Isaac is strengthening

By: Dr. Jeff Masters , 2:58 PM GMT on August 24, 2012

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Tropical Storm Isaac is strengthening. An Air Force Reserve hurricane hunter aircraft measured surface winds of 60 mph on the east side of the center, about 170 miles south of Santo Domingo in the Dominican Republic, at 8:40 am EDT this morning. Winds at the aircraft's flight level of 5,000 feet were hurricane force, 76 mph. The surface pressure remained fairly high, at 1000 mb. Tropical cyclones have a warm core, and the Hurricane Hunters typically find that a storm's lowest pressure is also where the warmest temperature are. However, this morning's flight found that Isaac was still disorganized, with the storm showing almost no evidence of a warm core. Isaac's warmest temperatures were displaced 75 miles to the west of where the lowest pressure was. There were no signs of an eyewall beginning to build. Infrared and visible satellite loops show that Isaac is somewhat asymmetric, with a large band of intense thunderstorms to the east, separated from the core region. This is interfering with both the storm's low-level inflow and upper-level outflow, but the band appears to be dying out. An analysis of upper level winds from the University of Wisconsin CIMSS shows an upper-level outflow channel well-established to the north, and an intermittent outflow channel to the south.


Figure 1. Evening shot of Tropical Storm Isaac taken on August 23, 2012, by the NOAA Hurricane Hunters.

Isaac's rains
Radar imagery from Puerto Rico shows that Isaac is dumping some very heavy rains to the south and east of the center. Ponce, Puerto Rico had a wind gust of 37 mph this morning as a heavy band of rain moved through, and radar-estimated rainfall amounts are in excess of 7 inches for the region just north of Ponce. Power outages to 2,000 homes have been reported in Puerto Rico this morning. NOAA buoy 42085 offshore from Ponce reported a wind gust of 54 mph near 9 am EDT this morning. Rainfall estimates from microwave satellite instruments suggest that Isaac's heaviest rains are to the south of the center, and that the Dominican Republic and Eastern Haiti will escape the worst of Isaac's rains. Haiti's southwest peninsula and Eastern Cuba should suffer the heaviest rains.


Figure 2. Radar-estimated precipitation from the Puerto Rico radar shows the region near Ponce has received up to 7" of rain as of 10 am EDT August 24, 2012.

Latest model runs for Isaac
The latest set of 00Z and 06Z (8 pm and 2 am EDT) model runs have come into better agreement, thanks to the dropsonde mission by the NOAA jet yesterday afternoon and evening. Isaac should move over Haiti's southwest peninsula and then eastern Cuba, then track along the spine of Cuba before popping off into the Florida Straits on Sunday. A trough of low pressure will then pull Isaac to the northwest, and then north, towards the Central Gulf Coast. Landfall locations range from Mississippi (06Z HWRF model run) to the Florida Panhandle south of Tallahassee (06Z GFDL model run.) It is possible that the trough of low pressure pulling Isaac to the north may not be strong enough to pull Isaac all the way to the northeast and out to sea, and the ECMWF model indicates that Isaac could stall out after landfall over the Tennessee Valley for several days.


Figure 3. Predicted 5-day rainfall total ending at 2 am EDT Wednesday August 29, from Tropical Storm Isaac. Graphics were generated from the 6Z (2 am EDT) August 24, 2012 run of the HWRF model. Image credit: Morris Bender, NOAA/Geophysical Fluid Dynamics Laboratory.

