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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Oh great my generator want crank. I really hope that I live far enough in not to lose power.
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Quoting Felix2007:


They showed the same exact thing with Isaac a week ago and we all know how that turned out.
It's been flip flopping between a recurve on the East Coast or a GOM storm.
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20+ inch now!!! wow
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Still looks like a replay.
Kirk at 228 hrs.

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Tornado warnings continue about 30 miles to my south (here at zip code 32934 - Melbourne, FL)... keep watching the warnings progress northward, and this next round is probably going to be HEAVY....

Been losing power off and on throughout the day. I'm here on the blog while possible, so I appreciate the warnings being posted - I might just happen to see it here before anywhere else.

*fingers crossed for friends in the gulf*
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Quoting RitaEvac:


Not me, her over there
..sorry i meant for the folks in that flood pic
Member Since: August 6, 2011 Posts: 4 Comments: 37044
252. 7544
Quoting leelee75k:
Is it me or does the blob over Florida seem to not to want to follow Isaac into the gulf?
I noticed that this morning..one on the east coast and one off the west coast..


its issacs tail and begining to fillin imo and moving north so lots more rain for fl east coast today but could the tail get stonger as it fills in just north of cuba anyone know do tails get stroger as isaac does thanks and stay safe everyone good luck to all on the gulf coast
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Quoting Felix2007:


They showed the same exact thing with Isaac a week ago and we all know how that turned out.


Yep, that many days out about anything could happen.
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SFC MAP


The shortwave trough has helped to erode the northern edge of the conus ridge with the central part of the high retreating northeastwards. The trough seems to be deepening a bit, extending a bit further south in to eastern Oklahoma. However, it's still leaving that fairly strong (1016mb) bubble, tongue of high pressure underneath it. And, that is what is driving the models nuts because the western edge of that tongue has flopped east and west repeatedly.

Currently, it's extending a bit further to the west at about the Galveston/Houston area. I don't think that will hold up, but its presence is evidenced by the currently trending more westwards movement of Isaac. I think that will continue for a bit and may allow the storm to traverse more of the warm loop eddy. Eventually, I believe the western periphery of the ridge will retreat, allowing Isaac an escape route inland. Where, I've said all along, for several days now at Mud Lake just swest of Grand Chenier in central coastal LA. But, it's a crap shoot really, since it's awfully difficult to determine exactly where the ridge recedes and exactly how strong Isaac intensifies to push harder against the ridge towards that somewhat deepening weakness of the trough. Probably Lake Charles to even Mobile is still well in play in the present.
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Quoting TreasureCoastFl:
Thanks!Definitely worst Ive seen yet with tons of lightening and thunder.
im seeing what is happening to you folks over there..this might be coming over to us on the west coast later on..and im thinking of those fools who keep saying..its ONLY a tropical storm...Flooding can be more devastating then wind when it comes in excess like this huh, hope you get thru this ok over there..my turn tonight for all that
Member Since: August 6, 2011 Posts: 4 Comments: 37044
Quoting LargoFl:
..no storm drains there by you ??


Not me, her over there
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Quoting floridaboy14:
12z GFS has hurricane irene all over again


They showed the same exact thing with Isaac a week ago and we all know how that turned out.
Member Since: July 12, 2008 Posts: 7 Comments: 383
I hope everybody in the path stays safe but please please please let that rainfall forcast for Illinois come true.
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GFS 114 hrs.

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Isaac may not be the most organized looking storm but it just looks so amazing for how large the range of effect it will have for the Gulf coastal regions! I still think Isaac has a small opportunity to go under rapid intensification.
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Expecting to hear within an hour whether NASP will close tomorrow and Wednesday. I will post if anyone's interested.

Also, my sister and her boyfriend work for an area marina. The owner took all their very large vessels (including a pirate ship!) waaayyy up Mobile River early Sunday morning. Not taking any chances.
Be safe, ya'll.
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Quoting WhoDat42:


ouch!!!

Yeah...I'm in the 400 block area ...9 feet above lake level...house raised 3 feet....kinda need to know
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Is Isaac shedding that mass of convection over eastern FL? Will that enable him to finally get his house in order, so to speak?
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Quoting HoustonTxGal:


You forgot one going to Alaska...LOL!!
And then from Alaska to Hawaii as cat.5
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Isaac been jogging more west last few frames.... Long term motion still WNW to NW.
Member Since: July 4, 2005 Posts: 204 Comments: 8806
12z GFS has hurricane irene all over again
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Quoting StormPro:
Really confused...NHC shows a slosh map that shows a possible 12 surge at Mandeville...NWS shows 5-8....
thoughts on a good source to actually see what to expect?


ouch!!!
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Did you know that they guy who played "Isaac" on the Love Boat was recently in New Orleans this summer...I think 3 weeks ago. Strangely coincidental!
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Quoting LargoFl:
stay safe over there, looks like the heavy bands are training up from the south
Thanks!Definitely worst Ive seen yet with tons of lightening and thunder.
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1142 AM EDT MON AUG 27 2012

...A FLASH FLOOD WARNING REMAINS IN EFFECT UNTIL 200 PM EDT FOR
CENTRAL PALM BEACH AND EASTERN BROWARD COUNTIES...

