Gulf of Mexico disturbance 93L a Lousiana flood threat; Katia a hurricane
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
Reader Comments
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I'd wager that has alot to do with that ULL. as it weakens and retrogrades, I'd assume flight level winds would pick up. I do agree that it is receiving some forcing from baroclinic processes currently. Alot of heat is building at the upper levels and I expect that will shunt/weaken the ULL and cause the system to take on more purely tropical characteristics.
Anyway, how does baroclinic instability explain this?
Yes! Wilma's eye went right over my house in Boca Raton,FL and broke our back slider(we now have shutters). That was a scary day.
Click on fronts here,
93L Floater - RGB Color Infrared Loop
Hey Tex. Hang in there there's still some hope. :)
There isn't a lot of mid-level shear right now, and it should continue to decrease. I could see how dry air may be a problem, but not wind shear.
I lived just to the East of Navarre and watched from a friends house in Fort Walton Beach as the eastern eyewall went over my house! I waded through 3 feet of water to check on my house after Dennis went through and I'm just thankful my house was elevated and built after Ivan hit in 2004 so it was built to a better code, others weren't so lucky!
Like a cold low (polar)? Shouldn't there be a pronounced vortmax at or about H 500? There is a vorticity maximum over the N central Gulf but it may have been too weakly analyzed. If the mechanism for development is indeed baroclinic instability, development will be slow. And yet may of the hi-res models show rapid intensification. Confusing!
NATIONAL WEATHER SERVICE TALLAHASSEE FL
412 PM EDT THU SEP 1 2011
.SHORT TERM (TONIGHT THROUGH SATURDAY NIGHT)...ALL EYES ARE ON
THE GULF OF MEXICO THIS AFTERNOON WHERE A POTENT TROPICAL WAVE IS
GRADUALLY BECOMING BETTER ORGANIZED. 12Z GUIDANCE HAS COME INTO
SOMEWHAT BETTER AGREEMENT WITH SLOW MOVING LOW PRESSURE DEEPENING
OVER THE NORTHWESTERN GULF TONIGHT INTO THE WEEKEND...BEFORE
LIFTING NORTH OF NORTHEAST INTO THE DEEP SOUTH BY LATE IN THE
WEEKEND OR EARLY NEXT WEEK. STEERING CURRENTS ARE FORECAST TO BE
VERY WEAK INTO EARLY NEXT WEEK SO THERE IS STILL QUITE A BIT OF
UNCERTAINTY AS TO HOW LONG THE SYSTEM REMAINS OVER WATER...HOW
MUCH IT DEEPENS...AND HOW FAR EAST THE EVENTUAL TRACK MIGHT BE.
REGARDLESS OF THE EXACT TRACK...CONFIDENCE IN THE GUIDANCE IS
INCREASING ENOUGH TO SPECULATE THAT THE SYSTEM WILL HAVE SOME
IMPACT ON THE WEATHER ACROSS THE TRI-STATE REGION...ESPECIALLY FOR
AREAS IN THE FLORIDA PANHANDLE AND THE COASTAL WATERS. WOULD
EXPECT TO SEE AN INCREASE IN SHOWERS AND THUNDERSTORMS ALONG THE
PANHANDLE BEACHES BEGINNING FRIDAY AFTERNOON AND CONTINUING INTO
SATURDAY NIGHT AND BEYOND. HEAVY RAIN WILL BE POSSIBLE FOR THIS
AREA...ESPECIALLY TOWARDS LATE SATURDAY INTO SUNDAY.
THE POPS WILL DROP OFF SHARPLY TO THE NORTHEAST OVER THE NEXT
SEVERAL DAYS AS THE INCOMING MOISTURE FROM THE TROPICAL SYSTEM
MEETS A VERY DRY AIRMASS CURRENTLY FILTERING IN FROM THE
CAROLINAS. DEWPOINTS IN SOUTHEAST GEORGIA HAVE DROPPED INTO THE
UPPER 50S THIS AFTERNOON AND THIS AIRMASS WILL SPREAD INTO THE
NORTHERN TWO-THIRDS OF THE FORECAST AREA FOR FRIDAY. FOR
FRIDAY...POPS WILL RANGE FROM AROUND 50 PERCENT ALONG THE COAST
TO LESS THAN 10 PERCENT ACROSS SOUTH GEORGIA. HIGHER POPS WILL
SLIDE SLOWLY NORTHEAST ON SATURDAY. HOWEVER...APPRECIABLE RAIN IS
NOT EXPECTED ACROSS SOUTH GEORGIA UNTIL SUNDAY.
