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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1059. NSB207
Entity, possible!
Member Since: August 17, 2010 Posts: 0 Comments: 32
Quoting Chicklit:

That's a lot of water.


Very odd..
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1057. flcanes
Quoting Chicklit:

That's a lot of water.

in isaac? or the gulf?
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1056. LargoFl
Quoting weathermanwannabe:
Assuming that Issac does not intensify above a Cat 1-2 at landfall, he is such a large storm and packing so much rain that the storm surge and flooding is going to be catastrophic........Especially if he slows down and lingers near the coast after landfall.

This ain't no fast moving system caught up in a trof.........He is a slow soaker and that is very dangerous.
yes everyone in the northern gulf needs to listen to him on this..this is a huge flooding event especially if he stalls out and sits there
Member Since: August 6, 2011 Posts: 4 Comments: 39025
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Quoting WunderBlogAdmin:
Please take the inappropriate sexual innuendo off of this blog. Let's keep the conversation clean.




am with you 1000% keep it clean and on topic guys
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1053. kwgirl
Quoting watchdog40:
Our local area (Pensacola) is still saying we are not out the woods yet, they still have evacuations on the coast and everything is closing down, yet on here everyone is thinking we are out of it completely?
All the weather has been on the NE/SE quadrant. If the storm is landing to your west, you will have some bad weather. Follow your emergency management people's advice. Keep safe.
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Quoting LargoFl:
I am wondering what happens to us..as this storm sort of stalls in the gulf..what happens to us if it pulls another debby?..coming back eastward and crossing florida like debby did..omg..too terrible to think about the added flooding huh


Lots of people missing checks in the mail, lots saying "Hey, the check is in the mail, honest!", and kids saying, "An alligator ate my homework!"
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Quoting TomballTXPride:

He meant he believes a hurricane at 5 PM, which is very plausible. He didn't say anything about a Cat 5 hurricane.


Thank you......some on here I just don't understand...
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Steering Layer for 984 mb. Isaac:

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Northeast quadrant is very disappointing; no SFMR readings exceeded 45kts. The strongest winds associated with Isaac are confined mainly in the southern semicircle(s), which is rather unusual.

192500 2631N 08551W 8404 01418 //// +166 //// 099049 051 042 008 01

Waiting on the dropsonde to see how the MSLP has evolved over the past hour or so.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
1048. flcanes
Quoting NSB207:
The rain is training in the plain...no this is East Coast Florida. Now almost 6" in the rain gauge today with 2" in the last hour and one-half. More to come. Interesting how it has detached itself from the TS and taking a path NNE.

could it become a entity?
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That's a lot of water.
Now if it would just trundle over to west Texas, Isaac might be appreciated.
Member Since: July 11, 2006 Posts: 14 Comments: 11345
1046. snotly
Is lake Okeechobee feeding the convection on the right feeder band?
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Quoting MiamiHurricanes09:
50+ respect points to Nea.


Absolutely. Wonder how many of those commenting on Stephanie's appearence should really be getting ready for a hurricane?
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1044. flcanes
Quoting HoustonTxGal:

intensifiying
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1043. NSB207
The rain is training in the plain...no this is East Coast Florida. Now almost 6" in the rain gauge today with 2" in the last hour and one-half. More to come. Interesting how it has detached itself from the TS and taking a path NNE.
Member Since: August 17, 2010 Posts: 0 Comments: 32
1042. flcanes
Quoting jskweather:


Well said, I personally have not watched her on TV or remembered watching her, but when people started making jokes about her, I googled her and I agree that her resume is VERY impressive, so I dismissed he jokes as ignorant jealously, but that isn't really fair either since like I said I haven't really watched her much.

Regardless, she deserves respect for her education and resume. She didn't get those degrees with good looks. However, it may be she is not a competent weather forcaster regardless of her education. i do not know, but at least that is a relavent argument - not her bust size.

bottom line is all meteorologists save lives. end-of-story
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Quoting Redbull77:
people just amaze me. I just had a lady come into my business and we asked her what they were doing to get ready, she said "they have warned us before and nothing happened so i don't think its gonna get to bad." this is the problem with people and this is how people lose there lives by getting lazy. Louisiana


That's why natural selection is going to be mainstream in the future. I can see it coming guaranteed for society as I guarantee the sun will rise again tomm.
Member Since: July 14, 2008 Posts: 1 Comments: 9630
1039. WunderBlogAdmin (Admin)
Please take the inappropriate sexual innuendo off of this blog. Let's keep the conversation clean.
Quoting jskweather:


Well said, I personally have not watched her on TV or remembered watching her, but when people started making jokes about her, I googled her and I agree that her resume is VERY impressive, so I dismissed he jokes as ignorant jealously, but that isn't really fair either since like I said I haven't really watched her much.

