FXUS64 KHUN 050257 AAA
AFDHUN
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
857 PM CST FRI DEC 4 2009
.UPDATE...TO LOWER POPS SLIGHTLY...REST OF FCST LOOKS ON TRACK.
&&
.DISCUSSION...MSAS SFC DATA HAS SFC LOW ABOUT 100 MILES SOUTH OF
LA (IN THE GULF OF MEXICO)...WHILE SFC HIGH EXTENDED FROM ERN TX
THRU MO AND INTO THE OHIO VALLEY. AXIS OF UPPER TROF WAS PUSHING
INTO MO/AR ATTM. RADARS WERE NOT SHOWING ANY PCPN IN OUR AREA.
CLOSEST RA/SN WAS OVER LA/SRN MS AND MOVING INTO SW AL. MEANWHILE
TO OUR NW...VERY COLD DRY AIR WAS OVER MO/AR/WRN TN WITH DEWPOINTS
IN THE TEENS WITH SOME SINGLE DIGITS OVER MO.
NEW NAM DATA COMING IN...SHOWS THE SFC LOW STAYING OVER THE GULF
OF MEXICO TONIGHT AND WILL MOVE ACROSS CENTRAL FL BY 12Z SATURDAY.
THINKING ATTM THAT AS THE SFC LOW MOVES EAST OVERNIGHT WILL DRAG
SOME OF THE DRY AIR INTO OUR CWA...ESPECIALLY NW AL. THIS WOULD
MEAN THAT THE CHC OF SNOW...WEST OF I65...WOULD BE IFFY AT BEST.
THE UPPER TROF AXIS IS PROGGED BY THE NAM TO MOVE INTO NW AL BY
SATURDAY MRNG AND THEN EAST OF THE CWA BY EARLY AFTN...AS A LARGE
AREA OF HIGH PRESSURE MOVES ACROSS THE TN VALLEY...WITH CLEARING SKIES.
AS FOR THE ZONES...WILL MAINTAIN CURRENT THINKING AND KEEP BEST CHC
OF SNOW OVER NERN AL/PORTIONS OF SRN MID TN. WILL LOWER POPS TO
40 PERCENT OVER NW AL...50 POPS IN HUN AREA AND 60 POPS OVER NERN
AL. MODEL SOUNDINGS WERE STILL SHOWING ALL SNOW...BUT AS MENTIONED
BEFORE...ANY SNOW WEST OF I65 WILL BE SLIM PICKINGS.
IF NAM IS CORRECT...BY MID MRNG IN NW AL TO ARND NOON IN NERN
AL...THE THREAT OF SNOW IS OVER..IF NOT A LITTLE SOONER.
&&
.PREV DISCUSSION... /ISSUED 555 PM CST FRI DEC 4 2009/
UPDATE...FOR 00Z TAFS
AVIATION...UPPER TROF NOW OVER THE SRN PLAINS WILL CONTINUE TO
MOVE EAST OVERNIGHT. LOWER LEVELS OF THE ATMOSPHERE STILL
RELATIVELY DRY ATTM ACROSS THE CWA BUT WILL QUICKLY MOISTEN UP
AFTER MIDNIGHT...ESPECIALLY FROM HUN AND POINTS EAST. ATMOSPHERE
OVER THE NW CORNER OF AL (INCLUDING MSL) WILL NOT BE AS MOIST.
THUS WILL KEEP THE BEST CHC OF -SN OVER HSV. EXPECTING ONLY LIGHT
SNOW AT HSV AND ONLY FLURRIES AT MSL IF THAT. THE CHC OF SNOW WILL
BEGIN ARND 09Z TONIGHT AND SHOULD BE OVER AT BOTH TAF SITES ARND
12Z/13Z SATURDAY MRNG. UPPER TROF WILL BE EAST OF THE CWA BY SATURDAY
AFTN WITH SKIES BECOMING SKC ARND 18Z AT BOTH MSL/HSV.
PREV DISCUSSION... /ISSUED 305 PM CST FRI DEC 4 2009/
DISCUSSION...
THE TENNESSEE VALLEY AREA IS EXPERIENCING A PARTLY SUNNY FRIDAY
AFTERNOON. DESPITE SOME BREAKS IN THE CLOUDS...CAA HAS HELPED HOLD
TEMPS DOWN INTO THE LOW TO MID 40S IN MOST AREAS. RIGHT NOW...ALL
EYES ARE ON THE GULF AS A SURFACE LOW PRESSURE SYSTEM DEVELOPS AND
MOVE EWRD TONIGHT. PRIMARY FORECAST CONCERN CONTINUES TO BE SNOWFALL
POTENTIAL OVERNIGHT INTO EARLY SATURDAY.
SHORT TERM... (THROUGH SUNDAY)
MODELS CONTINUE TO SHOW DECENT AGREEMENT IN THE SHORT TERM WITH
RESPECT TO THE PLACEMENT OF LARGE SCALE FEATURES. OF COURSE...ITS
THE SMALLER SCALE FEATURES THAT WILL ULTIMATELY DETERMINE THE
SENSIBLE WEATHER IMPACTS ON THE TENNESSEE VALLEY AREA. PAST
SEVERAL MODEL RUNS OF BOTH THE NAM AND GFS HAVE BEEN WAVERING
BETWEEN RELATIVELY LOW QPF /A FEW TENTHS OF AN INCH/ TO ZERO
QPF...WITH THE HIGHEST AMOUNTS OVER THE ERN CWA TAPERING TO LOWER
AMOUNTS OVER THE WRN CWA.
