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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Quoting AlRokerROCKS:


Please don't pimp your blog, especially when you don't have anything new to contribute


Normally my blogs are more extesnsive with custom birdseye view charts....like in this FULL update....

Those charts in the FULL update are something unique I create to help show bloggers how the atmoshpere is setup...and I write a FULL discussion...so hopefully unlike the SHORT statements, that does add to the knowledge here.

The update I just released is just a SHORT statement...because there are some things I said in my previous FULL update that have changed. If these SHORT statements are annoying to y'all...then I guess I can stop announcing them on this blog...

Member Since: September 15, 2009 Posts: 440 Comments: 3607
Quoting Megladon:


Heavy squally rain, thunder, dangerous lightning and tornado warnings here in Central Florida.

Is this blob over Florida still a part of Issac or can it be considered an orphan. A nasty, bratty one at that.
I dont know what it is, but definitely got worse real quick!
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Quoting leelee75k:


just saw your pic, wow!! nothing like that here in my streets, but if you step in the backyard, the standing water suddenly reaches past your ankles. I have ponding going on in my yard. Pretty nasty band over us right now too. Very fat heavy rain drops!!


Thanks for going to look at it; I cannot post it here for some reason..

our back yard is under water.. the water level is above the pool...

my neighbor's car is about to get water in it from driveway..

No regular vehicles can get down the street...
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The GFS has another storm in the eastern caribbean in 7 days.
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12z LOOP - Link

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Quoting TreasureCoastFl:


Had a power surge, back on now. Big booms of thunder and tornado warnings here in St Lucie.


Heavy squally rain, thunder, dangerous lightning and tornado warnings here in Central Florida.

Is this blob over Florida still a part of Issac or can it be considered an orphan. A nasty, bratty one at that.
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watch up to blue now over there....
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Quoting NCHurricane2009:

Yep...another thing I just stated in my Isaac update I just released...

"On an interesting note...despite the unusually high number of Atlantic storms so far this year...none have succeeded in become a major hurricane (115+ mph max winds)."

If you divided the total ACE number by the number of storms...I wonder if 2012 is at a record low...

I think Gordon may have been a Cat 3 for about 3 hours in between advisories.
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thanks for update doc good read
this one could of been worse
still a high rain total event
maybe make cat 1 maybe
its all good
don't want a monster anyway
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Quoting A4Guy:
I know all eyes are onthe Gulf Coast now...but I can't believe the squalls we are still getting in Broward County. Winds are as bad as any have been so far, and the rains are still torrential. They don't last as long as the ones from yesterday and last night...bit I am just amazed at the winds. Seems like the NHC windfield map needs to be bigger!! Maybe b/c the winds are gusts, and not sustained is why we are no longer in the windfield.
Yep, We are getting the worst we have gotten yet here in St Lucie.
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TWC is reporting latest air force recon has found 80 MPH surface winds NE Of the COC.
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guys there may yet be something trying to form off the east coast of florida..nws has a watch area for it..be careful over there.............
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Quoting Charliesgirl:


My husband has golfed during many a tropical storm, lol. usually when they do not amount to much of anything.



I played yesterday afternoon in Brooksville Fl, it was howling 25mph for the whole round, got to 18 and it started raining side ways.
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144. Skyepony (Mod)
Looking at model intensity error (scroll to bottom). of these models GFDI is doing best with average error in kts 0hr, 24hrs, 48hr & etc.

GFDI 0 6.9 7.2 11.0 12.3 13.1

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With the current NW movement, would the cone shift back east? I am in mobile Alabama, just wanted opinions on if the cone will shift?

Also since it is moving NW. Would this be more in line with the European models?
Member Since: August 3, 2012 Posts: 0 Comments: 45
Quoting TomballTXPride:
Interesting.



It looks like an eye is starting to clear out, but I highly doubt it based on Recon data and DVORAK estimates. Looks are deceiving.

LLC is likely east of the MLC and CDO.


on the image you posted it looks as if Isaac has left quite a large part of its convection over FL. I'm not 100% sure if it's still a part of the circulation or if it's left behind. On the animation on WU's Isaac page it seems as if it's left behind. Do you think Isaac is shedding 'excess baggage' in order to spin up faster?
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Here's hoping that this tracks more to the west like GFDL says it will. It would be better for NO, better for the midwest (who needs the rain) and better for me in upstate NY where I don't want rain this weekend and especially not three inches of the stuff.
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Quoting Neapolitan:
The entire 2012 Atlantic season has only amassed 31.935 ACE units...

Yep...another thing I just stated in my Isaac update I just released...

"On an interesting note...despite the unusually high number of Atlantic storms so far this year...none have succeeded in become a major hurricane (115+ mph max winds)."

If you divided the total ACE number by the number of storms...I wonder if 2012 is at a record low...

Member Since: September 15, 2009 Posts: 440 Comments: 3607
Quoting seflagamma:


When we get a few hours break inbetween like we did yesterday afternoon we will drain some..but when it comes consistently like it has been it does not have time to drain.

I took a picture and got it loaded but don't know how to get it here on this blog. will try..

