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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Anemometer on the LA Offshore oil platform reporting top sustained winds of 52 mph with recent gusts to 76 mph. The station has measured wind gusts as high as 90 mph, but since the anemometer is likely between 100 and 200 feet above the ground, there will be a reduction factor to these readings. Still, it is possible these kind of winds (and perhaps higher) can reach ground level as the eye of Isaac continues to edge closer to landfall.
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It's going to take another 24-36 Hrs for him to just get pass New Orleans. Too much rain!
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Quoting TX2FL:
That GFS long term ensemble is scary..

I'm up in Northeast PA..the same area that got evacuated with the remnants of TS Lee last year. It was just like a hurricane evacuation. Any danger of that kind of rainfall and flooding if it gets up this way?


Def not hoping for a widespread flood situation but this could be a significant drought buster for some folks in the Ohio valley.
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Quoting DavidHOUTX:


It will be interesting to see what they say. The 18z NAM is running now. Here it is as 27 hours..


The high scoots him W? Or, scoot half of him West?
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Quoting ncstorm:
I hope Isaac dont try to come back for round two..


Like this guy
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Quoting Thing342:
I think someone on here has a bad gif. I keep getting this message:

550 /ABI/HD/SRSOR/800x900_GOES_ B1_RSRSO_ISAAC_animated_2012241_120000_182_2012241 _130000_182_X.gif: No such file or directory

Yeah, it was someone in the last page apparently. Should be gone now (as long as you have your settings set to use 50 comments per page and not more).
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Quoting AllStar17:


Worthless comment.


And your's (and mine) is not?
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Quoting JeffM:
Looks like ol' Mike on TWC is having major issues with his comb-over.

Ha!!! Forgot to stock up on his errr rogaine?!?
Member Since: August 2, 2006 Posts: 0 Comments: 1240
katrina did the same thing,the river was flowing upstream now thats a lota water
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Quoting odinslightning:
anyone else here in mobile smell the air outside? it's not blowing hard here at mobile/tillmans corner but the clouds are very low and moving very fast in different directions.....

but it's the smell of the air....it makes me think that it either absorbed the smell of the oil in the GoM or maybe it is mixing out all of the pollution that is built up in the atmosphere in the upper levels....


it smells like being down in an industrial part of town where it smells like burning wood/car emissions, and chemicals used in the processing and manufacture of stuff....its a strange smell in the air, and the air is very dry...

im just asking cuz i wonder if it's just me smelling this or is anyone else as well?
Heard people say this in the past. I'm not noticing it, however.
Member Since: August 16, 2007 Posts: 0 Comments: 397
Quoting bigwes6844:
I know we talking about Isaac but look at this. We may have more trouble down the road big time! Here comes the peak of the season

Good Lord...this place is gonna be extra busy for a good while. We'll be putting in plenty of overtime on this sucker.
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I think someone on here has a bad gif. I keep getting this message:

550 /ABI/HD/SRSOR/800x900_GOES_ B1_RSRSO_ISAAC_animated_2012241_120000_182_2012241 _130000_182_X.gif: No such file or directory
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Quoting jeffs713:

Ike did that for Houston. "It is only a Cat 2"... that will clear an entire peninsula and cause $10+ billion in damage...


and in my opinion, that comment by Neil Frank caused folks to die.
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things are going down hill fast.... every gust seems to be stronger and stronger
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Quoting HrDelta:


Where are you that is happening exactly.

We real need a database recording stuff like this. It would be interesting to see the spatial pattern.

Also, the founder of Accuweather is trying to appeal the NCAA punishment given to his Alma Mater, Kid Fondle U.
Central Florida.
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1209. JeffM
Looks like ol' Mike on TWC is having major issues with his comb-over.
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I know we talking about Isaac but look at this. We may have more trouble down the road big time! Here comes the peak of the season
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We're getting gusts up to 33kt here at the scareport (KBPT). My buddy in LCH sez they're getting close to 40kt. They've been getting higher since around noon. As we're so far away from the center (thank goodness!!!), I don't know if that means the storm is gathering strength or not per se, but from the look of the satellite, I'd probably say so. And our altimeter has dropped a little more than I expected too, fwiw.
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English Version of Blog Update



Skip to 2:16
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This thing is just gonna dump an obscene amount of rain
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Quoting StormDrain:
Also re: the NWS Hou-Galv discussion someone posted that mentioned NAM and ECMWF.

