FXUS61 KRNK 120907
AFDRNK
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
407 AM EST SAT DEC 12 2009
.SYNOPSIS...
A LARGE HIGH PRESSURE SYSTEM WILL MOVE EAST OFF THE MID ATLANTIC
COAST BY THIS EVENING...AS AN AREA OF LOW PRESSURE TRACKS OUT OF
THE GULF COAST STATES TO THE NORTH CAROLINA COAST BY SUNDAY. THIS
SYSTEM WILL BRING LIGHT SLEET AND FREEZING RAIN TO PORTIONS OF
THE AREA LATE SATURDAY NIGHT INTO EARLY SUNDAY.
&&
.NEAR TERM /THROUGH SUNDAY/...
HIGH PRESSURE TO CONTINUE TO DOMINATE TODAY...WITH SOME MODERATION
IN TEMPERATURES EXPECTED...HOWEVER GUIDANCE HAS BEEN SOLIDLY TOO WARM
RECENTLY AND SO WILL MAINTAIN THAT TREND FOR HIGHS...GOING WITH
THE COOLER MET NUMBERS OR EVEN A DEG COOLER IN SOME SPOTS. WITH
BAND OF CIRRUS ACROSS THE SOUTH THIS MORNING...AND CLOUDS
INCREASING GRADUALLY EVERYWHERE ELSE LATER IN THE DAY AHEAD OF
APPROACHING LOW PRESSURE CURRENTLY NEAR THE GULF COAST...STAYING
WITH COOLER GUIDANCE SEEMS WARRANTED.
MAIN FCST ISSUE CONTINUES TO BE ADVANCING PRECIP WITH ISENTROPIC
LIFT OVERRUNNING COOL WEDGE...WHICH WILL ENHANCE BRIEFLY DUE TO
EVAP COOLING...BEFORE SFC HIGH MOVES FAR ENOUGH OFF COAST BY
MIDDAY SUNDAY TO ALLOW MOST OF WEDGE TO ERODE AT THE SFC. GUIDANCE
CONTINUES TO HOLD ON TO SHALLOW SFC WEDGE AS FAR NORTH AS LYH
THROUGH MID AFTERNOON. HOWEVER MODELS CONTINUE TO SHOW VERY STRONG
WARM NOSE AND WARM ADV ABOVE WEDGE...WITH INCREASING SOUTH TO
SOUTHEAST WINDS AND A SUBTLE TREND HAS BEEN TO ERODE WEDGE A
LITTLE FASTER. AT THE SAME TIME...PRECIP MAY MOVE IN A LITTLE
FASTER THAN EARLIER THOUGHT BUT LEAVE EARLIER AS WELL. SHOULD
REACH NW NC MTNS AS EARLY AS 03Z...AND FAR NORTHEAST PART OF CWA BY
06 OR 07Z. TREND IS FOR A LITTLE MORE QPF...ESPECIALLY IN VCNTY
OF BLUE RDG AND TO THE SOUTHEAST...BUT AT THE SAME TIME A LITTLE COOLER
ALOFT INITIALLY FOR PERHAPS A BIT LONGER PERIOD OF SLEET OR EVEN
LIGHT SNOW MIX IN THE MTNS TO START...BEFORE WARM ENOUGH ALOFT FOR
FRZG RAIN. AND THE TRANSITION FROM FRZG RAIN TO PLAIN RAIN MAY
OCCUR QUICKLY AS THE WARM AIR ALOFT MIXES DOWN IN THE HIGHER
ELEVATIONS OF NC AND SW VA. ONE EXCEPTION MAY BE RIGHT ALONG BLUE
RDG WHERE A PERIOD OF UPSLOPE COOLING MAY KEEP SFC TEMPS JUST COLD
ENOUGH FOR A FEW MORE HOURS. SO DESPITE HIGHER QPF POTENTIAL...THE
ICE ACCRETION MAY BE ABOUT THE SAME IN MOST PLACES GIVEN A LITTLE
