Flash Flooding and Flooding in Alabama, March 9 to 12, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
1209 AM CST WED MAR 9 2011

.UPDATE...
UPDATE FOR 09/06Z AVIATION FORECAST.

&&

.AVIATION...
THERE ARE NO MAJOR CHANGES TO PREVIOUS AVIATION FORECAST REASONING. A
MORE WIDESPREAD AREA OF LIGHT/MODERATE RA AND EMBEDDED TSRA IS NOW
BEGINNING TO SPREAD NEWD ACROSS NORTHERN ALABAMA...WITH CIG/VSBY
EXPECTED TO FALL RAPIDLY FROM VFR TO IFR BY 09/08Z. EMBEDDED TSRA
WILL INTERMITTENTLY IMPACT BOTH TERMINALS...BUT WILL ONLY INCLUDE CB
CLOUD GROUP ATTM...AS EXPECTED COVERAGE DOES NOT WARRANT VCTS/TSRA.
LLWS WILL ALSO REMAIN A CONCERN AT BOTH HSV/MSL THRU 09/13Z...WHEN
LOW-LEVEL JET WILL BEGIN TO WEAKEN AND VEER TO SW. SLIGHT
IMPROVEMENTS IN CIG/VSBY ARE EXPECTED BY MID/LATE MORNING...AS AXIS
OF HEAVY PRECIP MOVES EAST AND LOW-LEVEL WINDS GRADUALLY VEER TO SSW.
HOWEVER...SUBSTANTIAL IMPROVEMENTS IN VSBY WILL LIKELY HOLD OFF UNTIL
PREFRONTAL TROUGH AXIS/WIND SHIFT TO SW SWEEP ACROSS THE AREA EARLY
IN THE AFTERNOON. A BKN MVFR STRATUS DECK WILL REMAIN IN PLACE THRU
END OF THE FORECAST PERIOD.

70/DD

&&

.UPDATE DISCUSSION... /ISSUED 917 PM CST TUE MAR 8 2011/

BASED ON LATEST WATER VAPOR SATELLITE ANIMATION AND A DECREASING
TREND IN THE AREAL COVERAGE/INTENSITY OF PRECIPITATION...THE
TENNESSEE VALLEY APPEARS TO BE LOCATED BENEATH AN AREA OF VERY WEAK
MID-LEVEL SUBSIDENCE...IN THE WAKE OF A LOW-AMPLITUDE SHORTWAVE
TROUGH WHICH MOVED OVER THE REGION EARLIER TODAY. THIS ENVIRONMENT
WILL LIKELY PROVIDE A LULL IN THE PRECIPITATION LASTING THROUGH VERY
LATE TONIGHT...WITH WIDESPREAD RAIN/EMBEDDED THUNDERSTORM DEVELOPMENT
ANTICIPATED BETWEEN 09/06-12Z -- AIDED BY NOCTURNAL ENHANCEMENT TO
THE LOW-LEVEL JET AND STRENGTHENING LARGE SCALE ASCENT. HAVE MADE
ADJUSTMENTS TO POP/WEATHER GRIDS TO REFLECT THIS REASONING...AND ALSO
ADJUSTED HOURLY TEMPERATURE/DEW POINT/RH GRIDS TO REFLECT LOW
TEMPERATURES HAVING ALREADY OCCURRED AND TEMPERATURES STEADY OR
SLOWLY RISING OVERNIGHT. IT IS IMPORTANT TO NOTE THAT ALTHOUGH DEEPER
CONVECTIVE CELLS AFFECTING THE AREA TONIGHT WILL REMAIN ELEVATED
ABOVE A COOL/STABLE SURFACE AIR MASS...GUSTY WINDS AND OCCASIONAL
CLOUD TO GROUND LIGHTNING STRIKES WILL REMAIN POSSIBLE WITH STRONGER
STORMS. WIND ADVISORY AND FLOOD WATCHES APPEAR TO BE ON TRACK AT THIS
TIME...WITH NO CHANGES REQUIRED. UPDATED FORECAST PRODUCTS WILL BE
ISSUED SHORTLY.

