Flash Flood Warning 29, NWS Huntsville, AL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
548 PM CST TUE DEC 9 2008

.UPDATE...00Z AVIATION DISCUSSION.
&&

.AVIATION...MVFR TO OCCASIONALLY IFR CIGS/VISBYS ARE EXPECTED
THROUGH THE TAF PERIOD. LINE OF SHRA/TSRA HAS REACHED MSL AND WILL
APPROACH HSV WITHIN THE NEXT FEW HOURS. AFTER THE ONSET OF
RA...VISBYS WILL QUICKLY FALL INTO MVFR RANGE AND CIGS WILL
GRADUALLY FALL INTO MVFR RANGE OVERNIGHT. TEMPORARY REDUCTIONS TO
IFR/LIFR VIS ARE POSSIBLE IN +RA. PRECIP IS EXPECTED TO PERSIST
THROUGH THE ENTIRE TAF PERIOD...WITH ONLY OCCASIONAL BREAKS. +RA AND
TSRA CANNOT BE RULED OUT DURING THE DAY WED...BUT CONFIDENCE IS TOO
LOW TO INCLUDE IN TAF ATTM. LLWS WILL BE A CONCERN THRU
DAYBREAK...THEN 2 KFT WINDS SHOULD BEGIN TO SLACK OFF.

&&

.DISCUSSION... /ISSUED AT 354 PM CST TUE DEC 9 2008/
A BUSY WEATHER PATTERN IS IN PLACE ACROSS THE NORTH AMERICAN DOMAIN.
ANALYSIS OF EARLIER 12Z UPPER DATA INDICATED A POWERFUL 500MB JET AT
105 KTS OVER SOUTHERN ARIZONA ROUNDING THE BOTTOM OF SYNOPTIC SCALE
TROUGHING EXTENDED FROM THE NORTHERN PLAINS TO THE DESERT SW. EAST
OF THIS TROUGH...A COLD FRONT EXTENDED FROM THE GREAT LAKES TO THE
TEXAS HILL COUNTRY AND BIG BEND REGION. A WARM FRONT EXTENDED FROM
THE FLORIDA PANHANDLE TO NEAR THE EASTERN MISSISSIPPI GOLDEN
TRIANGLE TO SE ARKANSAS.

LOCALLY...VERY WARM...MUCH WARMER THAN NORMAL AND RECENT
TEMPERATURES HAVE BEEN NOTED THUS FAR ACROSS THE AREA. 3 PM TEMPS
HAVE RISEN TO THE UPPER 60S TO AROUND 70...WITH CORRESPONDING
DEWPOINTS IN THE LOWER 50S. SOUTHERLY WINDS IN THE 15-25 MPH RANGE
HAVE BEEN COMMON ACROSS THE AREA TODAY...WITH 25-35 MPH WINDS WITH
HIGHER GUSTS IN THE MOUNTAINS. THUS A WIND ADVISORY HAS BEEN ISSUED
MAINLY FOR THE HIGHER TERRAIN OF NE ALABAMA AND FRANKLIN COUNTY
TENNESSEE THROUGH 4 AM WED MORNING.

THIS MUCH WARMER THAN AVERAGE WARMTH...MOISTURE...AND THE APPROACH
OF THE COLD FRONT IS SETTING THE STAGE FOR A WET NIGHT ACROSS THE
REGION. RAINFALL AMOUNTS OF 1-3 INCHES ON THE BIG PICTURE HAVE BEEN
PRETTY CONSISTENT IF YOU CONSIDER THE PAST FEW DAYS MODEL OUTPUT.
THIS WILL HOLD FOR TONIGHT AND WEDNESDAY AS THE BULK OF THE RAIN AND
STORMS AFFECT THE AREA. A FEW ITEMS TO CONSIDER WITH HEAVY RAINS AND
POSSIBLE FLOODING...
- THE AREA IS STILL IN A DROUGHT...WE CAN TAKE A BOUT OF MODERATE TO
HEAVY RAINFALL.
- BUT TRAINING OF CELLS OVER A SAME AREA WILL CAUSE PROBLEMS.
- MANY OF THE AREAS DECIDUOUS TREES HAVE SHED THEIR LEAVES...WHICH
COULD CAUSE CLOGGING OF STORM DRAINAGE SYSTEMS AND CONTRIBUTE TO
PONDING OF WATER AND SLOW RUNOFF.
- THE HEAVIEST RAINS ARE EXPECTED FROM LATE TONIGHT INTO WEDNESDAY
MORNING AS THE STORMS MOVE FROM WEST TO EAST.

