
Maybe NHC is looking at the large size of the circulation. That will take quite a while to consolidate and may be the difference between at Cat 1 to a Cat 2 at landfall. That may be the saving grace but tomorrow will tell.


They and Dr. Masters have been saying this for the past two days.
Member Since: Posts: Comments:
2144. gator23
Quoting RTSplayer:
Anyone else see some weirdness with the CoC on radar?

Something is changing, and I'm not talking about intensity.

Maybe it's stalling or changing directions, I'm not sure.


I have been posting this for n hour now. The COC has stalled or is drifting north.

Member Since: Posts: Comments:
Member Since: Posts: Comments:
Quoting RTSplayer:
Anyone else see some weirdness with the CoC on radar?

Something is changing, and I'm not talking about intensity.

Maybe it's stalling or changing directions, I'm not sure.
Almost looks like it turned north
Member Since: Posts: Comments:
2141. WunderAlertBot (Admin)
JeffMasters has created a new entry.
Moving again...Live mobile video of eye moving over Key West FL:
Mobile Live feed - Key West FL
Member Since: Posts: Comments:
The 12z Euro ensembles and the 12z GFS ensembles are in much better agreement than their respective operational runs

Euro ensemble:



GFS ensemble:

Member Since: Posts: Comments:
Quoting MississippiWx:
So Pasch writes a two paragraph discussion for a system that could be a major hurricane making landfall in the next 3 days. Wow. Where is Stacy Stewart???
Good question. Stewart should be the designated discussion writer. Even if it isn't his shift and he's at home, he should e-mail it to them.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Recon's heading home.
Member Since: Posts: Comments:
Maybe NHC is looking at the large size of the circulation. That will take quite a while to consolidate and may be the difference between at Cat 1 to a Cat 2 at landfall. That may be the saving grace but tomorrow will tell.
Member Since: Posts: Comments:
It seems the major news networks are more interested in the convention schedule then Isaac and his potential affects on the U.S...
Member Since: Posts: Comments:
2133. gator23
Quoting Hurricanes101:


No...GFS did not have this solution until yesterday

Most of the time the GFS was off, nrtl posted the margin of error for the models and the GFS was way down the list...The Euro had this solution way before any other model swung west

but of course the GFS will continue to get the praise it does not deserve.


it deserved it with debby...
Member Since: Posts: Comments:
Quoting saltydog1127:
Pascagoula, MS here....what do you guys think?


Maybe not the best week to hold a Family Reunion at the Beach... You know...
Member Since: Posts: Comments:
Member Since: Posts: Comments:
look at his southern side. its covering him. he should become a hurricane possibly tonight
Member Since: July 25, 2011 Posts: 0 Comments: 1102
2129. airmet3
Quoting MississippiWx:
So Pasch writes a two paragraph discussion for a system that could be a major hurricane making landfall in the next 3 days. Wow. Where is Stacy Stewart???

Brevity is a lost art.
Member Since: Posts: Comments:
2128. GetReal
Member Since: Posts: Comments:
other thing i like too point out is that are storm has slow down too 16 mph
Member Since: Posts: Comments:
interesting, plaquemines parish president - levees are 30ft, surge predicted 5 to 10ft. Decision on schools this afternoon...
Member Since: Posts: Comments:

Viewing: 2176 - 2126

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
61 °F
Mostly Cloudy