Little change to Isaac, but storm poses a serious storm surge threat

By: Dr. Jeff Masters , 3:25 PM GMT on August 27, 2012

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Tropical Storm Isaac has changed little in strength or organization this morning, as the storm heads northwest at 14 mph towards the Central Gulf Coast. There are two hurricane hunter aircraft in the storm, and Isaac's central pressure held steady at 989 mb at the 8:30 am and 9:15 am center fixes. Top surface winds remain near 65 mph. Infrared and visible satellite loops show that Isaac is a very large storm, but isn't very symmetric. Heavy thunderstorm activity is lacking on the southeast side, where 10 knots of wind shear is driving dry air into the circulation. The center is surrounded by a ring of echoes now, which was not the case on Sunday. However, the echoes are weak. The 8:30 am center report from the Hurricane Hunters reported about half of a ragged eyewall, but the 9:15 am report did not mention any evidence of an eyewall. Isaac will have to form an eyewall in order to intensify significantly.


Figure 1. Morning satellite image of Isaac. Note the lack of heavy thunderstorm activity on the storm's southeast side, where dry air and wind shear are combining to interfere with development.

Isaac's rains
Isaac's heaviest rains have fallen along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. West Palm Beach received 7.57" of rain from Isaac as of 10 am EDT this morning. A trained spotter in Western Boynton Beach reported 10" of rain from midnight to midnight Sunday. 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 nineteen, and two died in the Dominican Republic.


Figure 2. Radar-estimated rainfall from Melbourne, Florida radar shows that Isaac has dumped a wide swath of 3+ inches of rain (orange colors) across the state.

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 major differences in what happens after that. It is still uncertain if a trough of low pressure passing to the north of Isaac will be able to turn the storm due north, as the ECMWF model is predicting, or bypass the storm, allowing a more west-northwest motion into western Louisiana, like the GFS model is predicting. In either case, Isaac is likely to slow down as it approaches the coast, which will increase the damage potential fro m its wind, storm surge, and rains. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. The ECMWF model predicts that these heavy rains will fall more over Mississippi. It appears likely that Arkansas will see some heavy rains from Isaac late in the week, which would help put a dent in the exceptional drought conditions there.


Figure 3. Predicted precipitation for the 8-day period from 2 am Monday August 27 to 2 am Tuesday September 4, from the 2 am EDT August 27 run of the GFS model. This model is predicting a wide swath of 5 - 10 inches of rain (orange colors) will affect portions of Louisiana. Additional very heavy rains are predicted for the Midwest, as moisture from Isaac interacts with a cold front. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac is currently crossing over a relatively cool eddy of water, which will keep intensification slow today. By tonight, the total heat content of the waters increases, which should aid intensification. Low wind shear of 10 knots or less is likely until landfall. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow to the north is not as strong as yesterday, which should also slow intensification today. The models forecast the upper-level outflow should improve by Tuesday. A storm this large will have trouble undergoing rapid intensification, and Isaac's most likely intensity at landfall will be as a Category 1 hurricane, which is what most of the intensity models are forecasting.


Figure 4. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Storm surge forecast for Isaac
Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac's large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1' this morning. Conversely, water levels have fallen by 2' this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac's counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac's storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac's predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5' high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extending pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

The affect of storm size and angle of approach on storm surge
A 2008 paper by Irish et al., The influence of storm size on hurricane surge, found that large storms like Isaac are capable to delivering a 30% larger storm surge to the coast than a smaller storm with the same maximum wind speeds. The angle with which the storm hit the coast is important, too--a storm moving due north or slightly east of north will deliver a storm surge about 10% greater than a storm moving NNW or NW. Consult our Storm Surge pages for detailed information on what the risk is for the coast. I expect that Isaac's storm surge will be about 30% higher than the typical surge one would expect based on the maximum wind speeds.