DESPITE THESE SMALL BUT IMPORTANT VARIATIONS IN QPF...THESE MODELS
HAVE GENERALLY BEEN CONSISTENT WITH THE TRACK OF THE SURFACE LOW
ACROSS THE GULF...AND INDICATING SOME PRECIPITATION OVER THE ERN
PORTIONS OF THE CWA. THUS...CONFIDENCE IS INCREASING THAT SOME
AMOUNT OF LIGHT SNOW WILL OCCUR OVERNIGHT AND EARLY SATURDAY...
ESPECIALLY OVER NE ALABAMA AND THE SRN CUMBERLAND PLATEAU IN SRN MID
TENNESSEE.
2 TO 4 INCH SOIL TEMPS REMAIN IN THE MID TO UPPER 40S. WITH AIR
TEMPS FALLING TO AROUND 30 THIS EVENING...IT WILL NOT TAKE VERY LONG
FOR SNOW ACCUMS TO BEGIN ON ELEVATED SURFACES AND IN GRASSY AREAS.
EVENTUALLY...SNOW MAY ALSO BEGIN TO COVER SOME SECONDARY
ROADWAYS...PARTICULARLY IN THE HIGHER TERRAIN. 12Z NAM BUFR
SOUNDINGS FOR HUNTSVILLE AND ALBERTVILLE CONTINUE TO SHOW AROUND A 6
HOUR WINDOW FOR LIGHT SNOW TO OCCUR BEGINNING SHORTLY AFTER MIDNIGHT
AND LASTING SHORTLY AFTER DAYBREAK SATURDAY. THOUGH WINDS WILL BE
RELATIVELY LIGHT...THERE COULD BE SOME OROGRAPHIC ENHANCEMENT IN
AREAS LIKE SEWANEE AND MENTONE.
GIVEN THIS RELATIVE CONSISTENCY AND INCREASED QPF FROM 12Z MODEL AND
HPC GUIDANCE...HAVE BUMPED SNOW ACCUM FORECAST FOR THESE AREAS TO
AROUND AN INCH...WITH AMOUNTS BETWEEN 1 AND 2 INCHES IN THE HIGHEST
TERRAIN. BASED ON THIS FORECAST...A WINTER WX ADVISORY HAS BEEN
POSTED FOR JACKSON...MARSHALL...AND DEKALB COUNTIES IN NE ALABAMA
AND FRANKLIN CO TENNESSEE FOR LATE TONIGHT THROUGH MID MORNING
SATURDAY.
ELSEWHERE ACROSS THE CWA...LESS SNOW IS EXPECTED FARTHER AWAY FROM
THE SURFACE LOW AND BETTER MOISTURE. A SPECIAL WEATHER STATEMENT
/SPSHUN/ HAS BEEN ISSUED FOR COUNTIES OUTSIDE OF THE WINTER WX
ADVISORY TO HIGHLIGHT LIGHT SNOW POTENTIAL THERE. FARTHER WEST...THE
SHOALS AREA MAY NOT SEE ANY SNOW AT ALL.
AS THE SNOW TAPERS OFF...SKIES SHOULD CLEAR QUICKLY ON SATURDAY. SO
ANY SNOW THAT DOES ACCUMULATE WILL LIKELY MELT QUICKLY...EXCEPT IN
SHADED AREAS.
HIGH PRESSURE WILL BUILD IN OVER THE WEEKEND...BRINGING DRY WEATHER
FOR LATE SATURDAY INTO SUNDAY. SOME AREAS MAY EXPERIENCE THEIR FIRST
HARD FREEZE ON SUNDAY MORNING AS TEMPS FALL INTO THE LOW TO MID 20S.
LONG TERM...(MONDAY THROUGH FRIDAY)
UPPER FLOW BECOMES MORE ZONAL NEXT WEEK. OUR NEXT RAIN CHANCE WILL
COME MONDAY AHEAD OF A WEAK FRONTAL SYSTEM. A MORE ROBUST STORM
SYSTEM WILL IMPACT THE AREA DURING THE MIDDLE PART OF NEXT WEEK.
THERE MAY ACTUALLY BE ENOUGH INSTABILITY WITH THIS SYSTEM FOR
TS...AND TS HAS BEEN INCLUDED IN THE FORECAST FOR WED MORNING.
AFTER A BRIEF BREAK ON THURSDAY...BOTH THE ECMWF AND GEFS INDICATE
AN INCREASINGLY UNSETTLED WX PATTERN HEADING INTO THE WEEKEND.
&&
.HUN WATCHES/WARNINGS/ADVISORIES...
AL...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 9 AM CST SATURDAY
FOR THE FOLLOWING ZONES: DEKALB...JACKSON...MARSHALL.
TN...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 9 AM CST SATURDAY
FOR THE FOLLOWING ZONES: FRANKLIN TN.
&&
$$
007