MORE SLEET AT START. BOOSTED ICE ACCRETION A LITTLE BIT ALONG
BLUE RDG BUT KEPT BELOW A QTR INCH...BUT OTHERWISE MOST LOCATIONS
WILL SEE AROUND A TENTH OF AN INCH...OR LESS. ANOTHER FACTOR TO
CONSIDER WITH THIS EVENT IS THE FACT THAT GROUND TEMPS ARE
CONSIDERABLY COLDER THAN WITH PREVIOUS EVENT...AND THIS MAY ALLOW
MORE ICING ISSUES ON ROAD SFC. SO DECIDED TO GO FORWARD WITH
WINTER WX ADV AT THIS TIME FOR TONIGHT AND EARLY SUNDAY...WITH
FOCUS ON AREAS NEAR BLUE RDG...AS WELL AS NORTH OF ROANOKE RIVER
IN THE FOOTHILLS AND PIEDMONT...AND THEN WESTWARD TO NEW RIVER
VALLEY AS WELL AS SUMMERS AND GREENBRIER COUNTY WV. ANY LIGHT
SLEET OR SNOW ACCUMS SHOULD ONLY AMOUNT TO A HALF AN INCH OR LESS
AND WILL DISSAPPEAR QUICKLY WHEN CHANGEOVER TO RAIN OCCURS EARLY
TO MID SUNDAY MORNING. CHANGEOVER TO RAIN MAY OCCUR WELL BEFORE
12Z IN NW NC MTNS...AND AS EARLY AS 14Z EVEN IN FAR NORTHEAST
GIVEN LATEST MODEL TRENDS. WEDGE WILL TRY TO HOLD ON BUT SFC TEMPS
SHOULD SLIDE ABOVE FREEZING MARK GIVEN VERY WARM AIR ALOFT. ALSO
PRECIP LOOKS TO BE HEADING EAST A LITTLE FASTER AS WELL...AND THIS
SHOULD HELP TEMPAS THEN BEGIN TO CLIMB AGAIN. THUS ENDED ALL FZRA
IN GRIDS BY 15Z.
&&
.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
5H TROF AXIS EXITS THE FORECAST AREA SUNDAY NIGHT AND AS WEAK SFC
HIGH BUILDS INTO THE REGION...EXPECT ANY LINGERING UPSLOPE SHOWERS
TO QUICKLY COME TO END. EVEN THE COLDEST GFS SOLUTION KEEPS PRECIP
AS RAIN...SO KEPT PRECIP LIQUID. DID BUMP UP WINDS A BIT AS A BRIEF
SHOT OF PRESSURE RISES BEHIND THE SYSTEM AND 850MB WINDS IN EXCESS
OF 30-35KT IS ANTICIPATED.
WEAK RIDING IN PROGRESSIVE UPPER FLOW WILL KEEP CONDITIONS DRY ON
MONDAY. TEMPERATURES SHOULD WARM NICELY UNDER SOUTHWEST FLOW AND
SURGING 1000-850MB THICKNESSES. LEANED TOWARD WARMER GFS MOS VALUES
FOR HIGHS AND WAS TEMPTED TO GO EVEN WARMER CONSIDERING THE MODEL
TRENDS HAVE BEEN UPWARD ON TEMPS...BUT INCREASING MID/HIGH LEVEL
CLOUDS ARE A POTENTIAL MITIGATING FACTOR.
WEAK SHORTWAVES PASSING AHEAD OF SOUTHERN STREAM SOUTHERN PLAINS
SYSTEM...AND INCREASING LOW LEVEL MOISTURE AND WARM AIR ADVECTION
WILL LEAD TO A CHANCE OF SHOWERS MONDAY NIGHT AND TUESDAY. THE NAM
WAS THE MOST AGGRESSIVE ON THE ARRIVAL AND AMOUNT OF PRECIP BY 12Z
TUESDAY...WHILE THE GFS...00Z/12 ECMWF AND 00Z/12 GEM KEEP PRECIP
SOUTH OF OUR AREA MONDAY NIGHT.
