Isaac makes its final approach towards Louisiana

By: Dr. Jeff Masters , 3:29 PM GMT on August 28, 2012

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The winds and water are rising all along the coast from Louisiana to the Florida Panhandle as Tropical Storm Isaac makes its final approach. Two hurricane hunter aircraft in the storm are measuring a steadily lowering pressure and increasing winds aloft, but hurricane-force winds have not yet been observed at the surface. The 8:30 am center fix found a pressure of 976 mb, which is very low for a tropical storm. Top surface winds measured with the SFMR instrument were 70 mph, but the plane measured 102 mph at an altitude of 5,000 feet. It's more typical to see surface winds of 85 mph with a storm with these characteristics. Infrared and visible satellite loops and hurricane hunter reports from this morning have shown that Isaac has developed a 25-mile diameter eye, though the eyewall has not yet formed a full circle around the eye. Heavy thunderstorm activity is lacking on the north side, where light wind shear of 5 -10 knots is still pumping some dry air into the circulation.


Figure 1. Morning satellite image of Isaac. Note how dry air has wrapped into the west side of the storm, causing a lack of heavy thunderstorm activity.

Isaac's rains
One of the most remarkable features of Isaac has been the huge spiral band that parked itself along most of the east coast of Florida and remained there for an entire day, despite the fact the center of the storm moved 400 miles away. This rain band was amplified by a weak trough of low pressure along the East Coast, which pulled away from the coast Monday night, taking the band of heavy rain out to sea (except for a few lingering showers near West Palm Beach.) Isaac's heaviest rains fell along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. The 2-day rainfall total of 9.03" at West Palm Beach brought their monthly rainfall total to 22.28", a new August record (old record: 20.12" in 1995.) Vero Beach's 6.48" of rain was a record for any August day. A possible tornado touched down there, damaging 20 mobile homes. In the Keys, rainfall totals as high as 7.94" (at Upper Matecumbe Key) were measured. 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 24, and two died in the Dominican Republic. The big concern in Haiti is the heavy damage that was done to crops, and the likelihood that the storm's rains will worsen the cholera epidemic that has killed over 7,000 Haitians.


Figure 2. Radar-estimated rainfall from Miami, Florida radar shows that Isaac has dumped a wide swath of 8+ inches of rain (orange colors) across the state. Rainfall amounts in excess of 20" may have fallen just west of West Palm Beach, though the highest amount reported by a rain gauge was 13.10" at Greenacres in Palm Beach County.

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 some differences in what happens after that. Isaac may scoot nearly west-northwest just inland along the coast into Texas, as predicted by the ECMWF model, or head straight inland to the northwest and into Arkansas, as predicted by the GFS model. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. It appears likely that Arkansas will see some heavy rains of up to five inches, which would help put a dent in the exceptional drought conditions there.

Intensity forecast for Isaac
Low wind shear of 10 knots or less is likely until landfall, along with very warm ocean temperatures. An analysis of upper level winds from the University of Wisconsin CIMSS shows that Isaac's upper-level outflow is the strongest we've seen, with a solid outflow channel to the south. These conditions favor continued strengthening of Isaac until landfall. However, we've observed in the past many instances of hurricanes suddenly weakening in the final 12 hours before making landfall along the Central Gulf Coast. Katrina, Gustav, Dennis, Ivan, and Rita all did so. A July 2012 paper in the Bulletin of the American Meteorological Society by Rosenfeld et al. titled, AEROSOL EFFECTS ON MICROSTRUCTURE AND INTENSITY OF TROPICAL CYCLONES, theorizes that this may happen because of the impact of small particles that get pulled into the outer circulation of hurricanes, seeding the clouds. These small particles, primarily from air pollution, serve as the seed around which water condenses, increasing the rain in the outer spiral bands. The increase in rain and heat energy at the periphery of the storm comes at the expense of the eyewall and inner core, where the winds tend to weaken. A detailed modeling study by Khain et al. (2010) of Hurricane Katrina in the final day before landfall was able to reproduce the storm's weakening only when this air pollution effect was included. This impact of small particles on hurricanes is not included in any operational hurricane model.


