Isaac makes its final approach towards Louisiana

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

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


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

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


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

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show some differences in what happens after that. Isaac may scoot nearly west-northwest just inland along the coast into Texas, as predicted by the ECMWF model, or head straight inland to the northwest and into Arkansas, as predicted by the GFS model. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. It appears likely that Arkansas will see some heavy rains of up to five inches, which would help put a dent in the exceptional drought conditions there.

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


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

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

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

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


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


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

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

Invest 97L in the Middle Atlantic
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1250 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 40% chance of developing by Thursday morning. None of the reliable models foresee that 97L will be a threat to any land areas.

Another tropical wave that moved off the coast of Africa on Sunday is located just southwest of the Cape Verde Islands, and is moving west at 15 mph. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Thursday morning. The disturbance could begin to affect the Northern Lesser Antilles as early as Saturday night, though our two best models, the GFS and ECMWF, predict the center of the disturbance will pass a few hundred miles north of the islands. The disturbance could be a long-range threat to Bermuda.

Jeff Masters

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Quoting alaina1085:
Is Isaac in any danger of downgrading due to upwelling since he's slowly moving/stalling?


At this point, it really wont make much different if he upwells or not...the damange is already in progress and he would not fall apart THAT quickly.
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Quoting LAlurker:
When was the last time recon found Hurricane force winds?
TS at next update????

Recon isn't out there..
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Isaac is almost cut off from the portion that went east up Florida and is now near the Carolinas/Georgia. The flow from South Florida seems to be leaning towards the Eastern portion and that portion isn't moving west towards the main body of Isaac anymore. Interesting to observe.
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1272. emcf30
Quoting TomballTXPride:

Bunker? No need to apologize. Enough said.


LOL, just passing on valuable information!
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Quoting Masquer08er:
Heard people say this in the past. I'm not noticing it, however.


I am not smelling anything right now. It started raining so maybe it washed the smell away.
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When was the last time recon found Hurricane force winds?
TS at next update????
Member Since: July 24, 2008 Posts: 0 Comments: 156
Quoting TampaFLUSA:
RT @capitalweather: 93 mph wind gust measured offshore LA at Apache Oil Platform (279 feet above water)


Thank you for that, just posted a comment about those readings from that platform :) 279 feet means a standard reduction of about 0.9 to get true 10 meter winds, so that 93 mph gust translates to 84 mph at the surface.
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Hour 45... Really?? I can't imagine this panning out, but if it does.. That is some torrential rain for LA

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1265. HrDelta
Quoting indianrivguy:


and in my opinion, that comment by Neil Frank caused folks to die.


I don't remember what he said, could you remind me?
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1264. Skyepony (Mod)
Looks like it's going to be slow going for a bit. NASA/GEOS-9 33hrs...insane rain.

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1263. TXCWC
Models showing a bit more westward componet than official NHC track showing - consistent with EURO and GFS

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soon to be 98L. watch out lesser
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Quoting HrDelta:


Part of the reason why the Saffir-Simpson Scale needs to be replaced. It doesn't not take into account either pressure, or size.


Exactly, it's not effective enough.
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1260. Jax82
Todays MODIS True color Image of Isaac!


Link
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Is Isaac in any danger of downgrading due to upwelling since he's slowly moving/stalling?
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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...



Yeah I know, Ike was one of the worst hurricanes to hit the U.S.
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1257. HiWay58
After seeing the green pop in on the funktop, if this thing stays slow and makes it to the diurnal maximum over the extremely warm coastal waters what can we expect? upwards of 90mph or just a continued broadening of the wind field?
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Quoting Kristina40:
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.


This is less intense than many thunderstorms.
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Quoting MAweatherboy1:
I think we have a NOAA plane heading out, but I don't think there are any more Air Force missions scheduled... too bad, because Isaac may be strengthening.

Isaac looks to be strengthening, things are getting interesting on the gulf coast.
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Areal View Of The Flooding In Wellington, FL below. Reports that septic tanks are backing up and snake infested *STAY OUT OF THE WATER*



Center visible in the bottom left of this image, not really going anywhere.

Member Since: August 19, 2005 Posts: 5 Comments: 5019
Quoting MississippiWx:
He wants to become a major hurricane so bad...



I have a feeling he knows what the significance of this day is...
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1251. flcanes
Quoting wxchaser97:

The engine that wanted to but couldn't. Luckily he wasn't a major coming in or we would have a way bigger problem.

tyeah, we dodged a CANNONBALL
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Seven years ago today

Member Since: July 12, 2008 Posts: 7 Comments: 383
1249. Patrap


New Video from 15 ago..in between Feeders
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Quoting DavidHOUTX:


Yea pretty much. It misses the trough and the High pushes him a little SW then West.

36 Hours:



Well, the large consensus of models tends to agree with that...we'll see what forecasters say. HGX usually releases their forecast discussion between now and the next hour, we'll see what they say about it.
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I think we have a NOAA plane heading out, but I don't think there are any more Air Force missions scheduled... too bad, because Isaac may be strengthening.
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All these post about Isaac moving west? The consensus models all have Isaac moving N/NW?
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1245. emcf30
I apologize if this has already been posted but Surge is rising FAST at Shell Beach, Louisiana, which is East of The Big Easy. 6 ft of surge now, water level at 7.2 ft when you include tide.