70/DD

.PREV DISCUSSION... /ISSUED 324 PM CST TUE MAR 8 2011/

ANALYSIS OF 12Z DATA REVEALED AN UPPER DISTURBANCE OVER THE CENTRAL
ROCKIES...WITH A SURFACE LOW FORMING OVER THE RED RIVER REGION. THE
WATER VAPOR VIEW SHOWED A SURGE OF DEEPER MID/UPPER ALTITUDE MOISTURE
MOVING FROM THE NW GULF OF MEXICO TO THE TENNESSEE VALLEY. ANOTHER
MORE SOUTHERN STREAM MOVING UNDER THE UPPER LOW CONTINUED FROM WEST
OF CA/BAJA TO CENTRAL TEXAS. THE RIGHT REAR REGION OF THIS JET COULD
PLAY A ROLE IN THE DEVELOPMENT OF STRONG/SEVERE CONVECTION FORMATION
LATER ON WEDNESDAY. EXTENSIVE CLOUDS COVERED MUCH OF THE SOUTHERN
STATES...WITH RATHER COOL EARLY MARCH TEMPS HOLDING IN THE LOWER 50S
WITH 5-15 MPH SE WINDS.

SHOWER ACTIVITY HAS ALREADY BEGUN ACROSS MUCH OF THE GREATER NWS
HUNTSVILLE COUNTY WARNING/FORECAST AREA...AS THE FIRST ROUND OF
CONVECTION INITIATING SW OF THE REGION MOVES TO THE NE AT 40-45 MPH.
AS OF THIS WRITING...MOST OF THE LIGHTNING ASSOCIATED WITH THESE
SHOWERS HAS PRETTY MUCH DISSIPATED AS IT NEARED THE TENNESSEE
RIVER...HOWEVER DID GET SOME INTERCLOUD AND A FEW C/G STRIKES OVER
CULLMAN COUNTY EARLY THIS AFTERNOON.

THUS A THREE FOLD THREAT (WINDS/FLOODING/STORMS) IS PRESENT FOR THE
REGION TONIGHT AND MUCH OF WEDNESDAY AS THE SYSTEM OUT WEST NEARS.
THE FIRST PARAMETER...WINDS. THE MODELS HAVE BEEN CONSISTENT SHOWING
BOUNDARY LAYER TO 850MB WINDS FROM THE SOUTH UP TO 70 MPH ESPECIALLY
FOR THE LATE NIGHT HOURS. A WIND ADVISORY HAS BEEN HOISTED FOR THIS
EVENING THROUGH WED MORNING. SE WINDS 15-25 MPH WITH GUSTS OVER 35
MPH ARE POSSIBLE...ESPECIALLY IN THE MOUNTAINS. SOME OF THESE WINDS
THROUGH THIS PERIOD COULD MIX TO THE SURFACE IN THE HEAVIER SHOWERS.

THE BIGGEST AND MORE CERTAIN THREAT IS FLOODING. PRECIP WATER
AMOUNTS IN THIS SYSTEM PER THE NAM/GFS RISE TO 1.3-1.4 INCHES...
WHICH IS PRETTY GOOD FOR THIS SEASON. THIS MOIST ATMOSPHERE ALONG
WITH EXPECTED SHOWER ACTIVITY WILL PRODUCE BIG PICTURE OF 1-3 INCHES
OF RAINFALL...WITH LOCALLY HIGHER AMOUNTS. GIVEN AN ALREADY WET
GROUND AND ELEVATED MINOR/MAINSTREAM RIVER ELEVATIONS...FURTHER
INCREASES ARE LIKELY OVER THE COURSE OF THE NEXT FEW DAYS. MOST AREA
TREES ARE STILL DORMANT...WHICH IN THE WARMER MONTHS HELPS ABSORB
EXCESS RAINFALL. FOR THE SHORTER TIMEFRAME...RUNOFF FROM THE SHOWERS
THIS EVENING AND DURING THE OVERNIGHT COULD PRODUCE FLASH FLOODING
AND FLOODING OF LOW LYING AREAS. NIGHTTIME FLOODING IS ESPECIALLY
DANGEROUS. THE SHOWERS...WITH THE HEAVIEST QPF WILL WIND DOWN FROM
WEST TO EAST DURING WED...MAINLY AFTER NOON. SHOWERS SHOULD END
COMPLETELY BY DAYBREAK THURSDAY. HOWEVER...LARGER STREAM/RIVERS
COULD CONTINUE TO BE AFFECTED FROM RUNOFF FOR A FEW MORE DAYS DOWN
THE ROAD.

