Isaac lashing the Keys; an eyewall is building

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

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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

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Euro has to flip back west for the 12Z doesn't it? It'll make the NHC's 21Z updated track a lot easier if it does. Even if not, if it still shows the FL solution, supported only by the UKMET, doesn't the NHC finally have to stop splitting the baby and buy into its own GFS's western solution?

My pure WAG, following many here, is that lightly populated South-Central Louisiana will be ground zero. Not to minimize Isaac's effects on those many tens of thousands of residents who would be near there, or the storm surge to NOLA. And a felix culpa could be the rain benefit to AR and maybe OK down the road.
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Quoting watchingnva:


I call bs...that water is too calm..this is a supercell elsewhere...hate what people lie about photos...


Hey watch... buddy long time no see!

Really interesting weather we had yesterday morning. As far as I've heard it was somewhat subtropical in nature? Interesting.
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that was from an archived site , nor the 2012 Isaac
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https://sphotos-b.xx.fbcdn.net/hphotos-ash4/295292_ 432818426759459_977235953_n.jpg
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Quoting robj144:


Aren't they usually one in the same? How does a tree fall over... from high winds.
Usually after the ground gets saturated from large amounts of rain the soil is not so compacted and the tree will fall from the wind and weight...
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Quoting Levi32:


I saw you talking about this earlier.

It's not the hit on the central gulf coast from the southeast that's unusual. We've had Georges, Betsy, 1947, Elena, you name it. It happens a lot. The uniqueness of this track is how it came out of the Caribbean, over the islands, and now straight NW towards the central gulf coast. None of the storms I mentioned above came from that far south. They all started either over the islands or east of Florida. They did not come up from the south. This kind of a track, if Isaac takes it, has not happened since the beginning of the hurricane database that now extends back to 1842.


That is one reason I rarely, if ever use one storm in comparison to another. If we look closely enough, we could find many unique storms and I am sure we will have new storms with different paths. Not to be poetic, but I always do compare storms to the Frost poem. "The Road Not Taken", ...and I, I took the Road less traveled by".

The trough has made all the difference in this one. You and I know each other pretty well, so most bloggers will know we are not bashing each other.

As we discussed the other night, I never saw this moving over the DR. I wrote numerous times that it would move over the Isthmus of Haiti and go through the Windward Passage,(much to the chagrin of Allstar17) then move to the NW, which is what the atmospheric conditions were going to be . Even most models were over analyzing Isaac from the beginning. Just my opinion, but sometimes if they over analyze these, they usually get it wrong. My initial "guess" that it would head towards New Orleans was logical. Hopefully, I am wrong and it dissipates. And know the one thing in which you and I, and I hope the other bloggers agree, is that this does not appear good for the Gulf Coast.
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euro very close to the gfs
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I think Frederic from 1979 is a decent precedent for Isaac. He spent more time over Cuba and emerged off of a different point, but still spent most of his life unable to reach 'big boy' status until getting into the Gulf.

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Key West NAF, Florida (Airport)
Updated: 7 min 33 sec ago
Heavy Rain
81 °F
Heavy Rain
Humidity: 84%
Dew Point: 75 °F
Wind: 25 mph from the East
Wind Gust: 37 mph
Pressure: 29.51 in (Falling)
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Quoting MiamiFL305:
Key West


I call bs...that water is too calm..this is a supercell elsewhere...hate what people lie about photos...
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12Z ECMWF - 48 hours

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Member Since: August 6, 2011 Posts: 4 Comments: 33465

Quoting zoomiami:


Baha -- how is the weather over there?
Still windy, though we haven't had any real rain since about 12:30... [lol only the fake kind lol]

I'm expecting we'll get some more squall lines through for the rest ofthe afternoon and evening, if Isaac holds true to pattern...

