Gulf of Mexico disturbance 93L a Lousiana flood threat; Katia a hurricane

By: Dr. Jeff Masters , 3:28 PM GMT on September 01, 2011

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Surface winds over the northern Gulf of Mexico are rising, pressures are falling, and heavy thunderstorms are building today thanks to a tropical disturbance (Invest 93L) that is the product of a tropical wave interacting with an upper-level low pressure system. At 8:35 am CDT, winds at the Mississippi Canyon 711 oil rig were south-southeast at 38 mph. This is just 1 mph below tropical storm force, but the wind instrument was 348 feet (106 m) above the ocean surface, and winds near the surface were probably considerably lower, near 30 mph. Long range radar out of Mobile, Alabama shows heavy rain showers building along the northern Gulf Coast, but these rain showers are not organized into spiral bands and show no signs of rotation. Strong upper-level winds out of the west-northwest are creating 30 knots of wind shear over 93L, keeping the storm's heavy thunderstorms disorganized. Strong onshore winds raising tides to 1 - 2 feet above normal are likely along the northern Gulf Coast through the weekend, and coastal flood statements have been issued for the region.


Figure 1. Predicted rainfall for the 5-day period ending at 8am EDT Sep 6, 2011. A large region of rains in excess of 15 inches is expected over Southeast Louisiana. Image credit: NOAA/HPC.


Figure 2. U.S. drought conditions on August 30. The rains from 93L have the potential to bring major drought relief to drought-stricken portions of the coast. Image Credit: U.S. Drought Monitor.

By late tonight, wind shear is expected to drop to the moderate range, below 20 knots, and 93L should begin to organize into a tropical depression. Wind shear is expected to remain moderate, 10 - 20 knots, into Monday. There is some cold, dry air aloft that will retard this process, and I think the earliest we would see a tropical depression is Friday afternoon. NHC is giving 93L a 70% chance of developing into a tropical depression by Saturday morning. All of the major models develop 93L near the Louisiana coast, and show a slow and erratic movement due to weak steering currents. Coastal Louisiana, Mississippi, Alabama, and the extreme western Panhandle of Florida will likely receive very heavy flooding rains beginning this afternoon and intensifying Friday and Saturday. The latest rainfall forecast from NOAA Hydrological Prediction Center (Figure 1) shows that a large area of 15+ inches of rain is expected over Southeast Louisiana. The region is under moderate drought, so flooding problems will be delayed compared to what we'd normally expect from heavy rains of over a foot. Nevertheless, minor to moderate freshwater flooding is likely from 93L, and flash flood watches are posted for New Orleans and surrounding areas, and rainfall amounts of 1 - 3 inches per hour are possible in some of the heavier rain squalls. Ocean temperatures are near record warmth, 88°F (31.3°C), which will provide plenty of moisture for heavy rains, and plenty of energy to help 93L strengthen into a tropical storm. Most of the models predict 93L will have some motion to the west by Saturday, which would bring rains to the Texas coast near the Louisiana border. Steering currents will be weak in the Gulf this weekend and early next week, making it difficult to predict where the storm might go. If 93L stays over water through Tuesday, like the ECMWF model is predicting, the storm would be a threat to intensify into a hurricane. Most of the other models predict 93L will move ashore over Louisiana by Sunday, limiting the storm's development to just tropical storm strength. I think it at least 50% likely 93L will be a tropical storm with 40 - 60 mph winds along the coast of Louisiana by Sunday.

Hurricane Katia
Hurricane Katia intensified into the 2nd hurricane of the 2011 season last night, and continues its long trek across the Atlantic Ocean today. Katia is expected to arrive at a position several hundred miles north of the Northern Lesser Antilles Islands by Monday. The islands are not in the cone of uncertainty, and it appears unlikely that they will receive tropical storm-force winds from Katia. Satellite images show that Katia is a well-organized storm with plenty of heavy thunderstorms, but the storm has been struggling with dry air and moderate wind shear of 10 -20 knots, and is looking less organized than it did last night. These problems will likely diminish by Friday night, as the upper low bringing the wind shear moves away. It is still unclear how much of a threat Katia may post to the U.S. Dr. Bob Hart's Historical Tropical Cyclone Probability web page suggests shows that tropical storms in Katia's current position have an 16% chance of hitting North Carolina, a 21% chance of hitting Canada, a 12% chance of hitting Florida, and a 54% chance of never hitting land. I suspect that Katia will turn north before reaching the U.S. and potentially threaten Bermuda and Canada, based on what past storms in similar situations have done, and assuming the jet stream maintains its current pattern of bringing frequent troughs of low pressure off the coast of the U.S. It will be another day or two before the models will begin to have a handle on the long-term fate of Katia, though.


