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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Quoting galvestonhurricane:
Latest computer model tracks = bad news for drought-stricken Texas.


No, that is good news.
Member Since: July 6, 2010 Posts: 113 Comments: 32508
1244. ackee
DOES not look like 93L will be upgraded have seen recon found even TD winds with this so far
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1243. Patrap
FunkTop

Member Since: July 3, 2005 Posts: 426 Comments: 129089
1242. Dennis8
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.


Alright brother..I am in Houston and mom in Corpus ..I am writing this posting this done to sing your praises as a met when this is over....TS 50-60 mph. We will now see.
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1241. P451
TS Katia

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Quoting DFWjc:


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

UPDATE 4.1 Earthquake in LA, CA
Awe. Well maybe they trough will dig down over you and pull some rain farther west.  Hope so. :)
Member Since: August 24, 2010 Posts: 0 Comments: 252
1239. Drakoen
P451, I think you have missed my point. The system will consolidate but should remain fairly large in nature in terms wind field, ROCI, and convection.
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1238. divdog
Quoting ackee:
KATIA seem like she be hurriace again tonight
gotta shed that shear first
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1237. DFWjc
Quoting 69Viking:


Just a matter of time before CA splits in two along that fault line and creates the Island of CA!


2.6 m 2011/09/01 20:53:44 34.339 -118.466 2.2 d GREATER LOS ANGELES AREA, CA

4.3 m 2011/09/01 20:47:07 34.343 -118.481 7.4 d GREATER LOS ANGELES AREA, CA

Member Since: July 19, 2011 Posts: 1 Comments: 1006
Quoting cat6band:


Sweet...My bad...I luv this weather stuff...I'd rather this than FB. I should have been a weather man!! LOL!! Ya'll play anywhere Fri or Sat?


I live on this site during Hurricane season. we suppose to play sunday nite but it's looking too good, suppose to have heavy rain all weekend.
ck out levi update, scary stuff!
Maybe yall should've taken the Dallas gig!! lol
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Quoting AllyBama:


yes it is and it is my afternoon off!..lol


This whole weekend will be a bust.....BBQ off for sure. :(
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Quoting FLGatorCaneNut:


Incorrect again..... Hurricane Wilma was a direct hit coming in from the west coast accross S.E. Florida
to be fully correct, Dennis, Katrina and Wilma ALL made direct landfall as a hurricane on the State of Florida.
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Will be interesting to see what the usually unreliable 18z GFS spits out today.
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wasnt katia supposed to be a cat 2 or 3 by now? its downgraded to a tropical storm wow gonna get real wild now
Member Since: August 28, 2011 Posts: 0 Comments: 362
1231. ackee
KATIA seem like she be hurriace again tonight
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Latest computer model tracks = bad news for drought-stricken Texas.
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Quoting TomTaylor:
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


Here is my guess. In a disorganized system we are really looking at a series of thunderstorms. Inside the storm is primarily vertical movement. The surface winds are being drawn in to fill the void but without vertical movement. Therefore, the horizontal component of the surface winds will be greater than the horizontal component of the flight level winds. Again, this is just a guess based on my one semester of meteorology in 1990.
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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..

I wouldn't bet on that, the park will be a mess, but there will be people drinking and dancing in the rain.
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1227. WxLogic
18Z GFS Init:

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18Z GFS running...
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1225. P451
Quoting Levi32:


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.


It does appear to have a rather large wind field associated with it already.

Over time I could see the system expand after first going through a consolidation/contraction near the system center.

I just think it has a lot of baby fat at the moment that it will look to shed - for lack of a better way to put things. :)

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1224. Patrap
Wet and Windy with chance of flooding in the Tidal Regions outside the LPS.
Member Since: July 3, 2005 Posts: 426 Comments: 129089
Quoting Progster:


It no expert, but in general when we have a surface condition where the boundary layer is being modified
by sensible and latent heat fluxes by a warm moist surface (like the Gulf), this produces convective destabilization and heavy precipitation. But the triggering of convection and heavy precip usually requires a preexisting synoptic-scale mechanism like a cold cut-off low at mid and high levels. Positive PV anomalies imply atmospheric instability increasing with height, i.e. the presense of colder air aloft, that often resides between standard analysis levels. So, in brief, its a mechanism that can encourage rapid convection, but only after sufficient convection has developed can the system transform from vertically baroclinic to barotropic, and that takes time.


Thank you very much.
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1222. divdog
Quoting P451:
RGB has been a great tool throughout the day to locate the low center. Although I think it's north of where they place the symbol when watching the loop.


RGB




VIS

lower pressure southeast of that location
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Quoting DFWjc:


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

UPDATE 4.1 Earthquake in LA, CA


Just a matter of time before CA splits in two along that fault line and creates the Island of CA!
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katia is now a tropical storm
Member Since: August 28, 2011 Posts: 0 Comments: 362
Quoting P451:


LOL. That just may prove to be the best forecasting method for this system.



stick arm out of window:

arm wet = rain
arm bent= wind
arm bent backwards = strong wind
arm gone = extreme wind or tornado
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Quoting Patrap:


So Pat ... what's yer gut tellin ya ( besides any of that gumbo left over ? )
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Quoting WhoDat42:


Scott from The Dingbats


Sweet...My bad...I luv this weather stuff...I'd rather this than FB. I should have been a weather man!! LOL!! Ya'll play anywhere Fri or Sat?
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Quoting JohninRal:
One of the historical tracks in the graphic on Dr. Master's site is Hugo.

Ugh.


