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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GOES-14 One Minute Imagery -- SSEC

Hurricane Isaac:



Link
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Meanwhile, in 174 hours.

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The rain is pounding here at the Federal Center in downtown Atlanta.
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No ant or bird behavior post today.
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Quoting TomballTXPride:
There are way to many folks on here excited that this thing has been designated a hurricane. What are wrong with people? So sad. Folks will lose their homes and all I see are !!! and cheerleading from the sidelines from people probably far enough away from the storm to get a cirrus cloud over their head. Sad.


This is a hurricane blog. It's not that people are excited. We have just been anticipating Isaac to reach Hurricane strength for days and it finally did play out. Judging by the pressure and satellite presentation of the storm we knew it was going to be announced soon anyhow. Regardless we have no say in what mother nature will do.
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I don't know if they made the right call at 11 but I wouldn't blame the NHC for employers/people downplaying Tropical Storms. The NHC does all their measurements and issues warnings and observations and predictions and give their readings but people will downplay it because it's called a TS and not an H.

The NHC shouldn't be expected to upgrade a storm just because people are silly when it comes to risk analysis and mother nature. Strong TS or borderline Cat 1 people will still drive through flooded streets, go play in dangerous surf, ignore tornado warnings, and wander about when lightning is striking or power / limbs are down.

Maybe they should just get rid of the designation TS and call storms that meet the measured criteria Cat 0 Hurricanes. I'm serious. Then everything is a hurricane and people might just wise up that nature has muscle and she can either bench press 200 lbs or do some cardio. Really depends on her mood, how much sleep she's been getting, and what's on her iPod. Either way, we got to pay attention to what she's doing or she'll bowl us over.
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watch it disapate


in 6 hrs

lol
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Live from Gulfport, MS.
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Quoting TomballTXPride:
There are way to many folks on here excited that this thing has been designated a hurricane. What are wrong with people? So sad. Folks will lose their homes and all I see are !!! and cheerleading from the sidelines from people probably far enough away from the storm to get a cirrus cloud over their head. Sad.


wrong,people pay more attention to a hurricane,therefore it saves lives.
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Quoting kmanislander:
Good morning

Just coming on for the day. I see that the last center fix is North of prior centers. This may be a wobble or a response to the blocking plains high which has pushed to the East since yesterday. As close as Isaac is to the coast this shift may reflect in the actual landfall point and bring the track to the right some. I mentioned this possibility last night, we just have to watch and see what the next center fix shows.
Good point. Still watching closely in destin. I aw wondering how to see this on maps
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Ok, I cannot seem to find these records, but I do recall in the archive that there have been storms that have gone from TS strength to category 2 strength in little time at all. Isaac, by all intents and purposes, should be upgraded. I am reading posts that he is now a low-end category one, but I believe he will continue to strengthen. Why are storms this year so incredibly baffling?
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Quoting groundgirl:

I lived in Gulfport/Biloxi for almost 2 years right after Katrina...I pray everyone there and in the "landmass" surrounding them is safe.
BUT like you I'm welcoming Issac and saying stay as long as you WANT!! My well water is starting to taste funny, I've laid off keeping trees alive...trees that have been there for 40 years BTW...central MO

Back to Storm Junkie live cam......

And moonlightcowboy...the secret is margarita mix....


my secret is minute maid limeade in the frozen dept, a little triple sec, and a tequila. Voila! i get compliments on my margaritas from anyone I make them for. throw in fresh berries and it's a real crowd pleaser!
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Quoting reedzone:
Isaac is the most strongest and intense Tropical; Storm on record.



Not anymore.
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In New Orleans (Midcity (70119))

Local Pressure: 1003mb

Longtime-Lurker here...
in Nola for Georges, Katrina, Gustav, Etc.
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there is going too be a lot of blow out windows in down town new New Orleans, LA
Member Since: May 21, 2006 Posts: 5091 Comments: 114719
Just received a FEMA text stating Isaac now a Hurricane...
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Quoting TomballTXPride:
There are way to many folks on here excited that this thing has been designated a hurricane. What are wrong with people? So sad. Folks will lose their homes and all I see are !!! and cheerleading from the sidelines from people probably far enough away from the storm to get a cirrus cloud over their head. Sad.


