Isaac slamming Gulf Coast with damaging floods, tornadoes

By: Dr. Jeff Masters , 4:44 PM GMT on August 30, 2012

Slow-moving Tropical Storm Isaac continues to hammer coastal Louisiana, Mississippi, Alabama, and Florida's Panhandle with tornadoes, torrential rains, high winds, and a damaging storm surge. Over the past 24 hours, destructive tornadoes have touched down in Biloxi and Pascagoula, Mississippi, and one person was killed by a tree falling on a car in Pearl River County, Mississippi. A major flood event is occurring in Slidell, Louisiana, where Isaac's storm surge filled Bayou Bonfouca and the W-14 Canal, inundating portions of the city with 1 - 5 feet of water. While Isaac is now a weakening minimal-strength tropical storm, it is still a potent rainmaker, and will cause damaging floods all along its path for the next three days. Major river flooding is occurring or is about to occur on a number of rivers in the landfall area. In north central Tangipahoa Parish in southeast Louisiana and southwestern Pike County in southern Mississippi, a mandatory evacuation has been ordered for all low-lying areas and along the Tangipahoa River, due to the potential failure of the Lake Tangipahoa dam. Audubon Park in New Orleans, recorded 11.19" of rain as of 7 pm Wednesday night. An earlier amount of 19" was found to be erroneous, and this is not a 24-hour precipitation record for the city. According to wunderground's weather historian, Christopher C. Burt, New Orleans' greatest 24-hour rainfall on record is 14.01" on July 24 - 25, 1933. The Louisiana official state 24-hour record is 22.00" on Aug. 29, 1962 at Hakberry, although U.S. Army Corps of Engineers `Storm Studies' mentions a 23.80" falling in a 24-hour period at Millers Island during a TS on Aug 7-8, 1940. Storm total was 37.50" over a 60-hour period there during that event.

A few other rainfall totals from Isaac, through 11 am EDT on Thursday:

15.02" Marion, MS
10.09" Hattiesburg, MS
10.15" Gulfport, MS
9.80" Slidell, LA
9.74" Biloxi, MS
8.52" Mobile, AL
5.57" Baton Rouge, LA


Figure 1. Isaac's winds and storm surge overcomes the seawall and floods South Beach Boulevard in Waveland, Miss., Wednesday, Aug. 29, 2012, the seventh anniversary of Hurricane Katrina hitting the Gulf Coast. (AP Photo/Rogelio V. Solis). Waveland experienced a storm surge in excess of 5' for 36 hours.

Isaac's storm surge winds down
Storm surge levels along the coast of Mississippi and surrounding areas are gradually receding, and the surge has finally fallen below 5' at Waveland, which experienced a storm surge in excess of 5' for 36 hours. Isaac's storm surge levels were characteristic of a Category 2 hurricane, and lasted for an exceptionally long period of time. Waveland, Mississippi experienced a peak surge of 8' and peak storm tide of 9' (surge plus the natural high tide), which beat the levels that occurred during Category 2 Hurricane Gustav of 2008 (7' of storm tide.) The peak 11.06' storm surge at Shell Beach, which is in Lake Borgne, 30 miles southeast of New Orleans, exceeded the 9.5' surge recorded there during Gustav. According to an article in nola.com, Isaac pushed a storm surge of 13.6' into Lake Borgne, on the east side of New Orleans. This is not far from the 15.5' storm surge Hurricane Katrina brought to the location. It is quite possible that Isaac's storm surge might have breached levees of the east side of New Orleans, flooding areas inhabited by tens of thousands of people, had the Army Corps of Engineers not completed their $14.5 billion upgrade to the New Orleans flood defenses this year. I estimate that storm surge damage from Isaac will exceed $2 billion. Isaac has likely caused $2.5 billion in insured damage not related to flooding, insurance firm Eqecat estimated yesterday. Here were some of the peak storm surge values that were recorded at NOAA tide gauges during Isaac:

11.1' Shell Beach, LA
8.0' Waveland, MS
3.5' Pensacola, FL
4.6' Pascagoula, MS
3.8' Mobile, AL


Figure 2. A TRMM satellite 3-D view of rainfall on Aug. 28 showed a few very powerful thunderstorms near Isaac's eye were reaching heights of almost 17 km (10.6 miles.) Intense bands of rain around Isaac were occasionally dropping rain at a rate of over 2.75 inches per hour. Image credit: SSAI/NASA, Hal Pierce.