Dora is on there as well...
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1215. IKE
COASTAL HAZARD MESSAGE
NATIONAL WEATHER SERVICE MOBILE AL
415 PM CDT THU SEP 1 2011

...HIGH RISK OF RIP CURRENTS THROUGH THE WEEKEND...

.MODERATE TO STRONG EAST TO SOUTHEAST WINDS ALONG WITH INCREASING
WAVE HEIGHTS AND LONGER PERIOD SWELLS WILL PRODUCE DANGEROUS RIP
CURRENTS THROUGH THE WEEKEND.

ALZ061-062-020515-
/O.CON.KMOB.CF.S.0002.000000T0000Z-110903T0000Z/
UPPER MOBILE-UPPER BALDWIN-
415 PM CDT THU SEP 1 2011

* COASTAL FLOODING...POSSIBLE COASTAL FLOODING FRIDAY THROUGH
EARLY NEXT WEEK. TIDES MAY RISE TO 1 TO 2 FEET ABOVE
PREDICATED LEVELS THROUGH THE WEEKEND IN RESPONSE TO AN AREA
OF LOW PRESSURE TRACKING NORTHWARD OVER THE WESTERN GULF OF
MEXICO.

* TIMING...MINOR COASTAL FLOODING MOSTLY DURING TIMES OF HIGH
TIDE WILL BE POSSIBLE FRIDAY THROUGH MONDAY. ASTRONOMICAL HIGH
TIDE FOR MOST AREAS ALONG THE NORTHWEST FLORIDA AND SOUTHWEST
ALABAMA COAST WILL OCCUR DURING THE EARLY MORNING HOURS EACH
DAY...GENERALLY BETWEEN 1200 AM AND 600 AM.

* IMPACTS...MINOR COASTAL FLOODING OR NUISANCE FLOODING LOOKS TO
BE THE MAIN CONCERN FOR NOW CONTINUING THROUGH THE WEEKEND.
THIS MEANS MINOR FLOODING COULD OCCUR OVER ROADWAYS ALONG THE
COAST AND ALONG BAYS AND SOUNDS. THE CAUSEWAY FROM MOBILE TO
BALDWIN COUNTY IN ALABAMA AND THE CAUSEWAY BRIDGE FROM THE
MAINLAND TO DAUPHIN ISLAND ARE SOME AREAS OF CONCERN. THIS
SITUATION COULD BE UPGRADED IF THE LOW PRESSURE SYSTEM OVER
THE WESTERN GULF DEVELOPS INTO A TROPICAL CYCLONE OVER THE
WEEKEND OR EARLY NEXT WEEK.

$$


PLEASE VISIT THE FOLLOWING WEBSITE TO SHARE YOUR THOUGHTS ON THE
NEW RIP CURRENT HAZARD...

HTTP://WWW.WEATHER.GOV/SURVEY/NWS-SURVEY.PHP?CODE =RCVTECCHM
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Quoting StormSurgeon:
Hello fellow WU bloggers. Guess I've been spending a little too much time on FB to join y'all. Rainy day in Mobile.


yes it is and it is my afternoon off!..lol
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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..


Would appear that might be the case.....
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Quoting BrockBerlin:
Hey Progster (or anyone) could you briefly explain the "PV anomaly" concept to me?


I'm no expert, but in general when we have a surface condition where the boundary layer is being modified
by sensible and latent heat fluxes by a warm moist surface (like the Gulf), this produces convective destabilization and heavy precipitation. But the triggering of convection and heavy precip usually requires a preexisting synoptic-scale mechanism like a cold cut-off low at mid and high levels. Positive PV anomalies imply atmospheric instability increasing with height, i.e. the presense of colder air aloft, that often resides between standard analysis levels. So, in brief, its a mechanism that can encourage rapid convection, but only after sufficient convection has developed can the system transform from vertically baroclinic to barotropic, and that takes time.
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1211. P451
Quoting notanothergoof:
im just gonna look out my window in a few days then ill know whats up with 93l


LOL. That just may prove to be the best forecasting method for this system.

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One of the historical tracks in the graphic on Dr. Master's site is Hugo.

Ugh.
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Quoting cat6band:


Who is that??


Scott from The Dingbats
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Mobile Radar.....


Link
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recon:

21:09:00Z 25.817N 90.883W 958.6 mb
(~ 28.31 inHg) 444 meters
(~ 1,457 feet) 1007.7 mb
(~ 29.76 inHg) - From 256° at 5 knots
(From the WSW at ~ 5.8 mph)

pressure still dropping
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1206. P451
Quoting TropicalAnalystwx13:


Like what happened with Irene? Remember all that excess convection it had on the west side for a while?


That was my general thought, yes.

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1205. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 129089
1204. P451
RGB has been a great tool throughout the day to locate the low center. Although I think it's north of where they place the symbol when watching the loop.


RGB




VIS

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1202. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 129089
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.



Like what happened with Irene? Remember all that excess convection it had on the west side for a while?
Member Since: July 6, 2010 Posts: 113 Comments: 32508
Quoting Patrap:


we pushing but itsa tuff ridge to budge


Tell me about it. I do appreciate your efforts though. ... Let me turn off some of these industrial heaters and we will see if that helps.
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im just gonna look out my window in a few days then ill know whats up with 93l
Member Since: August 28, 2011 Posts: 0 Comments: 362
1198. Levi32
Back after lunch.
Member Since: November 24, 2005 Posts: 635 Comments: 26682
Quoting WhoDat42:


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


Who is that??
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1196. Drakoen
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'm going to disagree and assert the notion that the system will remain fairly large.
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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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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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