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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GFDL 24 hr out



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Member Since: August 23, 2008 Posts: 0 Comments: 1918
I hope that, after the Alex-esque presentation we have gotten from Isaac, we would consider replacing the Saffir-Simpson Scale.
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473. JLPR2
As it was mentioned yesterday, 97L is basically Jose reborn. XD

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you guys need too stop down playing this


a cat 1 storm can do this has marh damg has a cat 3 or 5 hurricane can



storm suge is the # one killer in storms all so New Orleans is be low sea level

and there are a lot of old home that can be havy damg or gone

all so down town New Orleans could be havy damg with flying grass from the higher levels so there is likey going too be a lot of clean up
Member Since: May 21, 2006 Posts: 5089 Comments: 114072
HWRF 18 hrs.

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Quoting washingtonian115:
Does he live on the east coast?.Because if he does that's where I live.I don't want to see any destruction on no part of America's coast line.I hope Isaac does some freaky things to only your house...


this was not intended for you washington as you were not the original poster. I guess you didnt see who I quoted. and thanks for wishing bad things my way, I really appreicate the comment.. you seem like such a great person with a huge heart. have a nice day!
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Just wondering how many here are in New Orleans? I can see a few, and remember from past storms...Patrap I think you are uptown.
Member Since: September 1, 2007 Posts: 0 Comments: 249
It's been a nice change of pace here in Palm Beach County...that strange, shiny, yellow disc is visible in the sky. Some people are calling it "the Sun". Boy do we ever need it to stay out. Unbelievable how much water piled up out here in west Lake Worth / Wellington. :-(
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Quoting LargoFl:
guys maybe its a good idea when this does hit..to leave the blog to those that are under the gun there, like they did for us here in florida yesterday..they can tell each other whats going on in their neighborhood or city etc..just an idea..this is going to be alot worse than florida's hit, they get the 75 mph winds, and that huge storm surge etc..well anyway..just an idea....good luck to you folks up there, stay safe
OK, I'll hush now but the blog may get awfully quiet as I suspect power outages and such over there. Good luck to all!!
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Anyone here today in Lafayette? Leaving work at 3 today and they have us open for work tomorrow. I'm NOT coming in. I'd rather lose 8 hours of pay then not be at home watching my home.

What is the general consensus for Lafayette's weather from later tonight until Wednesday night??
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Any one know if the power is out in west mobile?
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Quoting GTcooliebai:
So far for the season 10 Tropical Storms, 4 Hurricanes, and no Major Hurricanes.


They've been saying how great conditions are to support intensification for days now... if Isaac couldn't take advantage of the near perfect conditions this time, I am not really expecting any storms to this season. Could we go the entire year without a major?
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Quoting kmanislander:
New center fix to the NNW of prior fix. Appears to be coming in slightly to the right of forecasted landfall track absent any further shift to the left.


Given current trends and possible eye relocation... it could make landfall sooner.
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Quoting MahFL:


The eye will proberbly contract, and landfall is when the center crosses not the eyewall.

I don't believe I declared landfall or anything about the center. I merely relayed what I saw on radar. Landfall of the center probably won't occur for many hours, probably in SW LA. The whole south coast of LA is going to be raked by this storm.
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Keeper, that's the first time i've seen this thing actually look like a hurricane...
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Quoting RayRayfromLa:


WOW, you almost sound jealous in some crazy way!!! I hate hurricanes and never ever want another one to land here in La. I have always wishcasted storms to go your way... so maybe one day my wish will happen. I wish it was today!!!
Does he live on the east coast?.Because if he does that's where I live.I don't want to see any destruction on no part of America's coast line.
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Click for loop


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guys maybe its a good idea when this does hit..to leave the blog to those that are under the gun there, like they did for us here in florida yesterday..they can tell each other whats going on in their neighborhood or city etc..just an idea..this is going to be alot worse than florida's hit, they get the 75 mph winds, and that huge storm surge etc..well anyway..just an idea....good luck to you folks up there, stay safe
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Quoting Waltanater:
Bartender Isaac wants to drop in on NOLA and give everyone a "drink"! He's definitely a chillin' storm!A round of drinks for everyone in New Orleans! LOL.


They're all in imminent danger of a hurricane hit with huge amounts of rainfall and flooding potential! LMAO! Let's make light of a dangerous situation! ROFL!
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Quoting TreasureCoastFl:
Looks so close! Get ready Gulf coast!! How close it that center to land point now?
Brace for impact!
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Quoting wunderkidcayman:


DUDE TAKE IT EASY

its too far out and too early to tell if it will hit the islands or head out to be truthfull I don't really buy the whole curving up into the Atlantic


Lol that's no surprise :) 40 W is your magic threshold.. so let's wait til it gets there!
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Quoting Patrap:


If Cantore's arms get any Bigger, Popeye is gonna take Big time offense me tinks as well.


well blow me down.... good to have ya back Pat... I shall hoist a Fresca in your general direction!
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Isaac looks to be wrapping dry air in on its N side.
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Bartender Isaac wants to drop in on NOLA and give everyone a "drink"! He's definitely a chillin' storm!A round of drinks for everyone in New Orleans! LOL.
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In Lakeview here...winds have picked up a lil. Oh this needs to be over already. Thanks all who post informative info and graphics!
Member Since: September 1, 2007 Posts: 0 Comments: 249
Today...no Banns will be shown..you shall just not be visible save for yourself.