DECIDED TO PLACE HIGHEST POPS ASSOCIATED WITH COLD FRONTAL PASSAGE
ON TUESDAY. VERY WARM 850MB TEMPS AHEAD OF THE FRONT SUGGEST PRECIP
WILL REMAIN RAIN...UNTIL STRONG CAA ARRIVES...WITH UPSLOPE SNOW
SHOWERS EXPECTED TUESDAY EVENING. GFS NOT AS COLD COMPARED TO ITS
PREVIOUS RUN BEHIND THIS FRONT WITH 850MB TEMPS FALLING TO AS COLD
AS -8 IN SOUTHEAST WV.
&&
.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
SURFACE HIGH PRESSURE PUSHES INTO THE AREA IN THE WAKE OF THE COLD
FRONT TUESDAY NIGHT INTO WEDNESDAY AND LASTING THROUGH THE END OF
THE WEEK. COLD AIR WILL REMAIN IN PLACE THROUGH THURSDAY NIGHT WITH
TEMPERATURES STAYING BELOW NORMAL.
&&
.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
VFR WEATHER WITH UNRESTRICTED FLIGHT CONDITIONS WILL CONTINUE OVER ALL
TERMINAL FORECAST SITES (KROA...KLYH...KDAN...KBLF...KLWB) THROUGH
THE DAY SATURDAY...BUT WILL GRADUALLY DETERIORATE LATE SATURDAY NIGHT INTO
SUNDAY AS AN AREA OF LOW PRESSURE DEVELOPS OVER THE GULF OF MEXICO
AND THEN LIFTS NORTHEAST ALONG THE SOUTH CAROLINA COAST.
HIGH AND MID LEVEL CLOUDS WILL BEGIN TO ADVECT NORTHEAST INTO THE
MID ATLANTIC REGION ON SATURDAY...BUT LOWER ATMOSPHERE IS NOT
EXPECTED TO BECOME SUFFICIENTLY SATURATED TO BRING CEILINGS DOWN
INTO MVFR RANGE UNTIL AFTER 06Z ON SUNDAY...BY WHICH TIME A WINTRY
MIX OF PRECIPITATION SHOULD BEGIN TO OVERSPREAD THE AREA FROM THE
SOUTHWEST. BY DAYBREAK SUNDAY...MOST TERMINAL FORECAST POINTS
SHOULD SEE CEILINGS AND VISIBILITIES IN THE IFR RANGE...WHICH MAY
VERY WELL PERSIST THROUGH MUCH OF THE DAY.
THE SYSTEM SHOULD EXIT THE REGION SUNDAY NIGHT WITH IMPROVED FLIGHT
CONDITIONS EXPECTED BY MONDAY. ANOTHER WEATHER SYSTEM WILL MOVE
THROUGH THE REGION LATER MONDAY NIGHT INTO TUESDAY...BRINGING AN
INCREASED THREAT OF PRECIPITATION ALONG WITH ANOTHER PERIOD OF
MVFR TO IFR CONDITIONS. STRONG COLD ADVECTION BEHIND THIS SYSTEM
WILL MAINTAIN A THREAT OF UPSLOPE CLOUDS AND MVFR WEATHER ACROSS
THE SOUTHEASTERN WEST VIRGINIA TERMINAL FORECAST POINTS INTO
WEDNESDAY...BUT POST FRONTAL DOWNSLOPING OF WINDS SHOULD ALLOW FOR
RETURN TO VFR WEATHER WITH UNRESTRICTED CONDITIONS TUESDAY NIGHT
INTO WEDNESDAY EAST OF THE BLUE RIDGE.
&&
.RNK WATCHES/WARNINGS/ADVISORIES...
VA...WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 9 AM EST
SUNDAY FOR VAZ010>020-022>024-032>035-045-046.
NC...WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 9 AM EST
SUNDAY FOR NCZ001>003-018-019.
WV...WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 9 AM EST
SUNDAY FOR WVZ043>045.
&&
$$
SYNOPSIS...SK/NONE
NEAR TERM...SK
SHORT TERM...PH/WP
LONG TERM...REB/PH
AVIATION...RCS/WERT