WILL NEED TO CONTINUE TO MONITOR THE DEVELOPMENT OF THE SYSTEM TO
DETERMINE IN OTHER IMPACTS ON COASTAL AREAS...SUCH AS HIGH SURF
AND COASTAL FLOODING. THE CURRENT FORECAST HAS THE HIGHEST SURF AND
STRONGEST ONSHORE FLOW NOT SETTING IN UNTIL LATER SATURDAY INTO
SUNDAY. THEREFORE...WILL HOLD OFF ON ANY COASTAL FLOOD PRODUCTS
UNTIL THE FORECAST BECOMES A BIT MORE CERTAIN.L
Dennis.
Interesting, but weird, shtuff.
Back L8R (phone's ringing).
I agree with you, I think we have a good chance of feeling something off of 93L at least here on the Tx/La border in southeast Texas Hopefully further west.
That is absolutely false. All 3 storms hit Florida directly. Go back and look at storm tracks. In fact, I'll post them here...
and a brush with Rita in the Keys:
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!
It does.
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.
Glad to hear that I'm not going crazy :P
Also, I believe a round of applause is in order for calling 94L a few days ago, and also saying it had a good chance this morning when it was given 10%.
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.
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).
Thanks, at least somebody was listening to me!
lol.
Houston averages 47" of rain a year...we have had 10.5", I have had 9.5" in Tomball. And I only had 33" last year!
The driest year on record is 1917 ( I think ) at 17-18"!
NATIONAL WEATHER SERVICE MOBILE AL
238 PM CDT THU SEP 1 2011
ALZ051>064-FLZ001>006-MSZ067-075-076-078-07 9-021045-
CHOCTAW-WASHINGTON-CLARKE-WILCOX-MONROE-CONECUH-B UTLER-CRENSHAW-
ESCAMBIA-COVINGTON-UPPER MOBILE-UPPER BALDWIN-LOWER MOBILE-
LOWER BALDWIN-INLAND ESCAMBIA-COASTAL ESCAMBIA-INLAND SANTA ROSA-
COASTAL SANTA ROSA-INLAND OKALOOSA-COASTAL OKALOOSA-WAYNE-PERRY-
GREENE-STONE-GEORGE-
INCLUDING THE CITIES OF...BUTLER...LISMAN...SILAS...CHATOM...
MILLRY...JACKSON...THOMASVILLE...GROVE HILL...CAMDEN...
PINE HILL...HOMEWOOD...MONROEVILLE...EVERGREEN...GREENV ILLE...
LUVERNE...BRANTLEY...ATMORE...BREWTON...EAST BREWTON...
ANDALUSIA...OPP...MOBILE...PRICHARD...SARALAND... BAY MINETTE...
TILLMANS CORNER...THEODORE...DAPHNE...FAIRHOPE...FOLEY...
SPANISH FORT...CENTURY...FLOMATON...MOLINO...PENSACOLA...
FERRY PASS...BRENT...WEST PENSACOLA...BELLVIEW...ENSLEY...
MYRTLE GROVE...JAY...PACE...MILTON...GULF BREEZE...CRESTVIEW...
WRIGHT...FORT WALTON BEACH...NICEVILLE...DESTIN...SEMINOLE...
EGLIN AFB...VALPARAISO...WAYNESBORO...RICHTON...BEAUMONT ...
NEW AUGUSTA...LEAKESVILLE...MCLAIN...WIGGINS...LUCEDAL E
238 PM CDT THU SEP 1 2011
...POTENTIAL EXTREME HEAVY RAINFALL TO MATERIALIZE FOR THIS LABOR DAY WEEKEND
ALONG THE NORTH CENTRAL GULF COAST...
AS A LOW PRESSURE SYSTEM GRADUALLY EVOLVES IN THE GULF OF MEXICO OVER
THE NEXT FEW DAYS...THE NORTH CENTRAL GULF COAST REGION IS UNDER A
THREAT FOR EXTREMELY VERY HEAVY RAINFALL...WITH THE MOST INTENSE PERIOD
EXPECTED TO BE FROM SATURDAY EVENING THROUGH MIDDAY MONDAY. IT IS NOT OUT
THE QUESTION FOR SOME AREAS OVER COASTAL ALABAMA AND NORTHWEST
FLORIDA TO RECEIVE AS MUCH AS 10 TO 15 INCHES OF RAIN...WITH LOCALLY
HIGHER ISOLATED AMOUNTS WERE RAINBANDS BECOME ESTABLISHED. A BREAK
DOWN IS GIVEN BELOW.