The storms hasnt changed just because the the NHC upped it 5mph....and cheering because hurricanes are cool to see doesnt mean we want anyone hurt.
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Very begrudgingly, I am backing off my statement that there is no way Houston will feel any effects from the storm. There might be a very, very small chance that we get some rain out of it if some of the models pan out with the storm running along the inland coast before turning North/Northeast. Very small chance. However our local NWS office has changed the forecast to include:

WHILE ISAAC IS NOT EXPECTED TO DIRECTLY THREATEN OUR CWA...ANY
FURTHER SHIFT WESTWARD IN TRACK WOULD NECESSITATE ADDITIONAL
CHANGES TO POPS AND TEMPS LATER TODAY OR TONIGHT BEFORE THE SYSTEM
MOVES INLAND.

So maybe a little rain and small gusts of wind, but I really doubt it.
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Quoting Asrock:



Flooding in downtown Charleston, SC


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since hes projected to head due west here shortly, i wonder if he will have further strengthening.
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FINALLY!!
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Wind gust at 12:00pm est was 68.9mph at the buoy...

http://www.ndbc.noaa.gov/station_page.php?station =burl1


finally!
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Quoting moonlightcowboy:
Water is also already over Hwy 90 in Biloxi!


Isaac couldn't hit at a more unfavorable angle for that area. The water piles up in that knotch and sloshes into Waveland, Bay St Louis, Gulfport, Bilox.. et al
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Quoting LongGlassTube:


Totally irresponsible for the NHC to call a storm with 976 millibars of pressure a TS.

What is up?

My wife is expected to go to work tomorrow in a nonessential business because it is ONLY a TS.

They are blowing it!

I'm a Fire Fighter and EMS responder here in Ascension Parish. We were told to expect 48 hours of TS force winds and 12 hours of Hurricane force winds within our Parish.
We filled sandbags till our fingers bled last night past midnight and even though winds are picking up here people are still getting sand bags. Most of the public fortunately has taken this seriously.

I just wish at 10:00 they would have called this thing what it is. A hurricane. A hurricane that is approaching CAT 2 strength according to pressure.


I don't get it. I'm sorry you worked so hard to fill sandbags but they may yet be needed, if not now, then for the next storm down the road. The peak of hurricane season is September 10. The central pressure of a tropical system has nothing to to do with surface winds. The NHC is calling it as they see all the information they have available. If your wife has to go to work tomorrow, that means conditions should not be as bad as once feared. She can always decide not go if she disagrees. I would be thrilled if I lived where you do that Isaac is not turning out as bad as the worst predictions. We certainly are in Alabama compared to what was forecast to be a direct hit on Mobile. Sounds like you need to forget the blog and get some rest after a full nights work.
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Good morning

Just coming on for the day. I see that the last center fix is North of prior centers. This may be a wobble or a response to the blocking plains high which has pushed to the East since yesterday. As close as Isaac is to the coast this shift may reflect in the actual landfall point and bring the track to the right some. I mentioned this possibility last night, we just have to watch and see what the next center fix shows.
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168 hours
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Flooding in downtown Charleston, SC


CHARLESTON, SC (WCSC) -

The Charleston County Sheriff's Office and the Charleston Police Department are reporting flooding in a number of areas across the area.

Captain Eric Watson said flooding is being reported on Savannah Highway near I-526, Dorchester Road at Meeting Street Road, and parts of Azalea Drive between Cosgrove Avenue and Leeds Avenue.

Various reports have surfaced of inaccessible streets in downtown Charleston after the heavy rain fall. Charleston police spokesman Charles Francis said motorists should avoid the following areas: The Market, Morrison and East Bay, Morrison and Meeting, Rutledge and the Crosstown, King and John and portions of Gadsden, Bennett, Wentworth, Beaufain and Pitt.

The Charleston County Sheriff's Office is asking motorist to use extreme caution while traveling on the roadways.

