Flash Flood Warning 108, NWS Birmingham, AL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BIRMINGHAM AL
200 PM CDT FRI OCT 9 2009

.UPDATE...MESOSCALE DISCUSSION.

&&

.MESOSCALE DISCUSSION...

THE ATMOSPHERE CONTINUES THE DESTABILIZATION PROCESS AS WE GO INTO
THE MID AFTERNOON HOURS...WITH TEMPS AS HIGH AS THE LOWER 90S AND
DEWPOINTS WELL INTO THE LOWER TO MIDDLE 70S.  THIS HAS DRIVEN CAPE
VALUES INTO THE 2000 J/KG RANGE...WITH THE POSSIBILITY OF GOING JUST
A BIT HIGHER.  HOWEVER...WITH WARMER TEMPS ALOFT...IT IS LIKELY THAT
INSTABILITY PARAMETERS WON'T SPIRAL OUT OF CONTROL.  ALL THIS AHEAD
OF AN APPROACHING COLD FRONT...WHICH AT THIS JUNCTURE IS APPEARING
MORE LIKE A PREFRONTAL TROUGH...AS THE CONVECTION IS BEGINNING TO
OUTRUN THE BEST WIDE SCALE LIFT AND FORCING.

SHEAR VALUES CONTINUE HIGH...WITH THE BEST LOW LEVEL TURNING AND
LOWER LEVEL JET MAX GENERALLY NORTH OF INTERSTATE 20.  THIS TREND OF
THE MOST PRODUCTIVE SHEAR WILL CONTINUE PUSHING OFF TO THE NORTH AS
THE MAIN CORE OF UPPER DYNAMICS LIFTS OUT AS THE SYSTEM PROGRESSES
FURTHER EAST.  AS IS TYPICALLY SEEN...CENTRAL ALABAMA IS IN THE
CROSS-HAIRS OF UNCERTAINTY WITH THE APPROACHING WEATHER.  WE DO HAVE
THE POTENTIAL TO SEE TORNADOES IN THE PREFRONTAL ENVIRONMENT...BUT
THEY WOULD LIKELY BE CONFINED TO THE MORE DISCRETE CELLS THAT ARE
DEVELOPING OUT AHEAD OF THE CONVECTIVE LINE.  ADDITIONALLY AS WE GET
LATER IN THE DAY...THE INSTABILITY WILL REMAIN HIGH...HOWEVER...THE
BEST TORNADIC SHEAR WILL BE FURTHER NORTH.

THERE IS ALSO SOME FORECAST UNCERTAINTY AS THE CONVECTIVE LINE MOVES
FURTHER EAST.  THE LINE ITSELF IS PUSHING OUT AHEAD OF THE MAIN
SURFACE CONVERGENCE AND BEST FORCING.  AS IT CONTINUES EAST...IT
WILL LIKELY WEAKEN SOMEWHAT UNTIL THE SURFACE FEATURE IS ALLOWED TO
CATCH UP.  WITH THAT IN MIND...THE LINE WILL LIKELY CONTINUE TO SLOW
DOWN WITH TIME AS IT MAKES ITS WAY ACROSS THE WESTERN HALF OF
CENTRAL ALABAMA THROUGH THIS AFTERNOON AND EVENING.  TIMING WILL
BECOME AN ISSUE...BUT FOR NOW WILL LEAVE THE HWO INTACT...UNLESS
TIMING NEEDS TO BE SPED UP OR SLOWED DOWN.  IF SLOWED ENOUGH...THE
THREAT FOR LOCALIZED FLASH FLOODING CAN BE REALIZED.

BOTTOM LINE EXPECTATION...I BELIEVE THERE IS A REAL THREAT OF A FEW
TORNADOES...ESPECIALLY IN THE NORTHWEST QUADRANT OF THE CWA OVER THE
NEXT SEVERAL HOURS.  THIS THREAT WILL CONTINUE TO DECREASE AS THE
DAY CONTINUES.  THIS LEAVES THE MAIN THREAT AT STRONG DAMAGING WINDS
FOR THE WESTERN TWO-THIRDS OF THE CWA FOR PRACTICALLY THE REST OF
FRIDAY.  THERE IS ALSO A SMALL HAIL THREAT WITH THE SEEMINGLY DRIER
AIR ALOFT...BUT NOT ENOUGH TO WARRANT A MAJOR CONCERN.  STAY TUNED
FOR UPCOMING WATCHES AND/OR WARNINGS.

17/KLAWS

&&

.AVIATION...18Z TAF DISCUSSION.