Figure 3. Tide gauge data from Shell Beach, located in Lake Borgne just east of New Orleans. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.)

Storm surge observations from Isaac
Isaac's storm surge has peaked along the west coast of Florida. As I explain in our Storm Surge Tutorial, we are most interested in the storm tide--the height above Mean Sea Level (MSL) of the tide plus the storm surge. The storm tide is the number given in NHC advisories for how much above ground level the ocean will be at the coast. The storm surge is the extra elevation of the water due to wind blowing on the water, and does not include the action of waves on top of the water, nor the tide. Tide gauges are specially constructed so that transient waves do not impact water level measurements. At Cedar Key on the West Florida coast north of Tampa, a storm surge of 3' and storm tide of 3.8' were observed early this morning. These were the highest water levels measured at any tide gauge along the Florida west coast. Higher storm surges are occurring in the Florida Panhandle. As of 9 am EDT, here were the storm surge/storm tide measurements along the Florida Panhandle:

Apalachicola, FL: 3.5' storm surge, 4' storm tide
Panama City, FL: 2.3' storm surge, 3.3' storm tide
Pensacola, FL: 1.5' storm surge, 2.5' storm tide

A storm surge of 3.5 feet was recorded at 10 am EDT at Shell Beach on the east side of New Orleans in Lake Borgne. This site will have one of the highest surge values during Isaac; a storm surge of 9.5' was measured at Shell Beach during Hurricane Gustav in 2008.


Figure 4. Category 2 Hurricane Gustav of 2008 followed a very similar path to Isaac, and brought a storm tide (the combined effect of the storm surge and tidal levels) of up to 14.5' above ground level to the east side of New Orleans. Isaac's surge may be similar, though probably a little less, than Gustav's.


Figure 5. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Isaac: similar to Category 2 Hurricane Gustav of 2008 in destructive power?
Isaac is a huge and slow-moving storm, with tropical storm-force winds that extend out 205 miles from the center. Isaac has cut its forward speed down from 14 mph yesterday to 10 mph today, and a large swath of the coast will be subject to high winds and a large storm surge for an usually long period of time for a hurricane--up to 24 hours. Long duration winds are much more damaging than short duration winds, and a long duration storm surge event allows damage to occur during multiple high tide cycles. The long duration storm event will also allow very high rainfall totals, resulting in greater fresh-water flooding problems than usual. As a result, I expect Isaac's to cause more damage than the typical Category 1 hurricane. The 9:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 2.3 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 4.1 on a scale of 0 to 6. For comparison, the storm surge destructive potential of Category 2 Hurricane Gustav of 2008 was rated at 4.2 on a scale of 0 to 6, and the wind destructive potential was 1.1--which is lower than Isaac's, even though Isaac was just a tropical storm at 9:30 am EDT. Gustav brought a storm tide (the combined height of the storm surge and high tide) of 14.5' to the east side of New Orleans, and 11' to Waveland, Mississippi. However, the destructive potential of Isaac's surge may be overrated by this analysis. Wave heights this morning from buoy and ships in Isaac have mostly been below 15', which is quite unimpressive. One ship report to the SE of the storm had a 19' wave height (thanks to meteorologist Steve Gregory for pointing this out.) With only another 12 - 18 hours over water, Isaac likely won't have time for its slowing increasing winds to build up a storm surge that will reach as high as 14', like Gustav did. The official NHC forecast of maximum storm surge height of 12' looks like a good one. The highest rainfall total observed in Gustav was 21" at Larto Lake, Louisiana, and I expect we'll exceed that for Isaac, since the storm is moving more slowly. Gustav spawned 41 tornadoes--21 in Mississippi, 11 in Louisiana, 6 in Florida, 2 in Arkansas, and 1 in Alabama. The strongest tornado was an EF2 in Evangeline Parish, Louisiana. Isaac will likely produce 10+ tornadoes. The total damage from Gustav in the U.S. was $4.5 billion (2012 dollars.) I expect Isaac's damage total will be in the $500 million - $4 billion range.

Invest 97L in the Middle Atlantic
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1250 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 40% chance of developing by Thursday morning. None of the reliable models foresee that 97L will be a threat to any land areas.

