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 reedzone:


Just receiving info from a friend. The plane is called "Kermit". He says it is surface winds..


According to Google Earth, the airport Kermit is flying out of is less than 500 miles from the center of the storm. At about 130 nm from the center of the storm it has contaminated surface winds of 49 kts. Uncontaminated winds are about 40 kts nearby.
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1373. Pastey
I'm being told parts of Chalmette, LA are starting to lose power.
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Quoting atmosweather:
From GetReal: Station KMDJ
Federal Aviation Administration
Location: 28.643N 89.794W
Date: Tue, 28 Aug 2012 20:15:00 UTC
Winds: NNW (330°) at 55.9 kt gusting to 76.0 kt
Air Temperature: 78.8 F
Dew Point: 78.8 F



That is almost 70 miles to the West of Isaac's center. Very powerful winds are going to continue to spread onshore quite a distance away from the center. The effects of Isaac, including wind, are going to be felt along a large portion of the northern Gulf Coast as well as points further inland. PLEASE do not take this Category 1 hurricane lightly. With extreme rainfall and Category 3 type storm surge, Isaac is likely to be the costliest Category 1 hurricane in United States history.

What is the most costly Category 1 at the moment? Irene from last year? Agnes from 1972?
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RPM radar on TWC says no landfall until 6 AM tomorrow!
Member Since: July 12, 2008 Posts: 7 Comments: 383


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.
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1369. Patrap
ESL by LSU kickin in the Imagery..

Mucho thanks to them.



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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.
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Big band moving onshore now.
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Quoting atmosweather:
From GetReal: Station KMDJ
Federal Aviation Administration
Location: 28.643N 89.794W
Date: Tue, 28 Aug 2012 20:15:00 UTC
Winds: NNW (330°) at 55.9 kt gusting to 76.0 kt
Air Temperature: 78.8 F
Dew Point: 78.8 F



That is almost 70 miles to the West of Isaac's center. Very powerful winds are going to continue to spread onshore quite a distance away from the center. The effects of Isaac, including wind, are going to be felt along a large portion of the northern Gulf Coast as well as points further inland. PLEASE do not take this Category 1 hurricane lightly. With extreme rainfall and Category 3 type storm surge, Isaac is likely to be the costliest Category 1 hurricane in United States history.



Excellent summation! Thanks! Folks, plz pay attention.
Member Since: July 9, 2006 Posts: 184 Comments: 29594
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...


And that's just the TIP of it!

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Eww. @80mph, I guess there's still time for 85 or 90mph now that he's closing off. :/ (imvho)
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1363. 7544
80mph now
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1362. 900MB
Quoting mobhurricane2011:
is it actually stalled or just a really slow movement


From the radar loop on TWC looks stalled ! ? !
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Link

4 PM advisory out
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Quoting will40:



Lockheed WP-3D Orion (Reg. Num. N42RF)

thats a NOAA flight


Well if they show Isaac strengthening more, can't that help the NHC nail the winds right at landfall later tonight?
Member Since: July 1, 2008 Posts: 13 Comments: 7334
1359. GetReal
Quoting reedzone:


Just receiving info from a friend. The plane is called "Kermit". He says it is surface winds..



Would not doubt it; there are automated stations on oil rigs reporting higher sustained winds, 150 ft from the surface.
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Santa Rosa Beach, FL
(Blue Mountain Beach)
2/28/12 15:00hrs

Waves 8-10ft with winds in the upper 30's...

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Quoting reedzone:


Just receiving info from a friend. The plane is called "Kermit". He says it is surface winds..


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Quoting Skyepony:
Waveland surge..

Not a good sign when low tide will be at 10:29PM and water levels keep rising rapidly.
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talked to kori hes doing fine right now
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1354. Patrap
Inside the NOAA P-3 Orion "Kermit





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is it actually stalled or just a really slow movement
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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...

There wouldn't even be room for Tip in there, lol.
Member Since: July 12, 2008 Posts: 7 Comments: 383
80MPH 975mb
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1350. Pastey
Baton Rouge here:

Still moderate cloud cover. Sun shining at the moment. Consistent breeze with a few mild gusts but nothing that hints at what lies a few hundred miles away. Not a drop of rain has fallen yet.
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Quoting TampaFLUSA:
4 pm update 80 mph now.
and it has slowed down more...could reach 95 - 100 mph
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It'll be a race to see if Isaac can hit Cat 2 before landfall. Not sure but he is looking much more impressive on satellite than before. Since he is moving slowly and will be over wetlands and Lake Ponchertrain, we may not see rapid weakening after landfall. Just a thought...
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And everyone think I was crazy for believing that 97L will be something he might even strength into Kirk that is why I never rip a storm or invest until is definitely out. might get Leslie of pre 98L and possible our first major.
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1346. JeffM
Anyone think it will surpass 80mph?
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1345. will40
Quoting reedzone:


Just receiving info from a friend. The plane is called "Kermit". He says it is surface winds..



