FXUS64 KBMX 081735
AFDBMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BIRMINGHAM AL
1130 AM CST TUE DEC 8 2009

.UPDATE...FOR AVIATION DISCUSSION.

&&

DISCUSSION...CONVECTION HAS BEGUN UPSTREAM...OVER WEST CENTRAL
MISSISSIPPI AND IS MOVING RAPIDLY TOWARD CENTRAL ALABAMA...WITH
WIDESPREAD SHOWERS SHIFTING OUT OF SOUTHWESTERN ALABAMA. DEWPOINTS
HAVE RISEN 5 TO 10 DEGREES ACROSS CENTRAL ALABAMA SINCE 12Z HAVING
CLIMBED INTO THE MIDDLE 50S IN SOUTHERN AND SOUTHWESTERN PORTIONS OF
CENTRAL ALABAMA. THIS MOISTURE RETURN IS THE KEY TO THE POTENTIAL OF
SEVERE WEATHER THIS EVENING AND OVERNIGHT. GUIDANCE CONTINUES TO
BRING IMPRESSIVE COLD SEASON/NIGHTTIME THERMODYNAMIC INSTABILITY TO
THE AREA OVERNIGHT WITH LI'S IN THE -1 TO -2 RANGE AND CAPES IN
EXCESS OF 500 J/KG. BASED ON WHAT WE HAVE SEEN SO FAR TODAY...FEEL
LIKE DEWPOINTS IN THE 62 TO 67 DEGREE RANGE WILL MAKE IT INTO AT
LEAST A POTION OF CENTRAL ALABAMA OVERNIGHT JUST AHEAD OF THE COLD
FRONT. SEVERE POTENTIAL WILL INCREASE PROGRESSIVELY SOUTHWARD OF A
LINE FROM TUSCALOOSA TO CLANTON TO ROANOKE. DYNAMICS ALOFT ARE
EQUALLY IF NOT MORE IMPRESSIVE. ONE FLY IN THE SEVERE POTENTIAL
OINTMENT IS MORE UNIDIRECTIONAL SHEAR EXPECTED BY MIDNIGHT AS
WINDS SHIFT MORE SOUTHWESTERLY. NEVERTHELESS...ANY WATCHES WILL MOST
LIKELY BE TORNADO WATCHES.

12/SIRMON
&&

.SHORT TERM...TODAY THROUGH WEDNESDAY.

ACTIVE WEATHER PERIOD IN STORE WITH SEVERAL HAZARDS POSSIBLE. THE
NAM IS STILL JUST A TAD SLOWER BUT IT SEEMS TO SPEED THINGS UP TOWARD
THE END OF THE EVENT. BUT THE REST OF THE 00Z GUIDANCE INCLUDING THE
ECMWF/GFS/SREF ARE ALL IN PRETTY GOOD AGREEMENT. STRONG SHORT WAVE
TROUGH NEAR THE FOUR CORNERS REGION QUICKLY MIGRATES INTO THE OHIO
VALLEY/GREAT LAKES TONIGHT AND BECOMES NEGATIVELY TILTED NORTH OF THE
AREA. SURFACE CYCLONE DEEPENS RAPIDLY AS IT MOVES FROM OKLAHOMA
TOWARDS MICHIGAN. SIGNIFICANT 500 MB HEIGHT FALLS STAY NORTH OF THE
AREA BUT WE STILL RECEIVE A 5 TO 9 DAM DROP TONIGHT. SURFACE
PRESSURES DROP SOME 5 TO 10 MB IN A 6 HOUR PERIOD BEGINNING THIS
AFTERNOON AND LASTING INTO TONIGHT. IT APPEARS THAT THE MOST
INTENSE DEEPENING OF THE LOW WILL OCCUR OVERNIGHT NORTH OF THE
REGION. BOUNDARY THAT IS PROGGED TO MOVE THROUGH OVERNIGHT DOES NOT
HAVE A GREAT DEAL OF THICKNESS PACKING AND LOOKS MORE LIKE A DRY
LINE...SOMETIMES ENHANCING THUNDERSTORM ACTIVITY ALONG IT.