A STRONG/SEVERE STORM THREAT RISK. AS MENTIONED ABOVE...THE STRONG
WIND FIELD ALOFT COULD MIX TO THE SURFACE THIS EVENING AND DURING
THE OVERNIGHT IN THE HEAVIER SHOWERS/STRONGER STORMS. CANNOT
ENTIRELY RULE OUT A STORM BRIEFING BECOMING SEVERE WITH DAMAGING
WINDS THE MAIN THREAT...MAINLY FROM WINDS MIXING TO THE SURFACE.
DURING LATE WED MORNING/AFTERNOON...THIS SYSTEMS MAIN COLD FRONT
SHOULD BE MAKING A MOVE ACROSS THE VALLEY. THE NAM AND OUR LOCAL WRF
IS PICKING UP ON INSTABILITY DEVELOPING WITH THIS FEATURE. THUS
STORMS THAT FIRE WED AFTERNOON COULD BECOME STRONG TO SEVERE...WITH
THE MAIN RISK SEVERE HAIL AND DAMAGING WINDS. THE STRONG/SEVERE
THREAT WILL END AFTER MIDNIGHT WED EVENING.

DRY WEATHER...WITH GENERALLY COOLER THAN AVERAGE TEMPS FOR THIS TIME
OF MARCH IS EXPECTED THE REST OF THE WORK WEEK...AS HIGH PRESSURE
BUILDS IN FROM THE WSW. GIVEN CLEARING SKIES BY THURSDAY NIGHT...LOW
TEMPS BY FRIDAY MORNING COULD BE FLIRTING WITH THE FREEZING MARK.
OUR AVERAGE LAST 'HARD' FREEZE IS DURING EARLY APRIL. HIGH TEMPS
HOWEVER BY THE WEEKEND WILL WARM TO NEAR NORMAL READINGS IN THE MID
60S.

ANOTHER FRONTAL BOUNDARY WILL BRING A CHANCE OF SHOWERS/STORMS BY
NEXT MONDAY...WITH DRY CONDITIONS RETURNING ON TUESDAY.

HYDROLOGY...WE ISSUED A FLOOD WARNING FOR THE TENNESSEE RIVER AT
FLORENCE EARLIER TODAY. THE LATEST STAGE WAS ABOUT 16 FT WITH THE
RIVER STILL RISING AND EXPECTED TO REACH FLOOD STAGE (18 FT) WED
AFTERNOON. IT IS EXPECTED TO RISE FURTHER...NEARING 20 FT BY FRI
MORNING. WITH THE AMOUNTS OF RAINFALL EXPECTED OVER THE NEXT 24
HOURS...IT IS POSSIBLE THAT SOME OF OUR FLASHIER STREAMS AND RIVERS
WILL ONCE AGAIN REACH FLOOD STAGE ON WED/THURS...SUCH AS THE INDIAN
CREEK IN MADISON...THE BIG WILLS CREEK IN FT PAYNE AND THE PAINT ROCK
RIVER.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...

AL...FLOOD WATCH THROUGH LATE WEDNESDAY NIGHT FOR THE FOLLOWING ZONES:
     DEKALB...JACKSON...MADISON...MARSHALL.

