Flash Flood in Alabama, September 22 to 23, 2009

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
939 PM CDT TUE SEP 22 2009

.UPDATE...
WILL INCREASE POPS TO LIKELY FOR FAR NW ALABAMA AND MENTION LOCALLY
HEAVY RAIN IN THE UPCOMING FORECAST.

&&

.DISCUSSION...
SHOWERS AND THUNDERSTORMS WERE IN PROGRESS MAINLY JUST WEST OF THE
NE MISSISSIPPI/NW ALABAMA BORDER. FLASH FLOODING WAS BEING PRODUCED
ACROSS PARTS OF FAR NE MISSISSIPPI AND ADJACENT MIDDLE TENNESSEE...
WITH RADAR AND SATELLITE SHOWING 1-3 INCHES OF RAINFALL PER HOUR.
THE HEAVIEST RAINS SO FAR APPEARED TO BE OVER RURAL FAR WESTERN
LAUDERDALE COUNTY AND THE TENNESSEE RIVER...WHERE UPWARDS OF 1-2
INCHES HAS ALREADY FALLEN.

ON A BIGGER PICTURE...BERMUDA HIGH PRESSURE WAS RIDGING WESTWARD TO
THE GULF COAST...WITH AN UNSEASONABLY DEEP UPPER LOW PRESSURE
SPINNING ACROSS KANSAS. THIS SETUP CONTINUES AS SOUTHERLY FLOW OF
RICH GULF MOISTURE MOVES ACROSS THE MISSISSIPPI AND TENNESSEE
VALLEYS. A COUPLE OF UPPER LEVEL DISTURBANCES MOVING NORTHWARD OVER
MS WILL PLAY A ROLE IN THE SENSIBLE WEATHER FOR NW ALABAMA FOR THE
LATE EVENING AND EARLY WED MORNING TIME FRAME.

GIVEN THIS LAYOUT...WILL INCREASE RAIN CHANCES TO LIKELY FOR NW
ALABAMA IN THE UPDATE. WILL BE HOLDING OFF ON A FLASH FLOOD WATCH AT
THIS TIME...GIVEN THAT ONLY A FEW COUNTIES (PRIMARILY LAUDERDALE...
COLBERT AND FRANKLIN) COULD BE AFFECTED. ELSEWHERE...CANNOT RULE OUT
SOME OF THE PRECIPITATION SOUTH AND SW OF HERE AFFECTING OTHER PARTS
OF THE TENNESSEE VALLEY DURING THE OVERNIGHT...THEREFORE WILL KEEP
THE REST OF THE FORECAST UNCHANGED FOR NOW.

&&

.AVIATION...   /ISSUED  647 PM CDT TUE SEP 22 2009/
VFR CONDITIONS PREVAILED ACROSS NORTHERN AL AND SOUTHERN MIDDLE TN
AT THE PRESENT. A LINE OF CONVECTION WAS DEVELOPING FROM NEAR TUPELO
MS TO BOWLING GREEN KY. THIS LINE WILL SLOWLY MOVE EAST...AND COULD
AFFECT THE KMSL AIRPORT DURING THE LATE EVENING/EARLY MORNING HOURS.
MVFR CEILING/VISIBILITY REDUCTIONS ARE EXPECTED ACROSS THE AREA
BEFORE DAYBREAK WED. VFR CONDITIONS SHOULD RETURN BY 14Z AND
CONTINUE FOR THE REMAINDER OF THE PERIOD. THERE IS A CHANCE FOR MORE
CONVECTION DEVELOPING WED AFTERNOON...HOWEVER CHANCES ARE TOO LOW
FOR INCLUSION IN TAFS AT THIS TIME.

&&

.PREV DISCUSSION...    /ISSUED  349 PM CDT TUE SEP 22 2009/
FORECAST EVOLVING AS EXPECTED FOR ABOUT A COUPLE OF DAYS NOW...AND
INDICATIONS ARE THAT A DEEP CONVERGENT ZONE WILL REMAIN ESTABLISHED
JUST TO OUR WEST OVER THE NEXT SEVERAL DAYS BEFORE AN INITIAL FRONT
FINALLY PUSHES ACROSS THE AREA DURING THE WEEKEND. THEN...A MUCH
COOLER POLAR AIRMASS LOOKS SET TO ENTER THE ERN CONUS BY THE EARLY
TO MIDDLE PART OF NEXT WEEK. WHILE OVERALL CONFIDENCE IS INCREASING
IN THE EVOLVING LONG WAVE PATTERN...SMALL TO MEDIUM SCALE FEATURES
WILL STILL BE A LITTLE PROBLEMATIC WITH THE DEEP CONVERGENT
BOUNDARY/MOISTURE AXIS POISED JUST TO OUR WEST THROUGH SAT. GIVEN
THE REGIONAL-SCALE SETUP...WRN PORTIONS OF THE HUN CWFA WILL BE
FAVORED TO RECEIVE THE MOST RAINFALL THROUGH THE REMAINDER OF THE
WEEK...ALTHOUGH HOW MUCH HIGHER REMAINS UNCERTAIN. DIFFERENCES ON
THE ORDER OF A FEW INCHES APPEAR CERTAINLY POSSIBLE THOUGH.

