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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don't know if this has been addressed, but NOLA and those along the Mississippi river, i'm thinking, are going to be buffered from the full effects of any storm surge due to very low river levels. a saving grace perhaps? for some, maybe..
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Weak wave coming off, getting into fantasy land now though.
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323. 7544
Quoting MAweatherboy1:
Way out to sea this run, models seem to be agreeing more on that solution but its still really early.



he finally made it lol'

thats what it showed for issac and look where he wind up lol long time to watch this one but i think the real worry will be the forms behind it
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Quoting wxchaser97:
I take a shower and then the NHC upgrades Isaac to a hurricane.


Don't take any more showers until November 1. :)
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264 hours
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Quoting wunderkidcayman:
I wounder guys what would happen if Hurricane Isaac rapidly tighten it circulation and he rapidly make a run to strong Cat 2 boarder cat 3


Look who returned....
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Insurance companies breathing a sigh of relief now that it's officially a hurricane. Now the claimant must pay a big hurricane deductible before their insurance providers incur any liability.
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Quoting divdog:
What's telling you the direction may change soon. Trying to learn. Cheers from destin



Weaker area of 1012mb of pressure is receding northwards to the weakness in the trough, and Isaac is continuing to slowly strengthen. All systems will move poleward if they can. The door just opened for Isaac and he will move towards it.
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252 hours, relatively quiet, seems suspicious...
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Finally Hurricane Isaac I see. If visual representation doesn't tell you how large Isaac is, the pressure to wind relationship will. Lowest pressure found by HH is 974 mb. That's more typical of a 95-100 mph hurricane, whereas Isaac is just now a 75 mph storm. Impressive.
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Quoting MAweatherboy1:
Way out to sea this run, models seem to be agreeing more on that solution but its still really early.



It may be weaker and get caught under that ridge.
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Quoting wunderkidcayman:
I wounder guys what would happen if Hurricane Isaac rapidly tighten it circulation and he rapidly make a run to strong Cat 2 boarder cat 3


Mass hysteria?
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Quoting lurkersince2008:
Is the storm expected to stay below cat 2 ....
The official NHC forecast keeps it a Cat 1
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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.
IMO they are cheering for two reasons.

One, it is generally thought by some that if the storm is labelled as a TS a good many people will not take it as seriously as if it were a hurricane. This is supported by employers that have non essential employees coming into work during this mess and not cancelling, IMO.

Two, I am not sure of this and somebody who knows for sure please correct me and expound on this but the label of hurricane in and of itself means a greater possibility of sustained damages being covered by insurance companies and also may mean that more people, without homeowners coverage, qualify for FEMA assistance after the storm. (Run on sentence, I know, lol).

Long story short, I do believe they are cheering because hurricane status v TS status means that "What's in a name" will now work to the benefit of the general public in the short and long run.
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240 hours
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Quoting MAweatherboy1:
Way out to sea this run, models seem to be agreeing more on that solution but its still really early.



Except for the little tidbit that it could be Fabian-izing Bermuda.
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Is the storm expected to stay below cat 2 ....
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Forecasting Kirk to be a fish, but too far out. 228
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IMO, the difference between a 70 mph tropical storm and a 75 mph Hurricane is greater than one may think. Main reason is now there is an eyewall which condense the strongest winds in closer to the center. Having an eyewall enables faster strengthening.
Member Since: August 19, 2005 Posts: 5 Comments: 5449
I wounder guys what would happen if Hurricane Isaac rapidly tighten it circulation and he rapidly make a run to strong Cat 2 boarder cat 3
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I'm surprised that it's forecast to be this quiet during the peak of the season.
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Quoting scott39:
Noticed it earlier also. I think Al to Fl line should have been left in the Hurricane warning area.


We will know soon enough
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Quoting Carnoustie:


wrong,people pay more attention to a hurricane,therefore it saves lives.


Exactly right! This storm is much more powerful than "just a TS" and so many people are not taking appropriate caution because of the psychological effect of it being a TS or a weak Cat 1. I have been following this blog for years and NOBODY here wishes any harm on anybody in the effected areas.
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Thought I would be first:

Why hasn't NHC upgraded to Cat 2 yet?
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I've seen way too many families walking around outside along the Gulf Coast while watching live feeds! This is a serious hurricane! Stay safe Gulf Coast Residents!
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Its a hurricane
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Way out to sea this run, models seem to be agreeing more on that solution but its still really early.

