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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55kt SFMR winds with some 60kt flight-level winds in this data set.

204830 2555N 08529W 8434 01421 //// +213 //// 231055 056 055 002 05
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What are the models looking like. Any chance this turns more northward toward ms/ls coast or ms/al line?
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1606. flcanes
Quoting jascott1967:


Not to take away from Nea, sadly my fellow comrades sunk so far today that for one of us to say something smart and with humility and sincerity that one person looks like he should be awarded a Noble Peace Prize.

Nea's outlook and sensitivity for women and their feelings should be the rule not the exception.

( eyes tear up)
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Thanks for correction... scale is in knots

85knots = aprox. 98mph....

So the forecast is Cat1 or no more than 100 mph at landfall....

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Quoting jascott1967:


And humidity.
Incorrect being on West side of a tropical system all the Dry Air is brought down with Northeast Winds, it will be nice and cool in the mornings and hot and dry in the afternoons.
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Quoting TomballTXPride:

I see that. I was just going by the discussion and forecast points. Isn't that the official?

Yes it is official from NHC site.
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10 mph higher than last pass in SE side of Isaac... whoa.
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Still a TS 60 kt.
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Quoting hurricanejunky:
I'm surprised Isaac isn't a hurricane but this is a large windfield/low pressure system like Irene which had what seemed like very low windspeed for such low barometric pressure.


Isaac...Irene...and Ike...yep...

What's up with these recent 'I' storms that wanna be this way....LOL
Member Since: September 15, 2009 Posts: 473 Comments: 3668
Quoting TomballTXPride:
NHC predicting a 90 MPH hurricane. Smart move. Glad they are not backing down from that just because it has taken this storm a bit longer to get together. At least they are scientific as opposed to reacting with every ebb and flow of each frame of NEXRAD/IR.


Actually, they're predicting 100mph now.
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...and now we wait for the discussion. LOL
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Quoting Bluestorm5:


*clapping*


Not to take away from Nea, sadly my fellow comrades sunk so far today that for one of us to say something smart and with humility and sincerity that one person looks like he should be awarded a Noble Peace Prize.

Nea's outlook and sensitivity for women and their feelings should be the rule not the exception.
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1595. 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.
Member Since: August 31, 2011 Posts: 0 Comments: 70
This dude is about to take off...

Member Since: July 15, 2006 Posts: 17 Comments: 10244
Recon passing through the SE side and finding the same 70+ knot flight-level winds as it had in the rest of southern semi-circle. Compare these to the last SW pass around 90 minutes ago, where the highest flight-level winds were 55-60 knots. Isaac is strengthening very quickly. SFMR shows winds haven't got down to the surface yet, but they will, and soon.

Would at all be suprised to see sub-980 on this pass.
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Isaac should be a hurricane before the night is out, and should peak as a Category 2 hurricane tomorrow afternoon. If it undergoes rapid intensification, a minimal Category 3 is possible.
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1590. flcanes
Quoting sar2401:


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

yeah, dry air is eating up his east side
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1588. barbamz
Quoting Hoff511:


Thanks for the concern, but no, it was not that either, unless my house got struck 10 time today.


Perhaps a rare ball lightning?
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Quoting Levi32:
Coolest loop in the world right now, set to fill the screen of a 15-inch laptop. You can play with the URL parameters.


amazing.
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1586. 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.
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Quoting TomballTXPride:
NHC predicting a 90 MPH hurricane. Smart move. Glad they are not backing down from that just because it has taken this storm a bit longer to get together. At least they are scientific as opposed to reacting with every ebb and flow or each frame or NEXRAD or IR.

100 MPH again!!
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Quoting TropicalAnalystwx13:

Stewart wrote this.
Then you know that there isn't sufficient data to warrant an upgrade.

Without a doubt we'll have a hurricane by 11p.m though.
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1582. bappit
Quoting kctinney:
I'm a "lay person" and I have been here lurking since 2005 and haven't posted much at all but the only other time I had a gut feeling that a storm was going to be more than what ppl were anticipating 3 days out to landfall was Katrina. That is how I found this particular website because I couldn't understand why Katrina at the time was being so downplayed. HOWEVER, I didn't become a memnber until 2009. I posted earlier today that regardless whatever the storm is classified as, it is going to be bad d/t how big it is and the amount of rain/moisture and NOBODY should downplay this storm!
]
Who downplayed Katrina?????
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(comes back on the blog).Oh lookey what we have here?.Good news for the gulf with this one.Rain will be a issue though.I'm sure some of the mid-west states wouldn't complain if Isaac could give them some rain.
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Quoting Thing342:
I wonder what the blog's reaction would be if the NHC doesn't make Isaac a hurricane.
The typical closet forecaster wannabees will think they know more than the NHC.
The level headed members will watch and wait.
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1579. flcanes
Quoting will40:
000
WTNT34 KNHC 272049
TCPAT4

BULLETIN
TROPICAL STORM ISAAC ADVISORY NUMBER 27
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
400 PM CDT MON AUG 27 2012

...RECONNAISSANCE AIRCRAFT INDICATES ISAAC GETTING STRONGER...
SIGNIFICANT STORM SURGE THREAT EXPECTED FOR THE NORTHERN GULF
COAST...

how strong?
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1578. sar2401
Quoting Elena85Vet:


L. Eye Character: Open in the north
M. Eye Shape & Diameter: Circular with a diameter of 20 nautical miles (23 statute miles)


That's actually quite an accomplishment for Isaac during the daytime. Nighttime has generally been Isaac's friend for organization, only to lose what he gained when the sun comes up. I'm thinking he finally achieves Cat 1 hurricane status sometime tonight..


