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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1425. JeffM
Mike Siedel on Orange Beach is in full on exaggeration mode.
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1424. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 128639
Old habits are hard to break. I can't help but think that Isaac will turn more due north as he gets stronger and 1012mb pressures recede northwards into central MS. Additionally, the plains high is or should be serving as a blocking high to the storm's nwest. I'm thinking there's a rut in the north GoM and Isaac will get on track shortly to follow Camille's and Katrina's path, maybe even a tad further eastwards eventually.

And, I hope all my rowdy friends are hunkered down for the evening! ;)

Member Since: July 9, 2006 Posts: 184 Comments: 29610
1422. Patrap
This is B-A-D Mojo...
Member Since: July 3, 2005 Posts: 426 Comments: 128639
Quoting weatherh98:
talked to kori hes doing fine right now
Are you guys out storm chasing?
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Quoting reedzone:


Well if they show Isaac strengthening more, can't that help the NHC nail the winds right at landfall later tonight?
A wind measured 500 miles from the center can help determine the size of the windfield, but it's not something that would help the NHC in determining the pressure 500 miles away in the center. Surprised you didn't know this.
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1419. ncstorm
Quoting Charmeck:


Check out the band that is connecting Isaac to that mess in SC. These areas are still trying to reconnect! Absurd - this storm is mega huge.


Earlier I showed those two blobs approaching our coast and they have now merge with each other and trying to merge with Isaac
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Kermit is flying at 12,000ft currently. Hopefully they will dip down and allow us to have a better idea of what's going on closer to the surface with Isaac. Kind of an epic fail to not have Air Force recon in the system right now.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
1416. Patrap
Quoting atmosweather:


Alright good to know. It's one thing to see the churning water and waves from a distance, but that's going too far!


That area is Barricaded off and has been for a Day,,they went around them on Foot..as per the Big Loop EOC discussion earlier.
Member Since: July 3, 2005 Posts: 426 Comments: 128639
Quoting Patrap:


TimeWave Zero..Terrance McKenna was right.

: )
yep thats right Pat
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Quoting ProgressivePulse:
Center is getting better and better defined as time passes.

Seems like it is pulling on itself,wrapping up tighter.
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1413. Patrap
Had a Low Voltage alert and the PC self shut down in Safe mode.

Backo still seems.

Member Since: July 3, 2005 Posts: 426 Comments: 128639
Quoting Patrap:


From user mountainpics: Noon on 8/28/12. Lake Pontchartrain as Hurricane Isaac approaches

Submit your photos of the weather in your neighborhood as Isaac bears down:

http://blog.nola.com/interact/2012/08/whats_the_w eather_like_in_your.html


I wanted to go to the lakefront and take some photos, but I was afraid they would close the CCC due to high winds and I would get stuck on the Eastbank. That happened to me before.
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Quoting Neapolitan:
Some have mentioned just how big Isaac is, and there's no doubt that he is larger than many cyclones. But for fun, here's how large 1979's Typhoon Tip would be were it placed at Isaac's location. The entire area under the circle would be subject to at least tropical storm-force winds.

Tip in America

Now, that's a big storm...
Here in Palm Beach county we are still flooded. Isaac was 400 miles away yesterday while the piece of feeder band broke off and sat it's butt over me all day and everyone else... Not fun.. School was closed again today in Palm Beach county and may be closed again tomorrow. Now that's BIG
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Quoting Patrap:


That was NOON and they we're escorted Home in a NOPD Unit.


Alright good to know. It's one thing to see the churning water and waves from a distance, but that's going too far!
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1408. ncstorm
Quoting UWalkTheMall:
Isaac has re-connected with the blob. Will it draw moisture in from it?


Wouldnt that be something for them to merge and become this super hurricane over the Southern states

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Quoting txag91met:
Gust to 18 here as far away as NW Houston. 98F.


21 mph gust near Hobby Airport....
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Quoting weatherh98:
usually the dry air kills the storm

now the storm is killing the dry air


Sounds like some Chuck Norris joke in weather form. Anyway I think 85-90mph is possible with Isaac.
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I'm skunker n a drunk lol. Winds a blowin purty good here and gotta sprinklin of rain. In my best redneck accent (which is pretty thick when I drink lol)
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1404. JeffM
Haven't seen Cantore all day while watching TWC.
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Quoting kwgirl:
When I clicked on the computer models for TD 10 here on WU a map came up showing TD 10 sitting on 0 degrees across from SA and under the Cape of Africa. I know my geography and thought that was real weird. So it was an error.


Try TD11.
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Quoting leftlink:
Trying to figure out the center location now. The station pstl1 is still dropping slowly, now at 981mb, 28.97in.

Wind is still from the northeast and pressure is not dropping as fast, so I am assuming it is almost due south of this position. Based on the NNW wind shift at other buoys to the west, I have the following estimate for a center fix:

28.75 N 89.1 W

Let's see how close this is to the NHC center fix at 5pm.


Well I was close... watch the wind direction at station LOPL1 for the next center fix.

Wind direction just changed back to NNW at PSTL1

Pressure dropping faster now, to 28.95 at 3:30pm. This indicates a wobble back to the north, and that the eye might go directly into this station.
Member Since: December 28, 2010 Posts: 0 Comments: 134
1400. K8eCane
unbelievable. Pat, SJ, Press and all stay safe as you witness this awesome power. BUT you can keep your rainband thingy that is over me up here in Wilmington
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Quoting bigwes6844:
winds up to 40 mph by me. out of the NNE
Gust to 18 here as far away as NW Houston. 98F.
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Isaac has re-connected with the blob. Will it draw moisture in from it?
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Quoting oracle28:


100 mph? Really? Is that after it fails to get west of 85W?