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Can someone post the ECWMF please?.
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2pm EST Mon
a)still tropical storm
b) 75mph
c) 80mph
d) 85mph
e) 90mph
f) Cat 3
g) Cat 4
h) Cat 5
i) Jupiter hurricane!!!!
j) open wave
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I really don't like to say things like this because I'm not one to hype, but Isaac could be a hurricane that is rapidly intensifying on its way inland. The major hurricanes of our recent past have been on a weakening trend at landfall. As long as dry air doesn't become a significant issue, I see no reason why Isaac couldn't rapidly intensify at some point in the Central Gulf. If it does, I'm afraid it will catch a lot of people off guard.
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hmm..
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from Key Colony Beach - I've got about 5+ inches of rain since yesterday (about 2 today). Power is on here, even my sat dish is working. Wind is approx 35knt at the tree tops.

took a walk around - just a few snall branches down, we'll see how that goes once the winds start shifting around.

My hot and spicy bean dip is ready - time to munch ;>)
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48 hrs, *slightly* weaker than the 00z, still near major hurricane status. Down to 974mb. Inline with NHC track.
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Quoting weatherman12345:
Did it go off from a tree fall or just really strong winds


Aren't they usually one in the same? How does a tree fall over... from high winds.
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Quoting MontanaZephyr:

Great Pic! If there was such a thing as weather-porn, this site would be the max!
Well there is such a thing, and you are looking at it right now. Hope your spouse or special-friend-whatever is OK with it.
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Quoting PalmBeachWeather:
Largo.It's not 100%, but I think you can breathe a little easier
ok thanks, this storm i just dont trust
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Quoting LargoFl:
so there is no chance issac comes up the coast of florida..for sure he goes westward, we dont wake up tomorrow and he's close to the coastline, making for the panhandle of florida?
Largo.It's not 100%, but I think you can breathe a little easier
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Product: Air Force Vortex Message (URNT12 KNHC)
Transmitted: 26th day of the month at 17:51Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 309)
Storm Number & Year: 09L in 2012
Storm Name: Isaac (flight in the North Atlantic basin)
Mission Number: 21
Observation Number: 10
A. Time of Center Fix: 26th day of the month at 17:09:20Z
B. Center Fix Coordinates: 23°50'N 81°17'W (23.8333N 81.2833W)
B. Center Fix Location: 59 miles (95 km) to the SSE (148°) from Key West, FL, USA.
C. Minimum Height at Standard Level: 1,390m (4,560ft) at 850mb
D. Estimated (by SFMR or visually) Maximum Surface Wind: 47kts (~ 54.1mph)
E. Location of the Estimated Maximum Surface Wind: 21 nautical miles (24 statute miles) to the ENE (74°) of center fix
F. Maximum Flight Level Wind Inbound: From 147° at 51kts (From the SSE at ~ 58.7mph)
G. Location of Maximum Flight Level Wind Inbound: 44 nautical miles (51 statute miles) to the ENE (65°) of center fix
H. Minimum Sea Level Pressure: 995mb (29.38 inHg)
I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 18°C (64°F) at a pressure alt. of 1,529m (5,016ft)
J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 22°C (72°F) at a pressure alt. of 1,394m (4,573ft)
K. Dewpoint Temp (collected at same location as temp inside eye): 19°C (66°F)
K. Sea Surface Temp (collected at same location as temp inside eye): Not Available
L. Eye Character: Not Available
M. Eye Shape: Not Available
N. Fix Determined By: Penetration, Radar, Wind, Pressure and Temperature
N. Fix Level: 850mb
O. Navigation Fix Accuracy: 0.02 nautical miles
O. Meteorological Accuracy: 5 nautical miles
Remarks Section:
Maximum Wind Outbound: 55kts (~ 63.3mph) in the northwest quadrant at 17:41:20Z
Maximum Flight Level Wind: 55kts (~ 63.3mph) in the northwest quadrant at 17:41:20Z
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Quoting MiamiHurricanes09:
Here we go again. Rain is absolutely POUNDING the windows facing east. Gusts definitely appear to be in the >40kt range.
guess this is what we can expect tonight or tomorrow
Member Since: August 6, 2011 Posts: 4 Comments: 33465
Quoting Miamigal:
Ook how is it possible that the FIU weather station basically 2 mile from me has a reading of 73 mph, the Westwood station right next to my house has it 12 mph is that station out or is it the individual band have such strong gust at 2 pm?