AT 1131 AM EDT...THE MEDIA REPORTED SIGNIFICANT FLOODING IN
LAUDERHILL AND PINE BOULEVARD IN BROWARD COUNTY. IN
ADDITION...STREET FLOODING IN THE AREAS WITH POOR DRAINAGE HAS ALSO
BEEN REPORTED IN BROWARD. THE LATEST RADAR TRENDS INDICATED THIS
PERSISTENT BAND OF HEAVY RAIN SLIGHT SHIFTING WEST AWAY FROM THE
IMMEDIATE COAST. AS A RESULT...THE WARNING COVERAGE AREA HAS BEEN
ADJUSTED ACCORDINGLY.

LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO NORTH COUNTY
AIRPORT...LOXAHATCHEE NATIONAL WILDLIFE REFUGE...WELLINGTON...ROYAL
PALM BEACH AND LION COUNTRY SAFARI
Member Since: August 6, 2011 Posts: 4 Comments: 37044
Quoting nrtiwlnvragn:


Not surface, 900 mb level.


A 966 mb pressure on the 900 mb level?

You can't have varying pressures on a constant pressure surface! :P
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Member Since: July 4, 2005 Posts: 204 Comments: 8806
Quoting TreasureCoastFl:
OMG I got power back now. What the hell?
stay safe over there, looks like the heavy bands are training up from the south
Member Since: August 6, 2011 Posts: 4 Comments: 37044
Really confused...NHC shows a slosh map that shows a possible 12 surge at Mandeville...NWS shows 5-8....
thoughts on a good source to actually see what to expect?
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Quoting weatherbeginner:
This might be a stupid question. Is it possible for a tropical system to spawn another tropical system?

Closest thing I can think of is Matthew and Nicole in 2010...

An upper trough increased divergence to the east of Matthew...resulting in surface pressure falls to the east of him that became a broad area of low pressure in Matthew's wake. That broad low became Nicole....
Member Since: September 15, 2009 Posts: 445 Comments: 3622
Bad time to be out on I-75 aka The Sawgrass Expwy....It's coming down in sheets of torrential rain right now!! rainfall rates are probably 3+ an hour in this band!
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Quoting LargoFl:
boy you folks are in for it, still coming up the coast...
OMG I got power back now. What the hell?
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Quoting MississippiWx:


Nope, 966.

Link


Not surface, 900 mb level.
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boy you folks are in for it, still coming up the coast...
Member Since: August 6, 2011 Posts: 4 Comments: 37044
This might be a stupid question. Is it possible for a tropical system to spawn another tropical system?
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Quoting DestinDave:
wow! where & when?
Last night palm beach county.
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WNW movement
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Member Since: August 6, 2011 Posts: 4 Comments: 37044
it looks to me like the dry air is getting cut off and issac is moistening the atmosphere. anyone want to elaborate on this? may i also ask if there is anyone who can just use their own opinion in answering if issac might still reach cat. 2 strength? tia!
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Quoting KEEPEROFTHEGATE:
mmm very dry air moving in!
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find all the latest tropical maps/info here at my blog page

Link
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Quoting seflagamma:


See that dark blob of rain is right over my area..

we cannot get a break here...

and what about something forming off the east coast???


I'm right with you under that blob, if I'm not mistaken, I think the swath of area from Sawgrass Malls down to you (Sunrise, Tamarac, Lauderhill) has received between 8-10 inches of rain since this event started, and it's still coming down.
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Quoting RitaEvac:


They need a boat to get out. Kayaking anyone?
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Quoting unknowncomic:
Video of Isaac feeder band storm. Basketball hoop almost
lands on car.
Link
wow! where & when?
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Quoting louisianaweatherguy:
Isaac not real impressive - will this lessen storm surge threat? or does the surge happen regardless?


The threat is not lessened in this case. Isaac still has a large area of winds moving an equally large area of water. The surge threat is very present...and depending on who's talking, perhaps longer lasting.
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Quoting Felix2007:

986mb, not 966.


Nope, 966.

Link
Member Since: July 15, 2006 Posts: 17 Comments: 10160
That will push water right up the bay like a funnel.
Quoting Bobbyweather:
Hmm... Because of Isaac no one's listening to me.
I hate you, Isaac (lol).

Typhoon Bolaven as well as Typhoon Tembin (TS by the time it gets here) is expected to impact the Korean peninsula.

Typhoon Bolaven forecast track:


Typhoon Tembin forecast track:


Bolaven is a big typhoon, with gale-force winds extending to 500km.

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Quoting HoustonTxGal:


I mentioned last night that about this latitude we could see Isaac move a bit west for a period before turning back wnw and then nw, etc.
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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.