High Wind in Alabama, December 25, 2012

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
557 PM CST TUE DEC 25 2012

.UPDATE...
FOR 00Z TAFS

&&

.DISCUSSION...
/ISSUED 307 PM CST TUE DEC 25 2012/
TEMPERATURES ACROSS THE TENNESSEE VALLEY ARE HOLDING STEADY IN THE
UPPER 40S TO AROUND 50 SO FAR ON THIS DREARY CHRISTMAS DAY. AN AREA
OF LIGHT/MODERATE SHOWERS WITH ISOLATED THUNDERSTORMS IS PUSHING
NORTHWARD ACROSS THE CWA AS OF THIS WRITING, WITH MUCH MORE
SIGNIFICANT THUNDERSTORMS TO THE SOUTH AND WEST. THIS ROBUST ACTIVITY
IS ASSOCIATED WITH A SFC LOW CENTERED ON THE E TX/WRN LA STATE LINES
THIS AFTN, WITH A WARM FRONT SPRAWLED NORTHEASTWARD ACROSS
CENTRAL/NORTH CENTRAL PORTIONS OF ALABAMA. THE COLD FRONT IS TRAILING
TO THE SOUTH AND WEST OF THE SFC LOW, WITH AN INTENSE DEWPOINT
GRADIENT AHEAD OF/JUST BEHIND THE FRONT (UPPER 6OS AND UPPER
20S/LOWER 30S, RESPECTIVELY). THE PARENT UPPER LOW IS APPROACHING THE
ARKLATEX (AS OF 18Z ANALYSIS) AND THE UPPER TROF WILL BECOME
NEGATIVELY TILTED AS THE SYSTEM EJECTS OFF TO THE NORTH AND EAST.

THIS SYSTEM IS CURRENTLY PRODUCING A LINE OF SEVERE STORMS ACROSS
MS/LA WITH DISCRETE CELLS DEVELOPING AHEAD OF IT, ALL OF WHICH WILL
SHIFT NEWRD ACROSS THE ARKLAMISS, NERN MS, AND INTO THE UPPER TN
VALLEY BY MORNING. THIS WILL TRANSLATE TO STRONG TO SEVERE
THUNDERSTORMS THIS EVENING AND TONIGHT ACROSS NORTHERN ALABAMA/SRN
MIDDLE TN, WITH NEARLY ALL MODELS IN AGREEMENT ON THE TRACK/TIMING
OF THIS SYSTEM. INSTABILITY IS THE ONLY COMPONENT THATS REALLY LACKING
WITH THIS STORM SYSTEM, WITH MUCAPE VALUES APPROACHING 500-600 J/KG
BETWEEN 03-06Z TONIGHT. IMPRESSIVE WIND SHEAR IS PRESENT, IN THE FORM
OF A JET STREAK NOSING INTO THE REGION, AS WELL AS 850MB WINDS
~50-60KTS. HELICITIES RANGE FROM 400-600 M2/S2, WITH HODOGRAPHS
DISPLAYING AMPLE SHEAR (ESPECIALLY IN THE LOWEST 0-1KM). BULK SHEAR
VALUES OF 40-45KTS (IN ADDITION TO OTHER PARAMETERS) INDICATE THE
POSSIBILITY OF SUPERCELLS LATER THIS EVENING AND TONIGHT, WITH THE
QLCS ALONG THE FRONT ITSELF RACING ACROSS THE AREA AROUND MIDNIGHT.
DAMAGING WINDS ARE THE PRIMARY THREAT, WITH A FEW TORNADOES POSSIBLE
(ESPECIALLY IN THE SOUTHERN/EASTERN PORTIONS OF THE CWA). PWATS
APPROACH 1.2" TONIGHT AS WELL, WHICH IS NEARLY 2SD ABOVE NORMAL FOR
THIS TIME OF YEAR. AS A RESULT, AND WITH WINDS TURNING PARALLEL TO
THE FRONT, HEAVY RAINFALL IS LIKELY (WITH ~2" OF QPF OR MORE EXPECTED
BY 12Z TOMORROW MORNING. THIS MAY LEAD TO SEVERAL INSTANCES OF FLASH
FLOODING, BUT WILL HOLD OFF ON FORMALLY ISSUING A FFWATCH WITH THIS
PACKAGE. AS PRESSURE GRADIENTS TIGHTEN ACROSS THE AREA WITH THE
SFC/UPPER LOW JUST TO THE WEST, WILL HOIST A WIND ADVISORY FROM 00Z
THIS EVENING THROUGH 00Z TOMORROW EVENING (ACCOUNTING FOR WINDS
~25MPH AND GUSTS APPROACHING ~30-35MPH).

AS THIS SYSTEM EXITS TO THE NORTHEAST TOMORROW MORNING, HAVE KEPT THE
FORECAST AS-IS REGARDING ANY SNOW MIXING IN WITH LINGERING SPRINKLES.
THE COLDEST OF AIR THAT WOULD SUPPORT ANY WINTRY WEATHER MIXING IN
WILL NOT MAKE IT INTO THE AREA FAST ENOUGH (DESPITE THE CLOSE
PROXIMITY OF THE UPPER LOW TO NW) AS PRECIP WILL END RATHER QUICKLY
ACROSS THE AREA BY MIDDAY TOMORROW.

