Little change to Isaac, but storm poses a serious storm surge threat

By: Dr. Jeff Masters , 3:25 PM GMT on August 27, 2012

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Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.


Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.

Isaac's rains
Isaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least nineteen, and two died in the Dominican Republic.


Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.


Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.


Figure 4. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Storm surge forecast for Isaac
Storm surge is the primary damage threat from Isaac. Isaac is a huge 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. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

The affect of storm size and angle of approach on storm surge
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.

Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.


Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.

New Orleans flood defense info
Army Corps of Engineers map of the new flood defenses
Army Corps of Engineers video showing the flood defenses
New York Times article, Vast Defenses Now Shielding New Orleans

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 miles west-northwest of the Cape Verde Islands. 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 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.

Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Wednesday morning.

Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.

Jeff Masters

Tropical Storm Isaac (chelina)
View from the north side of the Sunshine Skyway Bridge.
Tropical Storm Isaac
our street at noon today (seflagamma)
Aug 26, 2012: Isaac floods our street really bad. Now nearly 11
our street at noon today

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For those leaving nola.. Traffic on i-10 east in nola east is backed up due to a shooting.. Take an alternate route if possible
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and He didn't...

4:00 PM CDT Mon Aug 27
Location: 26.4°N 86.2°W
Moving: NW at 12 mph
Min pressure: 981 mb
Max sustained: 70 mph
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Quoting trHUrrIXC5MMX:
PRESENT MOVEMENT TOWARD THE NORTHWEST OR 305 DEGREES AT 10 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 981 MB
MAX SUSTAINED WINDS 60 KT WITH GUSTS TO 75 KT.
50 KT....... 50NE 15SE 15SW 50NW.
34 KT.......150NE 150SE 80SW 180NW.
12 FT SEAS..180NE 180SE 180SW 150NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

NO HURRICANE!!!!


It seems like it may make a run at being a strong category one at landfall.

At this point, it hasn't strengthened much over the past few days, but it has become much more better organized. The pressures are also extremely low for a strong tropical storm. It's really only a matter of time before the winds begin to match the barometric pressure.
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Quoting MiamiHurricanes09:
There hasn't been any outstanding data reported by Recon that would point towards the classification of a hurricane. Flight-level winds after the reduction, were still below 65kts, and there has only been 1 SFMR reading of 64kts. It's going to come down to the wire, and the only chance you have of it getting upgraded is if Stewart is writing the package lol.


Even Stewart wasn't convinced.

An update could come at any time, though, it really wouldn't take much.
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Quoting Sfloridacat5:
Dr Masters just now on TWC - "not going to be a cat 2 today or tomorrow."


NHC new advisory just upped their forecast to a Cat 2 prior to landfall....

Showdown...The Doc vs the NHC....
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Quoting jeffs713:

BNSF has been pulling trains from Galveston, but not from points inland.



I know I have heard and seen about 10 different trains. My husband said he listens and if they spend all that time and money to move them something is up. I guess they might be worried about the storm surge. We seen a huge ship leaving out yesterday and you could tell they where still working on it or something but they where getting it out of there.

sheri
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Quoting MiamiHurricanes09:
There hasn't been any outstanding data reported by Recon that would point towards the classification of a hurricane. Flight-level winds after the reduction, were still below 65kts, and there has only been 1 SFMR reading of 64kts. It's going to come down to the wire, and the only chance you have of it getting upgraded is if Stewart is writing the package lol.

Stewart wrote this.
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LAFAYETTE PARISH

Broussard

Sandbags are available for residents of Broussard at city hall, across the street from city hall and Deer Metal Subdivision. You have to bag them yourself, shovels are availlable.

Youngsville

The City of Youngsville has stocked four sand bag locations for the citizens of Youngsville. These sites have sand and sand bags for individuals to bag their own. Limit 15 bags each, please.

Bag-Your-own at the city Police Station 304 Fourth Street

Bag-Your-own the Public Works Building 310 Railroad Avenue

Bag-Your-Own at the Horse Arena across from Farmer's Bank - 412 Iberia Street (a.k.a. Hwy 92).

The fourth site is at the Youngsville sewer plant 416 Railroad Avenue. This site is reserved for elderly and individuals requiring assistance. There is assistance available until 4:00 PM to help load pre bagged sand into vehicles.
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Member Since: July 12, 2008 Posts: 7 Comments: 383
Quoting kwgirl:
With the water that is with this system and possible surge, is it wise to stay at a junction of three rivers?


Probably not but that's the only place we have to go. They didn't get ANY water with Katrina and like I said they have 3 generators. So I think we'll be fine. Thanks so much for you concern. :)
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Quoting CaicosRetiredSailor:
OK...

who is in charge of getting the pitchforks?