Isaac's storm surge will provide the first test of the newly-completed New Orleans levee system upgrade. In the wake of the disastrous storm surge flooding from Hurricane Katrina in 2005, Congress appropriated $14.5 billion to upgrade the New Orleans levee system to withstand a Category 3 hurricane storm surge. Katrina was a Category 3 storm at landfall, but the storm passed far enough to the east of the city that its storm surge was characteristic of a Category 1 - 2 storm at the places where the city's flood walls and levees failed. The new flood defenses were only partially completed in time for the arrival of Hurricane Gustav in 2008, which hit Central Louisiana as a Category 2 storm. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5' to Lake Borgne on the east side of the city. Since that time, the imposing 2-mile long IHNC Flood Barrier has been completed to block off the funnel-shaped pair of canals on the east side of the city. I expect New Orleans' new flood defenses will be able to hold back Isaac's surge, but areas outside the levees are at risk of heavy storm surge damage.


Figure 5. A portion of New Orleans' new $14.5 billion dollar flood defenses, as taken from an Army Corps of Engineers map.

New Orleans flood defense info
Army Corps of Engineers map of the new flood defenses
Army Corps of Engineers video showing the flood defenses
New York Times article, Vast Defenses Now Shielding New Orleans

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1050 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 30% chance of developing by Wednesday morning. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands, and high wind shear should begin to tear the disturbance apart on Tuesday.

Another tropical wave that moved off the coast of Africa on Sunday is located just south of the Cape Verde Islands. This disturbance is moving west at 10 - 15 mph, and could arrive in the Lesser Antilles around September 2. 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 Wednesday morning.

Angela Fritz will have a new post here by 6 pm EDT. I'm in Atlanta to help out The Weather Channel with their on-air hurricane coverage, and will doing a few 3-minute tropical updates at 30 minutes past the hour between 2:30 - 7:30 pm EDT today.

Jeff Masters

Tropical Storm Isaac (chelina)
View from the north side of the Sunshine Skyway Bridge.
Tropical Storm Isaac
our street at noon today (seflagamma)
Aug 26, 2012: Isaac floods our street really bad. Now nearly 11
our street at noon today

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@weatherchannel

BREAKING NOW: President Obama directs @FEMA to give LA, MS & AL governors the resources they need
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1708. help4u
Official forcast has it at cat 2 at landfall.100 mph gusts too 115.
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No mandatory evacuation for New Orleans, but those who want to urged to leave now
http://www.nola.com/hurricane/index.ssf/2012/08/n o_mandatory_evacuation_for_ne.html
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Quoting nrtiwlnvragn:


Looks like Daytona International Speedway

Yep. The text was posted after I quoted it... too fast on the "post" button.
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Quoting avthunder:
Yes I saw him. Never knew he was so tall. Or is Bryan Norcross just short?


He didn't look very comfortable..
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Quoting avthunder:
Yes I saw him. Never knew he was so tall. Or is Bryan Norcross just short?


Jeff is quite tall....I'm 6'2" and he dwarfs me...
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Bolaven is heading into the Koreas... He's not nearly the monster he was a few days ago but he's still a massive system and a typhoon... damage will be severe.

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Quoting MAweatherboy1:
I think he's slowing down... Could just be the way convection is changing, but I don't think he's moving nearly as fast as earlier.



He has slowed down 3 MPH since the last report, and I would expect it to slow down as it reaches the coast, which is due to the fact it will be feeling the pressure of the two highs.

I think that if it becomes slower it will allow it a bit more time to organize over water and overall will allow it to slowly get its act together.

A slow moving storm just makes this situation much worse.
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Isaac is the best looking Tropical Storm I've seen...
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Quoting NCHurricane2009:


There are 981 mb cyclones frequently across the globe in non-tropical form...yet they are not as windy as a hurricane. Its about pressure gradient...not central pressure. If a storm is spread out...it has a lower pressure gradient (and lower wind) for same central pressure.

So its not a hurricane alone by merit of central pressure value. But yeah...he is really close. Maybe 979 mb will finally get him to cat 1...




Just going by SS scale and referring to tropical cyclones
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1699. bappit
Quoting kctinney:


A TON of ppl on Fox Hurricane chat.......why I started googling about hurricanes and found this website. There were some ppl on here as well (some obviously trolls). Please don't ask me to recall from 2005 d/t I don't remember!

A forum is like an ensemble model but with even more outliers.
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Quoting jeffs713:

Where was that taken?


Looks like Daytona International Speedway
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Quoting linkays:
I know this storm is NOT coming near the Corpus Christi area but does anyone think it might send some rain? I can't figure out how big it is.....size not wind speed or anything like that. I'm sorry if anyone thinks this is a dumb question but we're just so desperate for rain.