THE OTHER ITEM OF CONCERN IS A SEVERE WEATHER POTENTIAL. THE SPC
OUTLOOK HAS BEEN PRETTY CONSISTENT OVER THE PAST FEW DAYS...KEEPING
THE HIGHEST POTENTIAL TO THE SOUTH AND WEST OF THIS AREA. HOWEVER
WITH HIGH LEVEL MOISTURE ADVECTION AND CORRESPONDING INSTABILITY...
CANNOT ENTIRELY RULE OUT SEVERE WEATHER. SEVERE WEATHER MODE THIS
EVENING WILL BE FROM STRONG TO MARGINALLY SEVERE WINDS. A BRIEF
TORNADO SPIN UP IS POSSIBLE TOO. STRONG WINDS 50KTS OR HIGHER...
MIXING DOWN FROM BOUNDARY LAYER TO A FEW THOUSAND FEET ABOVE THE
AVERAGE GROUND LEVEL THIS EVENING AND DURING MUCH OF THE OVERNIGHT.
WITH THESE WINDS...STORM RELATIVE HELICITIES STAY ABOVE 400 M/S
THROUGH DAYBREAK WEDNESDAY...PRECEDING THE FRONT. THE FORECAST AREAS
MOST LIKELY TO BE AFFECTED ARE MAINLY NW ALABAMA AND CULLMAN COUNTY.

THE FRONT SHOULD MOVE ACROSS NW ALABAMA AFTER MIDNIGHT...REACH THE
HUNTSVILLE AREA BEFORE DAYBREAK WEDNESDAY...AND REACH NE ALABAMA
DURING THE AFTERNOON. THE PASSAGE OF THE FRONT WILL PRETTY MUCH END
THE SEVERE WEATHER THREAT. HOWEVER...RAINS WILL CONTINUE ESPECIALLY
ACROSS OUR EASTERN AREAS.

A VERY TOUGH FORECAST ARISES ON THE DIFFERENT MODEL HANDLING OF NEW
LOW DEVELOPMENT OVER THE GULF (IN PART GENERATED BY THE STRONG JET
EXITING THE DESERT SW)...AND WHAT ROLE IT WILL HAVE HERE. THE 12Z
NAM WAS THE FASTEST MOVING THE FRONT AND RAINS EAST OF THE REGION BY
WED EVENING...WHILE THE GFS AND ESPECIALLY NGM WERE MUCH SLOWER. THE
EARLIER 00Z ECMWF WITH ITS FASTER TRACK WAS MORE PREFERRED...HOWEVER
THE NEWER 12Z RUN HAS SLOWED DOWN SOMEWHAT TO SOMETHING LIKE THE
GFS. THUS WITH THE DEVELOPMENT OF A GULF LOW...TRACKING TOWARD THE
MID ATLANTIC AND COLD AIR PLUS DYNAMIC COOLING...THERE IS A RISK OF
THE RAIN MIXING WITH OR BECOMING SNOW DURING THURSDAY. THE NEW 18Z
NAM IS TAKING A SLOWER LEAD WITH THE EVENTUAL TRACK OF THE GULF LOW.
HAVE DECIDED TO KEEP A MIX OF RAIN AND SNOW IN THE FORECAST. DO NOT
THINK ACCUMULATIONS ARE IN THE WORKS AT THIS TIME. A LOW CONFIDENCE
IN THIS PART OF THE FORECAST...STAY ABREAST FOR NEW UPDATES ON THIS.