LOW CLOUDS FINALLY BREAKING. BIG CONCERN TODAY WILL BE THE CLOUD
LEVELS AS THEY WILL BE UP AND DOWN BETWEEN 2500 AND 4000 FT.
QUESTION FOR THE TAFS WILL BE WHETHER OR NOT TO GO WITH PERSISTENT
MVFR FOR THE AFTERNOON. WILL MORE THAN LIKELY STICK WITH CURRENT
FORECAST AND BEGIN TO TIME THE THUNDERSTORM AND HEAVY RAIN THREAT
FOR THE TAFS. EXPECT SOUTHERLY WINDS TO INCREASE AHEAD OF AN
APPROACHING COLD FRONT...WITH GUSTS UP TO 20-23KTS DURING THE
AFTERNOON. AHEAD OF THE FRONT...ISOLATED TO SCATTERED TS DEVELOPMENT
IS POSSIBLE. AT THIS TIME WILL NOT PINPOINT VCTS FOR THE AFTERNOON
AS THE BULK OF THE RAIN WILL REMAIN NORTH AND WEST OF THE TERMINALS.
A LINE OF SHOWERS AND THUNDERSTORMS...SOME STRONG TO SEVERE WITH
STRONG WINDS...IS EXPECTED TO PUSH ACROSS THE AREA TONIGHT. THINK
THIS LINE WILL THEN MAKE IT INTO MGM AND TOI AFTER 12Z. WILL GO FOR
TIMING NOW AND FOCUS ON SPECIFICS ON THE NEXT SET OF TAFS.

16

&&

.BMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE BIRMINGHAM AL
1200 PM CDT FRI OCT 9 2009

ALZ011>015-017>050-101700-
MARION-LAMAR-FAYETTE-WINSTON-WALKER-BLOUNT-ETOWAH-CALHOUN-
CHEROKEE-CLEBURNE-PICKENS-TUSCALOOSA-JEFFERSON-SHELBY-ST. CLAIR-
TALLADEGA-CLAY-RANDOLPH-SUMTER-GREENE-HALE-PERRY-BIBB-CHILTON-
COOSA-TALLAPOOSA-CHAMBERS-MARENGO-DALLAS-AUTAUGA-LOWNDES-ELMORE-
MONTGOMERY-MACON-BULLOCK-LEE-RUSSELL-PIKE-BARBOUR-
1200 PM CDT FRI OCT 9 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE COUNTIES SERVED BY THE
NATIONAL WEATHER SERVICE OFFICE IN BIRMINGHAM.

.DAY ONE...THIS AFTERNOON AND TONIGHT.

A STRONG COLD FRONT WILL BRING A THREAT OF STRONG TO SEVERE
THUNDERSTORMS TO CENTRAL ALABAMA LATER THIS AFTERNOON...THROUGH THE
EVENING...AND POTENTIALLY INTO THE OVERNIGHT TIME PERIOD.  THE
THREAT SHOULD ENTER THE NORTHWESTERN COUNTIES BY 2 PM...WITH STORMS
CAPABLE OF STRONG DAMAGING WINDS...SMALL HAIL...AND ISOLATED
TORNADOES. THERE WILL ALSO BE A THREAT FOR LOCALIZED FLASH FLOODING.

AS THE CONVECTIVE LINE PUSHES FURTHER EAST THROUGH THE
AFTERNOON...COUNTIES MAINLY ALONG AND WEST OF THE INTERSTATE 65
CORRIDOR WILL BE AFFECTED AFTER 4 PM.  THE MAIN THREATS WILL
CONTINUE TO BE STRONG DAMAGING WINDS AND SMALL HAIL.

AFTER 8PM THE THREAT WILL BEGIN TO SHIFT EASTWARD...MAINLY ALONG AND
EAST OF THE INTERSTATE 65 CORRIDOR.  SOME STRONG TO POSSIBLY SEVERE
WINDS WILL REMAIN THE THREAT THROUGH THE REMAINDER OF THE EVENING
AND PERHAPS INTO THE OVERNIGHT TIME PERIOD.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY.

THUNDERSTORM CHANCES WILL REMAIN ON SATURDAY INTO SUNDAY...ALTHOUGH
THEY ARE NOT EXPECTED TO BE AS STRONG AS FRIDAY NIGHT.  THE MAIN
THREATS WILL BE GUSTY WINDS...LOCALLY HEAVY RAINFALL AND LIGHTNING.

.SPOTTER INFORMATION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT WILL BE
NECESSARY THIS AFTERNOON AND THIS EVENING...POSSIBLY INTO THE
OVERNIGHT.

17/KLAWS