STRONG JET MAXIMUM (100KT +) APPROACHES WESTERN ALABAMA OVERNIGHT AS
MODELS INDICATE A BIT MORE DIVERGENCE THAN PREVIOUS RUNS...ALTHOUGH
WE ARE NOT ORIENTED IN THE PREFERRED QUADRANT. MID LEVEL WINDS
INCREASE TO 50-70KTS LATE THIS AFTERNOON. LOW LEVEL JET DEVELOPING
AS WE SPEAK SOUTHWEST OF THE AREA WITH WINDS INCREASING ALL DAY.
MAX LOW LEVEL JET MOVES IN AROUND SUNSET.

THE MAIN CONCERN HERE IS THE TIMING OF THE WARM FRONT MOVING INTO
CENTRAL ALABAMA AND HOW FAR NORTH IT ACTUALLY GOES. THIS WILL PROVIDE
THE INSTABILITY AND MOISTURE FOR SURFACE BASED THUNDERSTORMS AND
INCREASE THREAT OF SEVERE WEATHER. WILL GO WITH THE IDEA THAT SURFACE
STABLE LAYER WILL HANG ON MUCH OF THE DAY...BUT RELATIVELY WARM RAIN
AND AN INCREASE IN SOUTHERLY WINDS ALOFT WILL EVENTUALLY SCOUR OUT
THE STABLE LAYER AND EXPECT A RATHER QUICK SURGE OF THE WARM FRONT
AROUND SUNSET. FAR NORTHERN AND EASTERN AREAS WILL BE THE LAST
AREAS TO INCREASE THE MOISTURE.

SHORT TERM MODEL CONSENSUS CONTINUES TO ADVERTISE PLENTY OF SURFACE
BASED INSTABILITY FOR WINTER TIME SEVERE CONVECTION. THEREFORE...
WILL GO WITH THE FOLLOWING SCENARIO. SOUTHERLY WINDS INCREASE JUST
ABOVE THE SURFACE THIS AFTERNOON AND WILL ADD MENTION OF ELEVATED
THUNDER AREA-WIDE...NO SEVERE. THE WARM FRONT SURGES NORTHWARD AROUND
SUNSET WITH A CHANCE OF ISOLATED SEVERE ALONG AND SOUTH OF THE WARM
FRONT INTO THE WARM SECTOR. THE MAIN FORCING OCCURS IN THE VICINITY
OF THE DRY LINE FRONT AND COINCIDING WITH THE LARGE SCALE PRESSURE
DROPS. THIS FORCING ENTERS NORTHWESTERN SECTIONS JUST AFTER SUNSET
AROUND 8 PM...MOVES TO NEAR THE INTERSTATE 20 CORRIDOR AROUND
MIDNIGHT...AND EXITS SOUTHEASTERN AREAS AROUND SUNRISE. LAPSE RATES
ARE A BIT BETTER ON THE 00Z RUNS AND CAPES ARE ALSO A TAD HIGHER
(300-800 OR SO) AND WILL RE-INTRODUCE MENTION OF SOME HAIL. THERE
WILL BE TREMENDOUS SHEAR AND SRH AROUND 300. MODEL HODOGRAPHS START
OFF CURVED AND TREND TOWARD STRAIGHT WITH THE DRY LINE. TORNADOES AND
DAMAGING WINDS REMAIN AS THREATS OVERNIGHT. MUCH LESSER CONFIDENCE ON
THE PLACEMENT OF ANY HEAVY RAINFALL. PRECIP WATER VALUES ARE A FEW
STANDARD DEVIATIONS ABOVE NORMAL AND SOME TRAINING THUNDERSTORMS MAY
OCCUR...BUT OVERALL A FAST MOVING SYSTEM.

