FXUS64 KHUN 220849
AFDHUN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
249 AM CST TUE DEC 22 2009
.SYNOPSIS...A VERY ACTIVE PATTERN DOMINATES MUCH OF NORTH AMERICA.
WITH THE PASSAGE OF THE TROUGH TO THE EAST...MUCH OF THE NEW ENGLAND
STATES HAVE FINALLY SEEN AN END TO THE SNOWFALL.
HOWEVER...ANOTHER POSSIBLY MORE POWERFUL TROUGH IS DIGGING OVER THE
WESTERN ROCKIES. THIS UPPER TROUGH WILL BE THE MAIN PLAYER FOR THE
TN VALLEY ON CHRISTMAS EVE AND POSSIBLY CHRISTMAS DAY.
MEANWHILE...FREEZING FOG WILL BE LIKELY AGAIN THIS MORNING BEFORE
TEMPS RISE ABOVE FREEZING AND FOG EVAPORATES BY MID MORNING.
&&
.SHORT TERM (TODAY THROUGH THURSDAY NIGHT)...ANOTHER FREEZING FOG
ISSUE WILL LIKELY REMAIN THIS MORNING. TEMPS WILL NOT BE AS
COLD...BUT SLICK SPOTS ON ROADS MAY STILL BE A PROBLEM UNTIL MID
MORNING. WITH SE FLOW DEVELOPING AND AMPLE SUNSHINE...EXPECTING
TEMPS TO WARM INTO THE 50S. FURTHERMORE...A WEDGE OF COOL AIR FROM
THE APPALACHIANS WILL CONTRIBUTE TO KEEPING TEMPS IN THE 50S. WITH
THE SOUTHERLY FLOW IN PLACE...TEMPS WILL MODERATE SOMEWHAT DROPPING
INTO THE LOW TO MID 40S TONIGHT.
MEANWHILE...AS THE UPPER TROUGH OVER THE ROCKIES SPREADS
EASTWARD...SFC CYCLOGENESIS WILL BEGIN OVER WEST TEXAS. THIS WILL
BRING THE POSSIBILITY OF SNOWFALL OVER THAT REGION AND VERY WINDY
CONDITIONS ACROSS ITS TRAJECTORY...DUE TO A HIGH PRESSURE
GRADIENT...AS THIS LOW STRENGTHENS AND HEADS EAST ON WEDNESDAY INTO
THURSDAY. THE NAM/GFS HAS BEGUN SLOWING THE PROGRESS OF THE SYSTEM
DUE TO THE TROUGH'S SHALLOWER PROGRESSION SOUTHWARD (I.E. NOT
DIGGING AS FAR SOUTH) AS OPPOSED TO THE ECMWF WHICH DIGS THE TROUGH
FURTHER SOUTH INTO CENTRAL TX. WHAT THIS MEANS IS THAT THE MAJORITY
OF THE RAINFALL MAY ACTUALLY OCCUR LATER THAN INITIALLY THOUGHT.
AS THE TROUGH LIFTS NORTHEAST ACROSS THE OZARKS...CLOUDS WILL BEGIN
TO OVERSPREAD THE AREA ON WEDNESDAY. TEMPS WILL LIKELY BE ABLE TO
RISE INTO THE 50S DURING THE DAY BEFORE DROPPING SOMEWHAT ON
WEDNESDAY NIGHT INTO THE MID TO UPPER 40S OVER NW AL AND UPPER 30S
IN NE AL (WEDGE EFFECT). AS WITH PREVIOUS SHIFTS...THINKING THAT THE
PRECIP MOVING ACROSS THE AREA WILL ACTUALLY BE DELAYED DUE TO THE
WEDGE OF COOL/DRY AIR IMPINGING OVER THE AREA ON WEDNESDAY. THIS
WEDGE COUPLED WITH THE OVERALL SLOWER MODEL SOLUTION HAS RESULTED IN
MY DECREASING POPS ON WEDNESDAY AND MAINTAINING CATEGORICAL POPS ON
THURSDAY.