Intensity forecast for Isaac
Isaac has not intensified as much as predicted, and I think that the storm's very large size is partially responsible for that. It's tough to spin up as much atmosphere as Isaac is attempting to do very quickly. Conditions remain favorable for intensification today, with wind shear low, 5 - 10 knots, ocean temperatures warm, 29°C, and dry air mostly mixed out of the storm's core. The large band of intense thunderstorms to the east, separated from the core region, appears to be dying out now, which will help the storm grow more organized. The storm's structure has improved considerably between 9 am - 10 am EDT, with a fairly tight center forming, exposed to view, on the north edge of Isaac's heaviest thunderstorms. A curved band of heavy thunderstorms is now trying to wrap around this center to the northeast, and this band will bring very heavy rains to Haiti and the Dominican Republic this afternoon. I expect that the Hurricane Hunters will observe a partial eyewall in their vortex reports between 2 - 4 pm EDT this afternoon. The storm's large size and disorganized structure suggests that Isaac will be able to intensify only slowly today, and will have top winds of 70 - 75 mph before encountering Southwest Haiti and Eastern Cuba tonight and Saturday. Isaac will likely be a 50 - 60 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate. The upper-level wind pattern favorable may be quite favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm--though the models disagree on whether or not this anticyclone will set up directly over Isaac or not. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane. It is possible that Isaac could be approaching Category 3 strength by the time it makes landfall on Tuesday on the Gulf Coast, as suggested by the latest 06Z run of the HWRF model.

Impact on Tampa, Florida
The Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 17% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. Tampa is in the NHC cone of uncertainty, though near the edge of it. At a minimum, Tampa will receive very heavy rains and wind gusts in excess of 40 mph. Isaac is going to be hard-pressed to bring hurricane-force winds to the city, though, since any path that takes it close to Tampa would keep the storm too close to land for significant intensification to occur. I put the odds of a mass evacuation being ordered for Tampa during the convention at 1%. I have detailed information on Tampa's storm surge vulnerability in a post from last week.

Invest 97L off the coast of Africa
A tropical wave that moved off the coast of Africa on Thursday has been designated Invest 97L by NHC this morning. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear of 10 - 20 knots. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Sunday morning. The 8 am EDT SHIPS model forecast predicts that 97L will track west-northwest over the next few days, and encounter a region of high wind shear associated with an upper-level low on Monday and Tuesday. This low may be capable of tearing the storm apart, as happened to Tropical Storm Joyce. None of the models currently foresee that 97L will affect the Lesser Antilles Islands, but 97L may pass near Bermuda 7 - 8 days from now.



20th anniversary of Hurricane Andrew
Today, August 24, is the 20th anniversary of Hurricane Andrew, which hit South Florida as a Category 5 hurricane with 175 mph winds--one of only three Category 5 hurricanes ever to hit the U.S. With Isaac churning through the Caribbean this week, I didn't have time to prepare a special post on Andrew, but our Hurricane Andrew archive page has links to satellite and radar images, newspaper headlines, and 49 YouTube videos. Here's an additional link for an Andrew damage video shot by wunderblogger/storm chaser Mike Theiss, when he was 14 years old.

Jeff Masters

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Any guesses on the 5pm intensity and track. My guess is 65mph and no change in track (since GFS went right and Euro went left). Anyone else have thoughts?
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Quoting wunderkidcayman:
steering at Isaac's pressures shows still a W-WNW track no NW movements yet

but that will be fore Recon to decide

lol.

you never give up, i give you that.

recon has already proven it's been moving NW. all you have to do is go back and see where they put the center early this morning while flying in the storm.
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1309. Levi32
Quoting divdog:
Levi. Is your current thinking this will stay east of destin Florida. I realize that is asking a lot to pinpoint but just wondered what your current thoughts were. New condo near the beach has me a little more anxious than usual.
Thanks


My forecast track is on my blog page. It is east of Destin but Destin is right in the middle of the range of possibility at 5 days out.
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Quoting Articuno:

|o|
Out of all this time I still have no clue what all these numbers mean. I only know TS and 995 pressure and 50 knots..? Or is it 55?


55kts.
wind speed is left of the pressure.

55kts ~= 63.25mph...

Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting Articuno:

|o|
Out of all this time I still have no clue what all these numbers mean. I only know TS and 995 pressure and 50 knots..? Or is it 55?



Link
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we can not disregard the ECMWF Model because it has been the best model
for the last two years and for the fact that it has been very consistent
with the track to the west near the LA/MS Border. The other models
including the second best model the GFS has been flip floping all over
the place. here is my take on what TS Isaac may do. Issac may not go
over the northern part of Cuba at all and end up moving real close
skirting the coast of the southern coast of Cuba and make a run for the
hot waters of the Yucatan Channel. the reason that I say this is because
the canter of Isaac has a history of his center reforming to the
south. if you look at the recent and past Satellite Loops you will see
that this storm will continue to fire blowups of storms to the southwest
causing the center to reestablish itself in that direction IMO.
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If Isaac were to intersect Hispañola through the thin sliver of land on the southwestern side of Hispañola, I have a hard time believing it'll weaken anymore than 5 knots.

