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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I just want to thank all the people who share information here, my parents live in key largo and during hurricane season I am always glued the this blog and wunderground...never posted anything before, but I needed to say thank you for everyone sharing their thoughts, opinions, and information.
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As of 9 a.m. Isaac is finally detaching from the Caribbean low.
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anyone know how to post a picture on the blog? LOL
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Link This is a great tool to view track. You can overlay most models, sat image (gom is a good view), and other "toys". If you haven't seen it already, enjoy.
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Quoting luvtogolf:


And we're sitting here in a dry slot since early this morning. Heck, I may get out and play a quick 9 this afternoon. The sun is popping in and out and it's breezy but nothing too bad.
It's all over on east coast right now but as he moves west i think it is coming your way.
Member Since: August 18, 2008 Posts: 0 Comments: 474
Quoting bohonkweatherman:
Longer Isaac stays a tropical storm, the better for everyone.


The more west it goes as well.
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Quoting mcluvincane:
80mph winds being repoted sustained


I heard gust in the N.W. quad.

But could see upgrade to cat 1.
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Quoting luvtogolf:


And we're sitting here in a dry slot since early this morning. Heck, I may get out and play a quick 9 this afternoon. The sun is popping in and out and it's breezy but nothing too bad.
this could..turn out to be Bad for us later on, with the sun heating up the air here, when those bands finnaly get to the west coast we might be in for a tornadic evening..we surely dont need tornado's here thats for sure
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It is apparent that some here (not all by any means) seem to "write off" the risks associated with a Storm just because the Category is not high and, therefore, winds are not extreme.
There are many examples of tragedy and loss in recent history that are down to water (rain, surge etc)- please, never underestimate the power of that water whether or not you have a wind impact - I say this as one who saw Honduras very soon after Mitch.
Meanwhile - my thoughts and best wishes to all in the cone of Isaac and to aid work colleagues in Haiti who now face the very real possibility of an extended cholera outbreak
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Quoting SherwoodSpirit:
Thanks Dr. Masters

I live in Missouri, near St. Louis where we're in Extreme Drought conditions as of two days ago. Yesterday we FINALLY got some decent rain (.5" to 1" approximately) but not nearly enough to actually bust the drought%u2026 today I see this in my local forecast.

Link

Hey Sherwood I'm in the middle of the state by Sedalia....we are in extraordinary drought conditions....we MIGHT have got .5 an inch but I doubt it..I can't open your link but I see a chance of rain toward the end of the week here...best case scenario is Issac doesn't hurt the coast badly and comes up here and DUMPS!! LOL
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gamma - do you have a stratus rain gauge?
if not, wu-mail me. I want you to have one. No cost.

this is ridiculous- your rain amounts- and I wanna make sure
you are measuring right.



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Quoting SherwoodSpirit:
Well dang. Three different uploads to three different places and I STILL couldn't post my image. Then when I finally give up and just post a link, the blog adds gobbledygook to my text. haha
I give up.

The jist of it was, St. Louis is going to have RAIN from Isaac. Which will be SO WELCOME! :)



Yes you will not sure if you saw my other post. You will prob be on the east side of the System so i think you will def get rain
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Quoting StormHype:
No Cat1 @ 11am still... maybe by 5pm. Where's taz to hype his imaginary cat4 doomcast?


Reed was predicting cat4 yesterday.
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80mph winds being repoted sustained
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16 to 18inch now for me!!!!
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Quoting StormHype:
No Cat1 @ 11am still... maybe by 5pm. Where's taz to hype his imaginary cat4 doomcast?
Longer Isaac stays a tropical storm, the better for everyone.
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Quoting seflagamma:
I put this on prev blog right before this new one opened. copy here.


Hi everyone!!! Thanks for the new blog and info.
______________________________________________


Check in from Broward County. That one long band that is a line from South Dade thru Palm Beach county training north.. is right over us.

We now have OVER 10" of rain, you can float a boat down the street, except so windy and rainy it is white capping!

Water is now in neighbor's garage and starting to get in ours.. patio is almost flooded..
Back yard and pool is under water and water now up half way to front of house.. Side walk is about 7" deep....


Our area has gotten way more than our share of rain from Isaac, just not fair.. other areas needs some of this.
We were already soggy before Isaac.

Hope you are all good and safe...
wow the flooding predictions are coming true in florida
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WNW movement heading towards La.
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Quoting coffeecrusader:
Is this thing EVER going to become a hurricane??


I think it's expected. For the most part, the environment is working for it, but it seems that inner-structural problems have been plaguing this system throughout its life. It has been gradually strengthening, though.
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Quoting LargoFl:
geez..when he finally gets up close to the northern gulf coast..these storm bands might be all the way up to south carolina!!..he is Huge in area that he affects wow.....


And we're sitting here in a dry slot since early this morning. Heck, I may get out and play a quick 9 this afternoon. The sun is popping in and out and it's breezy but nothing too bad.
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Really confused...NHC shows a slosh map that shows a possible 12 surge at Mandeville...NWS shows 5-8....
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Well dang. Three different uploads to three different places and I STILL couldn't post my image. Then when I finally give up and just post a link, the blog adds gobbledygook to my text. haha
I give up.