Enjoy.
Member Since: July 3, 2005 Posts: 415 Comments: 125699
Quoting KEEPEROFTHEGATE:
STORM 09L



Beautiful. Glad it won't probably be nothing more than just a category one hurricane.
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any ways are hurricane could still make a run for cat 2 has it has about 14 too 18hrs still overe water
Member Since: May 21, 2006 Posts: 5089 Comments: 114072
Quoting KEEPEROFTHEGATE:
STORM 09L



I see now,, Thanks
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New center fix to the NNW of prior fix. Appears to be coming in slightly to the right of forecasted landfall track absent any further shift to the left.
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Quoting opal92nwf:
No one in or West of the Mississippi coastline has any reason whatsoever to wishcast a storm anymore, you all have gotten so many hits in the past six years since 2006 (including part of this year with Isaac). Bash me if you wan't to, but there's no denying that regions in the East-Central/Eastern Gulf have not had a hurricane from 2006 onward, while people in the West-Central/Western Gulf have gotten 5 hurricane impacts with many tropical storms to boot.


WOW, you almost sound jealous in some crazy way!!! I hate hurricanes and never ever want another one to land here in La. I have always wishcasted storms to go your way... so maybe one day my wish will happen. I wish it was today!!!
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STORM 09L

Member Since: July 15, 2006 Posts: 165 Comments: 52353
Soon to be 98L!!
Quoting Tazmanian:




there is no 98L
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Glad to see you still hanging around here Floodman
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Quoting CaribBoy:
WTF WITH PRE 98L NOW PREDICTED TO PASS FAR TO THE NE OF ME ON SUNDAY!
Where is that ridge that caused ISSAC to bring only trace over the N Leewards!!


DUDE TAKE IT EASY

its too far out and too early to tell if it will hit the islands or head out to be truthfull I don't really buy the whole curving up into the Atlantic
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Quoting WeatherfanPR:
People:

Ohh it's just a Tropical Storm with 70 mph winds, lets go to the beach to see the waves and feel the wind !!!

then

Ohh it's a Hurricane with 75 mph, lets go home to be safe !!!


haha yup
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Quoting BlxMS:
GOT TIME FOR LIGHTER MOMENT? Dr. Doom Cantore became a very hot topic here when earlier this week when referred to "Mississippi" as the "land mass" bewteen New Orleans and Mobile. Folks around here took a lot of offense...We call that land mass "home" and Mississippi. Didn;t take long for this song to come out...Very entertaining...Frankly, in MS Isaac is more welcomed than Dr. Doom...Linked here:Link


If Cantore's arms get any Bigger, Popeye is gonna take Big time offense me tinks as well.
Member Since: July 3, 2005 Posts: 415 Comments: 125699
\.
Member Since: May 21, 2006 Posts: 5089 Comments: 114072
Quoting WeatherfanPR:
People:

Ohh it's just a Tropical Storm with 70 mph winds, lets go to the beach to see the waves and feel the wind !!!

then

Ohh it's a Hurricane with 75 mph, lets go home to be safe !!!


That is very true to the amateur, although they are very much similar in strength and capability to destroy.
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Quoting fulltimerver:


I quite often do damage assessment for FEMA.. It doesn't matter if the storm is a TS or a Hurricane, or something less. There are many factors involved in getting a Federal Declaration but type of storm isn't one of them.
Thank you for the clarification, truly.

I wonder if many people are under the same misconception as I was (until now)or if because of Katrina most folks in LA and Miss. are "old hands" at this and know the ins and outs like the backs of their hands, so to speak.

Thanks again, truly, truly. :)
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Wow - it's finally a hurricane!
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433. JLPR2
Quoting Waltanater:
It's about FREAKIN' time! Finally, it has made Hurricane status! He did it! Everyone applaud. It's like it is the "under-achiever" of storms! This one should go down in history just because of its struggles. Unreal. Isaac is truly one to remember for this season!


Finally! Jeez! at first I saw Dr.M's entry and I was like, no way, it still isn't a hurricane?!!! XD

So Isaac is the fourth of the season, though it wont be a major, we still have not seen one of those.
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Is it moving North now, or just looks that way?
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Pat no rain over here yet and winds out of the N to NNE at 10 with gusts to 20; hows your Wx?
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Quoting CaribBoy:
WTF WITH PRE 98L NOW PREDICTED TO PASS FAR TO THE NE OF ME ON SUNDAY!
Where is that ridge that caused ISSAC to bring only trace over the N Leewards!!




there is no 98L
Member Since: May 21, 2006 Posts: 5089 Comments: 114072
Quoting seminolesfan:
Animals don't wait for the NHC to start their preps; I imagine finishing preps before conditions begin deteriorating would help survival rates...


My hummingbirds were still looking for food this morning. To bad I took them down last night.
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College of DuPage Meteorology
Severe Weather and Flash Flood Warnings
Member Since: July 3, 2005 Posts: 415 Comments: 125699
Quoting fredric1979:
@ 11:20 160 miles away its moving @ 10 mph if that so its going to take a while
Oh ok, well I guess its strong bands starting soon then.. lots of rain too!
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425. BlxMS
GOT TIME FOR LIGHTER MOMENT? Dr. Doom Cantore became a very hot topic here when earlier this week when referred to "Mississippi" as the "land mass" bewteen New Orleans and Mobile. Folks around here took a lot of offense...We call that land mass "home" and Mississippi. Didn;t take long for this song to come out...Very entertaining...Frankly, in MS Isaac is more welcomed than Dr. Doom...Linked here:Link
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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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