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Curvature of the earth is between 7-9 inches per mile, so over 150 miles is 1200 inches, or 100 extra feet. Levi's approximation of flat earth is acceptable IMO.
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1303. guygee
Looks like Isaac is finally stacking up just in time to strafe the Massif de la Hotte over Haiti's southern peninsula, then probably entering the Windward Passage.
From watching past storms that one-two punch is very unfavorable for tropical cyclones, so Isaac will have some re-organizing to do after that.

I decided I am not going to jump too far ahead in time on this one...a lot of unknown factors still coming into play...
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vis.eye.wall.pinching.off?
Member Since: September 11, 2010 Posts: 1 Comments: 4995
Quoting Levi32:


Curvature affects it a bit but a flat earth is an ok approximation at that distance.
At 140 miles? Yes it is.

Just look at the NOAA radar beam height calculator cat7 posted earlier, it will calculate it all for you. Link. It's nowhere near 6500 feet. Even if it were, recon was still flying lower, which was something several of us were trying to bring up yesterday.
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Quoting RTSplayer:


That seems like a pretty good fix. Still isn't perfectly stacked, but this is about the most realistic fix I've seen in terms of data not conflicting with itself and such...

Suggests an eye-like feature is developing, but has a slight misalignment at the surface.


But slowing down is the best thing that has happened to Isaac. It is allowing itself to realign the MLC with the LLC. Won't take much longer for it to be perfectly aligned ;).
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Quoting Levi32:


I want FSU, but we'll have to see how that goes.



If you do plan to be at FSU when would you potentially start? I plan to start there in this upcoming Spring.
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Putting in perspective.......(?)

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1265:

Minus 15% rule suggests it's 65mph at the surface. Good agreement with other data.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting bocahurricane:


I know I am probably late on reading this but thanks for the laugh. It is much needed with this storm. That was hilarious!


This sounds like a Bastardi forecast
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Looks like he just built an eyewall
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steering at Isaac's pressures shows still a W-WNW track no NW movements yet

but that will be fore Recon to decide
Member Since: June 13, 2009 Posts: 2 Comments: 12712
Quoting Noodoggy:



That's poor statistics when you have a 150+ yr span and say maybe twenty (?  cant and will not count those lines  lol) storms.  150 yr span is poor sample of say 2billion years plus of weather.
Remember those really hot summers 1.7394 billions years ago, and all those d*** volcanoes...I reminisce.
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1292. centex
Quoting wunderkidcayman:
hmm seem like HH found it at 16.8N 71.4W
Are they going to start using the HH positions or still use a "mean" location?
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1291. 7544
Quoting seflagamma:
Hi everyone, still lurking for awhile....

wondering what the 5pm will show. I was ready to write this off for SE Fla this morning; now I
am watching again.



knowing the nhc they go left lol
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Still a lot of dry air in there:



Somehow he's strengthening despite that though, which is a bit worrying... Imagine if the dry air wasn't there.
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Quoting floridaboy14:
GFS is further east at the begining but isaac looks to be bending back wnw on the run


You keep hoping!
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Quoting Tazmanian:
\



no

18:41:00Z 16.783N 71.367W 842.9 mb
(~ 24.89 inHg) 1,446 meters
(~ 4,744 feet) 994.8 mb
(~ 29.38 inHg


Whoops... missed that other barb. Thanks taz!
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1287. pottery
Quoting stormpetrol:


Thats where it is, but it has commenced a westward movement again IMO, we'll have to wait for 2-3 more fixes to confirm.

A little of that, yes.
Important though.
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Until the EURO comes into agreement, if it ever does, or other models shift west, don't think anyone from MS/AL to Panhandle should let their guards down.