12ZGFS Crawls along the LA coast. Not much N movement till 42 hrs. Link


It will be interesting to see what they say. The 18z NAM is running now. Here it is as 27 hours..

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1203. HrDelta
Quoting Jedkins01:
Isaac might be one of the weirdest tropical cyclones ever, and also one of the most violent "minimal" hurricanes ever. Who ever heard of a 975 mb category 1? All I know is, with a circulation center this large, and a pressure this low, and moving as slow as it is, unfortunately I have a feeling that it will remind people to never again say "its only a category 1"


Part of the reason why the Saffir-Simpson Scale needs to be replaced. It doesn't not take into account either pressure, or size.
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Quoting TX2FL:
That GFS long term ensemble is scary..

I'm up in Northeast PA..the same area that got evacuated with the remnants of TS Lee last year. It was just like a hurricane evacuation. Any danger of that kind of rainfall and flooding if it gets up this way?

Hey Fellow Pennsylvanian!!!!!!!!!!!!!!!!! lol nothing nearly as bad as Lee but there culd b minor flooding
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Quoting Jedkins01:
Isaac might be one of the weirdest tropical cyclones ever, and also one of the most violent "minimal" hurricanes ever. Who ever heard of a 975 mb category 1? All I know is, with a circulation center this large, and a pressure this low, and moving as slow as it is, unfortunately I have a feeling that it will remind people to never again say "its only a category 1"

Ike did that for Houston. "It is only a Cat 2"... that will clear an entire peninsula and cause $10+ billion in damage...
Member Since: August 3, 2008 Posts: 16 Comments: 5885
anyone else here in mobile smell the air outside? it's not blowing hard here at mobile/tillmans corner but the clouds are very low and moving very fast in different directions.....

but it's the smell of the air....it makes me think that it either absorbed the smell of the oil in the GoM or maybe it is mixing out all of the pollution that is built up in the atmosphere in the upper levels....


it smells like being down in an industrial part of town where it smells like burning wood/car emissions, and chemicals used in the processing and manufacture of stuff....its a strange smell in the air, and the air is very dry...

im just asking cuz i wonder if it's just me smelling this or is anyone else as well?
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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.
In the Acreage here.  Canal water is not even moving.  Just got word National Guard has been requested to assist with sand bagging if necessary.  Forecast is for 4-6 more inches of rain in the near term.  Hopefully not.  Most of my neighbors cannot leave their premises.  Fortunately my Tundra has enough clearance to get me out.
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Shell Beach tide gauge at 7 feet. I am guessing that it will peak at 9 or 10 feet based on where the storm is and how much time it takes the surge to build.
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Isaac might be one of the weirdest tropical cyclones ever, and also one of the most violent "minimal" hurricanes ever. Who ever heard of a 975 mb category 1? All I know is, with a circulation center this large, and a pressure this low, and moving as slow as it is, unfortunately I have a feeling that it will remind people to never again say "its only a category 1"
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1196. TX2FL
That GFS long term ensemble is scary..

I'm up in Northeast PA..the same area that got evacuated with the remnants of TS Lee last year. It was just like a hurricane evacuation. Any danger of that kind of rainfall and flooding if it gets up this way?
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It looks like it could possibly be another 6 hours or so till landfall. Not good at all!!!!!!
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1194. HrDelta
Quoting seer2012:
Ants aren't the only creatures trying to get up high.Several days this week I have had baby toads in my carport trying to find higher ground.I have never seen that behavior before.


Where are you that is happening exactly.

We real need a database recording stuff like this. It would be interesting to see the spatial pattern.