I got it on my blog in header.


just saw your pic, wow!! nothing like that here in my streets, but if you step in the backyard, the standing water suddenly reaches past your ankles. I have ponding going on in my yard. Pretty nasty band over us right now too. Very fat heavy rain drops!!
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138. A4Guy
I know all eyes are onthe Gulf Coast now...but I can't believe the squalls we are still getting in Broward County. Winds are as bad as any have been so far, and the rains are still torrential. They don't last as long as the ones from yesterday and last night...bit I am just amazed at the winds. Seems like the NHC windfield map needs to be bigger!! Maybe b/c the winds are gusts, and not sustained is why we are no longer in the windfield.
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Quoting bucyouup68:
Sitting in St Pete, Winds have picked up again and starting to rain. Hold on for this afternoon.
yes later on its our turn here
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What strikes me as an interesting analog observation is that the cone shifted West with Katrina as well back in 2005; originally slated as a Florida storm and the cone shifted towards LA in the span of a few days.

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Sitting in St Pete, Winds have picked up again and starting to rain. Hold on for this afternoon.
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its gonna rain maybe a little wind with some dark clouds moving real fast


lol

hows everyone today just in on lunch

morning shift is done

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Quoting leftlink:
Here you go: in last 20 minutes the "funktop" view of the storm looks so much more organized... as the convection of the storm is believe it or not just an hour or two from the SE tip of Louisiana.



Yep...this is what I had also stated in my new update on Isaac...

"......With this information...we are highly certain that the center of Isaac should make landfall near Buras, Louisiana (in the SE corner of the state) sometime Tuesday afternoon...with rainfall and gusty winds arriving well-in-advance of that time due to the large size of the storm."
Member Since: September 15, 2009 Posts: 440 Comments: 3607
Quoting MississippiWx:


I believe you are seeing it correctly. Isaac has shown this a few times, but it collapsed each time. Let's see if he can hold onto it without dry air killing it again.

looks almost like there are 2 storms..one in the gulf, one over on the east coast of florida
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Quoting Skyepony:
Isaac ace so far is 5.43.

Bolaven has amassed an ace of 34.5975...
The entire 2012 Atlantic season has only amassed 31.935 ACE units...
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Good afternoon.

Blog update:

Tropical Tidbit for Monday, August 27th, with Video
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Quoting TomballTXPride:
Interesting.



It looks like an eye is starting to clear out, but I highly doubt it based on Recon data and DVORAK estimates. Looks are deceiving.

LLC is likely east of the MLC and CDO.


I believe you are seeing it correctly. Isaac has shown this a few times, but it collapsed each time. Let's see if he can hold onto it without dry air killing it again.

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I just did a special update on Isaac...I think he's going to make landfall over or very near Buras, LA (at the SE corner of the state) Tuesaday....

Basically my update says some things that Dr. M also says. The cool thing is I wrote that update statement before I saw Dr. M's...so it looks like I have learned quiet a bit over the years from my meteo minor and this blog....
Member Since: September 15, 2009 Posts: 440 Comments: 3607
124. Skyepony (Mod)
Gamma~ Here's hoping the flood stays out of your house!



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Longtime lurker.... GFS Tightened Up to east slightly. Bottoms out at 985mb.. Would it make it to the 90MPH forecast?
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Quoting HTownJitters:
Big pipeline of dry air mixing in from over Cuba which seems to have some staying power...it's going to limit Isaac's ability to strengthen.

This storm has been battling dry air intrusion throughout it's journey......big time development inhibitor
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Station 42003
NDBC
Location: 26.044N 85.612W
Date: Mon, 27 Aug 2012 14:50:00 UTC
Winds: N (360°) at 27.2 kt gusting to 36.9 kt
Significant Wave Height: 17.1 ft
Dominant Wave Period: 10 sec
Mean Wave Direction: ENE (58°)
Atmospheric Pressure: 29.21 in and falling rapidly
Air Temperature: 81.5 F
Water Temperature: 85.1 F
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11:41 GMT this morning
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12z GFS is a good bit further east after landfall. No longer goes into Texas. Landfall is around Buras, Louisiana.
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Quoting leelee75k:


Hope it dries out soon for you Gamma. I'm west of you and got similar rainfall rates, but no problems in my neighborhhood, we're draining properly.


When we get a few hours break inbetween like we did yesterday afternoon we will drain some..but when it comes consistently like it has been it does not have time to drain.

I took a picture and got it loaded but don't know how to get it here on this blog. will try..

I got it on my blog in header.
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Quoting waterskiman:
I like this map because it puts warning boxes up in real time

...looking at this map..guys is it possible..those bad storm bands never make it to the west coast of florida?
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Quoting Waltanater:
Thanks, but no where does it say URL. It just says Folder path. Note: this is a file on my hard drive, not on the web.


Oh nvm

You'll have to upload the image to a different source first.
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Quoting Waltanater:

LOL...does the image have to be in a particular format?

Copy the entire link to the image....
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Quoting seflagamma:
I took pictures with my cell phone, street is probably 10" deep, water coming into our garage now..

will try to get these picture on here ...or my blog anyway...

I don't think any other area has got the rain we have received..

this is about as serious as it gets except it is not in our house yet..
but getting close.


Hope it dries out soon for you Gamma. I'm west of you and got similar rainfall rates, but no problems in my neighborhhood, we're draining properly.
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Big pipeline of dry air mixing in from over Cuba which seems to have some staying power...it's going to limit Isaac's ability to strengthen.
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Quoting Waltanater:
Thanks...I wonder why you can't directly upload a pic that is on your local machine! Wierd.


You could alway upload it to Wunder Photos, and then link that. haha
Why didn't I think of that myself?
Derp.
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Here you go: in last 20 minutes the "funktop" view of the storm looks so much more organized... as the convection of the storm is believe it or not just an hour or two from the SE tip of Louisiana.

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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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