Another tropical wave that moved off the coast of Africa on Sunday is located just southwest of the Cape Verde Islands, and is moving west at 15 mph. 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 Thursday morning. The disturbance could begin to affect the Northern Lesser Antilles as early as Saturday night, though our two best models, the GFS and ECMWF, predict the center of the disturbance will pass a few hundred miles north of the islands. The disturbance could be a long-range threat to Bermuda.

Jeff Masters

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Quoting hydrus:
Godspeed to all..keep us here posted when possible..:)
I sure will do hydrus
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use 70374 for twc zip to check for local radar and what not
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Quoting mobilebayal:
Getting windy here in Mobile. Not much rain though. Power has blinked a few times.



Over here in Daphne, Pretty windy... lots of little branches down... nothing to bad though
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Quoting RTSplayer:
Check regional radar, or Mobile and Tallahassee radar and get a load fo this back side feeder band which is going to be training over SELA for the next several days.

This thing is a monster.



Big, but not monster-looking, to me.
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We are down to watching the wobbles now....
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This is the F. Quarter Cam....one of the CAMs here...


Link

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Updated rainfall rates with this band in Eastern FL to 1-3 inches per hour....

Next band coming in soon

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1017. Patrap
Quoting Chicklit:
Now here are some major pumps! And they will work this time.

Link Pump Station Repairs and Stormproofing

Excerpt:
Nearby safe rooms are being strengthened to withstand hurricane force winds up to 250 MPH, wind-driven water, and loss of power. Stormproofing pump stations will ensure that pumps remain operable and that station operators can safely stay on the job during a storm event. The Corps worked closely with local governments to ensure that all repair and maintenance issues were considered during the repair and stormproofing processes.



Those are Very Secure..and we lost something just now off the roof here or next door in the Highest gust yet..that was 40 plus gust easily.

Son is looking into it..pic will be taken.
Member Since: July 3, 2005 Posts: 426 Comments: 128630
1016. GetReal
There maybe something to that theory of the northern circulation interacting with the friction of the coast, causing an increase in convection as Isaac makes landfall. The radar is indicating an increase in coverage and intensity of the convection as it is being crushed up against the coast of MS and LA. Those stronger winds found in the upper levels of Isaac could very well be force to the surface.




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Getting windy here in Mobile. Not much rain though. Power has blinked a few times.
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Now here are some major pumps! And they will work this time.

Link Pump Station Repairs and Stormproofing

Excerpt:
Nearby safe rooms are being strengthened to withstand hurricane force winds up to 250 MPH, wind-driven water, and loss of power. Stormproofing pump stations will ensure that pumps remain operable and that station operators can safely stay on the job during a storm event. The Corps worked closely with local governments to ensure that all repair and maintenance issues were considered during the repair and stormproofing processes.

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Quoting CaneHunter031472:
Have the NHC realized that Isaac is actually moving to the north of the official forecast?


It jogged north, headed north of the mouth of mississippi. Then it jogged south and it will end up south of the mouth now if you look at the radar closely.
Member Since: December 28, 2010 Posts: 0 Comments: 134
Storm surge is supposed to be 12 feet in New Orleans area, 8 feet in Mississippi, and 6 feet in West Louisiana and Alabama. Stay safe from the surge, folks!
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Quoting Skyepony:
Wellington got dumped on yesterday, 1 in a 100 year event for them. Bottom video here is the Wellington Showgrounds underwater. People are also looking for places to move horses to. Only takes a few days of standing in water & many of their hooves will kinda falloff.

Awful to see a line of showers settling up this afternoon there.


Oy, I've had to urgently move horses in a flood before. 10 PM emergency, actually, we didn't know the creek was going to overflow so badly -- it changed course completely in that storm.

Got the phone call that whoever could needed to get out there at least to get them out of the worst flooding paddocks, so suddenly we all descend on the poor terrified guys in the dark, about ten of us, wearing slickers and big hats. As far as they were concerned, not only were the waters crashing everywhere around them in pouring rain, but _aliens had landed_.

Much luck to folks -- hope they all find spots.