Also, the founder of Accuweather is trying to appeal the NCAA punishment given to his Alma Mater, Kid Fondle U.
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Has anyone seen any real net movement in the last 2 hours?
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Quoting charlottefl:
Friction with land is causing the eye to tighten up, probably making the "engine" slightly more efficient. We saw it with Ike also at landfall...
I think this friction may cause (may not) Isaac to jog around Grand Isle. There was already a slight jog following the contour of the continental shelf.
Member Since: August 16, 2007 Posts: 0 Comments: 397
hmm so much for El Nino

I'm rasing my numbers to 17-18 named 6-7 hurricanes and 4 majors
Member Since: June 13, 2009 Posts: 2 Comments: 12157
1189. Gino99
Isaac has almost made landfall... the best wishes for the people who live in Louisiana and other affected states from The Netherlands! :)
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evere one in down town new orlean thinks this this is a norml day and there out and a about


what you people doing get indoors get off the rds things are going down hill
Member Since: May 21, 2006 Posts: 5091 Comments: 115241
1187. Patrap
Quoting 1900hurricane:
The Mississippi River just SE of New Orleans is rising in height while dropping in discharge, a sure sign of storm surge.







Betsy in 65 had the River Moving North at NOLA and up in Baton Rouge a Chlorine Barge Sank..I believe.
Member Since: July 3, 2005 Posts: 426 Comments: 128667
Quoting leftlink:


Is there a chance of a "tidal bore" in the narrow channel headed towards new orleans? As in the bay of Fundi (I am located in New England.) Or is the water going to be high enough that the all of the channels that could direct water will have their banks run over?

Tidal Bore like what happened during Katrina is much less likely, due to the MASSIVE Lake Borgne Surge Barrier (Wiki Link and US COE Link). The IHNC barrier essentially cuts off the "point" of the funnel that caused the tidal bore that weakened the levees on Industrial Canal during Katrina.
Member Since: August 3, 2008 Posts: 16 Comments: 5885
Wow, the dry air is pretty much gone. If he had another day out there he would probably become a real monster.

Member Since: February 11, 2012 Posts: 83 Comments: 7837
Quoting jeffs713:

Isaac is doing the same thing that Ike did right before landfall... tightening up.

No kidding. The radar of those two are uncannily similar.



Ike at landfall

*Edit to put the Ike radar loop in a link since it was slowing down the blog.
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Quoting Levi32:
New 12z ECMWF still has Isaac offshore at 7am CDT tomorrow morning.

Also re: the NWS Hou-Galv discussion someone posted that mentioned NAM and ECMWF.

12ZGFS Crawls along the LA coast. Not much N movement till 42 hrs. Link
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Quoting hurricanehanna:

lmao!!! Oh we SOOOO needed that!
Loved it..had to share it on my facebook..thanks...to whoever made it..Stay safe all my friends..we are STILL getting rain here in east central fla from that outer band that broke away from it...and according to the track it looks like it will go visit the rest of my family in Michigan before it's all said and done...lol
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My first hurricane was Betsy. I can tell you from living here in NOLA for over 50 years, this is not a typical tropical system. That distintive smell and feel is not present. The clouds are not racing across the sky. The rain is that of an afternoon shower. I know this will be a big rain event, but this does not have the characteristics of a hurricane that I am very familiar with. It must be the dry air that has plagued this thing.

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1179. Patrap
Quoting leftlink:
Pressure has decreased to 29.02in, or 982.7mb at mouth of the Mississippi, (Pilot's Station East, SW Pass, LA). The eye is defintely going to go over this station, and the storm is going left of the station just to the east (PILL1).
\

Venice Marina is having some Boats rack up ..

Not confirmed.
Member Since: July 3, 2005 Posts: 426 Comments: 128667
1178. 7544
Quoting KEEPEROFTHEGATE:
STORM 09L



hmmm whats up with those bands froming over the fla pinisular part of isacc tia
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Quoting coondini:

When was the last time we went from code orange straight to TD? Don't seem to remember doing that before.
Jose went from yellow to a TS :/
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They were just showing some fool out hitting golf balls as Isaac comes in on HLN. People with kids and dogs wandering around as huge waves are breaking and the winds gusting. This is not a game, get inside folks! People are nuts.
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The Mississippi River just SE of New Orleans is rising in height while dropping in discharge, a sure sign of storm surge.





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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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