It will hit the Keys or S. Florida in my opinion. From there, guess time will tell. I don't think it is going west of New Orleans tho. Leaning more towards a Pensacola to Panama City Beach landfall personally.

Just my opinion though.
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center is right where the NHC put it at 2pm per the HH.
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i think the remains could hit iowa next thursday
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Quoting cchssweatherman:


Okay, after reviewing the data from the Gulfstream-IV along with my own personal instrumentation, I'm able to report my observations and my forecast for this system.  There are a few anomalies that the models don't seem to be picking up on, but which should seem rather obvious to the trained eye.  

First, there is a rather potent riptide traversing the coastline from Maine to just west of Florida, where a substantial area of cooler water is now present.  As my graphic indicates this will serve to weaken Isaac from a category 4 to 3 briefly; however, Isaac will quickly gain category 5 status due in part to the sudden change from cold to extremely warm temperatures as well as a large area of cumulus clouds south of Cuba.  These clouds act to “supercharge” storms due to their shape, which creates the perfect amount of turbulence needed to reach category 5.

The blocking low, along with an area of concern in the Eastern Pacific (though expected to enter the Gulf of Mexico as early as Sunday), will serve to turn Isaac sharply north in a “V” motion.  Shortly thereafter, the Saharan Air Layer over the Eastern Gulf/Texas will cause a slight weaking, albeit only to Category 4.  The Wind, along with the encroaching High over Texas, will cause Isaac to make several right angles, after which the blocking high over Florida will allow Isaac to move north towards the Louisiana coast.  Because this motion will occur rather slowly, the waters off the northern Gulf coast will have churned for quite some time, cooling them substantially.  As a result, combined with the perfect amount of Saharan Air Layer over Florida, shortly after landfall in Louisiana on September 13, when Isaac re-enters the Gulf after a brief eastward trajectory, Isaac will transition to a Noreaster (depicted by blue dots on my graphic).

This Noreaster status could potentially cause more problems for the northern Gulf coast than even Isaac’s landfall as a Category 4 hurricane.  Nor’Isaac is expected to produce substantial hail and snow, with Louisiana and Mississippi possibly experiencing blizzard-like conditions for upwards of a week.  Residents should prepare accordingly.  

In addition to Isaac, forecasters should note the additional two areas of concern I have highlighted.  I will update my forecast as additional information rolls into the instruments in my parents’ basement.  

CCHS signing off for now!


I know I am probably late on reading this but thanks for the laugh. It is much needed with this storm. That was hilarious!
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1282. BigTuna
My theory is that all the hot air in here is reaching Isaac's core and causing the pressure to drop. If the web server goes down it will drop to a minimal depression almost immediately.
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Quoting Neapolitan:
ATCF says Isaac is just 10 knots from hurricane status:

AL, 09, 2012082418, , BEST, 0, 168N, 714W, 55, 995, TS, 50, NEQ, 60, 0, 0, 0, 1008, 250, 60, 0, 0, L, 0, , 0, 0, ISAAC, D,

|o|
Out of all this time I still have no clue what all these numbers mean. I only know TS and 995 pressure and 50 knots..? Or is it 55?
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1280. pottery
Quoting Ameister12:
Afternoon everybody!

Isaac's center has become much better defined today, which explains why recon has found a stronger storm.

So, what's recon finding right now?

See post 1265.
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Quoting tropicfreak:
Time: 18:42:00Z
Coordinates: 16.8N 71.4167W
Acft. Static Air Press: 843.0 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,448 meters (~ 4,751 feet)
Extrap. Sfc. Press: 995.1 mb (~ 29.39 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 64° at 1 knots (From the ENE at ~ 1.1 mph)
Air Temp: 18.0°C* (~ 64.4°F*)
Dew Pt: 18.0°C* (~ 64.4°F*)
Peak (10s) Flt. Lvl. Wind: 3 knots (~ 3.4 mph)
SFMR Peak (10s) Sfc. Wind: 18 knots (~ 20.7 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)
(*) Denotes suspect data


That seems like a pretty good fix. Still isn't perfectly stacked, but this is about the most realistic fix I've seen in terms of data not conflicting with itself and such...