It sure wasn't last night. He looked worse than any time since getting into the Gulf at about 0300 CST.
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Quoting LargoFl:
SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE TAMPA BAY AREA - RUSKIN FL
355 PM EDT MON AUG 27 2012

FLZ050-051-055-272045-
HILLSBOROUGH-PINELLAS-MANATEE-
355 PM EDT MON AUG 27 2012

...A SIGNIFICANT WEATHER ADVISORY HAS BEEN ISSUED FOR EAST CENTRAL
PINELLAS...NORTHEASTERN MANATEE AND WESTERN HILLSBOROUGH COUNTIES FOR
A STRONG THUNDERSTORM WITH STRONG WIND GUSTS VALID UNTIL 445 PM
EDT...

AT 351 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATES A
STRONG THUNDERSTORM LOCATED NEAR DUETTE...OR 22 MILES WEST OF
WAUCHULA...MOVING NORTHWEST AT 40 MPH WILL AFFECT DUETTE...LITTLE
MANATEE RIVER STATE PARK...SUN CITY CENTER AND WIMAUMA.

THIS INCLUDES INTERSTATE 75 BETWEEN EXITS 240 AND 256.

THIS INCLUDES INTERSTATE 275 BETWEEN EXITS 39 AND 52.

GUSTY WINDS OF 45 TO 55 MPH CAN BE EXPECTED WHICH CAN CAUSE UNSECURED
OBJECTS TO BLOW AROUND...SNAP TREE LIMBS AND CAUSE POWER OUTAGES.
TORRENTIAL RAINS WILL REDUCE VISIBILITY TO NEAR ZERO AND WILL CAUSE
PONDING OF WATER ON ROADWAYS.


&&


Thank you. Put the lawn furniture in to the pool.
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1576. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 38436
I'm surprised Isaac isn't a hurricane but this is a large windfield/low pressure system like Irene which had what seemed like very low windspeed for such low barometric pressure.
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Quoting catastropheadjuster:



I know I have heard and seen about 10 different trains. My husband said he listens and if they spend all that time and money to move them something is up. I guess they might be worried about the storm surge. We seen a huge ship leaving out yesterday and you could tell they where still working on it or something but they where getting it out of there.

sheri


Yep. I live near some BNSF tracks, and my neighbors across the street both work for the BNSF... when a storm even thinks of threatening, they empty out galveston.
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Quoting TomballTXPride:
NHC predicting a 90 MPH hurricane. Smart move. Glad they are not backing down from that just because it has taken this storm a bit longer to get together. At least they are scientific as opposed to reacting with every ebb and flow or each frame or NEXRAD or IR.

They are predicting 100mph not 90
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Quoting auburnguy:
For those leaving nola.. Traffic on i-10 east in nola east is backed up due to a shooting.. Take an alternate route if possible


It has started.
Member Since: September 18, 2007 Posts: 0 Comments: 1074
1570. flcanes
Quoting WALMARTPROFITSAGAIN:
wow what a storm again... so far since last monday theres been 58 track changes and 18 different landfalls predicted and now they will use this storm to increase gas prices because of the oil rigs are being affected in the gulf someone is making a fortune off this storm

who, nobody is in the gulf
other than maybe some clams
do clams make money?
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NHC trying to make Dr. Masters eat his words that it won't be a cat 2...well the nhc says it will again! LOL!
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The pressure is at 981mb, one deep storm.
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Tweet form Dr. Knabb:

@NHCDirector

Large size and slow fwd motion of #Isaac as it comes ashore, regardless wind intensity, means big storm surge, inland flood threat
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1566. will40
000
WTNT34 KNHC 272049
TCPAT4

BULLETIN
TROPICAL STORM ISAAC ADVISORY NUMBER 27
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
400 PM CDT MON AUG 27 2012

...RECONNAISSANCE AIRCRAFT INDICATES ISAAC GETTING STRONGER...
SIGNIFICANT STORM SURGE THREAT EXPECTED FOR THE NORTHERN GULF
COAST...
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Quoting TropicalAnalystwx13:
The NHC is waiting for surface winds to come up slightly before declaring this a hurricane.


HH coming up for another center fix. If the pressure is down again or even holding steady we should have Cat 1 winds sustained for 8 PM. The weak NE quad still poses a risk to the system but if the eyewall closes off on the North the winds should come up fairly quickly. The eye may still be open to the N on the next pass based upon available satellite presentation but real time may be different.
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Link
Member Since: August 18, 2008 Posts: 0 Comments: 469
Quoting trHUrrIXC5MMX:
PRESENT MOVEMENT TOWARD THE NORTHWEST OR 305 DEGREES AT 10 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 981 MB
MAX SUSTAINED WINDS 60 KT WITH GUSTS TO 75 KT.
50 KT....... 50NE 15SE 15SW 50NW.
34 KT.......150NE 150SE 80SW 180NW.
12 FT SEAS..180NE 180SE 180SW 150NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

NO HURRICANE!!!!

Yeah!!!

I wonder why some folks are so frustrated. I wouldn't want a storm to bomb out on my worst enemy....right? Maybe if this was over open waters out of shipping lanes its worth cheering on...but not this one IMO...but that's just me...

He's definelty an Ike-type kina dude...large and broad with more lax pressure gradient and no tight core. Slow strengthening...and lower than normal pressures are required to get the same amount of wind...Dr. M called this out yesterday....
Member Since: September 15, 2009 Posts: 473 Comments: 3668
Seriously? NHC being too tight vested here.
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Quoting CosmicEvents:
lol...you think so?
He needs to loosen up but it's tough to be thrown into the mix in the middle of a weather event like this.
.
Do appreciate his level headed opinion as always...much preferred over the hopesters and hypesters.


Brian kept throwing him softballs, and all he did was agree with him. With time, he will do fine.
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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.