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Quoting LostTomorrows:
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?
Rina (from last season) intensified from a TS to a C3 in about 12 hours.
Member Since: August 1, 2012 Posts: 0 Comments: 436
204 hours
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Quoting DavidHOUTX:


Area forecast discussion
National Weather Service Houston/Galveston Texas
1010 am CDT Tuesday Aug 28 2012

Update...
see morning update.
Discussion...

Isaac still prognosticated by TPC to make landfall over southeast Louisiana. The
12z NAM is trending further west with Isaac and it is looking more
and more like the 00z European model (ecmwf). There could be some significant
changes in the forecast package this afternoon if the 12z GFS supports
the the 12z NAM. The NAM is advertising that the inflow for Isaac
will come through east/southeast Texas on Thursday night and
Friday. This band of higher moisture could cause significantly
higher rain chances than is currently projected. The current TPC
track would keep the heavy rain axis east of southeast Texas. 43
Yikes that would be even worse for NO and points east no?
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Quoting moonlightcowboy:



More poleward motion is about to begin.
What's telling you the direction may change soon. Trying to learn. Cheers from destin
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Quoting nolajet:
An insurance CEO was reading this blog, realized that a blogger mentioned the fact that there would be no "hurricane deductible" and called his friends over at the NHC to demand that he listen to the wunderground blog.

Kidding of course. At least it's finally confirmed. People that weren't taking the "tropical storm" idea seriously may make smarter planning decisions because of the "hurricane" part being out.


If people in the path haven't already made "smarter" decisions, it's a little late now. Not much planning to do when landfall is less than a day away.
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Quoting Felix2007:
Meanwhile, in 174 hours.



Looking pretty fishy, but that's 8 days out.
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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.
Noticed it earlier also. I think Al to Fl line should have been left in the Hurricane warning area.
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Beginning to recurve at 192.
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Quoting uptxcoast:
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.


Area forecast discussion
National Weather Service Houston/Galveston Texas
1010 am CDT Tuesday Aug 28 2012

Update...
see morning update.
Discussion...

Isaac still prognosticated by TPC to make landfall over southeast Louisiana. The
12z NAM is trending further west with Isaac and it is looking more
and more like the 00z European model (ecmwf). There could be some significant
changes in the forecast package this afternoon if the 12z GFS supports
the the 12z NAM. The NAM is advertising that the inflow for Isaac
will come through east/southeast Texas on Thursday night and
Friday. This band of higher moisture could cause significantly
higher rain chances than is currently projected. The current TPC
track would keep the heavy rain axis east of southeast Texas. 43
Member Since: August 18, 2011 Posts: 0 Comments: 635
I take a shower and then the NHC upgrades Isaac to a hurricane.
Member Since: March 16, 2012 Posts: 127 Comments: 7948
Hi sean
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180 hours, Kirk the only game in town
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285. leias
Just received a news alert from AP that Isaac has strengthened to a hurricane.
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Water just rose 6 inches at mississippi river gauge in the last hour... not much:

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Quoting divdog:
Good point. Still watching closely in destin. I aw wondering how to see this on maps


Here is the current steering. You can see the plains high very strong to the NW in a blocking position. The weakness between it and the Atlantic high is now to the NNE of the system instead of the North yesterday.

The center shift may be in response to the position of the weakness or just a temporary wobble. More data needed from the HH in the next two passes.

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I don't understand the comments on here lately.

Since when does pressure determine storm strength?

I would think pressure differential would be the key indicator of strength, isn't that what wind speed represents?

Everyone says it's a Cat 2 pressure, but what's the big deal if the surrounding environment is TS pressure?

I always thought pressure was relative and wind speeds are absolute.

Am I missing something here?
Member Since: July 25, 2008 Posts: 0 Comments: 596
Quoting WetBankGuy:
Congratulations everyone. NCH has declared 'sacc a hissicane.


Thank the Lord. It's up a whole 5 mph from the earlier 70 mph TS. Now maybe we can get over the NHC bashing for a while and see what Isaac finally does.
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An insurance CEO was reading this blog, realized that a blogger mentioned the fact that there would be no "hurricane deductible" and called his friends over at the NHC to demand that he listen to the wunderground blog.

Kidding of course. At least it's finally confirmed. People that weren't taking the "tropical storm" idea seriously may make smarter planning decisions because of the "hurricane" part being out.
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Quoting moonlightcowboy:
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.



More poleward motion is about to begin.
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GOES-14 One Minute Imagery -- SSEC

Hurricane Isaac:



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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