Could be a number of things. The band they went through weakened, an isolated downburst, bad placement of the instrument, bad instrument lol. I'm sure they have a pretty reliable station at FIU...
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Quoting hurricanehunter27:
What are you talking about? The temps in the gulf are boiling. Have you looked at a SST map?
27.Makes one wonder where these people come from
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Fwiw...which isn't much, it looks like Bastardi's quadracane of doom isn't gonna verify. Gonna' slide south of Key West. 3 landfalls only on this one.
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My forecast
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Quoting MiamiHurricanes09:
POWER'S OUT. Dammit. Posting this on my phone. Will keep you updated; very bad weather.
Stay Safe Miami and Best Wishes from Texas
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ECMWF 24hrs, down to 988mb.
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Quoting Hurricanes101:


There is a good bet he goes west, but something is still bothering me about what the models are showing, even with the west path it seems like Isaac just misses having the weakness, a small deviation in the short term track to the north could really impact the track down the road
ok thanks so we here need to keep an eye on him..thanks
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789. Sasha
Nice...!!! Felt just like I was in the car driving thru KW!!!  I need to make another trip down there sometime...  Stay safe!
Quoting StormHype:
Live mobile chaser video driving the streets of Key West:
Live mobile Key West stream

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Radar shows that a tighter 15 mile radius center of circulation is forming in the extreme southwest portion of what was a larger center of circulation.



This has halted the drop in pressure at Key West, at least temporarily. The pressure dropped 4Mb in 18 minutes, then in the last 18 minutes it has remained constant at 29.51 inches.
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Here we go again. Rain is absolutely POUNDING the windows facing east. Gusts definitely appear to be in the >40kt range.
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Quoting photomunkey:

Ah, my mistake with that term. But I don't see rapid intensification happening with the relatively cool water temps and dry air in the Gulf.

Without strong intensification, Isaac will not, cannot turn north and break the ridge without help from some sort of low pressure trough or jet stream, and it isn't going to get that help. This is looking more and more like a eastern TX/western LA Cat1 hurricane with a late Wednesday landfall. Just MHO...
What are you talking about? The temps in the gulf are boiling. Have you looked at a SST map?
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Quoting stoormfury:
both the GFS AND the ECMWF are consistently showing a stong storm perched off the central antilles come sunday Sept ist. This could be the big cape verde system. the islands should watched this one very closely. THis again could be a cental gulf hurricane
gee one after the other..patrap..where are you my friend..the mayan doom is here
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Quoting BahaHurican:
LOL... I keep hoping the winds will stay high enough to make my employer say "No... you should stay home today!".... doubt that'll happen, though... lol



LOL, yeah another day off would be nice.
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Quoting MiamiFL305:
Key West


Fantastic picture! Thanks for posting it.
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Quoting LargoFl:
so there is no chance issac comes up the coast of florida..for sure he goes westward, we dont wake up tomorrow and he's close to the coastline, making for the panhandle of florida?


There is a good bet he goes west, but something is still bothering me about what the models are showing, even with the west path it seems like Isaac just misses having the weakness, a small deviation in the short term track to the north could really impact the track down the road
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Quoting MiamiFL305:
Key West


Great Pic! If there was such a thing as weather-porn, this site would be the max!
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Member Since: August 9, 2009 Posts: 10 Comments: 4504
Member Since: August 6, 2011 Posts: 4 Comments: 33465
both the GFS AND the ECMWF are consistently showing a stong storm perched off the central antilles come sunday Sept ist. This could be the big cape verde system. The islands should watched this one very closely. This again could be a cental gulf hurricane
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Nice band working into the space coast right now, breezy but not too bad...actually a rather pleasant break from the heat unless a tornado spins up nearby.
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.