Isaac's storm surge on the Mississippi River
A storm surge estimated at 12' moved up the Mississippi in Plaquemines Parish near Port Sulphur, LA, near 8:30 pm EDT Tuesday, causing overtopping of the levees and flooding of homes in the mandatory evacuation areas behind the levees. These levees were not part of the $14.5 billion levee upgrade New Orleans got after Hurricane Katrina, and were not rated to Category 3 hurricane strength, like the levees protecting New Orleans are. Since salt water is more dense than fresh water, the surge travelled along the bottom of the river, with the fresh water flow of the river lying on top. The surge continued upriver, and before reaching New Orleans, encountered an underwater barrier in Plaquemines Parish. This barrier was constructed by the Army Corps of Engineers beginning on August 15, in order to keep salt water from moving upstream and contaminating drinking water for Plaquemines Parish and New Orleans. Salt water had made it 90 miles upriver to the outskirts of New Orleans, due to the low flow rate of the river (which had dropped 7' below average in height due to the drought of 2012.) According to a spokesperson for the National Weather Service River Forecast Office, this barrier was probably able to completely block the flow of salt water upriver due to Isaac's storm surge, and no salt water made it as far as New Orleans. However, the massive intrusion of ocean water into the river channel caused the mighty Mississippi's fresh water flow to back up for hundreds of miles. Water levels were elevated by 10' in New Orleans (103 miles upstream from the mouth of the Mississippi), 8' in Baton Rouge (228 miles upstream), and 1.4' at Knox Landing, an amazing 314 miles upstream.

Hurricane Kirk in the Central Atlantic
Hurricane Kirk intensified into a 75 mph Category 1 hurricane this morning, becoming the busy 2012 Atlantic hurricane season's fifth hurricane. With the season's mid-point of September 10 still almost 2 weeks away, we've already had 12 named storms and 5 hurricanes, which is close to what an entire season experiences in an average year (11 named storms and 6 hurricanes.) Kirk should stay well out to sea and not trouble any land areas.


Figure 3. Morning satellite image of Hurricane Kirk.

Tropical Storm Leslie forms in the Central Atlantic
Tropical Storm Leslie has formed in the Central Atlantic. Leslie's formation on August 30 puts 2012 in 2nd place for earliest formation date of the season's 12th storm. Only 1995 had an earlier formation date of the season's 12th storm. With records dating back to 1851, this year is only the second time 8 total storms have formed in August. The other year was 2004, when the first storm of the season formed on August 1 (Alex), and the 8th storm (Hermine)
formed on August 29th. Leslie is organizing quickly, and appears destined to become a hurricane before the week is out. Fortunately, Hurricane Kirk is weakening the ridge of high pressure to the north of Leslie, and Leslie is expected to turn to the northwest and miss the Lesser Antilles Islands. In the long term, it remains unclear if Leslie will follow Kirk and fully recurve out to sea. The latest 2 runs of the GFS model have predicted that Leslie will recurve out to sea and not threaten any land areas, but the latest 2 runs of the ECMWF model have predicted that the trough of low pressure pulling Kirk to the northeast will not be strong enough to recurve Leslie out to sea. Instead, the ECMWF predicts that a ridge of high pressure will build in early next week, forcing Leslie more to the northwest, making the storm a potential threat to Bermuda, then to the Northeast U.S. and Canada in 8 - 11 days.

Jeff Masters

Hurricane Isaac Louisiana (apphotos)
Two men walk in the storm surge from Isaac, on Lakeshore Drive along Lake Pontchartrain, as the storm approaches landfall, in New Orleans, Tuesday, Aug. 28, 2012. (AP Photo/Gerald Herbert)
Hurricane Isaac Louisiana
west palm beach flood isaac (alishu)
West Palm Beach flood from Isaac
west palm beach flood isaac
Hurricane Isaac Impacts Navarre Beach & Pier10 (jennjeff1)
Hurricane Isaac versus Navarre Beach Pier, the longest concrete pier on the Gulf of Mexico
Hurricane Isaac Impacts Navarre Beach & Pier10

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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Quoting SouthDadeFish:
It's incredibly obvious TO ME that this is not a tropical depression:



More like a 50-60mph TS.


fixed that for ya
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Quoting Twisterman555:


I wonder what is going on here. . . . can this be right?

looks like the sensor died a horrible death. I mean it is 2012...the water could've hit 90 feet if say, a meteorite hit somewhere on the river... but otherwise...no.
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50 to 60 thousand need to be evacuated because the damn is letting go AND it is going to Lake Pontchartrain
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It's incredibly obvious that this is not a tropical depression:



More like a 50-60mph TS.
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34. MTWX
Quoting Twisterman555:


I wonder what is going on here. . . . can this be right?


