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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Hello fellow WU bloggers. Guess I've been spending a little too much time on FB to join y'all. Rainy day in Mobile.
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1194. Levi32
Quoting P451:


Yes, and my point is that I don't think you're going to see the entire mass of convection in the GOM spin up into a gigantic circulation.

I expect the smaller region associated with the low to spin up and generate convection overhead as the shear relaxes and you would see a good deal of that mass to the east and south east fade away with time as the system spins up.



I agree that we won't see a storm circulation covering the entire breadth of the Gulf of Mexico. However, light westerly to stationary winds may already extend up to 200 miles south of the center, which is impressive for an invest.
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oh yay...TWC just used the term "disconcerting" as far as the models with Katia...and they just answered my question about 93L and Katia...i need a tums
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Quoting AtHomeInTX:

Hey DFW. Yeah, just posted a update from out locals said the same thing. Looks like the NAM wanted to send some rain up the length of east TX/west LA. That would be nice. :)


you go mail..
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1190. Motoko
Quoting BrockBerlin:


Dennis was a fringe storm yes, and Katrina was a minimal hurricane, but it is hard to argue Wilma was not a direct hit by a moderate hurricane.


Right on Brock! We lived in the Fort Myers, Florida, area (65 miles north of Wilma's landfall) and Wilma took off half our roof. No minor storm.
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1189. Patrap
Quoting Some1Has2BtheRookie:


Pat, do me a favor, please. Grab your radar and move about 300 miles west. Bring the echoes with you. Thanks!


we pushing but itsa tuff ridge to budge
Member Since: July 3, 2005 Posts: 426 Comments: 128764
0430 CDT Marine Forecast Synopsis for the GOM:
.SYNOPSIS...TROUGH FROM 30N91W TO 24N91W WITH EMBEDDED LOW PRES
1010 MB...POSSIBLE TROPICAL CYCLONE...AT 27N91W. MODEL SOLUTIONS VARY GREATLY IN TRACKING AND TIMING OF LOW PRES AS IT MEANDERS OFF LOUISIANA COAST...WITH DIFFERENCES IN LANDFALL FROM FRI NIGHT ON THE GFS TO MON NIGHT ON THE ECMWF. WINDS INCREASE TO NEAR GALE FORCE WITHIN 150 NM OF E LOW PRES CENTER WITH SEAS TO 12 FT. LOW PRES ACCOMPANIED WITH EXTENSIVE AREA OF HEAVY RAINFALL EXPECTED TO EVENTUALLY MOVE INLAND.
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Quoting Levi32:
Sounding this morning from SW Louisiana shows the dry air northwest of 93L. This should move away to some extent as deep moisture invades the northern GOM, but dry entrainment off of the land mass will likely remain at least somewhat of an issue for 93L.


that is true only its not going to weaken or even stop intensification. the dry air will prevent RI. Irene became a cat 3 and she didnt go through RI. if lee stalls out in the gulf for 4 to 5 days is possible he becomes one although a cat 1 is more realistic but still not out of the book. we shouldnt discount this situation of lee becoming a major just yet right?
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Well, whoever ends up with it, I hope they need it rather than someone who's already had more than they need.
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Quoting cat6band:
Ok.....anyone familiar please feel free to answer....My band is supposed to be closing out the Shrimp and Petroleum Fest tomorrow night in Morgan City, La. Is it pretty safe to say that it's gonna be a wash out? Morgan City is about 30 miles west of Houma...TIA..


What up Kris? not looking good for your show Fri nite! We considering canceling our show sunday nite in Madisonville.
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1184. DFWjc
Quoting AtHomeInTX:


Lol. I'm gonna need dramamine now. :)


AtHome - think about me, i won't get any of it up here

UPDATE 4.1 Earthquake in LA, CA
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Quoting notanothergoof:
florida hasnt had a hurricane since 2004

Wilma? 2005
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Quoting Patrap:


Pat, do me a favor, please. Grab your radar and move about 300 miles west. Bring the echoes with you. Thanks!
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1180. doubtit
Quoting SPLbeater:
once again i disagree with the NHC advisory. if they say its a TS now, why didnt they issue it as a TS in the last one where Katia looked worse?

I'd give them a call.
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depending on exactly WHERE 93L goes...and of course how long it lingers...couldn't it dig a hole for Katia to miss being picked up?
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1178. Levi32
Sounding this morning from SW Louisiana shows the dry air northwest of 93L. This should move away to some extent as deep moisture invades the northern GOM, but dry entrainment off of the land mass will likely remain at least somewhat of an issue for 93L.