Figure 3. Morning satellite image of Katia.

94L
A well-organized low pressure system with a surface circulation and limited heavy thunderstorm activity has developed between Bermuda and the Canadian Maritimes. This disturbance, (94L), is headed out to sea, and is being given a 10% chance of developing into a tropical depression by NHC. 94L is under a very high 30 - 40 knots of wind shear, and will not be able to intensify very much. However, Tropical Storm Jose formed from a similar type of system, and we might get surprised by 94L.

I'll have more on Irene in tomorrow's post.

Jeff Masters

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2795. DFWjc
Quoting KEEPEROFTHEGATE:
T.C.F.W.
012/H/K/C1
MARK
16.95N/50.95W




you've got WU mail...
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2794. KEEPEROFTHEGATE (Mod)
Tropical.Cyclone.Formation.WARNING
013/TD/L/CX
MARK
25.95N/89.91W


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I'm out for tonight.
Member Since: August 19, 2006 Posts: 0 Comments: 15948
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Algiers did well in Katrina.
Quoting SavannahStorm:
To you NOLA folks- my sister and her family just moved from Metairie to a new house in Algiers near Brechtel Park- Google Earth says its -4 feet below sea level. Did the levees in that area get breached during Katrina?
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Quoting GTcooliebai:
Andrew, Isabel, and Hugo should start ringing a bell if it hasn't.


Don't forget Dora-



and Gloria-

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2789. KEEPEROFTHEGATE (Mod)
T.C.F.W.
012/H/K/C1
MARK
16.95N/50.95W


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2788. ncstorm
Quoting Clearwater1:
Right. And the NHC, say wnw track starting from day five. You know what that means don't you?


I'm guessing round two..
Member Since: August 19, 2006 Posts: 13 Comments: 16083
Quoting atmoaggie:
You can drive an airboat down the right lane of most of Gause with any decent rainfall...

Now, we're not talking about floating cars, but the tendency for the road to hold water for a time after a rainfall and if that is any indication of a water-in-house flood threat. And that with someone apparently new to the area thinking of evacuating due to Gause holding water after today's rains.

Truly, if that is the indication of the need for evacuation, one would have to leave 20 times a year.


Yea, sorry, I'm new here. I'm new everywhere I go. Thanks again for the advice.

I'll be heading north tomorrow, I will not stick around to drive through 6" of water through a panicked environment, no matter how used to it the locals say they are.

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Quoting kmanislander:


Could have been worse, you might have had a colonoscopy.
I suppose. lol
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2783. floyd99
I'm wondering what the veterans think about the bend back to the left for Katia in the 11pm NHC discussion. Will the ridge just funnel the storm towards the SE coast?
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Quoting whepton3:


And heeere we go!

The westcasting will be on by morning...

Katia is looking a little too cozy getting near the SEUS.
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Quoting ncstorm:
bend back to the west..

Right. And the NHC, say wnw track starting from day five. You know what that means don't you?
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2779. ncstorm
Member Since: August 19, 2006 Posts: 13 Comments: 16083
Quite low this new wave...

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2777. Skyepony (Mod)
Quoting Drakoen:


No it is not attached to a front.


Is on the surface map..


here's the sat pic..it never cut off.

& not all that warm core either..
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Quoting ncstorm:
bend back to the west..



And heeere we go!

The westcasting will be on by morning...

Katia is looking a little too cozy getting near the SEUS.
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Quoting Patrap:



I am not making fun but there will be 100,000 or so upset people this weekend in New Orleans. Rainbow flags floating down Burbon Street, that will be a sight...
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Quoting Clearwater1:


Those images you posted resemble the xrays of the last lower G.I. series of medical tests I had. lol


Could have been worse, you might have had a colonoscopy.
Member Since: August 19, 2006 Posts: 0 Comments: 15948
Quoting ncstorm:
bend back to the west..

Andrew, Isabel, and Hugo should start ringing a bell if it hasn't.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
2772. DFWjc
Quoting KEEPEROFTHEGATE:
what i do this time


no not you, my good friend, i just could do the button thing this(bows)
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Quoting AussieStorm:








Those images you posted resemble the xrays of the last lower G.I. series of medical tests I had. lol
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To you NOLA folks- my sister and her family just moved from Metairie to a new house in Algiers near Brechtel Park- Google Earth says its -4 feet below sea level. Did the levees in that area get breached during Katrina?
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Quoting Patrap:


I hope you know how to get around on stilts Pat... gonna be a gullywasher at the very least.