Regardless, she deserves respect for her education and resume. She didn't get those degrees with good looks. However, it may be she is not a competent weather forcaster regardless of her education. i do not know, but at least that is a relavent argument - not her bust size.


You should watch her...I think she does pretty well at her job. I'm not generally a fan of TWC but she does very well for herself...and it's very well earned.
Member Since: June 24, 2012 Posts: 0 Comments: 472
1037. GetReal
This will answer a lot of questions about direction, organization, and strength.






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Assuming that Issac does not intensify above a Cat 1-2 at landfall, he is such a large storm and packing so much rain that the storm surge and flooding is going to be catastrophic........Especially if he slows down and lingers near the coast after landfall.

This ain't no fast moving system caught up in a trof.........He is a slow soaker and that is very dangerous.
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1033. JeffM
Quoting MrNatural:


You need to get back to reality....


Kman was just here and felt it's close to reaching hurricane status as well.
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Quoting WalkingInTheSun:


I don't think it's past troubled history will decide its future. lol. It does seem to be intensifying now, however, and also seems headed more W/NW than NW, to me. As they kept moving it's track westward, I think we should still consider the possibility that it might continue to do that, especially as it is headed into the warmest GOM waters in that more Westward path. IF, if, IF...it doesn't turn ...& should decide to morph & upscale dramatically, areas from New Orleans to Houston need to be ready.


I think the HGX need not be concerned as much about a landfall but rather the effects of a more westward track. I think that at this point Central LA coast is probable.
Member Since: June 24, 2012 Posts: 0 Comments: 472
1031. flcanes
Quoting hurricanehanna:
I'm just not seeing the NW movement...been staring at it too long I guess. Seems to be more WNW or W

wnw yes, west no
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Quoting Neapolitan:
I'm not the blog police, and far be it from me to stomp on anyone's fun, but I personally find the repeated jokes about Stephanie Abrams' body to be tasteless and disrespectful. I understand that some may not like her, and that's their choice. I also understand that some may find her attractive. That, too, is their choice. But the woman is a professional, not some bimbo posing for the cover of Maxim; she's got degrees in meteorology and geography (along with a minor in mathematics, fer cryin' out loud). As often as met students in this forum talk about how difficult those courses are, I'd think Ms Abrams would deserve at least here to be talked about for what she's accomplished more than the physical attributes gifted to her by her DNA. Not to mention: there aren't just hormonal males frequenting this forum. So can you guys maybe save the female body worship for WU mails or your own blogs? Please?


Well said, I personally have not watched her on TV or remembered watching her, but when people started making jokes about her, I googled her and I agree that her resume is VERY impressive, so I dismissed he jokes as ignorant jealously, but that isn't really fair either since like I said I haven't really watched her much.

Regardless, she deserves respect for her education and resume. She didn't get those degrees with good looks. However, it may be she is not a competent weather forcaster regardless of her education. i do not know, but at least that is a relavent argument - not her bust size.
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1029. LargoFl
Quoting Redbull77:
people just amaze me. I just had a lady come into my business and we asked her what they were doing to get ready, she said "they have warned us before and nothing happened so i don't think its gonna get to bad." this is the problem with people and this is how people lose there lives by getting lazy. Louisiana
yes you would be amazed at how many people in florida said the same thing
Member Since: August 6, 2011 Posts: 4 Comments: 39025
Quoting MississippiWx:


Levi had nearly 60 pluses last week when we first started tracking Isaac. Lol.

Yeah, definitely missed that one. For what it's worth, it's up to 48 now, which is more than Dr. Masters at the moment.
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Quoting wxmanjarod:
Convection looks like it's finally trying to wrap in on the southern half. Pretty bold effort by Isaac. If it's going to do something, now is the time. This thing only has about 12-18 more hours to put the pedal to the metal.


....unless it decides to slow down on us, which could give it more time.
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Quoting MrNatural:


You need to get back to reality....



Let me guess....you think it's falling apart??
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Good looking TD in the E-Pac...