BY WEDNESDAY NIGHT...THE TROUGH WILL CLOSE OFF AND LIFT FURTHER
NORTH RESULTING IN AN OCCLUDED SFC LOW PRESSURE SYSTEM CENTERED OVER
IA AT THE SFC. DEPENDING ON THE TIMING OF THIS OCCLUSION...THE
OVERALL PROGRESS OF PRECIPITATION WILL SLOW. DESPITE THE EXPECTED
SLOWDOWN...AS PRECIP INCREASES AND CONTINUES THROUGH
THURSDAY...EXPECTING THE CHANCE OF FLASH FLOODING TO INCREASE. FOR
ONE THING...PWATS ARE ALMOST 1.25 INCHES (FROM BUFR SOUNDINGS).
COUPLE THIS WITH THE ENORMOUS AMOUNT OF WIND SHEAR WITH 0-3KM SRH AT
300-400 M2/S2 AND A STRONG WAA PROFILE...A QLCS LOOKS LIKELY AHEAD
OF THE SFC COLD FRONT. ANOTHER ITEM TO NOTE IS THAT 850 HPA WINDS
ARE AROUND 70-80 KTS AS THE FRONT MOVES OVER THE REGION. INSTABILITY
IS RATHER NEGLIGIBLE...BUT EVEN IF ANY THUNDERSTORMS WITH THE QLCS
DO NOT CAUSE DAMAGING WINDS...ANY DUCTING OR MOMENTUM TRANSFER
DOWNWARD MAY CAUSE HIGH WINDS TO TOPPLE TREES.
THEREFORE...HAVE KEPT THE MENTION OF HEAVY RAINFALL AND ISOLATED
THUNDERSTORMS FOR THURSDAY INTO THURSDAY NIGHT.
.LONG TERM (FRIDAY THROUGH MONDAY)...BY CHRISTMAS DAY...PRECIP
SHOULD BEGIN TO TAPER OFF WITH RESIDUAL SHOWERS DISSIPATING WEST TO
EAST DURING THE MORNING. CLOUDS WILL BE SLOW TO CLEAR DUE TO THE
STACKED/CLOSED UPPER LOW MEANDERING ACROSS MUCH OF THE OHIO RIVER
VALLEY. THIS WILL KEEP CLOUDS FROM CLEARING AT LEAST THROUGH FRIDAY
NIGHT. OTHERWISE...A COLD/RAW DAY IS LIKELY ON CHRISTMAS DAY.
BY SATURDAY...ANY RESIDUAL CLOUDS SHOULD GRADUALLY CLEAR WITH NW
FLOW KEEPING TEMPS IN THE LOW 40S...IF THAT. TEMPS MAY STRUGGLE TO
RISE MUCH ABOVE THE UPPER 30S. NEVERTHELESS WITH CLOUDS CLEARING AND
LIGHT WINDS...TEMPS WILL ONCE AGAIN DROP BELOW FREEZING. IN
FACT...TEMPS MAY ACTUALLY APPROACH THE TEENS ON SUNDAY MORNING IF
THE WINDS COMPLETELY DROP OFF.
SUNDAY THROUGH MONDAY SHOULD BE GENERALLY CLEAR AND COLD WITH SOME
WARMING POSSIBLE ON TUESDAY AHEAD OF APPROACHING FROM THE
GULF OF MEXICO.
&&
.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE 58 43 56 44 58 / 0 0 20 40 80
SHOALS 60 44 57 46 60 / 0 0 40 50 80
VINEMONT 57 44 55 45 58 / 0 0 20 30 80
FAYETTEVILLE 57 42 55 43 56 / 0 0 20 30 70
ALBERTVILLE 58 40 54 41 56 / 0 0 20 30 70
FORT PAYNE 58 37 55 39 56 / 0 0 10 20 70
&&
.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&
$$
SL.77