I doubt much of TX (aside from the very far eastern end) will get any rain from Isaac. His exact landfall point will make all the difference, as that influences his track post-landfall.

Sadly though, I think Corpus Christi won't get lucky.
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Quoting Pirate999:


How about SI... Slow intensification.


hahahaha

SI FLAG: ON
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Quoting linkays:
I know this storm is NOT coming near the Corpus Christi area but does anyone think it might send some rain? I can't figure out how big it is.....size not wind speed or anything like that. I'm sorry if anyone thinks this is a dumb question but we're just so desperate for rain.


Not a dumb question... I doubt you'll see anything.
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Quoting TheHurricaneDundee:
Hey! anyone else watching Dr.Masters on TWC?
Yes I saw him. Never knew he was so tall. Or is Bryan Norcross just short?
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1693. acl8610
Quoting STXHurricanes2012:
I still see this becoming a 105 mph Hurricane! Mid cat 2 who agrees??

I agree, the core of the storm is disassociating from the activity over FL and the Atlantic, leaving a smaller COC that could spin up faster than it would have otherwise...
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Quoting Dsntslp:
Frances Jeanne, (Treasure Coast and others local to me)

Do you have power?
Do you need anything?

I have lots of ice and a deep freeze if you need it to put your food in here.
I have a generator too if it starts to look like your power will be off for any length of time.

You can email me at (my screen name here) @yahoo.com if you want.

Still have power (and a/c again, thank god, after it being out since Saturday!)and all is well - that's very kind of you, though, thank you!
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I'm a little worried that SOME people have kind of dismissed Isaac as potentially a non-issue. The facts are that the storm will intensify right up until landfall...and it is worth reiterating that intensity forecasts are extremely vulnerable to large errors, even in the short term.
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1690. linkays
I know this storm is NOT coming near the Corpus Christi area but does anyone think it might send some rain? I can't figure out how big it is.....size not wind speed or anything like that. I'm sorry if anyone thinks this is a dumb question but we're just so desperate for rain.
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Quoting weatherh98:


Yes

No
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i think we could see 120 too 130mph at winds at land fall its slowing down wish would all so give it more time overe water
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Quoting STXHurricanes2012:
I still see this becoming a 105 mph Hurricane! Mid cat 2 who agrees??


maybe not mid cat 2, but i am on the cat 2 train with ya.
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Quoting wxchaser97:
Structure is improving and winds increasing, looks great.



It still looks a little ragged in the Northeastern quadrant due to dry air most likely, but if that anticyclone can slip down Southwest over the center of circulation then this storm can become mighty powerful.

This thing could potentially erupt if it can consolidate a strong inner core, although I am expecting it to be a more steady process of strengthening.

It still needs to fill in that Northeastern Quadrant before we can even consider Rapid intensification.
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1684. nolajet
Jeff beat me, but my post:

Quoting NCHurricane2009:

Is there a mandatory evac for NOLA? I haven't been checking any news sources all day here at work....(except for this blog)...


No there isn't. Link>From Nola.com

As ready as I will be. Now the waiting, which sucks more. Can't believe I am getting called in to work tomorrow. A coffee shop is not going to generate revenue in 40mph winds. And I'm the lucky guy that gets to walk home in it. Just like 7 years ago... exactly the same situation. Better results this time though. :)
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Quoting STXHurricanes2012:
I still see this becoming a 105 mph Hurricane! Mid cat 2 who agrees??

Last afternoon...I thought he'd bomb out to cat 4...then I changed my mind this morning and agreed with Dr. M on nothing past cat 1...so I don't know what to think anymore....

All I know is I need to learn more....having 2 forecasts one way and the other means only 1 is right and 1 is wrong...so that is less skill....
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ISAAC WOBBLED WESTWARD THIS AFTERNOON...BUT A REPRESENTATIVE INITIAL
MOTION ESTIMATE IS 305/10


OK lets give the wobble-casters some love!
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Quoting CJ5:
If I had a dollar for every time someone said and RI was coming, I would be rich. It has been said for the past 4 days with nothing. He just has been unable to close his core and until he does that he will not gain much strength.