Link
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from KATC in Lafayette:

Eighty buses are on stand-by outside of Blackham Coliseum in anticipation of Hurricane Isaac's Louisiana landfall. The buses are part of a collaborative effort to rescue residents of the New Orleans area who decided to ride out the storm at home if the need arises. An estimated 500 buses are on call in Lake Charles, Baton Rouge, and Houma in the event of a catastrophe in New Orleans.
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Quoting MississippiWx:
He wants to become a major hurricane so bad...


The engine that wanted to but couldn't. Luckily he wasn't a major coming in or we would have a way bigger problem.
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RT @capitalweather: 93 mph wind gust measured offshore LA at Apache Oil Platform (279 feet above water)
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1241. HarryMc
Quoting bigwes6844:
try lightning speed on here in a week!


I have an extra set of DiLithium crystals on order just in case hyperdrive gets tired. The CV train looks like it's getting going.
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1240. GetReal
First squall producing TS force winds and heavy rain blowing sideways on Westbank.... Barometer now down to 29.32 inches.
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I am in Central LA....not much going on....yet...
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Quoting redwagon:

The high scoots him W? Or, scoot half of him West?


Yea pretty much. It misses the trough and the High pushes him a little SW then West.

36 Hours:

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Quoting MississippiWx:
He wants to become a major hurricane so bad...

doesnt appear to be moving to much either, at least not to me
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1236. Patrap
This is going to get out of control..easily, toward Dark.

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1235. ugajag
It almost looks like Isaac is stationary..Don't most storms that stall in the gulf move east when movement starts up again?
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Quoting coondini:

Good Lord...this place is gonna be extra busy for a good while. We'll be putting in plenty of overtime on this sucker.
try lightning speed on here in a week!
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1233. Patrap
Quoting leftlink:


Pressure down to 28.98in at 3pm LA time, and winds have shifted from a 30 heading to 40, or NE at PSTL1. This is 981.3mb.

LINK

The wind shift for 3 straignt readings confirms that the center is staying offshore and could continue to strengthen to 970mb as predicted. Can someone post funktop again?



Rogo'
Note the "Lime" in da CoConut'



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He wants to become a major hurricane so bad...

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Isaac appears to have really slowed up or eve stalled. It's also re-organizing itself.. Becoming more round and looks to have mixed out that dry air.

I'm predicting 100 mph. at landfall based on slow movement and strengthening.
These flight level winds will probably collapse down to the surface as Isaac makes landfall sometime tonight or maybe tomorrow morning. The steering pattern has a very slow erratic movement for a good amount of time, which isn't good. Praying that the levees hold in New Orleans or we will have a major disaster in our hands.



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1230. Patrap
Quoting HrDelta:


Part of the reason why the Saffir-Simpson Scale needs to be replaced. It doesn't not take into account either pressure, or size.


Plus 4.5 to the 5th Power indeed.
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Quoting Patrap:
\

Venice Marina is having some Boats rack up ..

Not confirmed.


Pressure down to 28.98in at 3pm LA time, and winds have shifted from a 30 heading to 40, or NE at PSTL1. This is 981.3mb.

LINK

The wind shift for 3 straignt readings confirms that the center is staying offshore and could continue to strengthen to 970mb as predicted. Can someone post funktop again?
Member Since: December 28, 2010 Posts: 0 Comments: 134
Quoting indianrivguy:


and in my opinion, that comment by Neil Frank caused folks to die.

Neil Frank was one of those that was pushing for more preparedness, and prompt evacuations... as would befit a former director of the NHC. People lost their lives because of complacency. The mayor of Galveston delayed evacuation - that cost lives. People stayed in their homes in Gilchrist... homes that ceased to exist post-Ike (one home was found on the other side of Galveston Bay, 20 miles away).

If you're told to evacuate, get the heck out. If you're told to prepare, get your stuff together. If you're told to implement your hurricane plan, get to work on it. It isn't hard... but some people think they are invincible... and frequently, they pay for it - with their lives.
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Quoting uptxcoast:
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
1235 PM CDT TUE AUG 28 2012

ISAAC STILL PROGGED BY TPC TO MAKE LANDFALL OVER SE LOUISIANA. THE
12Z NAM IS TRENDING FURTHER WEST WITH ISAAC AND IT IS LOOKING MORE
AND MORE LIKE THE 00Z ECMWF. THERE COULD BE SOME SIGNIFICANT
CHANGES IN THE FORECAST PACKAGE THIS AFTN 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

Well, I was wrong. Looks like we will get some rain after all.


Yes, I want to see what the 12Z GFS says about this. If all three models agree I might have to change my evening plans from setting up to water the grass to other activities.

Another problem is West Nile Virus is HUGE in Texas right now. We don't need a bunch of rain that has water just standing around and pooling.
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Quoting PRweathercenter:
English Version of Blog Update

another GOM storm and maybe better than Isaac?? seriously?
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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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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.