SURFACE WINDS WILL BE GUSTY AND A LAKE WIND ADVISORY HAS BEEN ISSUED
AREA-WIDE FROM 18Z TODAY THROUGH 18Z WEDNESDAY. SUSTAINED WINDS OF
10 TO 20 MPH ARE EXPECTED WITH WIND GUSTS OF 25 MPH POSSIBLE. THIS
MAY NEED TO BE UPGRADED AS A FEW HIGHER GUSTS MAY MATERIALIZE.

.LONG TERM...THURSDAY THROUGH MONDAY.

ONCE THIS FIRST SYSTEM GETS OUT OF THE AREA ON WEDNESDAY...CLEARING
SKIES AND MUCH COOLER TEMPERATURES ARE IN STORE FOR THURSDAY THROUGH
FRIDAY MORNING. WILL LIKELY SEE 20S AREA WIDE AGAIN FRIDAY MORNING.

ATTENTION NOW TURNS TO THE NEXT SYSTEM THAT WILL BE MOVING INTO THE
AREA ON FRIDAY AFTERNOON. THIS SYSTEM WILL BE A GULF LOW THAT WILL
DEVELOP. CONTINUE TO FEEL AS THOUGH THIS LOW WILL SLIDE NORTH INLAND
TOWARD CENTRAL ALABAMA. THIS SOLUTION IS SUPPORTED BY THE EUROPEAN
AND OVERALL GFS ENSEMBLES. THE GFS ITSELF IS MUCH FURTHER NORTH THAN
YESTERDAYS RUN...SO FEEL COMFORTABLE ABOUT THE COLD AIR REMAINING TO
THE NORTH. IN FACT WILL START THE MENTION OF RAIN AFTER NOON ON
FRIDAY. THIS IS HEDGING TOWARD THE EUROPEAN WHICH IS A GOOD 12 HOURS
SLOWER ON THE START TIME.

OVERALL THE SYSTEM ON FRIDAY INTO SATURDAY IS TRENDED TOWARD THE
EUROPEAN...THUS TEMPERATURES ARE A TOUCH HIGHER THAN GFS MOS THROUGH
THE WEEKEND. POPS ARE ALSO TRENDED THAT WAY AS WELL. WILL KEEP A
SLIGHT CHANCE OF RAIN THROUGH SUNDAY AFTERNOON WITH THIS SLOWER
SOLUTION.

ANOTHER ROUND OF SHOWERS MOVE IN MONDAY AS THE SOUTHERN STREAM
REMAINS ACTIVE OVER THE AREA.

16

&&

.AVIATION...18Z TAF DISCUSSION.

ACTIVE WEATHER PERIOD EXPECTED AS STRONG STORM SYSTEM MOVES ACROSS
LATER TONIGHT. THROUGH 00Z EXPECT SCATTERED TO NUMEROUS SHOWERS
WITH CEILINGS MAINLY BELOW 1000 FEET. AFTER 00Z THUNDERSTORMS
DEVELOP ACROSS THE WEST AND SPREAD EASTWARD OVERNIGHT. SOME STRONG
TO SEVERE STORMS POSSIBLE WITH WIDESPREAD SHOWERS. EXPECT IFR
CONDITIONS AT ALL SITES. STRONG SURFACE WINDS EXPECTED WITH MAINLY
SOUTHEAST 15 TO 25 MPH THROUGH 00Z...THEN SHIFTING TO SOUTHWEST 15
TO 30 MPH FROM THE SOUTHWEST...GRADUALLY SHIFTING TO THE WEST AFTER
06Z FROM WEST TO EAST.

&&

.BMX WATCHES/WARNINGS/ADVISORIES...
LAKE WIND ADVISORY IN EFFECT FROM NOON TODAY THROUGH NOON
WEDNESDAY FOR ALL OF CENTRAL ALABAMA.

&&

$$

12/50