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Ok.....anyone familiar please feel free to answer....My band is supposed to be closing out the Shrimp and Petroleum Fest tomorrow night in Morgan City, La. Is it pretty safe to say that it's gonna be a wash out? Morgan City is about 30 miles west of Houma...TIA..
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Quoting DFWjc:



20+ inches for NO/MS is what one model forecasts, and BAMS has the Low going west to 100 miles from Corpus then go south parallel with Brownsville headed back east and then curve back up through central LA...LOL!!!

Lol. I'm gonna need dramamine now. :)
Member Since: August 24, 2010 Posts: 0 Comments: 245
1175. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 128764

Quoting weatherganny:



This is very interesting...moves from la to Texas. hmmmm
Kinda looks like it's showing a strong small ridge to the east? Interesting thanks. :)
Member Since: August 24, 2010 Posts: 0 Comments: 245
1173. DFWjc
Quoting AtHomeInTX:

Hey DFW. Yeah, just posted a update from out locals said the same thing. Looks like the NAM wanted to send some rain up the length of east TX/west LA. That would be nice. :)



20 inches for NO/MS is what one model forecasts, and BAMS has the Low going west to 100 miles from Corpus then go south parallel with Brownsville headed back east and then curve back up through central MS...LOL!!!
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if conditions are going to be "good" in the gulf and Lee is in the water for 4 to 5 days he could easily become a cat 1 maybe a 3 if he really stalls out. conditions wont be bad but wont be EXCELLENT like RI so gradual stregnthining overall. ive seen majors that didnt go through rapid intensification.. (IRENE) just gotta wait and see although at least a cat 1 is lookin likely
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Quoting Levi32:


I dunno about you but I've seen many, many times that the recon will find faster SFMR readings than flight-level in a system that has not yet been classified or strengthened above a weak tropical storm. Slower surface winds are nearly a pure result of friction in the PBL. A tropical cyclone's circulation generally weakens with height. In a particularly weak system, it may not be too surprising to see slightly faster winds at the surface right where the inflow into the thunderstorms is coming from. It's also unclear to me how much of an error the SFMR instrument has in weak wind situations (minimal TS force or less).
Exactly what I'm thinking. This seems to have happened a lot this year with all out weak storms, and not to mention, weak systems usually have a stronger vorticity signature at the low levels than at the upper levels in the first place. Maybe not from the 850mb level down to the surface, due to frictional effects, but regardless, the flight level winds aren't even at the 850mb level, they're at 1500ft, or 960mbs.

What I'm not sure about is his explanation behind the stronger surface winds being a result of baroclinic instability. The only baroclinic process I can think of is the ULL to the north inflicting strong divergence over the system...or is that not a baroclinic process? idk, someone feel free to teach me a lesson, I'm not met
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once again i disagree with the NHC advisory. if they say its a TS now, why didnt they issue it as a TS in the last one where Katia looked worse?
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Quoting weatherganny:



This is very interesting...moves from la to Texas. hmmmm


geez...wish these things would make up their mind.
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Quoting AtHomeInTX:



Granted. All of Texas needs rains. Never said any different. But I wouldn't begrudge anyone who needs the rain getting some. Was hoping some of the models taking it into south TX would come true. But our grass is just as dry. Our lakes and ponds are just as empty. Making it harder to put out forest fires. Hell I hope everyone gets drenched. And I wouldn't complain about that even if I wasn't getting a drop.
It is very unusual for southeast texas to be this dry but not so much other parts of the state. In my 50 plus years of living here I dont remember East Texas being dry like the rest of the state.
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Quoting DFWjc:
I love how TWC won't say WEST for Katia, but all they keep saying is that it's moving "Left"


roflmbo...i know right?! i found it a bit disturbing that the discussion on Katia said that a few models keep it going west when the other models have her being picked up...this time of year keeps my stomach in knots i swear....
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Quoting DFWjc:


TWC said is could be a TD or TS at 5pm EST
Hey DFW. Yeah, just posted a update from out locals said the same thing. Looks like the NAM wanted to send some rain up the length of east TX/west LA. That would be nice. :)
Member Since: August 24, 2010 Posts: 0 Comments: 245
TWC suk
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1161. JGreco
Quoting 69Viking:


Guess again, just check your forecast, shows Heavy Rain and Wind for Sunday and Monday, Happy Labor Day! I'd like to see it go West so I can chill out on the boat at Crab Island!