There will be extra patrols in the James Island and West Ashley area to monitor road conditions, Watson added.
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Congratulations everyone. NCH has declared 'sacc a hissicane.
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162 hours
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I can't tell you how many times I have seen the NHC update say something like this:

The aircraft reoprted flight level winds of X, and reliable SFMR readings of Y. Since it is unlikely that the aircraft sampled the strongest winds, this data supports an initial intensity of Z.

I have seen that type of statement more than once, but I don't have an old advisory to reference at the moment.
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11:20 AM CDT Tue Aug 28
Location: 28.1°N 88.6°W
Moving: NW at 10 mph
Min pressure: 975 mb
Max sustained: 75 mph
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Quoting AtHomeInTX:
dr knabb this a hurricane now


New 12Z Nam is west too skirting the coast to SE Texas!
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BBC announce Hurricane,12:19 EDT.
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ISAAC IS FINALLY A HURRICANE http://www.nhc.noaa.gov/
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Starting to go all downhill now, folks. Make a pass with the elderly, help if you can, then get hunkered down! It's not going away anytime soon either.
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Looks like the NHC has been reading the blog.
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156 hours, what's left of Isaac finally leaving.
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Quoting MississippiWx:


Lol. I don't even care anymore. The strength of the storm won't be determined by the NHC in reality anyway.


The area will be experiencing hurricane conditions. I on't think anyone could argue that.

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It's about time!

...RECONNAISSANCE DATA INDICATE ISAAC FINALLY ACHIEVES HURRICANE STATUS... ...U.S. Warnings in Effect...
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NHC has finally succumbed to pressure from this blog and has designated it HURRICANE ISAAC !!!!!!!
To be downgraded at 4PM.
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Quoting Felix2007:

They were all previously hurricanes though, and Isaac wasn't.


I thought the same thing when looking at that table. He's been a very unique storm. Amazing, really.
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Quoting jeffs713:

Right... but Gustav had a smaller core of stronger winds. The destructive potential of Isaac's winds are not necessarily related to strength, but rather duration and area. An individual house has a better chance of faring well in Isaac - but because the possible number of houses that could be damaged are much greater, the damage level is higher.

Also, in regards to the duration, a well-built wood frame house can withstand 70mph winds under normal circumstances. But it puts strain on the house. The longer those winds blow, the greater the chance of that strain finding a fault in the construction, or just giving out.

The best analogy I can think of is when someone is water skiing... they can go all day at 25-30mph, but every once in a while, a unique combination of wakes and waves will cause them to fall. The longer they are going, the greater the chances are of that fall happening.


right. and that's what i'm not liking. locally people are thinking it's no big deal because it is only just now making Hurricane status. I don't think people realize how long we are going to be under TS winds. 30 hours is a LONG time for TS winds.

the rain is even more of an issue.

overall, people aren't expecting much, but it's starting to look like it's not going to be a cakewalk when you factor in the slow movement and size. Isaac is a big fat slug
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Def a cane now I say. Cat 1 at eye landfall
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Quoting reedzone:


How strong do YOU think it is and what its peak will be?


With flight level winds approaching 110mph, I have to believe that Isaac will be able to approach 90mph eventually. Some gusts to 100mph are possible as well when the stronger bands reach the coast and mix down due to friction.
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Quoting SherwoodSpirit:


I'm terrified in the collective American-PTSD-because-of-Katrina sense because this storm is hitting New Orleans...

But I live in severely drought stricken Missouri, and Isaac is coming here by the end of the week. I'm welcoming him with open arms. Give us RAIN!

I lived in Gulfport/Biloxi for almost 2 years right after Katrina...I pray everyone there and in the "landmass" surrounding them is safe.
BUT like you I'm welcoming Issac and saying stay as long as you WANT!! My well water is starting to taste funny, I've laid off keeping trees alive...trees that have been there for 40 years BTW...central MO

Back to Storm Junkie live cam......

And moonlightcowboy...the secret is margarita mix....
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Quoting stormchaser19:
I think Isaac will don´t do nothing in New Orleans this City is well prepared after Katrina


normally i dont nit pick a ppl sentences but geez... i know i dont have the best grammar, but you may want to try that again
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Bermuda may need to keep an eye out on this one.
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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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