FXUS64 KHUN 220549 AAC
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
1149 PM CST MON DEC 21 2009

.UPDATE...
UPDATED FOR THE 06Z AVIATION DISCUSSION.

&&

.AVIATION.../06Z TAFS/
CONFIDENCE IS INCREASING THAT MVFR VISIBILITIES DUE TO FOG WILL
OCCUR AT BOTH KHSV AND KMSL THROUGH 15Z. WITH LOW TEMPERATURES
NEAR FREEZING...LIGHT ICE ACCUMULATIONS WILL ALSO BE POSSIBLE
MAINLY BETWEEN 08-13Z DUE TO THE FOG. VFR CONDITIONS WILL RETURN
AFTER 15Z AS ONLY HIGH CLOUDS CAN BE EXPECTED.

&&

.DISCUSSION.../ISSUED 946 PM CST MON DEC 21 2009/ 
CLEAR SKIES CONTINUE ACROSS THE TENNESSEE VALLEY THIS EVENING AS
PATCHY FOG BEGINS TO DEVELOP OVER PORTIONS OF THE AREA. EVEN
THOUGH SOME HIGHER CLOUDS ARE MOVING EAST NEAR THE MISSISSIPPI
RIVER VALLEY...DECIDED TO GO AHEAD AND INCLUDE PATCHY FOG AS WELL
AS FREEZING FOG IN THE FORECAST OVERNIGHT. ONCE AGAIN...FOG
FORMATION WILL NEED TO BE MONITORED AS THIS COULD CAUSE SOME ICY
SPOTS IN AREAS WHERE TEMPERATURES FALL BELOW FREEZING TOWARDS
DAYBREAK.

TEMPERATURES HAVE FALLEN RAPIDLY ONCE AGAIN THIS EVENING AND LOWS
WERE ADJUSTED TO REFLECT THIS TREND. DO THINK ONCE THESE HIGH
CLOUDS MOVE INTO THE AREA OVER THE NEXT SEVERAL HOURS...
TEMPERATURES WILL REMAIN IN THE LOWER 30S. OTHERWISE...NO
ADDITIONAL CHANGES WERE MADE AT THIS TIME.

&&

.PREV DISCUSSION.../ISSUED 332 PM CST MON DEC 21 2009/ 
PRIMARY FORECAST CONCERNS CENTER AROUND IMPACTS FROM A POTENT
STORM THAT WILL AFFECT THE FORECAST AREA OVER THE MIDDLE AND
LATTER PORTIONS OF THIS WEEK.

SHORT TERM (THROUGH WEDNESDAY)...
THE LARGE EXPANSE OF STRATUS TO OUR NORTH IS EXPECTED TO REMAIN TO 
THE NORTH THROUGH THIS EVENING AS LOW LEVEL SUBSIDENCE PERSISTS. 
HIGH CLOUDS WILL REMAIN IN PLACE THOUGH...AND WILL LIKELY THICKEN 
OVER THE NEXT 24 TO 36 HOURS. OUR NEXT WEATHER MAKER IS CURRENT 
TAKING SHAPE IN THE FORM OF A DEEPENING TROUGH OVER THE ROCKY 
MOUNTAIN STATES. AS IT MOVES EAST...A VAST WAA REGIME WILL TAKE HOLD 
OVER THE GREAT PLAINS AND MID SOUTH REGION AS LEE SIDE TROUGHING 
ENSUES. DRY CONDITIONS WILL PERSIST THROUGH TUESDAY NIGHT AS THE 
SURFACE HIGH HOLDS SWAY OVER THE REGION. 

BY WEDNESDAY THE SURFACE LOW WILL BEGIN TO MAKE EASTWARD PROGRESS 
ACROSS TEXAS. SOUTHERLY WINDS WILL INCREASE THANKS TO A TIGHTENING 
PRESSURE GRADIENT ALONG THE MS RIVER VALLEY. WAA SHOWER ACTIVITY 
WILL BE POSSIBLE DURING THE DAY...ESPECIALLY IN THE WESTERN HALF OF 
THE CWA CLOSER TO THE SURFACE LOW AND THE BETTER LOW LEVEL MOISTURE 
FETCH. AS MENTIONED BY THE PREV SHIFT...THE ONSET OF PRECIP IN 
EASTERN SECTIONS MAY BE DELAYED BY COOL DRY AIR BEING WEDGED AROUND 
THE APPALACHIANS BY THE SURFACE HIGH POSITIONED OVER THE MID 
ATLANTIC. LEANED CLOSE TO SREF POPS FOR THIS TIME PERIOD.

LONG TERM...(THURSDAY THROUGH MONDAY)...
RAIN CHANCES WILL RAMP UP IN EARNEST ON CHRISTMAS EVE AS THE SURFACE 
LOW ADVANCES NEWD AND OCCLUDES OVER THE MIDWESTERN STATES. FOR THIS 
FORECAST PACKAGE...BELIEVE THE SLOWER ECMWF SOLUTION IS MOST 
REALISTIC. THUS THE BEST CHANCE OF RAIN IS EXPECTED FOR THE 
AFTERNOON AND EVENING HOURS CHRISTMAS EVE. THE PRIMARY CONCERN WITH 
THIS SYSTEM IS THE THREAT OF HEAVY RAINFALL...WITH GFS AND ECMWF QPF 
OUTPUT AROUND 1 TO 2 INCHES AREA WIDE. LOCALLY HIGHER AMOUNTS WILL 
BE POSSIBLE. AS FAR AS SEVERE WX IS CONCERNED...A FAMILIAR HIGH 
SHEAR/LOW INSTABILITY ENVIRONMENT WILL BE IN PLACE AHEAD OF THE 
SYSTEM. PROGGED LI VALUES ARE MINIMAL AND EVEN ELEVATED CAPE VALUES 
WERE LESS THAN IMPRESSIVE. NEVERTHELESS GIVEN THE STRONG FORCING...A 
SQUALL WITH OCCASIONAL THUNDER IS ANTICIPATED TO MOVE WEST TO EAST 
ACROSS THE AREA THURSDAY EVENING...WITH DAMAGING WINDS POSSIBLE. 
EVEN OUTSIDE OF CONVECTION...GRADIENT WINDS WILL INCREASE ON 
THURSDAY MORNING WITH GUSTS OVER 40 MPH POSSIBLE DURING THE 
DAY...ESPECIALLY IN THE HIGHER ELEVATIONS. WITH THE GROUND ALREADY 
SATURATED BY HEAVY RAINFALL...EVEN GUSTY WINDS OUTSIDE OF STORMS 
WILL BE CAPABLE OF DOWNING TREES.

SHOWERS WILL TAPER OFF FROM WEST TO EAST EARLY CHRISTMAS MORNING AS 
A COLD FRONT APPROACHES. LINGERING ISOLATED SHOWERS WILL BE POSSIBLE 
ON CHRISTMAS DAY...WITH HIGHS ONLY IN THE UPPER 40S TO AROUND 50. 
STRONGER CAA WILL MAKE THE WEEKEND AFTER CHRISTMAS EVEN COLDER. 
HIGHS ON SATURDAY MAY STRUGGLE TO GET OUT OF THE 30S. GENERALLY DRY 
CONDITIONS ARE EXPECTED FROM SATURDAY THROUGH MONDAY.

&&

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

$$

AVIATION/DISCUSSION...KNS 
PREV DISCUSSION...DL