The jist of it was, St. Louis is going to have RAIN from Isaac. Which will be SO WELCOME! :)
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Quoting LargoFl:


Had a power surge, back on now. Big booms of thunder and tornado warnings here in St Lucie.
Member Since: August 18, 2008 Posts: 0 Comments: 474
Quoting Elena85Vet:
There will be heavy inland rainfall trying to drain into that surge as well. Hope Isaac is a fast mover
Isaac is predicted to be over La. for 2 or 3 days, slow mover, not good. La. does not need rain, so major flooding will be an issue.
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Quoting JeffM:
Just seems this storm can't get over the hump.



Yeah travelling the straits, over the GOM and not even a hurricane. Hmm fishy.

I think wxmod has all the answers ;)

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Why relate storm surge to the Saffir-Simpson Scale? They removed mention of storm surge from that scale for good reasons.
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No Cat1 @ 11am still... maybe by 5pm. Where's taz to hype his imaginary cat4 doomcast?
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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.


Congratulations with your new TV personality status!

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Thank,Doc looks like New Orleans will prove his new $14.5 billion dollar flood defenses with Isaac
Member Since: June 20, 2012 Posts: 0 Comments: 2169
I put this on prev blog right before this new one opened. copy here.


Hi everyone!!! Thanks for the new blog and info.
______________________________________________


Check in from Broward County. That one long band that is a line from South Dade thru Palm Beach county training north.. is right over us.

We now have OVER 10" of rain, you can float a boat down the street, except so windy and rainy it is white capping!

Water is now in neighbor's garage and starting to get in ours.. patio is almost flooded..
Back yard and pool is under water and water now up half way to front of house.. Side walk is about 7" deep....


Our area has gotten way more than our share of rain from Isaac, just not fair.. other areas needs some of this.
We were already soggy before Isaac.

Hope you are all good and safe...
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geez..when he finally gets up close to the northern gulf coast..these storm bands might be all the way up to south carolina!!..he is Huge in area that he affects wow.....
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TWC keeps reporting 80mph winds recored by HH.
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Last vortex data message said the center was at:

26 1' N, 85 22' W -- 10:37am this morning


Last buoy observation:

26 3' N, 85 37' W -- 10:50am this morning


It is amazing how close to the buoy this was! The stats on the storm, from buoy readings, are:

a. 29.21in pressure, or 989.16 MB
b. 17.1 ft wave height (SE eye wall)
c. 85.1F water temperature
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There will be heavy inland rainfall trying to drain into that surge as well. Hope Isaac is a fast mover
Member Since: August 18, 2008 Posts: 0 Comments: 628
"Hurricane Hunters reported about half of a ragged eyeball"

Lol.

XD

Thanks for the laugh Dr Masters along with an excellent blog.
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Thank you Dr. Masters
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Quoting kwgirl:
Thank you Dr. Masters. I saw you on TWC on Saturday night speaking about Isaac. Norcross said this blog was THE most popular weather sites on the internet. Looking at the satelitte, it does appear that Isaac has a ragged eyeball. But I think you meant eyewall. Have a good day.
or there was a grizzly murder aboard the plane.
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Is this thing EVER going to become a hurricane??
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Quoting FookyMonster:
Thanks! A couple inches of rain in Pasco county, FL. Some gusts of probably 20mph but that's it.


Yes, still pretty quiet south of Tampa bay too. TS Debby was 5x worse than Isaac for us so far.
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Quoting SherwoodSpirit:
Thanks Dr. Masters

I live in Missouri, near St. Louis where we're in Extreme Drought conditions as of two days ago. Yesterday we FINALLY got some decent rain (.5" to 1" approximately) but not nearly enough to actually bust the drought… today I see this in my local forecast.



i think you will be on the east side of the system so you could see quite a bit of rain
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Just seems this storm can't get over the hump.
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Thank you Dr. Masters. I saw you on TWC on Saturday night speaking about Isaac. Norcross said this blog was THE most popular weather sites on the internet. Looking at the satelitte, it does appear that Isaac has a ragged eyeball. But I think you meant eyewall. Have a good day.
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Thanks for update Dr. Good luck to NO.
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Typhoon Bolaven now affecting the southern coast of South Korea...
Expected to be near Seoul by 0600 UTC. Winds in Seoul are expected to be about 20 m/s, corresponding to about 38.8 kt (43.7 mph)
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"the Hurricane Hunters reported about half of a ragged eyeball" nice mistake DR. some much needed humor here in LA.
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Thanks Doc.
Member Since: September 27, 2007 Posts: 1 Comments: 22583
"Hurricane Hunters reported about half of a ragged eyeball"

Eyeball Mk1 recon is the best recon. Not sure how an eyeball could be ragged, though..
Member Since: August 3, 2008 Posts: 16 Comments: 5891

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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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