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people just amaze me. I just had a lady come into my business and we asked her what they were doing to get ready, she said "they have warned us before and nothing happened so i don't think its gonna get to bad." this is the problem with people and this is how people lose there lives by getting lazy. Louisiana
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Quoting flcanes:

so 60 mph wind speed? right?
that is weakening
o_o
Well they're still ways away from the circulation. The data that they're finding right now is not necessarily indicative of Isaac's current intensity.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
1021. erisred
Quoting gulfbeachgal:
NASPen starting to bus out the students.
No "official" word on base closure as yet...
Pensacola State College closed until Thursday. UWF closed also.

I was leaving PSC Warrington at 1 pm today and saw a dozen big Trailways buses caravaning down Navy Blvd past the Hyway 98 traffic light. I wondered if they were heading for NAS to evac the students down there.
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1020. flcanes
Quoting dmh1026:

I agree....Everyone to the right of the center needs to watch out as that major rain may be coming their way. Isaac dumped a ton of rain on DR, Haiti, and PR long after he moved away to the west...JMO

because his circulation is so big
and big circulations+ gulf coast= ike= should have been cat4
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I'm just not seeing the NW movement...been staring at it too long I guess. Seems to be more WNW or W
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Quoting flcanes:

so 60 mph wind speed? right?
that is weakening
o_o



Not yet in the core yet.
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Quoting LurkyMcLurkerson:


Yep. And the movement to _its_ NE is not what I would expect to see in a normal chunk of band. I really think it's being fed by Isaac's bands, but is becoming its own little world of mess over there.

And I say that with all sorts of "I don't necessarily know what I'm talking about" implied, bear in mind -- the movement in that blob is just not something I've seen from watching these things, the way that thing is behaving is odd to me.

Wordy today, McLurkerson. Back to watching and seeing, but good luck to all.
Been watching that thing all day and it is just not right!
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Quoting 1900hurricane:
929. Neapolitan 2:13 PM CDT on August 27, 2012 +29

I think someone just won the blog. Never seen such a highly rated post before!

Totally agree, by the way.

29 is nothing.
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Quoting g5harlan:
Would you then expect a more westerly track with this "bridge" in place?



No, because there is another Ridge over Texas, although it could move further West if that Ridge slightly weakens or is outdone by the Bermuda high, but it seems like its all going to work out enough where Isaac continues moving NW.
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1014. dmh1026
Gotta go..Bye all and be safe in preparations!
Member Since: August 16, 2008 Posts: 0 Comments: 144
Quoting GTcooliebai:


Link? Thanks!
Member Since: February 2, 2010 Posts: 0 Comments: 1522
Quoting Category5HitsFl:

It looks like shear and dry air has taken a toll on Isaac, so it will never be able to reach anything more than a strong Cat 1 to weak Cat 2, but I have noticed it is still steadily getting its act together.

I would not be surprised if there were a few minor outbursts of intensification before this storm made landfall. The problems with Isaac in terms of it not strengthening much is just due to the fact that the conditions are not ideal, its so large, and it has gone through quite a troubled history.

I do expect that when/if it gets its act together it could be one potent force to contend with. Other then that I just expect steady strengthening as it nears the Louisiana coast.


I don't think it's past troubled history will decide its future. lol. It does seem to be intensifying now, however, and also seems headed more W/NW than NW, to me. As they kept moving it's track westward, I think we should still consider the possibility that it might continue to do that, especially as it is headed into the warmest GOM waters in that more Westward path. IF, if, IF...it doesn't turn ...& should decide to morph & upscale dramatically, areas from New Orleans to Houston need to be ready.
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Quoting 1900hurricane:
929. Neapolitan 2:13 PM CDT on August 27, 2012 29

I think someone just won the blog. Never seen such a highly rated post before!

Totally agree, by the way.


I got ~80 pluses on a massive rant last year to a troll claiming to be a met who believed that 2011 would not see a single hurricane. I believe that's the record for most pluses on a comment. Still, his post is excellent and deserves just as many.
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1010. Jstn568
Quoting CaneHunter031472:
Since Isaac is going to start slowing down, should I be worried about a surprise intensity wise?


Unlikely, and here's why: If the system begins to slow or stall, the deeper (colder) ocean waters will become churned and come to the surface. If this happens, it is highly likely that Isaac will either just reach the peak intensity currently forecasted, or even remain somewhat weaker, depending on how long it takes to move ashore.

That said, it never hurts to be over-prepared.
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1009. flcanes
Quoting MiamiHurricanes09:
Getting there; 54kt flight-level winds. The lack of extrap. pres. readings is rather disheartening.

192430 2632N 08550W 8412 01415 //// +176 //// 100052 054 043 009 01

so 60 mph wind speed? right?
that is weakening
o_o
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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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