     WIND ADVISORY UNTIL NOON CST WEDNESDAY FOR THE FOLLOWING ZONES:
     DEKALB...JACKSON...MADISON...MARSHALL.

     FLOOD WATCH THROUGH LATE WEDNESDAY NIGHT FOR THE FOLLOWING ZONES:
     COLBERT...CULLMAN...FRANKLIN AL...LAUDERDALE...LAWRENCE...
     LIMESTONE...MORGAN.

     WIND ADVISORY UNTIL 9 AM CST WEDNESDAY FOR THE FOLLOWING ZONES:
     COLBERT...CULLMAN...FRANKLIN AL...LAUDERDALE...LAWRENCE...
     LIMESTONE...MORGAN.

TN...FLOOD WATCH THROUGH LATE WEDNESDAY NIGHT FOR THE FOLLOWING ZONES:
     FRANKLIN TN...LINCOLN...MOORE.

     WIND ADVISORY UNTIL NOON CST WEDNESDAY FOR THE FOLLOWING ZONES:
     FRANKLIN TN...LINCOLN...MOORE.

&&

AVIATION...70/DD
UPDATE...70/DD
PREV DISCUSSION...RSB

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
533 PM CST TUE MAR 8 2011

ALZ001>010-016-TNZ076-096-097-091100-
LAUDERDALE-COLBERT-FRANKLIN AL-LAWRENCE-LIMESTONE-MADISON-MORGAN-
MARSHALL-JACKSON-DEKALB-CULLMAN-MOORE-LINCOLN-FRANKLIN TN-
533 PM CST TUE MAR 8 2011

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

.DAY ONE...TONIGHT...

HEAVY SHOWERS AND THUNDERSTORMS WILL BECOME MORE NUMEROUS TONIGHT
ACROSS NORTHERN ALABAMA AND SOUTHERN MIDDLE TENNESSEE. THE THREAT FOR
FLASH FLOODING WILL INCREASE ESPECIALLY AFTER MIDNIGHT WHEN RAIN AND
THUNDERSTORM ACTIVITY WILL BE THE HEAVIEST. TOTAL RAINFALL AMOUNTS
AROUND TWO TO THREE INCHES ARE POSSIBLE. A FLOOD WATCH HAS BEEN
ISSUED FOR THIS POTENTIAL FROM LATE TONIGHT INTO LATE WEDNESDAY
NIGHT. IN ADDITION...WIND GUSTS MAY APPROACH SEVERE THRESHOLDS IN A
FEW OF THE STRONGER STORMS.

OUTSIDE OF THE THUNDERSTORM ACTIVITY...GENERAL WINDY CONDITIONS WILL
DEVELOP...WITH SOUTHERLY SUSTAINED WINDS OF 20 TO 30 MPH...AND GUSTS
AROUND 40 MPH. A WIND ADVISORY HAS BEEN ISSUED FOR TONIGHT INTO
WEDNESDAY MORNING.

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

A WIDE BAND OF HEAVY SHOWERS AND THUNDERSTORMS WILL PROGRESS
EASTWARD...FINALLY EXITING THE AREA TO THE EAST BY LATE WEDNESDAY
MORNING. AN ADDITIONAL ROUND OF SCATTERED THUNDERSTORMS MAY ALSO
DEVELOP ALONG A COLD FRONT WEDNESDAY AFTERNOON. A FEW OF THESE
THUNDERSTORMS MAY BECOME SEVERE...WITH LARGE HAIL AND DAMAGING WINDS
THE PRIMARY THREATS.

ANY ADDITIONAL HEAVY RAINFALL COULD FURTHER EXACERBATE THE FLASH
FLOODING OR FLOODING POTENTIAL. GIVEN THE HEAVY RAINFALL EXPECTED
OVER THE PERIOD...MINOR TO MODERATE FLOODING MAY OCCUR IN SOME
STREAMS AND RIVERS.

.SPOTTER CALL TO ACTION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT PERSONNEL
IS POSSIBLE TONIGHT AND WEDNESDAY.

SL