FXUS61 KRNK 110901
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
401 AM EST FRI DEC 11 2009

.SYNOPSIS...
A LARGE HIGH PRESSURE SYSTEM WILL MOVE EAST ACROSS THE AREA TODAY 
AND TONIGHT...THEN OFF THE COAST SATURDAY. LOW PRESSURE WILL TRACK 
SOUTH OF THE REGION OVER THE WEEKEND AND WILL LIKELY SPREAD WINTRY 
PRECIPITATION INTO OUR AREA LATE SATURDAY NIGHT INTO SUNDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
WATER VAPOR LOOP SHOWS A BROAD TROUGH ACROSS MUCH OF THE EASTERN 
U.S...BUT WITH SHORT WAVE ENERGY LIFTING NORTHEAST AND LEAVING 
STRONG ZONAL FLOW OVER THE MID ATL REGION.  IN THIS ZONAL 
FLOW...HIGH PRESSURE OVER THE TN VALLEY THIS MORNING SHOULD FINALLY 
MOVE OVER THE AREA TONIGHT. WINDS WILL CONTINUE TO BREEZY 
TODAY...AND WITH AN ARCTIC AIRMASS IN PLACE OVER THE AREA...IT WILL 
BE A COLD DAY. LOOKS LIKE HIGHER ELEVATIONS OF GREENBRIER AND BATH 
COUNTIES WILL CONTINUE TO SEE WIND CHILLS BELOW MINUS 5 DEGREES 
THROUGH DAYBREAK...SO WILL KEEP THE WIND CHILL ADVISORY GOING THRU 
1000 AM. HOT SPRINGS AIRPORT...ELEVATION NEARLY 3800 FEET...HAS HAD 
WIND CHILLS AROUND MINUS 10 DEGREES ALL NIGHT. THERE IS A THREAT FOR 
SOME THICKER CIRRUS ON THE SOUTH SIDE OF THE UPPER JET...SO HAVE 
PARTLY CLOUDY SKIES ACROSS NW NC. OTHERWISE...SHOULD BE A SUNNY DAY. 

SFC WINDS WILL DECOUPLE TONIGHT FOR ALL BUT THE RIDGES...AND OTHER 
THAN SOME THINNER CIRRUS POSSIBLE...LOOKS LIKE THERE WILL BE GOOD 
RADIATIONAL COOLING CONDS. DEWPOINTS WILL STILL BE IN THE SINGLE 
DIGITS. WITHOUT SNOW COVER...I DONT THINK WELL SEE ANY LOWS DROP
INTO THE SINGLE DIGITS...BUT EASILY THE TEENS. HAVE GONE CLOSER TO
THE COLDER MET GUIDANCE FOR LOWS.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
SFC HIGH PRESSURE CENTERED OVERHEAD EARLY SATURDAY WILL GIVE WAY TO
APPROACHING WEAK LOW PRESSURE CENTER MOVING NORTHEASTWARD OUT OF
THE GULF COAST REGION BY THE EVENING. PLENTY OF SUN DURING THE DAY
AND LOW DEW PTS SHOULD LEAD TO MILDER TEMPS BUT STILL A BIT BELOW
NORMAL. WITH EAST FLOW GIVEN SFC HIGH POSITION OVER ERN VA...KEPT
TEMPS AT OR BELOW COOLER MET GUIDANCE.

FAIRLY GOOD AGREEMENT IN MODEL TIMING OF ONSET OF OVERUNNING
PRECIP FROM SOUTHWEST...WITH WIDESPREAD LIGHT PRECIP REACHING THE
NW MTNS BY AROUND 06Z..THEN ACROSS ENTIRE CWA BY 12Z. MEASURABLE
MAY BE A LITTLE SLOWER TO OCCUR THAN MODELS SUGGESTING GIVEN VERY
DRY SFC DEW PTS...BUT NOT CONFIDENT ENOUGH AT THIS POINT TO
INDICATE MUCH OF A DIFFERENCE FROM WHAT BOTH NAM AND GFS ARE
SHOWING. DID INCREASE POPS TO CATEGORICAL OVER THE SW HALF OF CWA
BY 12Z AND THEN EVERYWHERE FOR FIST PART OF SUNDAY. SECOND S/WV
IN WRLY FLOW HELPS TO KICK PRECIP EAST FAIRLY QUICKLY NOW BY SUN
AFTERNOON...WITH ONLY LINGERING UPSLOPE SHWRS IN FAR WEST BY END
OF THE DAY.