SHORT TERM (TONIGHT THROUGH FRIDAY)...
HAVE BROKEN UP THE SHORT TERM PORTION OF THE DISCUSSION TO DEAL
WITH CONDITIONS AHEAD OF THE LONG-HERALDED FRONT FOR SAT.
ANYWAY...FOR THIS EVENING...WHILE SOME CONVECTION HAS DEVELOPED IN
RESPONSE TO DAYTIME HEATING...THIS WILL TEND TO DISSIPATE AFTER
SUNSET. CONSIDERING LACK OF WIDESPREAD CONVECTION...WILL GO AHEAD
AND CANCEL THE FFA. LATER TONIGHT...THINKING THAT THE BULK OF
DYNAMIC FORCING WILL REMAIN TO OUR WEST. THE GFS 12-24 HR FCST
APPEARS TO BE SUFFERING FROM SIGNIFICANT CONVECTIVE FEEDBACK
ISSUES...AND ITS SOLN WAS THUS DISCOUNTED FOR THIS TIME PERIOD.
REMAINING GUIDANCE GNRLY INDICATES A SHORT WAVE...PARTLY
RESPONSIBLE FOR CURRENT DEEP CONVECTION ALONG THE GULF COASTAL
PLAIN...WILL MOVE NWRD INTO THE REGION AFTER MIDNIGHT.
LIFT/MOISTURE CONVERGENCE IN ASSOC/W THIS FEATURE WILL BRING A
CHANCES FOR SHRA MAINLY TO WRN PORTIONS OF THE AREA. ELEVATED
INSTABILITY MAY BE SUFFICIENT TO ALLOW FOR ISOLD DEEP CONVECTION
AND TSRA ACTIVITY...SO WILL LEAVE MENTION IN THE FCST. DRIER AIR
ADVECTING IN FROM THE EAST WILL ALSO HELP LIMIT POTENTIAL FOR
DOWNSTREAM MAINLY STRATIFORM -RA IN ERN AREAS. BUT...WILL STILL
LEAVE IN SLGT CHANCE POPS IN ERN COUNTIES.

POPS/QPF FOR TMRW WILL LIKEWISE FOLLOW THIS SPATIAL TREND...WITH THE
POTENTIAL FOR ANOTHER SHORT WAVE RIDING NEWD ALONG THE TROUGH/RIDGE
INTERFACE. SHRA/TSRA LOOK LIKELY IN THE WRNMOST COUNTIES GIVEN THE
DEGREE OF INSTABILITY ESPECIALLY IN THE AFTN. RIDGING EXTENDING
ALONG THE ERN/SRN FLANK OF THE APPALACHIANS MAY LEAD TO SUFFICIENT
LOW-LVL DRYING IN OUR ERN COUNTIES. HOWEVER...GIVEN CONTINUED
UNCERTAINTY IN THE PRECISE PLACEMENT OF FEATURES AND THE MAKE-UP OF
LOW-LVL MOISTURE...WILL KEEP MDT CHANCE POPS IN THE EAST.

THE FCST FOR THE REMAINING PORTION OF THE SHORT TERM WILL CONTINUE
TO FOLLOW A SIMILAR SPATIAL AND TEMPORAL TREND...INDICATING HIGHER
POPS IN THE WEST A DEFINITE DIURNAL SIGNAL. FOR THOSE THAT MAY BE
NEEDING TO DO WORK OUTSIDE...THURS OR PERHAPS FRI MAY OFFER THE
LEAST CHANCES FOR RAINFALL...ESPECIALLY IN THE EAST.