ALONG THE COASTAL COUNTIES OF ALABAMA AND NORTHWESTERN FLORIDA...SHOWERS
AND THUNDERSTORMS WILL BEGIN TOMORROW MORNING AND GRADUALLY INCREASE
IN COVERAGE AND INTENSITY DURING THE DAY. BY SUNSET TOMORROW... TWO TO FOUR
INCHES OF RAIN COULD FALL LOCALLY....MAINLY WEST OF PENSACOLA AND
VERY CLOSE TO THE COAST.
LOOKING INTO THE FRIDAY NIGHT THROUGH SATURDAY EVENING TIME
FRAME...RAIN WILL CONTINUE. AN ADDITIONAL ONE TO THREE INCHES OF
RAIN COULD FALL...BRING TOTALS UP TO A FORECAST OF FOUR TO SIX INCHES
FROM PENSACOLA WEST TO MOBILE...AND MAINLY SOUTH OF I-10.
FINALLY...THE SITUATION COULD BE FURTHER EXACERBATED SATURDAY NIGHT THROUGH
EARLY MONDAY AS RELATIVE HEAVIER RAINFALL BECOMES ESTABLISHED ON THE
EAST SIDE OF A GULF SURFACE LOW THAT IS EXPECTED TO DEVELOP. GIVEN
THE CURRENT UNCERTAINTY ASSOCIATED WITH THE DEVELOPING SURFACE
LOW...EXACTLY WHERE THE HEAVIEST RAINFALL SETS UP WILL REQUIRE
CONSTANT MONITORING. DESPITE THIS...IT IS REALIZED THAT OUR REGION
WILL MOST LIKELY BE ON THE EAST SIDE OF THE STORM SYSTEM. WITH
THIS...WE COULD EITHER BECOME SITUATED UNDER A DETACHED HEAVY
RAINBAND IF THE SURFACE LOW MOVES FURTHER WEST...OR WITHIN HEAVY
RAINBANDS IF THE SYSTEM MOVES CLOSER. EITHER WAY...OUR REGION IS IN
FOR POTENTIALLY EXTREMELY HEAVY RAINFALL. IT MUST BE EMPHASIZED THAT
5 TO 10 INCHES OF RAIN COULD FALL JUST IN THE SATURDAY EVENING TO
EARLY MONDAY TIME FRAME ALONE.
THIS TYPE OF RAINFALL WILL LIKELY BRING ABOUT ISOLATED FLASH
FLOODING BY SATURDAY NIGHT...IF NOT EARLIER. THIS COULD EXPAND INTO
AREAL FLOODING EVENTUALLY EVOLVING UPSCALE TO LOCAL COASTAL RIVER
FLOODING BY LATE WEEKEND THROUGH EARLY NEXT WEEK...MAINLY AFFECTED
THE RIVERS OF SOUTHEAST MISSISSIPPI...COASTAL ALABAMA AND THE WESTERN
FLORIDA PANHANDLE. A FLASH FLOOD WATCH WILL LIKELY BE ISSUED BY EARLY
FRIDAY FOR THE PERIOD FROM MID-WEEKEND THROUGH MONDAY.
PLEASE STAY TUNED TO LOCAL MEDIA OUTLETS...NOAA WEATHER RADIO...AND
YOUR LOCAL NWS WEB PAGE (SEE HTTP://WWW.SRH.NOAA.GOV/MOB) FOR LATER
UPDATES...GRAPHICAL FORECASTS AND FAST BREAKING WATCH OR WARNING
INFORMATION. THIS IS A POTENTIAL VERY SERIOUS SITUATION AND NOW IS
THE TIME TO BEGIN PREPARATIONS.
JMM 23 / DS 12
I understand....
TWC said is could be a TD or TS at 5pm EST
It depends. Impacts might be greater if the storm takes a track towards Alabama putting us on the east side of however strong the storms become.
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.
Maybe - weak positive anomaly on the 330K surface directly over the disturbance. Interesting.
Link
... 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.
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.
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