If this thing hangs in a while it's going to be a mess.

Keep safe.
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Quoting Skyepony:
Wow 15 " on NOLA..you are doom... & curve the Katia cone toward FL..

I talked about this yesterday 94L formed too far north...this is why i argued it would stay too cold core. It's just not deep enough, attached to that front. Had it happened farther south as expected there would be a gaping hole in the ridge for Katia to follow. Now the front is our only hope or FL will be doom too.
cold water and shear killed 94L's hopes.

Convection was deep enough this morning, but with shear values of around 30kts the convection couldn't be held over the LLC.
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2767. KEEPEROFTHEGATE (Mod)
Quoting DFWjc:


not this time KOG, i'm putting my foot down, this has to stop and stop now...
what i do this time
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2766. MZT
Yeah, not liking the tracks for Katia. Getting a little too neighborly with the southeast USA.
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2765. will40
00z GFS could be very interesting
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2764. ncstorm
bend back to the west..

Member Since: August 19, 2006 Posts: 13 Comments: 16083
Quoting bayeloi:


I'm not even sure that Gause flooded in the May '95 flood.
You can drive an airboat down the right lane of most of Gause with any decent rainfall...

Now, we're not talking about floating cars, but the tendency for the road to hold water for a time after a rainfall and if that is any indication of a water-in-house flood threat. And that with someone apparently new to the area thinking of evacuating due to Gause holding water after today's rains.

Truly, if that is the indication of the need for evacuation, one would have to leave 20 times a year.
Member Since: Posts: Comments:
Quoting Skyepony:
Wow 15+" on NOLA..you are doom... & curve the Katia cone toward FL..

I talked about this yesterday 94L formed too far north...this is why i argued it would stay too cold core. It's just not deep enough, attached to that front. Had it happened farther south as expected there would be a gaping hole in the ridge for Katia to follow. Now the front is our only hope or FL will be doom too.


Great. I returned my helmet to the store when Irene missed us here in S. FL.

Hopefully I'll find one on Labor Day sale.

I'm keeping one eye on Katia... I just don't quite think the suspense is over yet for CONUS on this one... and of course a system getting its groove back is bad news for the islands in any event.
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2760. GatorWX
Quoting Patrap:


You've had that radar up constantly since 0600, lol.
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Quoting Clearwater1:
Well, if on day five, she heads wnw, (per nhc 11pm) under the sub trop ridge, then the whole east coast from cental FL to NC will be in the cone. If the ridge really builds in strong, it could send Katia even more westerly. imo.


How likely do you think this scenario would be?
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2757. THL3
TX- We Want NOGAPS, We Want NOGAPS, We Want NOGAPS, We Want NOGAPS. :)
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Quoting wxobsvps:
I see "doom" here a lot. Who started that?


I've wondered the same myself, actually. And The Homer Simpson Doomcon graphics are a beaut.
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2755. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 129093
Quoting whipster:

The Governor can go it alone and issue a "State of Emergency" declaration and the EOP is up and running regardless-

Don't worry, those governors/mayors tend to flee the state at a moment's notice.


Pretty sure that Bobby Jindal isn't going to make the same mistakes the previous LA governor did. Idk who is mayor of New Orleans, but hopefully s/he learned from the mistakes of the idiot Nagin.
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Ahh Ooh! The latest NHC forecast for Katia seems to be turning her towards the CONUS with the high building in behind it.
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Quoting MississippiWx:
84 hour rainfall total from the NAM






Cheese and crackers, that's what Mama calls a "frog-strangler."
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2751. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 129093
I dont like Katias' cone.......Irene is enough!!! GO AWAY KATIA!!!THE NORTH ATLANTIC LOWS ARE HAVING A PARTY, GO JOIN EM UP THERE!!
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2749. GatorWX
Very surprised Katia wasn't re-upgraded! She's looking ripe and the coc is certainly under that convection; it's no longer displaced. Pre-Lee looks ugly and looks to have a LONG way to go.
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2748. KEEPEROFTHEGATE (Mod)
ides is rising in the gulf gotta a bad feelin on the sudden size on this one
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Quoting SavannahStorm:



This Link is highly relevant on many levels...


I thought so also...:)
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2746. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 129093
2745. DFWjc
Quoting tiggeriffic:
im done...in the last 24 hours this blog has been more about politics, arguing moot points about the NHC, and someone trying to start a race riot... i will return when the 3 billy goats eat the trolls under the bridges and weather is the main topic...


night tigger
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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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