Sure hope not!! Think the sensor on the gage may have just broke...
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Quoting VR46L:
Kirk , looking quite the dapper Captain



and in Rainbow



Looking at him, I wonder if he could somehow pull out a Major status.
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Thank you Dr. Masters
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I wonder what is going on here. . . . can this be right?
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Has anyone checked to see if the Chandeleur Islands still exist? After Katrina, their disappearance has been quite accelerated.
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Quoting HopquickSteve:



There you go....that's why politicians are not emergency managers.
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Kirk is the cutest hurricane in awhile. xD



And to think the original disturbance was huge.
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Quoting HopquickSteve:
Gov. says intact, but might blow it anyway?


Sounded to me in the press conf like they're going to try an intentional breach _away_ from the river to reduce pressure on the dam.

Which is probably a good idea if possible.

Dam is badly damaged but not fully breached yet. MS is not treating it as mandatory evac, LA is. A lot is still fairly unclear.
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Kirk , looking quite the dapper Captain



and in Rainbow

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The CIMSS/ADT model really likes both cyclones currently churning in the Atlantic. Here's the read on Kirk:

----- Current Analysis -----
Date : 30 AUG 2012 Time : 154500 UTC
Lat : 27:17:51 N Lon : 49:34:31

CI# /Pressure/ Vmax
4.4 / 983.5mb/ 74.6kt

And it's hinting that 5pm will bring us strong TS Leslie:

----- Current Analysis -----
Date : 30 AUG 2012 Time : 154500 UTC
Lat : 14:09:57 N Lon : 43:36:03 W


CI# /Pressure/ Vmax
2.2 /1010.7mb/ 32.0kt


Final T# Adj T# Raw T#
2.2 2.4 3.4

With proto-Leslie, the key is the Raw T# - the constraint flag currently in place restricts the rise to 0.2/hour. But given that the Raw number is already up to 3.4, the Final and Adjusted T# will each creep upward by 0.2/hour at least through 5pm, bringing the reading well within upgrade status.
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Quoting fireflymom:
Isaac has deposited a bunch of frigate
birds in Pensacola (Cuba being the nearest portion of their range). A
bunch are hanging like kites over Bayfront Parkway downtown.
Shrimpers in LA say when they see those offshore they know it's time to leave....

Birdcasting


Frigate birds nest offshore Cedar Key Florida.
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Greenland glacier disintegrating.Breached ice dam is draining a lake behind it. MODIS today

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Good source for storm damage photos.
Link
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Audubon Park in New Orleans, recorded 19.30" of rain as of 7 pm Wednesday night, and 18.7" of rain in a 24-hour period. This is the greatest 24-hour rainfall event at any official New Orleans site, with weather records extending back to 1871, according to wunderground's weather historian, Christopher C. Burt.
The NWS has now thrown out the Audubon Park readings as likely erroneous, and updated its totals:

IN REGARDS TO THE EARLIER REPORTED RAINFALL AMOUNTS FROM AUDUBON
PARK...THERE SEEMED TO BE AT LEAST TWO ERRONEOUS REPORTS FROM THE
SYSTEM THAT SHOWED EXCESSIVE AMOUNTS OF RAINFALL IN A ONE HOUR
PERIOD THAT DID NOT LINE UP WITH OTHER REPORTS THAT WERE MUCH MORE
CONSISTENT WITH EACH OTHER AT THOSE TIMES.

HERE ARE THE RAINFALL TOTALS ALONG WITH ANY NOTES REGARDING THE
TOTALS. THESE TOTALS ARE FOR AUGUST 28 AND 29.

LOUISIANA

AUD / AUDUBON PARK / 11.19 / LARGE ERRONEOUS AMOUNTS REMOVED
NEW / LAKEFRONT AIRPORT / 9.19 / SOME MISSING DATA
MSY / NEW ORLEANS INTERNATIONAL / 9.08 / NONE
ASD / SLIDELL AIRPORT / 7.62 / NONE
BVE / BOOTHVILLE / 5.60 / SOME MISSING DATA
BTR / BATON ROUGE AIRPORT / MMMM / VERY ERRATIC INFORMATION FROM ASOS
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Thanks Jeff. Not a lot to discuss in Navarre, (edited) but I still like the pic...
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[Updated 12:39 p.m. ET] Looks like the confusion is cleared up. The Mississippi Emergency Operations Center says the dam near the 700-acre Lake Tangipahoa has been damaged by heavy rains, but water has not breached the dam.
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He was pretty much calling doomsday...glad he was wrong.
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"Isaac's storm surge on the Mississippi River" That was an excellent explanation! Thank you!
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Isaac has deposited a bunch of frigate
birds in Pensacola (Cuba being the nearest portion of their range). A
bunch are hanging like kites over Bayfront Parkway downtown.
Shrimpers in LA say when they see those offshore they know it's time to leave....