WITH SFC HIGH OVER MID-ATLANTIC COAST AS PRECIP BEGINS AND VERY
LOW SFC DW PTS...STILL LOOKING AT SOME WINTRY P-TYPES AS PRECIP
MOVES IN AFTER MIDNIGHT AND AT LEAST THROUGH THE MORNING HOURS
SUNDAY. VERY WARM ALOFT HOWEVER...AND FCST SOUNDINGS SUGGEST ONLY
BRIEF PERIOD OF SLEET AT THE START...MAYBE SOME SNOW MIXED IN
ACROSS THE NORTH...BEFORE CHANGING TO FREEZING RAIN OR PLAIN RAIN.
DETAILS ADN TIMING OF WEDGE BREAKING ARE HARD TO DEPICT EXACTLY AT
THIS POINT...BUT TRIED TO INDICATE SUB-FREEZING TEMPS HOLDING ON
THROUGH MID MORNING NEW RIVER VALLEY TO ABOUT NOON ACROSS FOOTHILL
REGION NORTH OF ROANOKE RIVER...SUCH AS LYH AREA. LQUID EQUIV
VALUES A QUARTER OF AN INCH OR LESS AND GIVEN SO WARM ALOFT...NOT
EXPECTING VERY EFFICIENT ICE ACCRETION. SOME UPSLOPE COOLING ALONG
BLUE RDG BUT THIS IS FAIRLY SHORT-LIVED SO ONLY SUBTLE ENHANCEMENT
OF ANY LIGHT ICING EXPECTED THERE. OVERALL...ICE ACCUM AMOUNTS
EXPECTED TO REMAIN BELOW WARNING CRITERIA...BUT WILL HAVE TO
CONSIDER ISSUANCE OF ADVISORY. WITH TEMPS WARMING BY END OF SUNDAY
INTO LOW 40S MOST PLACES...ANY ICE SHOULD QUICKLY MELT. NOT
EXPECTED MUCH WIND BEHIND THIS SYSTEM EITHER THIS TIME...MAYBE
10-15 MPH. WARM ADV ACTUALLY BEGINS BEHIND THIS BY SUNDAY NIGHT
WELL IN ADVANCE OF NEXT SYSTEM...AND AS A RESULT INCREASED THE
LOWS FOR SUN NIGHT A FEW DEG. ANY LINGERING UPSLOPE SHWRS IN FAR
WEST QUICKLY FADE AS FLOW TURNS WEST OR WSW OVERNIGHT AND ANTICYCLONIC
FLOW ALOFT RETURNS. MOST OF THESE WILL BE RAIN SHWRS GIVEN WARMING
850 TEMPS.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
AREA OF LOW PRESSURE MOVING OUT OF THE GULF OF MEXICO...AND
PROGGED TO AFFECT THE CWA ON TUESDAY TAKING A MORE SOUTHWARD JOG
ON THE ECMWF/GFS/GEFS. HOWEVER...WITH A WEAK 50/50 LOW IN PLACE AND
UPPER RIDGING OVER THE NORTH ATLANTIC...THIS LOW HAS A BETTER
CHANCE OF TAKING MORE OF SW-NE TRACK INTO THE MIDDLE ATLANTIC THAN
THIS WEEKENDS SYSTEM. HOWEVER...DOES NOT APPEAR TO BE MUCH COLD
AIR IN PLACE...UNTIL AFTER THIS SYSTEM EXITS TO THE EAST. IN
FACT...THE BIGGEST CHANGE TO THE LONG TERM WAS TO WARM TEMPS
MONDAY AND TUESDAY...AS H85 ADJUSTMENTS ARE SEVERAL DEGREES WARMER
THAN CURRENT MEX NUMBERS AND GOING FORECAST. COLDER AIR WILL NOT
BE ABLE TO PENETRATE THE MIDDLE ATLANTIC AND SOUTHEAST....UNTIL
ONE OF THE SHRTWVS FROM THE SW CONUS IS ABLE TO PHASE/MERGE WITH
THE POLAR VORTEX MOVING INTO HUDSON BAY. ATTM...APPEARS THAT THE 
PREDOMINATE PRECIP TYPE WOULD BE RAIN FOR TUESDAY'S SYSTEM...WITH 
SNOW IN SE WEST VA WITH UPSLOPE FLOW INTO WED MORNING. BOTH ECMWF/GFS
ALLOW SUB -10C H85 AIR TO ARRIVE WEDNESDAY ACROSS SE WEST 
VA...ALTHOUGH THE BITE WILL BE TAKEN OFF OF THIS CHILL THANKS TO 
DOWNSLOPING FURTHER EAST. ONCE THE TROUGH AXIS MOVES INTO OUR 
CWA...THE DOWNSLOPE COMPONENT WILL WEAKEN...ALLOWING PERHAPS SOME 
LOWER 40S MAX TEMPS TO REACH INTO THE PIEDMONT AND SOUTHSIDE 
THURSDAY. CONSIDERING THE HIGH LATITUDE BLOCKING IN PLACE...THE 
EASTERN TROUGHINESS COULD BE THE BEGINNING OF A PATTERN 
CHANGE...WITH THE COLDER H85 VALUES OFF THE OP GFS FAVORED OPPOSED 
TO THE WARMER ECMWF PAST THURSDAY.