DRYING SHOULD FINALLY TAKE PLACE DURING FRIDAY AS HIGH PRESSURE FROM
THE WEST MOVES EAST. THIS SHOULD PRODUCE NEAR SEASONABLE
TEMPERATURES FOR THE REGION. ANOTHER SYSTEM MOVING TOWARD THE REGION
FROM THE NW WILL BRING A SLIGHT CHANCE OF SHOWERS TO THE AREA FOR
NEXT MONDAY AND TUESDAY.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...WIND ADVISORY UNTIL 4 AM CST WEDNESDAY FOR THE FOLLOWING ZONES:
     CULLMAN...DE KALB...JACKSON...MADISON...MARSHALL...MORGAN.

TN...WIND ADVISORY UNTIL 4 AM CST WEDNESDAY FOR THE FOLLOWING ZONES:
     FRANKLIN TN.

&&

AVIATION...DL
DISCUSSION...RSB

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
851 PM CST TUE DEC 9 2008

ALZ001>010-016-TNZ076-096-097-101100-
LAUDERDALE-COLBERT-FRANKLIN AL-LAWRENCE-LIMESTONE-MADISON-MORGAN-
MARSHALL-JACKSON-DE KALB-CULLMAN-MOORE-LINCOLN-FRANKLIN TN-
851 PM CST TUE DEC 9 2008

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

.DAY ONE...TONIGHT...

SOUTHERLY WINDS OF 15 TO 25 MPH WILL OCCUR AHEAD OF AN
APPROACHING STORM SYSTEM...WITH GUSTS AS HIGH AS 40 MPH IN THE
HIGHER ELEVATIONS. IN ADDITION...SHOWERS AND THUNDERSTORMS WILL
OVERSPREAD THE AREA THIS EVENING. SOME STORMS MAY BE STRONG...
CAPABLE OF PRODUCING HEAVY RAIN AND BRIEF WIND GUSTS TO 50 MPH.

WIDESPREAD RAINFALL OF 1 TO 3 INCHES CAN BE EXPECTED THROUGH
DAYBREAK...WITH LOCALLY HIGHER AMOUNTS UNDER TRAINING THUNDERSTORMS.
PONDING OF WATER ON ROADS AND MINOR FLOODING OF LOW-LYING AREAS WILL
BE POSSIBLE WITH ANY PERSISTENT HEAVY RAIN. FOR MORE
INFORMATION...PLEASE SEE THE FLASH FLOOD WATCH /HUNFFAHUN/.

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

AS THE COLD FRONT PUSHES EASTWARD...THUNDERSTORMS WILL EXIT EASTERN
PORTIONS OF THE CENTRAL TENNESSEE VALLEY EARLY WEDNESDAY. COLDER
AND DRIER AIR WILL FILTER INTO THE REGION ON WEDNESDAY.

ANOTHER STORM SYSTEM DIGGING INTO THE GULF COASTAL REGION...IS
FORECAST TO TRACK FROM THE NORTHERN GULF OF MEXICO AND THROUGH
THE SOUTHEAST STATES WEDNESDAY NIGHT THROUGH THURSDAY. THIS WILL
BRING ANOTHER BATCH OF MODERATE TO HEAVY RAINFALL...WITH THE
POTENTIAL FOR A MIX OF RAIN AND SNOW THURSDAY.

UNCERTAINTY REMAINS QUITE HIGH WITH REGARDS TO STRENGTH AND TIMING
OF THIS PARTICULAR STORM. RESIDENTS ARE URGED TO MONITOR FUTURE
UPDATED FORECASTS AS THIS SYSTEM DEVELOPS.

.SPOTTER CALL TO ACTION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT PERSONNEL
IS POSSIBLE TONIGHT. SPOTTERS ARE ENCOURAGED TO REPORT ANY STORM
DAMAGE OR RAINFALL AMOUNTS TO THE NATIONAL WEATHER SERVICE IN
HUNTSVILLE.

AMP/DJN/JLL