And if this thing really is stalling much, I fear it's going to be a repeated flooding story around the whole friggin' gulf. About the last thing most places right around there need at the moment is Isaac just sort of kicking it for a day and throwing water everywhere.
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1010. Patrap
Land friction will begin to spin down those spinners at some point

Member Since: July 3, 2005 Posts: 426 Comments: 128630
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1008. VR46L
Quoting RTSplayer:
Check regional radar, or Mobile and Tallahassee radar and get a load fo this back side feeder band which is going to be training over SELA for the next several days.

This thing is a monster.



Very good point !!

Seems to me some folk are forgetting that NW Florida , Alabama, Mississippi and other parts of Louisiana will be affected ... not forgetting the flooding in both the Carolina s and Florida ..





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1007. JeffM
When will recon be back?
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Quoting CybrTeddy:
Also, I'd like to say that the blog hasn't really degraded at all - it is the amount of people that come and visit us during events like Isaac that has gone up, it goes up hugely every year. Look back say, 2007. There iwas loads of misinformation, drama and such even then, why? Because tropical cyclones are bloody hard to predict. Now though even more data is thrown around and people simply don't agree with ones interpretation of it, including me. I come back every year because the community of regulars here are great and I have learned so much about cyclone forecasting. Back later.


It definitely has, especially when Dr. Masters' blog is mentioned on other forums, it seems that there is near-universal disdain for the comments and lots of recommendation to skip over them entirely
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Check funktop.

Even though symetry isn't the greatest, this is possibly the most widespread, intense convection we've seen with this storm.



shortwave



Visible



Unenhanced IR

Member Since: January 25, 2012 Posts: 33 Comments: 1520
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Uh-oh, time to break out the tinfoil hats!

Quoting whetherboy:


Check out the image with figure 2 in the Doc's post. There appears to be numerous chemtrails associated with the convection over Florida. Perhaps there was a mad made effort to prevent moisture from being drawn into Isaac by pre-emptively triggering precipitation. This enabled the moisture to escape Isaac and be drawn north to a weak area of low pressure. It is well documented that numerous governments, including our own, engage in weather manipulation, and even the Doc noted that this event was the most "remarkable" aspect of Isaac.
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Quoting CybrTeddy:
Also, I'd like to say that the blog hasn't really degraded at all - it is the amount of people that come and visit us during events like Isaac that has gone up, it goes up hugely every year. Look back say, 2007. There iwas loads of misinformation, drama and such even then, why? Because tropical cyclones are bloody hard to predict. Now though even more data is thrown around and people simply don't agree with ones interpretation of it, including me. I come back every year because the community of regulars here are great and I have learned so much about cyclone forecasting. Back later.


Agree - I just filter out the belw average comments.
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Quoting Chicklit:


Link US Army Corps of Engineers Team New Orleans Site

Lots of money spent...hope it works!
Member Since: September 1, 2007 Posts: 0 Comments: 249
In the "Never Take It For Granted" Department...

From this blog 6AM August 29, 2005:

Katrina is due south of the Mississippi-Louisiana border, and moving northward at 15 mph. On this course, the western edge of the eyewall will pass some 20 miles to the east of New Orleans, sparing that city a catastrophic hit. As the eye passes east of the city later this morning, north winds of about 100 mph will push waters from Lake Pontchartrain up to the top of the levee protecting the city, and possibly breach the levee and flood the city. This flooding will not cause the kind of catastrophe that a direct hit by the right (east) eyewall would have, with its 140 mph winds and 15-20 foot storm surge. New Orleans will not suffer large loss of life from Katrina.

You just never know. Be safe.
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Patrap and nola gang :

I don't know how long you'll have comms, so I'm sending this now.

Stay safe and as comfortable as possible. More on the other side...
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Quoting Skyepony:
Wellington got dumped on yesterday, 1 in a 100 year event for them. Bottom video here is the Wellington Showgrounds underwater. People are also looking for places to move horses to. Only takes a few days of standing in water & many of their hooves will kinda falloff.

Awful to see a line of showers settling up this afternoon there.
We can't take any more rain runoff here in Port Saint Lucie, FL, IMO. The blessing will be that there is no school today so all those parents stuck on roads with flooded engines and children in the car won't happen today like yesterday. TY for the link. :)
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Quoting Felix2007:

Worse AKA Jose
Well, it has sustained convection and a closed circulation, It's just being sheared.
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Quoting Levi32:
If Isasac moves north of the mouth of the Mississippi River, he will have an extra stretch of water and a favorably-shaped coastline that will help him continue tightening until final landfall.