Birdcasting
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12. LBAR
Kirk is one handsome dude.
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Thanks for the update.
Like I said this has been one crazy storm for it to be a Cat1....




Taco
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Thanks, Doc
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Thanks Doc!

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thanks for the update doc
more news and stories
over the coming days
iam sure
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Quoting CocoaLove:
Dealing with the wrath of Isaac. My family near Hattiesburg is without power, so I have to text them weather updates. Wow.


Prayers to them! It's just windy rain up here in Columbus...
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Dealing with the wrath of Isaac. My family near Hattiesburg is without power, so I have to text them weather updates. Wow.
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From the last blog:

Quoting aislinnpaps:


I am lake property, but not lakefront. Our lake is always lowered prior to a hurricane or TS. I wonder if they did that with this lake prior to Isaac?



It's not a lake dam that can be opened and closed. Most of our state park lakes are creek fed and have and overflow culvert on the downstream end.

When rains like this occur, the culvert cannot release the water as fast as it's coming in.... creating the problems mentioned.
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News statement as of 12:30 pm ET

Link
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Thanks for the update Dr. M... that was(is) a sobering storm for many undoubtedly!
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All times in GMT. Derived from NHC_ATCF data for HurricaneIsaac for 30Aug.12pm
28Aug.12pm , 27.8n88.2w
28Aug.06pm , 28.5n88.9w , 318.6*NW@ 17.3km/h(10.2mph)8.9knots , 60knots , 975millibars , HU
29Aug.12am , 28.9n89.5w , 307.2*NW@ 12.2km/h (7.6mph) 6.6knots , 70knots , 968millibars , HU
29Aug.06am , 29.0n90.1w , 280.9*West@ 9.9km/h (6.2mph) 5.4knots , 70knots , 968millibars , HU
29Aug.12pm , 29.4n90.5w , 318.8*NWest@9.8km/h(6.1mph)5.3knots , 70knots , 970millibars , HU
29Aug.06pm , 29.8n90.8w , 326.8*NNW@ 8.8km/h (5.5mph) 4.8knots , 60knots , 974millibars , TS
30Aug.12am , 30.1n91.1w , 319.0*NWest @7.3km/h(4.6mph)4.0knots , 50knots , 980millibars , TS
30Aug.06am , 30.6n91.4w , 332.6*NNW@10.4km/h(6.5mph)5.6 knots , 45knots , 982millibars , TS
30Aug.12pm , 31.3n91.9w , 328.5*NNW@ 15.2km/h(9.4mph)8.2knots , 40knots , 985millibars , TS

AEX-Alexandria :: LS46-Monterey :: BTR-BatonRouge
HUM-Houma :: LA86-Cocodrie :: KGNI-GrandIsle :: 5LA6-SouthwestPass,MississippiRiver

The kinked line represents the 18hour-path thu 4 positions in which Isaac is listed as a Hurricane.

29Aug.7:26am: H.Isaac made it's first Louisiana landfall directly upon PortFourchon before re-entering the Gulf a few minutes later.
(As to why there's an emphasis on first, see the previous mapping)
29Aug.12pm: While H.Isaac appears to have made a "second landfall" earlier, there's a lot less land and a lot more Gulf within the HUM-LA86-KGNI triangle than suggested by the map: mostly bayou with small islands less than a meter above sea-level.
29Aug.~12:50pm: H.Issac made landfall directly upon Montegut: ~1/3rd of the distance from the 12pm position toward Houma (NHC hadn't downgraded Isaac to TropicalStorm before the second landfall)
30Aug.12pm: TS.Issac still dawdling NNWestward between Alexandria and Monterey

Copy&paste bpt, aex, ls46,hum, la86, btr, kgni-29.215n90.022w, 5la6, 28.5n88.9w-28.9n89.5w- 29.0n90.1w- 29.4n90.5w, 29.4n90.5w-29.8n90.8w, 29.8n90.8w-30.1n91.1w, 30.1n91.1w-30.6n91.4w, 30.6n91.4w-31.3n91.9w into the GreatCircleMapper for a larger map and other information
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather