FXUS61 KRNK 100234
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
934 PM EST WED DEC 9 2009

.SYNOPSIS...
A LARGE HIGH PRESSURE SYSTEM WILL BUILD EAST INTO THE MID ATLANTIC
REGION OVERNIGHT INTO FRIDAY BRINGING STRONG WINDS AND MUCH
COLDER WEATHER.
&&

.NEAR TERM /THROUGH THURSDAY/...
85H FRONT NOW CROSSING THE MOUNTAINS ALONG WHICH WINDS HAVE 
DIMINISHED QUITE A BIT FROM EARLIER. HOWEVER AS ADDED PRESSURE RISES 
START TO PUSH EAST WITH THE DEEPER COLD ADVECTION EXPECT THE 
INVERSION LAYER TO LOWER ALLOWING ANOTHER SURGE OF W/NW WINDS THRU 
LATE TONIGHT ESPCLY SW. ALTHO MARGINAL WARNING SPEEDS LIKELY THINK 
NOCTURNAL TIMING WITH COLD ADVECTION A GOOD BET TO PUSH SPEEDS ABOVE 
40 KTS NW NC AFTER MIDNIGHT. ELSW WILL LEAVE THE GOING ADVISORY IN 
THE WEST AS STILL SEEING GUSTS NEAR CRITERIA ON THE RIDGES. 

LOW LEVEL MOISTURE BEHIND THE FRONT LOOKS PRETTY SHALLOW AND GIVEN
MORE OF A WESTERLY COMPONENT THINK LESS -SHSN COVERAGE IN THE
UPSLOPE REGIONS THRU MORNING. MAY LEAVE IN A LOW END CHANCE POP NW
OTRW REMOVING CHANCES FOR THE MOST PART UNTIL AFTER MIDNIGHT WHEN
THE COLD AIR IS DEEPER. UPPER JET AND MOUNTAIN WAVE ACTION ALSO
CREATING QUITE A BIT OF A MID/HIGH CLOUD SWATH SOUTHERN HALF AND
EXPECT SOME OF THIS TO SHIFT NORTH AS THE VORT AXIS LIFTS OUT
LATER ON. THIS IN COMBO WITH MIXING LIKELY TO HOLD TEMPS UP SOME
ESPCLY EAST AS THE COLDER AIR GETS SLOWED UP ALONG THE BLUE RIDGE
OVERNIGHT. THUS RAISED LOWS A FEW DEGREES IN SPOTS WHICH IS MORE
IN LINE WITH SOME OF THE LATEST WARMER LAV MOS. OTRW MAINLY PC FOR
HIGH CLOUDS INITIALLY THEN MOSTLY CLOUDY WEST PER INCREASING
UPSLOPE LATER ON.

COLD ADVECTION CONTINUES OVER THE ENTIRE AREA ON THURSDAY...WITH
ANY UPSLOPE CLOUDS AND SPOTTY SNOW SHOWERS/FLURRIES EXPECTED TO
END BY/SHORTLY AFTER NOON AS SURFACE RIDGE NUDGES CLOSER TO AREA.
GOOD INSOLATION THROUGHOUT THE DAY IN MOST LOCATIONS SHOULD ALLOW
FOR TRANSFER OF MODERATELY STRONG WESTERLY WINDS ALOFT TO IMPACT
THE SURFACE...SO GUSTY WINDS ARE LIKELY...BUT NOT LIKELY TO REACH
ADVISORY LEVELS.

MAVMOS TEMPERATURES (COLDER WEST AND WARMER EAST) PREFERRED OVER
METMOS GUIDANCE THROUGH THE PERIOD. BOUNDARY LAYER TEMPERATURES
EXPECTED TO COOL SUFFICIENTLY TO JUSTIFY COLDER READINGS ACROSS
THE WEST TONIGHT...WITH UPSLOPING CLOUDS ALSO EXPECTED TO
SUFFICIENTLY INHIBIT EXTENT OF DAYTIME HEATING ON THURSDAY.
INFLUENCE OF STRONG DOWNSLOPING AND LATER ARRIVAL OF COLDER
BOUNDARY LAYER AIR EAST OF THE BLUE RIDGE SHOULD HELP TO KEEP
FOOTHILLS AND PIEDMONT WARMER THAN THOSE DEPICTED BY METMOS
PROJECTIONS.
&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/...
THE BIGGEST CHANGE TO THE FORECAST FOR THE THURSDAY NIGHT THROUGH 
SUNDAY TIME PERIOD IS THE TIMING OFF THE ARRIVAL TIME OF 
PRECIPITATION THIS WEEKEND.

CONDITIONS STILL LOOK TO BE DRY AND COLD THURSDAY NIGHT THROUGH 
FRIDAY NIGHT WITH A VAST AREA OF CANADIAN ORIGIN HIGH PRESSURE 
BUILDING OVER THE REGION. WILL CONTINUE TO FAVOR HIGHS AND LOW 
DURING THE PERIOD BELOW THE COLDEST GUIDANCE.

WE HAVE BEEN FORECASTING THE ARRIVAL PRECIPITATION FOR THIS WEEKEND 
IN ASSOCIATION WITH BOTH A SHORTWAVE TROUGH MOVING THROUGH THE 
OHIO VALLEY...TEAMING UP WITH MOISTURE AND OVERRUNNING IN 
ASSOCIATION WITH A SYSTEM PASSING TO OUR SOUTH. WHILE THIS SCENARIO 
STILL LOOKS PROMISING...WHAT HAS CHANGED IS THE TIMING THESE TWO 
EVENTS OCCURRING. WE ARE NOW LOOKING AT THE ONSET OF PRECIPITATION 
ABOUT 12 HOURS LATER THAN CONSIDERED EARLIER. WILL NOW HAVE A 
COMPLETELY DRY SATURDAY FORECAST WITH SATURDAY EVENING DRY AS WELL. 
WILL WORK SLIGHT TO CHANCE POPS OF PRECIPITATION INTO THE REGION 
FROM THE SOUTH AFTER ABOUT MIDNIGHT SATURDAY NIGHT. A SATURATED WARM 
NOSE CENTERED AT 800 MB IS CURRENTLY PROGGED VIA THE GFS TO 
ACCOMPANY THIS PRECIPITATION ALONG WITH MUCH OF THE AREA SEEING 
SUB-FREEZING TEMPERATURES AT THE ONSET OF THE PRECIPITATION. THIS 
SUGGESTS A RAIN...FREEZING RAIN OR SLEET FORECAST LATE SATURDAY 
NIGHT. GIVEN THE POTENTIAL FOR SIGNIFICANT MODEL RUN TO RUN 
VARIABILITY PAST DAY 3 OF THE FORECAST...AND DUE TO OFFICE 
POLICY...WILL HAVE A FORECAST SATURDAY NIGHT WITH EITHER RAIN...SNOW 
OR RAIN AND SNOW MIX AS THE PRECIPITATION TYPES...AND THE TYPES WILL 
BE SURFACE TEMPERATURE BASED...32 AND BELOW...SNOW...32 TO 33...RAIN 
SNOW MIX...ABOVE 33 RAIN.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
MODELS CONTINUE TO TREND TOWARD SLOWER SOLUTION FOR THE WKND
WEATHER SYSTEM...WHICH IS ESSENTIALLY AN UPPER WAVE IN GENERAL SW
FLOW OVERRUNNING A WEAK WEDGE. THE ECMWF AND MOST OF GFS ENSEMBLE
MEMBERS TRACK BULK OF PRECIP TO OUR SOUTH WITH A STRONGER HIGH
PRESSURE OVR MID ATLANTIC...WHILE 12Z OPERATIONAL GFS SPREADS
OVERRUNNING PRECIP INTO OUR WRN CWA BY EARLY SUNDAY COMING OUT OF
TN/KY. GIVEN ONGOING UNCERTAINTY...WILL CONTINUE TO GO WITH A
COMPROMISE BTWN LATEST ECMWF AND GFS...BUT IN LINE WITH THE SLOWER
TRENDS. SO STILL NOTHING HIGHER THAN CHC POPS FOR NOW...WITH NEAR
LIKELY ACROSS THE SOUTH FOR SUNDAY. AS FOR P-TYPE ISSUES...WEDGE
EXPECTED TO BE IN PLACE GIVEN SFC HIGH POSITION AND CERTAINLY A
CHANCE FOR MORNING MIX OF FREEZING RAIN OR SLEET FOR CENTRAL TO
NORTHERN PORTION OF CWA...THEN LIKELY TO CHANGE TO RAIN EVERYWHERE
BY AFTERNOON AS THE HIGH SHIFTS OFF THE COAST TO UNFAVORABLE
POSITION TO SUPPORT ONGOING WEDGE. GIVEN THIS IS STILL DAY 4 TIME
FRAME AND UNCERTAINLY EXISTS AS TO PRECIP EVEN REACHING MUCH INTO
THE CWA...WILL HESITATE TO SPECIFICALLY MENTION THESE MIXED
PTYPES...AND INSTEAD WILL CONTINUE WITH SIMPLE CHC SNOW OR RAIN TO
INDICATE SOMETHING WINTRY POSSIBLE. IF CONFIDENCE IS HIGHER BY
TOMORROW WE WOULD THEN BEGIN TO ADD SPECIFIC MENTION OF THE OTHER
LIKELY PTYPES. LIKELIHOOD OF ANY SIGNIFICANT IMPACTS FROM THIS
APPEAR VERY MINIMAL AT THE MOMENT HOWEVER.

GENERAL SW FLOW TO CONTINUE INTO EARLY NEXT WEEK WITH NO SIGNIFICANT 
COLD AIR ADVECTION BEHIND WKND SYSTEM YET LINGERING MOISTURE AND 
POSSIBLE WEAK WAVES OF ENERGY IN THE FLOW COULD KEEP SOME SPOTTY 
SHWRS IN THE WRN UPSLOPE AREAS. NEXT POTENTIAL SYSTEM TO IMPACT 
REGION FOR LATE MONDAY NIGHT OR TUESDAY IS A SLIGHTLY DEEPER WAVE IN 
SPLIT FLOW...AND RECENT MODEL TRENDS ARE NOW TO WARM UP AIR MASS 
AHEAD OF THIS ENOUGH SUCH THAT WINTRY PRECIP NOT AS LIKELY AS IT 
ONCE APPEARED...OTHER THAN PERHAPS HIGHEST ELEVATIONS IF PRECIP 
COMES IN OVERNIGHT. ECMWF ACTUALLY TRACKS THIS WELL SOUTH OF AREA 
BUT WEAK SFC FRONTAL PASSAGE WOULD STILL BRING SOME LIGHT UPSLOPE 
SHWRS BACK TO THE WESTERN MTNS. NOTHING HIGHER THAN CHC POPS WITH 
THIS SYSTEM FOR NOW...AND GIVEN SPLIT FLOW REGIME THERE IS LIKELY 
TO REMAIN QUITE A BIT OF UNCERTAINTY WITH THIS AS WELL UNTIL WE GET 
WITHIN A COUPLE OF DAYS. LIKELY A STRONGER COLD FRONTAL PASSAGE WITH 
THIS SYSTEM COMPARED TO WKND ONE...SO AT LEAST A BRIEF RETURN OF 
COLDER AIR BEHIND IT BY MID WEEK...BUT LARGE SCALE PATTERN 
SUGGESTING LOWER AMPLITUDE FLOW COMPARED TO CURRENT REGIME...SO 
TEMPS NOT TOO FAR BELOW NORMAL DESPITE IMPRESSIVE TROUGH OVER THE 
GREAT LAKES.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
DID INCLUDE A FEW HRS OF A WIND SHEAR GROUP THIS EVENING...BUT
SHOULD SUBSIDE BY LATE EVENING. COLD AIR/PRESSURE RISES WILL
ESCAPE INTO THE SOUTHSIDE AND PIEDMONT LATER TONIGHT...SO ALTHOUGH
WINDS ARE MEEK ATTM IN THOSE LOCATIONS...THERE COULD BE SOME 25KT GUSTS
THERE IN THE NEXT SEVERAL HRS. ALSO..LOWERING INVERSION WITH
CONTINUED CAA WILL ALLOW GOOD MOMENTUM TRANSFER LWB/BLF/ROA SO DID
KEEP SOME 25-30KT GUST THERE THROUGH TONIGHT. CURRENT SAT PIC AND
CANADIAN SKY COVER GUIDANCE INDICATE A 3K-5K DECK TO ARRIVE
BLF/LWB CORRIDOR AFTER MIDNIGHT...AND BREAK UP INTO THURSDAY.
TYPICALLY GUIDANCE IS A LITTLE TOO OPTIMISTIC WITH THE RETREAT OF
UPSLOPE CLOUDINESS SO AT LEAST KEPT A BKN LAYER INTO THE PM
THURSDAY THERE. DECENT WINDS REMAIN JUST OFF THE SURFACE INTO
TOMORROW...SO CONTINUED SOME 25-30KT WIND GUSTS LWB/BLF/ROA
THROUGH THURSDAY AFTERNOON.

MAINLY VFR WEATHER WITH UNRESTRICTED FLIGHT CONDITIONS ARE EXPECTED
OVER ALL TERMINAL FORECAST SITES (KROA...KLYH...KDAN...KBLF...KLWB)
THURSDAY INTO SATURDAY AS LARGE SURFACE RIDGE OF HIGH PRESSURE
BUILDS EAST INTO MID ATLANTIC REGION. ANOTHER WEAK STORM SYSTEM
MAY LIFT NORTHEAST INTO THE MID ATLANTIC REGION LATER SATURDAY
INTO SUNDAY...BRINGING AN INCREASED THREAT OF MIXED PRECIPITATION
AND A LOWERING OF FLIGHT CATEGORIES BACK INTO MVFR...AND POSSIBLY
IFR RANGE.

&&

.HYDROLOGY...
RAINFALL BETWEEN 1 AND 2 INCHES OVER THE PAST 24 HOURS HAS
COMBINED WITH LEFTOVER SNOW MELT IN SPOTS TO PRODUCE MINOR
FLOODING ALONG AREA RIVERS. MINOR FLOODING IS NOW EXPECTED ALONG
THE DAN RIVER AT SOUTH BOSTON AS WELL AS ON THE LOWER ROANOKE
RIVER AT RANDOLPH...AND AT BREMO BLUFF ON THE JAMES. SOME MINOR
FLOODING IS ALSO EXPECTED ALONG THE GREENBRIER RIVER AT ALDERSON
THROUGH THURSDAY MORNING.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...WIND ADVISORY UNTIL 4 AM EST THURSDAY FOR VAZ007-009>014-
     016>020-022>024-035.
     HIGH WIND WARNING UNTIL 4 AM EST THURSDAY FOR VAZ015.
NC...WIND ADVISORY UNTIL 4 AM EST THURSDAY FOR NCZ002.
     HIGH WIND WARNING UNTIL 4 AM EST THURSDAY FOR NCZ001-018.
WV...WIND ADVISORY UNTIL 4 AM EST THURSDAY FOR WVZ042>045.

&&

$$
SYNOPSIS...JH/WERT
NEAR TERM...JH/WERT
SHORT TERM...DS
LONG TERM...SK
AVIATION...KM/WERT
HYDROLOGY...