&&

.AVIATION /09Z FRIDAY THROUGH TUESDAY/...
VFR CONDITIONS AT ALL AIRPORTS THROUGH FRIDAY NIGHT UNDER DRY HIGH
PRESSURE EASING UP FROM THE SOUTHWEST. CENTER OF HIGH REMAINS TO
THE SOUTHWEST HOWEVER...AND STRONG WESTERLY GRADIENT STILL EXISTS
ALOFT. SOME DECOUPLING TOWARD MORNING IN THE PIEDMONT...OTHERWISE
WINDS WILL REMAIN GUSTY THROUGH THE NIGHT. LWB/LYH/DAN MAY SEE
GUSTS DIMINISH AT TIMES EARLY THIS MORNING...BUT THEN INCREASING AGAIN
DURING THE DAYLIGHT HOURS AS GRADIENT AT 850MB STILL FAIRLY STRONG
AND GOOD MIXING EXPECTED. BY THIS EVENING...SFC HIGH SETTLES IN MORE
OVERHEAD AND MORE COMPLETE DECOUPLING EXPECTED AT ALL SITES SO
FINALLY WILL SEE WINDS DROP OFF TO LIGHT WESTERLY. SOME VERY THIN
CIRRUS POSSIBLE BY MORNING...BUT NOT WORTHY OF MENTION IN TAFS.

VFR WEATHER WITH UNRESTRICTED FLIGHT CONDITIONS ARE EXPECTED OVER
ALL TERMINAL FORECAST SITES (KROA...KLYH...KDAN...KBLF...KLWB)
AGAIN SATURDAY...BUT THEN EXPECTED TO DETERIORATE SATURDAY NIGHT
INTO SUNDAY. AN AREA OF LOW PRESSURE IS EXPECTED TO DEVELOP OVER
THE GULF OF MEXICO SATURDAY...THEN SLIDE TO ALONG THE SOUTH
CAROLINA COAST BY SUNDAY. THIS WILL BRING AN INCREASE IN CLOUD
COVER SATURDAY NIGHT AND A THREAT FOR A WINTRY MIX OF
PRECIPITATION OVER THE MID ATLANTIC REGION SUNDAY AND LIKELY
RETURN OF IFR CONDITIONS FOR MUCH OF SUNDAY. THE SYSTEM SHOULD EXIT
THE REGION SUNDAY NIGHT WITH IMPROVED FLIGHT CONDITIONS MONDAY.

&&

.HYDROLOGY...
MINOR FLOODING IS EXPECTED TO END THIS MORNING ALONG THE JAMES RIVER 
NEAR BREMO BLUFF...AND THE ROANOKE RIVER AT RANDOLPH...WHILE MINOR 
FLOODING ALONG THE DAN RIVER AT SOUTH BOSTON IS EXPECTED TO END LATE 
THIS AFTERNOON. THE FLOOD STATEMENT (FLSRNK) WILL BE UPDATED BY 630 
AM.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...WIND CHILL ADVISORY UNTIL 10 AM EST THIS MORNING FOR VAZ020.
NC...NONE.
WV...WIND CHILL ADVISORY UNTIL 10 AM EST THIS MORNING FOR WVZ045.

&&

$$
SYNOPSIS...PM/JJ
NEAR TERM...JJ 
SHORT TERM...SK 
LONG TERM...KM
AVIATION...KM/SK 
HYDROLOGY...