LONG TERM (FRIDAY NIGHT THROUGH MONDAY)...
THE CLOSED LOW IN THE CENTRAL PLAINS WILL FINALLY GET THE NECESSARY
PUSH AS CAA DEVELOPS IN THE NRN PLAINS AND CENTRAL CANADA BY LATE IN
THE WEEK. AS THE LOW GETS PICKED UP BY MAIN BELT OF MID-LAT
WRLIES...AN ASSOC SFC FRONT SHOULD PUSH ACROSS THE TN VALLEY EARLY
IN THE WEEKEND. ALTHOUGH SPECIFIC TIMING REMAINS UNCERTAIN...CAN
REASONABLY NARROW THE FRONTAL TIMING WITHIN ABOUT A 12 WINDOW AT
THIS POINT...SAY SOMETIME BETWEEN 12Z AND 00Z SAT. ALTHOUGH THE BEST
KINEMATIC FORCING MAY REMAIN JUST TO OUR NORTH...SUFFICIENT
INSTABILITY AND SHEAR AHEAD OF THE FRONT MAY ALLOW FOR SOME STRONG
TSRA ACTIVITY WITH THE FRONTAL PASSAGE.

TEMPS/DEW POINTS WILL DROP A LITTLE BEHIND THIS FIRST FRONT...BUT
NOT SUBSTANTIALLY. THE FCST WILL STILL INDICATE HIGH TEMPS AROUND 80
ON SUN...WITH DEW POINT TEMPS IN THE LOWER 60S (ALTHOUGH THIS WILL
BE SUBJECT TO LATER REVISION). NEVERTHELESS...GOOD AGREEMENT EXISTS
AMONG MED RANGE GUIDANCE IN BUILDING A VERY HIGH AMPLITUDE
RIDGE/TROUGH PATTERN ACROSS NOAM BY EARLY NEXT WEEK. THIS SHOULD
ALLOW A SUBSTANTIALLY COOLER AIRMASS OF POLAR ORIGIN TO DROP IN THE
ERN CONUS AT THAT TIME. 40S DEW POINTS LOOK TO BE POSSIBLE BY TUES
OR PERHAPS WED...WITH FINALLY A FALL-LIKE FEEL IN THE AIR.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

UPDATE/AVIATION...BOYD
PREV DISCUSSION...KDW/03

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
941 PM CDT TUE SEP 22 2009

ALZ001>010-016-TNZ076-096-097-231030-
LAUDERDALE-COLBERT-FRANKLIN AL-LAWRENCE-LIMESTONE-MADISON-MORGAN-
MARSHALL-JACKSON-DEKALB-CULLMAN-MOORE-LINCOLN-FRANKLIN TN-
941 PM CDT TUE SEP 22 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH ALABAMA AND PORTIONS
OF SOUTHERN MIDDLE TENNESSEE.

.DAY ONE...TONIGHT...

SHOWERS AND THUNDERSTORMS WILL CONTINUE TO SLOWLY PUSH NORTH ACROSS
PORTIONS OF NORTHWESTERN ALABAMA TONIGHT...BEFORE DISSIPATING LATE.
SOME LOCALLY HEAVY RAINFALL COULD CAUSE SOME ISOLATED INSTANCES OF
FLASH FLOODING...PRIMARILY NEAR THE ALABAMA AND MISSISSIPPI STATE
LINE...BUT WIDESPREAD FLASH FLOODING IS NOT EXPECTED.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY...

A GENERALLY UNSETTLED WEATHER PATTERN WILL CONTINUE FOR THE REST OF
THE WORK WEEK...AS A MOIST AND UNSTABLE AIRMASS REMAINS ACROSS THE
AREA. ALTHOUGH WIDESPREAD FLOODING IS NOT ANTICIPATED...SCATTERED
THUNDERSTORMS MAY LEAD TO LOCALIZED HEAVY RAINFALL AND ISOLATED
INSTANCES OF FLASH FLOODING. STRONGER STORMS THROUGH THE PERIOD WILL
BE CAPABLE OF PRODUCING FREQUENT LIGHTNING AND GUSTY WINDS.

A COLD FRONT IS FORECAST TO MOVE THROUGH THE AREA DURING THE
UPCOMING WEEKEND...WHICH SHOULD FINALLY PUT AN END TO THE RAINY
CONDITIONS.

.SPOTTER CALL TO ACTION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT PERSONNEL IS
NOT ANTICIPATED AT THIS TIME. SPOTTERS AND EMERGENCY MANAGERS ARE
STRONGLY ENCOURAGED TO PROVIDE REPORTS OF FLASH FLOODING OR CLOSED
ROADS TO THE NATIONAL WEATHER SERVICE.

KTW