Are you sure of this? I remember when the storm coasted along the edge of cuba... it basically rolled along the coast. Since there is no small area of low pressure to get caught in a bay, I don't think going north of the mouth of the MS river will help. It will actually force the storm into land prematurely and weaken it quickly.

I think it equally likely that the storm center, since it is not concentrated, will keep relocating south every couple of hours to the area of strongest thunderstorms, which might ONLY exist over water once its center is to the west of New Orleans' longitude.

This could keep the storm hugging the coast for up to a day.
Member Since: December 28, 2010 Posts: 0 Comments: 134
Quoting GetReal:


Its blowin'up......bet the shear is like 0.
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Quoting MississippiWx:
Dry air looks to be less of an issue. The core has remained solid for a while. Of course, now we don't have recon. Lol.

It also looks like on radar that Isaac is moving slower.
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Quoting Abacosurf:
Really looks to have slowed. I wouldn't doubt if this just makes it inland and then crawls to the WNW paralleling the coast of LA. for 24 hours.


Hahaha -- that fits my doomsday scenario for popping it back out offshore near SE TX, rebuilding, & slamming the Houston metro area as it fences people in all the while. That's just a "worse-case scenario". This thing looks like it wants to beef up now, but is short on space as it pushes for land. That will save a lot of people some trouble. Maybe it'll keep going NW & give Oklahoma some some rain -- they sure need it bad up that way.
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Is the recon going to go back in or is that it?
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TD#11...
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Quoting WxNerdVA:
I know the focus is on Isaac, but 97L was given an upgrade at the 5PM advisory... Maybe Kirk, cause I've seen worse get names before.



8/1745 UTC 23.7N 43.5W T2.0/2.0 97L

EDIT: In the time it took me to write the post, it got the renumber...

BEGIN
NHC_ATCF
invest_RENUMBER_al972012_al112012.ren
FSTDA
R
U
040
010
0000
201208281902
NONE
NOTIFY=ATRP

Worse AKA Jose
Member Since: July 12, 2008 Posts: 7 Comments: 383
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Quoting MississippiWx:
Dry air looks to be less of an issue. The core has remained solid for a while. Of course, now we don't have recon. Lol.



For the first time in its life it looks like a classic tropical storm. Took long enough.
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Quoting cajunkid:


Wow, not cool.
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Check regional radar, or Mobile and Tallahassee radar and get a load fo this back side feeder band which is going to be training over SELA for the next several days.

This thing is a monster.

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


Wow can you imagine this happening a few days ago? Impressive. Any new strengt report?
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Since conditions have been more favorable in the sub-tropics then the tropics themselves this year It'll be interesting to see what T.D 11 does.Never would've expected Gordon to be near(or was) major hurricane status so far north.
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981. 7544
looks like hes getting stronger hmmmm
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WeathernerdPR called it yesterday that it had a chance lol great call!
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Quoting ProgressivePulse:
*Rest Assured* Help is on the way. This image only caught a small portion of the staging area, hundreds of electric trucks at the SFL Fair Grounds



The power situation in the Western Communities seems pretty good. Glad they are going to be able to help those who need it.
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Quoting 9030mike:
Just walked to Lake Pontchartrain, the lake level is lower than I would have thought, almost like a high tide plus 3-4 foot waves. Maybe the new massive Lake Bourne structure is doing it's job?



That 2 mile REACH is the Key.

Member Since: July 3, 2005 Posts: 426 Comments: 128630
Quoting Patrap:


Good Luck tkeith, my Sista is on Williams and 12th street over dere.


God Bless You and Yours----We helped our friends to the east before and if needed we will do it again.
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Quoting ncstorm:
Texas, Mississippi, LA, Florida, NC and SC, Alabama, GA, VA .nine states currently being affected by Isaac..


I see a little in Tennessee also. And there's still some in Cuba!
Member Since: July 12, 2008 Posts: 7 Comments: 383

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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.