Suggests an eye-like feature is developing, but has a slight misalignment at the surface.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting Levi32:
12z JMA hugs the west coast of the peninsula and then up into Apalachicola.

,Actually the most realistic solution yet imo,if it doesn't go up the east coat of Fl that is.
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Quoting wunderkidcayman:
hmm seem like HH found it at 16.8N 71.4W


Thats where it is, but it has commenced a westward movement again IMO, we'll have to wait for 2-3 more fixes to confirm.
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994.8 MB!!
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ok now waiting for HH to make a few more passes
Member Since: June 13, 2009 Posts: 2 Comments: 12712
1274. Brock31
Impressive to see all this towards the end of D-MIN.

Isaac should be interesting to watch tonight.
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Why does it look like we keep nudging further and further east. Also, latest sat images look like a N-NW motion? I don't think the east coast of Fla or Ga and Car. Can let their guard down yet...IMO





blockquote class='blogquote'>Quoting SFLWeatherman:
Not going be good for S FL
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1272. divdog
Levi. Is your current thinking this will stay east of destin Florida. I realize that is asking a lot to pinpoint but just wondered what your current thoughts were. New condo near the beach has me a little more anxious than usual.
Thanks
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Quoting WxGeekVA:
I've updated my forecast after the latest model runs and surface observations, and made my graphics a little better. I don't really buy the eastward shift much yet, and I am putting a little bit more stock in the Euro with an (possibly rapidly) intensifying hurricane heading west of Florida.



A 120 mph cat 3 making a direct hit on Panama city? That'll be a news story.
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Hi everyone, still lurking for awhile....

wondering what the 5pm will show. I was ready to write this off for SE Fla this morning; now I
am watching again.

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Afternoon everybody!

Isaac's center has become much better defined today, which explains why recon has found a stronger storm.

So, what's recon finding right now?
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Quoting Levi32:


I edited it to include 130-150 miles and for some reason it dropped that edit after I opened it a 2nd time.

Ok...math...

150 miles * 5280 feet/mile = 739200 feet

Setting up the triangle from the radar tower in San Juan to a point on the ocean 140 miles away and the 3rd point directly above that at the radar beam. Beam angle is 0.5 degrees, here theta. "y" is the height of the far side of the triangle from the ocean to the beam. "x" is the horizontal distance from the radar tower to the point beneath the beam:

We know that: tan(theta) = y/x

We know everything except y:

y = x*arctan(theta)

y = (739200 feet) * arctan(0.5 degrees) = 6450 feet

Well, not my argument, but you are assuming Earth is flat...you left out the Earth's curvature.
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pressure 995
Member Since: June 13, 2009 Posts: 2 Comments: 12712
Quoting Chema44:
"A mal Tiempo buena Cara" Para los que no lo han visto he aquí el video desde La Guancha en Ponce, tenemos a Chema Weather Cantore, Jasleen Ginelle, Janet Ortiz, Joel Lugo, Julio Santos, El Corillo del CMC con nuestras camisas. Ada que lo disfrutes.

http://www.youtube.com/watch?v=8sqL_K7K1a8&fe atur e=youtu.be


¿Cómo va allí con inundaciones?
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1265. Chiggy
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I see a Mobile landfall in there.
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Quoting CybrTeddy:
Bang.
994.8 mb
(~ 29.38 inHg)

Winds died off too as they passed through the CoC and started to pick up as they exited, indicative of a possible developing eye.


coordinates?
Member Since: January 25, 2012 Posts: 33 Comments: 1520
1262. snotly
Yea and the relativistic effects are probably in the ball park of a few thousandths of a centimeter or less

Quoting RTSplayer:


Was going to mention that, but figured why bother.

His formula is true for a flat earth, but within radar range I don't think the curvature makes more than a few hundred feet difference anyway...
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Quoting fire635:


I think youre somewhat off base.... If the new GFS verified, we would be looking at some very serious consequences here in Tampa Bay. Maybe not "worst case scenario" but much more than the "usual" storms we are accustomed to.



Alright far enough, you may disagree with me, and I will not get angry, it's ok to disagree :)
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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.