I can get ahold of a few stun guns
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1547. JeffM
NHC now saying Cat 2 at landfall.
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NHC can always issue a special update to increase winds/declare Isaac a hurricane. They are correct that Isaac is not showing much in the way of hurricane force winds at the surface. However, it will not be long.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Quoting sunlinepr:
Intensity forecast....

Not more than 90mph....



actually its 90 knots...105 mph
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1544. sar2401
Quoting Grothar:
I'm too tired to go back to post 929 and plus Nea. If I put one here and said it belonged to him, does it count?


Gro, glad to see you back. We missed you on the night shift. :)
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Just got 8" at my house in Port Saint Lucie. WOW flooded everywhere.
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Quoting nrtiwlnvragn:
Think they are going to get the Doc an "on air" coach.
lol...you think so?
He needs to loosen up but it's tough to be thrown into the mix in the middle of a weather event like this.
.
Do appreciate his level headed opinion as always...much preferred over the hopesters and hypesters.
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There hasn't been any outstanding data reported by Recon that would point towards the classification of a hurricane. Flight-level winds after the reduction, were still below 65kts, and there has only been 1 SFMR reading of 64kts. It's going to come down to the wire, and the only chance you have of it getting upgraded is if Stewart is writing the package lol.
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Quoting CJ5:
L. Eye Character: Open in the north
M. Eye Shape & Diameter: Circular with a diameter of 20 nautical miles (23 statute miles)


This is not much better than any of the other recon reports we have had the past 96 hours. At least "ragged" is not in there. And it is open only on one side. Maybe he can eventually close up. It is the smallest diameter report we have had do that shows some improvement in that regard.


L. Eye Character: Open in the north
M. Eye Shape & Diameter: Circular with a diameter of 20 nautical miles (23 statute miles)


That's actually quite an accomplishment for Isaac during the daytime. Nighttime has generally been Isaac's friend for organization, only to lose what he gained when the sun comes up. I'm thinking he finally achieves Cat 1 hurricane status sometime tonight..
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cat 2 forecast again!
Member Since: July 12, 2008 Posts: 7 Comments: 383
1538. bappit
Quoting Wiiilbur:
Maybe some of you who are more experienced than me can help me understand something. According to the Cumulative Wind Map, all of south, southeast and southwest Florida are shown to have experienced winds in excess of tropical storm level (>39 mph). However, looking at the past history of the storm, with a NHC intensity of 60 mph as it passed Key West, I can't find a single National Weather Service station or offshore buoy that shows sustained winds that reached tropical storm intensity in any part of Florida, let alone anywhere near 60 mph. Maybe someone can help clarify this for me.

The cumulative wind map is created kind of arbitrarily by plotting the maximum radius of tropical storm force winds and connecting the dots. That maximum radius is an estimate to begin with. I love that phrase "connecting the dots" ... as if that is how logic works.
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I knew you were in here Pat! Hope you are having a relaxing and wonderful time :)
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1536. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 42264
PRESENT MOVEMENT TOWARD THE NORTHWEST OR 305 DEGREES AT 10 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 981 MB
MAX SUSTAINED WINDS 60 KT WITH GUSTS TO 75 KT.
50 KT....... 50NE 15SE 15SW 50NW.
34 KT.......150NE 150SE 80SW 180NW.
12 FT SEAS..180NE 180SE 180SW 150NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

NO HURRICANE!!!!
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1534. flcanes
Quoting Hurricane12:
It's only a matter of time before the winds begin to coincide the system's pressure.

very much true
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Isaac has gotta be one of the most frustrating storms of all time.
Member Since: July 12, 2008 Posts: 7 Comments: 383
Most interesting part of the advisory to me is the fact that he has slowed to 10 knots...........Not good in terms of longer time over water and potential for catastrophic flooding at landfall.
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OK...

who is in charge of getting the pitchforks?
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Ridiculous... period.
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Quoting TropicalAnalystwx13:
Isaac is only up to 70 mph.

SWEET...what did I tell ya...

See post 1500. He's a larger storm with lax pressure gradients...so the pressure is going to have to be lower than normal to get this to a hurricane. Not sayin it won't happen...but he's got more work to do...
Member Since: September 15, 2009 Posts: 539 Comments: 3712
Quoting Pirate999:


I think Texas is going to get a lot of......heat.


And humidity.
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Quoting AtHomeInTX:


Naturally. But the north winds will knock the humidity down a little. Take what I can get. :)
Texas had alot of heat last Sept too so this wont be any different
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Quoting Charliesgirl:


Hi! I am from Pearl River! sheltering in place. wish I had a generator.