Lockheed WP-3D Orion (Reg. Num. N42RF)

thats a NOAA flight
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Bay St. Louis MS update: Gusting to 35 knots, steady at 27, almost six feet of water above mean low water. Pressure at 1000.7. 3-4' swells.
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Center is getting better and better defined as time passes.

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1342. LargoFl
are wave heights on top..of surge
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Hurricane Isaac: 4 PM CT, 80 mph winds, Cat 1, 975 mb, moving NW at 8 mph.
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Quoting ThisEndUp:
Every time I see that comparison of Typhoon Tip to the US, I get shudders...could you imagine a super-cyclone like that striking the U.S. mainland? That'd be some zombie-apocalypse stuff right there...


Well, if you take a look at the SE regional radar composite, Isaac and his spawn are impacting a pretty darn large geographic area, from North Carolina over to Ms., and down to Florida panhandle. That's a pretty large footprint right there. Georgia and SC are getting slammed.
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1339. Dakster
Quoting reedzone:


Just receiving info from a friend. The plane is called "Kermit". He says it is surface winds..


Thanks -- and I have heard of Kermit.

That is some far away TS winds...
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From GetReal: Station KMDJ
Federal Aviation Administration
Location: 28.643N 89.794W
Date: Tue, 28 Aug 2012 20:15:00 UTC
Winds: NNW (330°) at 55.9 kt gusting to 76.0 kt
Air Temperature: 78.8 F
Dew Point: 78.8 F



That is almost 70 miles to the West of Isaac's center. Very powerful winds are going to continue to spread onshore quite a distance away from the center. The effects of Isaac, including wind, are going to be felt along a large portion of the northern Gulf Coast as well as points further inland. PLEASE do not take this Category 1 hurricane lightly. With extreme rainfall and Category 3 type storm surge, Isaac is likely to be the costliest Category 1 hurricane in United States history.
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1337. 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
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Quoting atl134:
I have a real hard time seeing Isaac strengthen much even with the stalling, anything higher than 85mph would surprise me. The main issue with stalling is the extension of time these areas will get heavy rains.
His major impediment for the last few days has been dry air. With that gone, he should strengthen until landfall.
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1335. Michfan
Rapid Scan of Issac
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30.283N 86.083W
Flight Level Wind (30 sec. Avg.)
From 153° at 45 knots
(From the SSE at ~ 51.7 mph)
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winds are up too 80mph

4:00 PM CDT Tue Aug 28
Location: 28.7°N 89.2°W
Moving: NW at 8 mph
Min pressure: 975 mb
Max sustained: 80 mph
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1332. GetReal
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4:00 PM CDT Tue Aug 28
Location: 28.7°N 89.2°W
Moving: NW at 8 mph
Min pressure: 975 mb
Max sustained: 80 mph

winds are up
Member Since: September 5, 2007 Posts: 0 Comments: 3480
1329. atl134
Quoting kwgirl:
And weird. I looked at computer models and they have it sitting on 0 degrees. Isn't that the equator? Didn't think anything would form on the Equator.


The equator? It's at 23.8N. 43.9W
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Quoting kwgirl:
And weird. I looked at computer models and they have it sitting on 0 degrees. Isn't that the equator? Didn't think anything would form on the Equator.


Probably a model glitch, TD11 is at 23 degrees north.

Might be another weak and inconsequential system, if it gets named at all.
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Isaac growing!
4:00 PM CDT Tue Aug 28
Location: 28.7N 89.2W
Moving: NW at 8 mph
Min pressure: 975 mb
Max sustained: 80 mph

well people here goes the next one
5:00 PM AST Tue Aug 28
Location: 23.8N 43.9W
Moving: WNW at 10 mph
Min pressure: 1008 mb
Max sustained: 35 mph
new TD official
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4 pm update 80 mph now.
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Quoting reedzone:


Just receiving info from a friend. The plane is called "Kermit". He says it is surface winds..


Those are flight winds SFMR is turned off.
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.