I'm bummed also, my Aunt has a huge boat and we all were going to get together for a family part at crab island. I guess it possibly won't happen.
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Quoting AtHomeInTX:
While waiting on NWS from ch 12

A
broad area of low pressure south of Louisiana is producing
thunderstorms and gusty winds over the Central Gulf of Mexico this
afternoon.  So far, shear has kept this system from developing, but that
is expected to change by Friday.  A reconnaissance plane will
investigate the system later this afternoon.  Late tonight after the
models have been run, we should start to have better data on where this
system will go and how strong it will be.
The
usually reliable European Model now develops this system into a
depression Saturday south of Sabine Pass.  By Sunday, we may see
tropical storm Lee form.  By Labor Day, Lee could become a hurricane. 
Tuesday, the European takes Hurricane Lee into Southeastern Louisiana.
Let
me remind you that nothing has formed at this point and it may or may
not, but there is a pretty good indication that it will.  Also, the
exact path is not set in stone and the strength of the system is not
known.  So, keep abreast of the weather through the weekend and
remember, that the forecast could change for the better or worse.



hmmm
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This is very interesting...moves from la to Texas. hmmmm
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Quoting atmosweather:


Deep layered shear is still 25-30 kts and even if we see the ULL exit the northern Gulf Coast to the NW sometime soon the location of the upper ridging to its SW will still impart around 15 kts. There isn't a lot of mid-level shear, but once the system becomes a moderate tropical storm this doesn't have as much of an effect as the deep layered shear when the system is trying to grow into the upper levels. I think this is what will prevent 93L from becoming anymore than a 50-60 mph tropical storm at best.


Good 'cuz it's forecast to come right over my house and sit there.
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1156. ackee
recon found west wind with 93L
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1155. Seastep
Nice burst in Katia. Think she got mad at being downgraded.



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from Recon:

20:52:30Z 25.100N 91.583W 959.1 mb
(~ 28.32 inHg) 447 meters
(~ 1,467 feet) 1008.5 mb
(~ 29.78 inHg) - From 299 at 12 knots
(From the WNW at ~ 13.8 mph)
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Quoting notanothergoof:
i believe i said yesterday that 93l would hit around the louisiana florida coast in a day or 2 maybe 3


YOU WIN.............NUTHIN!
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1152. DFWjc
I love how TWC won't say WEST for Katia, but all they keep saying is that it's moving "Left"
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While waiting on NWS from ch 12

A
broad area of low pressure south of Louisiana is producing
thunderstorms and gusty winds over the Central Gulf of Mexico this
afternoon.  So far, shear has kept this system from developing, but that
is expected to change by Friday.  A reconnaissance plane will
investigate the system later this afternoon.  Late tonight after the
models have been run, we should start to have better data on where this
system will go and how strong it will be.
The
usually reliable European Model now develops this system into a
depression Saturday south of Sabine Pass.  By Sunday, we may see
tropical storm Lee form.  By Labor Day, Lee could become a hurricane. 
Tuesday, the European takes Hurricane Lee into Southeastern Louisiana.
Let
me remind you that nothing has formed at this point and it may or may
not, but there is a pretty good indication that it will.  Also, the
exact path is not set in stone and the strength of the system is not
known.  So, keep abreast of the weather through the weekend and
remember, that the forecast could change for the better or worse.

Member Since: August 24, 2010 Posts: 0 Comments: 245
1150. GetReal
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1149. Jax82
August 2011 climate summary for Jacksonville...

... August was warmer and drier than normal...

There were 2 big weather stories in August. For the first time since
the unbroken weather record began in Jacksonville... 1871... the maximum
temperature hit 90 degrees or higher every day in August. However...
the average temperature for the month ranks 2011 as only the 7th warmest
on record.

The second big story was Hurricane Irene. Irene passed some 250 east
of Jacksonville on August 25th. A few rainbands made it onshore...
but rain amounts were mostly low. The St Augustine Airport reported
a wind gust to 39 mph and surf was reported as 7 to 10 feet along
the beaches.

Temperatures...

The average temperature for August was 83.8 degrees which is 2.0
degrees above normal. The average daily high was 94.4 degrees and
the average daily low was 73.2 degrees. The high temperature every
day reached at least 90 degrees during August... which is the first
time this has ever happened in the month of August.
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Quoting atmoaggie:
What we've been figuring, with the fast model development, is a potential vorticity inversion.

Interesting, but weird, shtuff.

Back L8R (phone's ringing).


Maybe - weak positive anomaly on the 330K surface directly over the disturbance. Interesting.

Link
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1146. Drakoen
Quoting P451:


There's a trof mixed in there though this isn't just a single disturbance.

I wouldn't expect that whole area to spin up and wrap around a surface center.



What exactly do you mean by a trough out there? There is a trough axis meridionally oriented south of Louisiana along 81W. Low pressure is forming on the poleward end this trough axis where the curvature is best.
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Quoting Gearsts:
Doesnt look close yet.
HH found some west winds.
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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.