How about SI... Slow intensification.
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I think he's slowing down... Could just be the way convection is changing, but I don't think he's moving nearly as fast as earlier.
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Quoting CaicosRetiredSailor:

Where was that taken?
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1676. drs2008
Quoting TomballTXPride:

Great work is all I have to say. The world needs more people like you.
Hi Storm Junkie. Im on the ride out team at the VA. Good luck and Thanks!
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Quoting help4u:
100 mph winds is a cat2,i will go with the national hurricane center,has anyone ever see a hurricane stall and sit in one place for 24 hrs. This is not good for New Orleans.
Danny in 1997 sat an spun in Mobile bay for a long time.
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Quoting TropicalAnalystwx13:

As long as it doesn't intensify too fast, in which case it would pull in dry air and completely collapse its core.


When hasn't this been ingesting dry air?

My point exactly
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Quoting sar2401:


It sure wasn't last night. He looked worse than any time since getting into the Gulf at about 0300 CST.


Yeah. You are right, last night is an exception.

I still think Isaac get there tonight. Like Kman says maybe even the 8pm
Member Since: August 18, 2008 Posts: 0 Comments: 628

It looks like some sort of eye feature is trying to develop, which is definitely an indicator of it intensifying and of the possibility of future intensification.

The Northeastern and Southeastern section of the storm seem a little ragged still, but I am sure they will fill in slowly, at least this seems to be indicated on the radar.

I would not be surprised if this storm becomes a 80-85 mph storm by late tonight or tomorrow, and then perhaps become a 95-100 mph storm later tomorrow afternoon before landfall.
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Structure is improving and winds increasing, looks great.
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Quoting MAweatherboy1:

I don't think so, the NE quad is to weak and dry to allow anything more than slow, steady intensification until landfall. I'm with Doc on this one, Cat 1 peak.



That same dry air has been present during this 3mb drop in the past hour. It's a fragile situation for sure, but the core seems well cut-off from any dry air for now.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
1668. flcanes
Quoting weathermanwannabe:
STRENGTHENING IS EXPECTED TO CONTINUE RIGHT UP UNTIL LANDFALL OCCURS DUE TO THE VERY WARM WATERS THE
CYCLONE WILL BE PASSING OVER


Folks on/near the coast in LA/MS/AL should pay close attention to this NHC statement and plan accordingly and not wait until the last minute if you are still on the bubble and expecting a weak Cat 1.......

keep up on the attention
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Next vortex is going to say "eye open to the NW or W"..

The pressure is going to be interesting on this one.. it doesn't usually take long for a storm to intensify from 1 to 2.
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I still see this becoming a 105 mph Hurricane! Mid cat 2 who agrees??
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1664. flcanes
Quoting WxNerdVA:

eyewall?
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Quoting WCSCTVCharleston:
Yea issac is a hurricane has been for a few hours 981 pressure normall equates to a strong cat 1 this thing is about to ramp up


There are 981 mb cyclones frequently across the globe in non-tropical form...yet they are not as windy as a hurricane. Its about pressure gradient...not central pressure. If a storm is spread out...it has a lower pressure gradient (and lower wind) for same central pressure.

So its not a hurricane alone by merit of central pressure value. But yeah...he is really close. Maybe 979 mb will finally get him to cat 1...
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STRENGTHENING IS EXPECTED TO CONTINUE RIGHT UP UNTIL LANDFALL OCCURS DUE TO THE VERY WARM WATERS THE CYCLONE WILL BE PASSING OVER

Folks on/near the coast in LA/MS/AL should pay close attention to this NHC statement and plan accordingly and not wait until the last minute if you are still on the bubble and expecting a weak Cat 1.......
Member Since: August 8, 2006 Posts: 0 Comments: 9329
Im going to post pictures of the lake with reference points that some of you who live around me know of.
Gimme 30 minutes at least
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well i guess if you consider a 10mph increase over almost 3 days is RI you will say RI when it makes landfall as a cat 1...not my idea of RI, but whatever this storm has been a disappointment since day 1
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1659. airmet3
Quoting NCHurricane2009:

Is there a mandatory evac for NOLA? I haven't been checking any news sources all day here at work....(except for this blog)...


This morning on TWC, one of the city officials said they do not order mandatory evacuation unless it is a high end Cat 3.
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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.