TEMPERATURES WILL HOVER IN THE UPPER 30S AND LOWER 40S TOMORROW
BEFORE PLUNGING INTO THE UPPER 20S THURSDAY MORNING (WITH WIND CHILL
VALUES IN THE UPPER TEENS).

RIDGING AT THE SFC/ALOFT BUILDS INTO THE TENNESSEE VALLEY THROUGH THE
END OF THE WEEK AS ANOTHER UPPER LEVEL SYSTEM DIPS SOUTHEASTWARD
FROM THE INTERMOUNTAIN WEST. THIS WILL DEVELOP A SFC LOW IN THE WRN
GOMEX, WHICH WILL TRACK ALONG THE GULF COAST STATES FRIDAY INTO
SATURDAY. TEMPERATURES WILL REMAIN TOO WARM FOR ANY WINTRY WEATHER
ACROSS OUR AREA, BUT A COLD RAIN IS LIKELY NEXT WEEKEND WITH DAYTIME
TEMPS IN THE LOWER/MIDDLE 40S. BEYOND THE WEEKEND TIMEFRAME,
GFS/ECMWF DIVERGE SIGNIFICANTLY WITH THE PLACEMENT OF UPPER LEVEL
FEATURES, SO LEFT THE FORECAST AS-IS FOR NOW.

12

&&

.AVIATION...
FOR 00Z TAFS...A BROAD LINE OF RA COVERING MUCH OF NRN AL WITH
EMBEDDED TSRA WILL CONTINUE TO MOVE ACROSS KHSV/KMSL TAF SITES THIS
EVENING. CURRENTLY...THE BACK EDGE OF THE LINE OF CONCENTRATED RA IS
EXPECTED TO MOVE THRU KMSL BY ~02Z AND AT KHSV BY ~03-04Z. MORE
SHOWERS MAY DEVELOP ALONG/AHEAD OF A CDFNT BEFORE IT MOVES ACROSS THE
AREA...BUT LOW CONFIDENCE IN FURTHER DEVELOPMENT AT THIS TIME. THE
FROPA AND ASSOCIATED SHIFT TO SWLY WINDS WILL TAKE PLACE ARND 06-08Z.
IFR CIGS ARE EXPECTED TO PREVAIL THROUGH ABOUT 10-12Z...BEFORE
LIFTING TO MVFR CONDITIONS.

KDW

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...WIND ADVISORY UNTIL 6 PM CST WEDNESDAY FOR ALZ001>010-016.

TN...WIND ADVISORY UNTIL 6 PM CST WEDNESDAY FOR TNZ076-096-097.

&&

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
549 PM CST TUE DEC 25 2012

ALZ001>010-016-TNZ076-096-097-260800-
LAUDERDALE-COLBERT-FRANKLIN AL-LAWRENCE-LIMESTONE-MADISON-MORGAN-
MARSHALL-JACKSON-DEKALB-CULLMAN-MOORE-LINCOLN-FRANKLIN TN-
549 PM CST TUE DEC 25 2012

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

.DAY ONE...TONIGHT...

SHOWERS AND EMBEDDED THUNDERSTORMS WILL ONCE AGAIN OVERSPREAD THE
TENNESSEE VALLEY THIS EVENING AHEAD OF AN APPROACHING STRONG COLD
FRONT. CURRENT INDICATIONS ARE THAT THE MOST WARM AND UNSTABLE AIR
WILL REMAIN CONFINED TO CENTRAL AND SOUTHERN ALABAMA THROUGH
TONIGHT...WHERE THE HIGHER THREAT OF SEVERE WEATHER WILL RESIDE.

HOWEVER...THERE WILL BE A NARROW THREAT WINDOW FOR SEVERE
WEATHER...PRIMARILY SOUTH OF THE TENNESSEE RIVER AND EAST OF
INTERSTATE 65 THROUGH MID TO LATE EVENING. THE MAIN SEVERE WEATHER
THREATS WOULD BE DAMAGING WIND GUSTS TO AROUND 60 MPH ALONG WITH A
SMALL POSSIBILITY FOR TORNADO DEVELOPMENT. OTHERWISE...LOCALLY HEAVY
RAINFALL WILL BE A CONCERN AREAWIDE WITH AMOUNTS OF 2 TO 3 INCHES
LIKELY BY THE OVERNIGHT HOURS.

IN ADDITION TO THIS...A WIND ADVISORY IS IN EFFECT FOR ALL OF
NORTHERN ALABAMA AND SOUTHERN MIDDLE TENNESSEE FROM 6 PM THIS
EVENING THROUGH 6 PM WEDNESDAY.

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

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER CALL TO ACTION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT PERSONNEL IS
POSSIBLE...ESPECIALLY FOR OUR EASTERN AND SOUTHERN COUNTIES THIS
EVENING. THE THREAT FOR SEVERE WEATHER WILL LIKELY DIMINISH ACROSS
MUCH OF THE AREA BY MIDNIGHT.