It's gained 10 mph in 2 days, and it's going to go from 75-to-100 in 6 hours?


Hey oracle. You and I have been on here long enough to realize that is reed for ya. Mr. Hyped-up doom-caster. Yet when someone calls him on his ridiculousness, he basically cries about people picking on him. He should really grow a pair. If you put yourself out there like that -- expect to deal with people calling you on it.
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Quoting Patrap:


TimeWave Zero..Terrance McKenna was right.

: )


how are yall fairing
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My original forecast of 80kts at landfall may become reality considering Isaac's core continues to improve, as well as the fact it should have until late tonight to intensify.

Even though Isaac is a category 1 hurricane, expect damage of that consistent with a major hurricane moving onshore along the northern Gulf coast.
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Quoting 1900hurricane:

What is the most costly Category 1 at the moment? Irene from last year? Agnes from 1972?


I looked up Irene on wikipedia and the damage statistics are actually a lot higher than I remember...between 15 and 16 billion dollars in the United States.

Unless major storm surge and flooding in the city of New Orleans was to occur, the cost of damage from Isaac will likely be below that of Irene's, but will still be high enough to make it the second costliest Category 1 storm. Agnes caused 2.1 billion dollars in damage (1972 dollars), and at the time became the costliest hurricane in U.S. history.
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usually the dry air kills the storm

now the storm is killing the dry air

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1392. robj144
Quoting Jedkins01:


Exactly, it's not effective enough.


I was discussing this last year at one point. A better metric would be an area integrated kinetic energy calculation. This would take into account both the size of the storm and the winds as well as the hurricane wind radius.

I would actually love to calculate this for various storms in the past to know how it would work, but I need to have a very accurate wind field map with a proper length scale so I can see how far the contours of the wind extend from the center. Does anyone know where I can find these maps or if they exist?
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WINDS AFFECTING THE UPPER FLOORS OF HIGH-RISE BUILDINGS WILL BE SIGNIFICANTLY STRONGER THAN THOSE NEAR GROUND LEVEL. AT ABOUT THE 30TH STORY...WINDS WOULD LIKELY BE ONE SAFFIR-SIMPSON CATEGORY STRONGER THAN AT THE SURFACE.
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Quoting HiWay58:



That's an awesome image. I thought they were completely broken apart.
Link
They were earlier today - if you look at the "flash" image on this link you'll see the connection getting bigger.
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winds up to 40 mph by me. out of the NNE
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1388. flcanes
Quoting Patrap:


TimeWave Zero..Terrance McKenna was right.

: )

who is terrence mckenna
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Quoting atmosweather:


Get those kids outta there! My goodness that is so reckless


its fun
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1386. Patrap
Quoting bigwes6844:
same time K made landfall 7 years ago


TimeWave Zero..Terrance McKenna was right.

: )
Member Since: July 3, 2005 Posts: 426 Comments: 128639
1385. kwgirl
Quoting atl134:


The equator? It's at 23.8N. 43.9W
When I clicked on the computer models for TD 10 here on WU a map came up showing TD 10 sitting on 0 degrees across from SA and under the Cape of Africa. I know my geography and thought that was real weird. So it was an error.
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1384. HiWay58
Quoting Charmeck:


Check out the band that is connecting Isaac to that mess in SC. These areas are still trying to reconnect! Absurd - this storm is mega huge.



That's an awesome image. I thought they were completely broken apart.
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Its looking much more organized. the wind will be eastfor at least ten hours and push mass amounts of water into the lake
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Quoting 1900hurricane:

What is the most costly Category 1 at the moment? Irene from last year? Agnes from 1972?
Agnes, I would assume. Although, Stan killed around 1500 people in Central America.
Member Since: August 1, 2012 Posts: 0 Comments: 436
Quoting Felix2007:
RPM radar on TWC says no landfall until 6 AM tomorrow!
same time K made landfall 7 years ago
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1379. Patrap
Quoting atmosweather:


Get those kids outta there! My goodness that is so reckless


That was NOON and they we're escorted Home in a NOPD Unit.
Member Since: July 3, 2005 Posts: 426 Comments: 128639
Quoting 900MB:


From the radar loop on TWC looks stalled ! ? !
may be gonna start the westward turn some of the models were predicting
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HURRICANE CENTER LOCATED NEAR 28.7N 89.2W AT 28/2100Z
POSITION ACCURATE WITHIN 15 NM

PRESENT MOVEMENT TOWARD THE NORTHWEST OR 310 DEGREES AT 7 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 975 MB
MAX SUSTAINED WINDS 70 KT WITH GUSTS TO 85 KT.


Upgraded to 80 MPH at 4PM
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Quoting Patrap:


From user mountainpics: Noon on 8/28/12. Lake Pontchartrain as Hurricane Isaac approaches

Submit your photos of the weather in your neighborhood as Isaac bears down:

http://blog.nola.com/interact/2012/08/whats_the_w eather_like_in_your.html


Get those kids outta there! My goodness that is so reckless
Member Since: Posts: Comments:

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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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