Hey neighbor. Stay safe out there. hugs :)
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On visible it's apparent that Isaac has wrapped a strand of dry air all of the way to his core...he gets an A for effort but he's racing with a restrictor plate...
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1523. flcanes
Quoting NCHurricane2009:

Didn't we have a quick pressure drop to 990 mb last night...and we thought RI was starting...when in fact it seemed like a drop in the bucket after he only went to 65 mph winds?

Last night I was wondering if he was shrinking in size. I say nay...he is better modeled as a bigger storm with a more lax pressure gradient...so the pressure drops may perhaps push him to 70 mph...

nope, hurricane
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It's only a matter of time before the winds begin to coincide with the system's pressure.
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Quoting Hypemachine:
What we here in the vernacular call a tropospheric vortex inducing a Burnuli effect in conjunction with the Coriolis effect due to Abram's huge rack causing a cat5 storm surge in my groin equalization of pressure caused by the evaporation transport of heat above 850 millibars layer inducing a relative wind speed of or above 3308.1 centimeters per second.




Hahaha
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The NHC is waiting for surface winds to come up slightly before declaring this a hurricane.
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I'm a "lay person" and I have been here lurking since 2005 and haven't posted much at all but the only other time I had a gut feeling that a storm was going to be more than what ppl were anticipating 3 days out to landfall was Katrina. That is how I found this particular website because I couldn't understand why Katrina at the time was being so downplayed. HOWEVER, I didn't become a memnber until 2009. I posted earlier today that regardless whatever the storm is classified as, it is going to be bad d/t how big it is and the amount of rain/moisture and NOBODY should downplay this storm!
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70MPH ts isaac
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1517. Grothar
I'm too tired to go back to post 929 and plus Nea. If I put one here and said it belonged to him, does it count?
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Quoting TropicalAnalystwx13:
Isaac is only up to 70 mph.


This is frustrating
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Quoting bappit:
#1458 Water vapor showing the warming above the eye, too.


Matches the vortex message stating that the eye is open to the N.
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1514. GetReal
Quoting Sfloridacat5:
Dr Masters just now on TWC - "not going to be a cat 2 today or tomorrow."


Funny what sometimes happens when you have to step away from a computer monitor for a television interview... LOL
Member Since: July 4, 2005 Posts: 204 Comments: 8898
Quoting Thing342:
I wonder what the blog's reaction would be if the NHC doesn't make Isaac a hurricane.
Relieved for me. I think he does become a Hurricane but he won't be very strong. Right now he will be mainly a rainmaker and for areas that dont need rains they will get way too much.
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Quoting Pirate999:


I think Texas is going to get a lot of......heat.


Naturally. But the north winds will knock the humidity down a little. Take what I can get. :)
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1511. Dsntslp
Quoting Hoff511:
Hey Everyone, on topic but a little bit of a sidetrack. I know storms, but just experienced in Port Saint Lucie, FL thunder and lightning like never before. It was a weird flash, not a bolt, followed by what could only be described as multiple sonic booms occurring simultaneously. To the point of shaking my patio doors. Not thinking conspiracies or anything crazy just wondering if anyone else is aware of or has experienced this phenomenon. I have been through plenty of storms and rolling thunder but this is beyond that exponentially.
Usually lightning striking a transformer and the resulting backfeed. It has an almost aura quality to it like a blue or green hovering type light that wells up and goes boom (your super sonic boom). Sometimes more than one boom. Hope this helps. :) I don't know all of the terminology but have observed many transformers being struck over the years. Sometimes the backfeed is so bad it blow people electronics and refrigerators etc... The reason behind power bar protection, IMO.
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1510. LargoFl
Quoting FrancesJeanne:
Port St. Lucie this afternoon. So glad to see this band finally moving offshore - crossing my fingers that it keeps right on going; enough is enough already!
there are going to be some amazing flooding pics coming out of eastern florida i bet..i for one cannot believe the rainfall amounts, we all thought..over here on the west coast..that it would be happening to US today..schools were all closed, states of emergency enacted..and boom..you folks got it...this sure is one strange storm alright, throw out the predictions right..this storm is going to do what HE wants to do
Member Since: August 6, 2011 Posts: 4 Comments: 42264
ESTIMATED MINIMUM CENTRAL PRESSURE 981 MB
MAX SUSTAINED WINDS 60 KT WITH GUSTS TO 75 KT.
50 KT....... 50NE 15SE 15SW 50NW.
34 KT.......150NE 150SE 80SW 180NW.
12 FT SEAS..180NE 180SE 180SW 150NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

Oh boy...
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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.

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