Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 280516
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
1216 AM EST SUN DEC 28 2014

.SYNOPSIS...
A WEAK COLD FRONT WILL DRIFT SOUTHEAST INTO THE MOUNTAINS TODAY
BEFORE SPILLING SOUTH OF THE REGION LATER SUNDAY NIGHT. WEAK
WAVES OF LOW PRESSURE TRACKING ALONG THE BOUNDARY TODAY INTO
MONDAY MORNING WILL BRING PERIODS OF MOSTLY LIGHT RAIN TODAY THROUGH
EARLY MONDAY. COLD HIGH PRESSURE WILL SLOWLY BUILD EAST INTO THE
REGION BEHIND THE FRONT FOR THE MIDDLE OF NEXT WEEK.
&&

.NEAR TERM /THROUGH SUNDAY NIGHT/...
AS OF 830 PM EST SATURDAY...

CLOUDS HAVE QUICKLY OVERSPREAD THE REGION THIS EVENING ASSOCIATED
WITH A LEAD FAINT MID LEVEL IMPULSE IN ADVANCE OF THE MAIN PIECES
OF SOUTHERN STREAM SUPPORT TO THE SW. HOWEVER MOSTLY OF THE MID
DECK VARIETY WITH ANY -RA OR SPRINKLES CONFINED JUST WEST OF THE
CWA CLOSER TO THE UPSTREAM FRONT/JET. LATEST HRRR SHOWS SOME
INTERMITTENT -RA PERHAPS MAKING IT TO THE BLUE RIDGE AFTER
MIDNIGHT BUT WITHOUT MUCH ORGANIZATION GIVEN LINGERING DRY AIR
ALOFT PER EVENING SOUNDINGS...AND BEST LIFT WEST-NW OF THE REGION
UNTIL DAYBREAK. THUS SLOWED DOWN MAIN CORRIDOR OF POPS A BIT MORE
OVERNIGHT AS MAY BE DURING SUNDAY BEFORE BETTER COVERAGE ARRIVES
AS SEEN VIA THE LATEST NAM. OTHER CONCERN WITH LOW TEMPS AS THE
CURRENT CLOUD CANOPY HAS TENDED TO STOP FALLS FOR THE MOMENT WITH
MANY LOCATIONS STILL IN THE 50S. IR PICS DO SHOW SOME BREAKS
HEADING THIS WAY OUT OF TN SO STILL SOME POTENTIAL TO SEE SOME
SPOTS SNEAK BACK DOWN GIVEN DRY AIR. HOWEVER APPEARS EVEN WITH
SOME ADDED COOLING BY LATE RAINFALL THAT WILL NEED TO RAISE LOWS
A CAT OR TWO MOST SPOTS.

PREVIOUS VALID DISCUSSION AS OF 340 PM EST SATURDAY...

THE AFTERNOON MILD TEMPERATURES ALONG WITH INCREASING CLOUD COVER
FROM THE SOUTHWEST MAY RESULT IN SOME MILD TEMPS OVERNIGHT
TONIGHT...ESPECIALLY IN THE WEST WHERE CLOUD COVER WILL BE
THICKEST SOONER. IN THE EAST...CLOUDS WILL NOT THICKEN UP UNTIL
SOME LATE AFT/EARLY EVENING RADIATIONAL COOLING...SO MAY END UP
BEING COOLER IN PARTS OF THE PIEDMONT THERE THAN IN MTNS. WENT
WITH OR SLIGHTLY WARMER THAN GUIDANCE AT SOME OF THE HIGHER
ELEVATION LOCATIONS...AND WITH COOLER MAV GUIDANCE IN EAST.

OTHERWISE...MAIN ISSUE OVERNIGHT IS TIMING OF PRECIP HEADED
NORTHEAST OUT OF GULF COAST AREA WITH SHORT WAVE AHEAD OF THE
APPROACHING COLD FRONT FROM THE WEST. A LITTLE CONCERNED ABOUT
AMOUNT OF CONVECTION DOWN THERE NOW ACTING IN SOME WAY TO ROB SOME
OF THE MOISTURE...AS WELL AS VERY DRY AIRMASS WE CURRENTLY HAVE IN
PLACE THAT COULD HELP SLOW DOWN ARRIVAL. MODEL TRENDS...AND HIGH
RES CONVECTIVE-ALLOWING VERSIONS...NOW HOLD PRECIP OFF UNTIL MAYBE
JUST BEFORE 12Z IN FAR SW AND FAR WEST PART OF AREA. SO LOWERED
POPS SLIGHTLY...MAINLY FROM 06-12Z TIME FRAME....AND ESPECIALLY
ACROSS NORTHEAST PART OF AREA WHERE IT LOOKS LIKELY THAT NO PRECIP
WILL REACH PAST ROANOKE BEFORE THEN. WHATEVER DOES REACH FAR SW BY
EARLY MORNING WILL BE QUITE LIGHT.

FOR SUNDAY...MAIN BAND OF STEADIER RAIN WILL MOVE IN FROM MID
MORNING THROUGH EARLY AFTERNOON. LEANING TOWARD NAM/ECWMF/SREF
IDEA THAT BEST CHANCES WILL BE FROM BLUE RIDGE WEST...WITH MORE
SCATTERED LIGHTER PRECIP INITIALLY IN PIEDMONT...ALTHOUGH 12Z GFS
WOULD DISAGREE. THEN THIS WAVE MOVES OUT BY LATER AFTERNOON BUT
WITH LINGERING MOISTURE KEEPING HIGH POPS IN WEST AND REDUCED TO
CHANCE IN PIEDMONT FOR LATE IN DAY INTO EARLY EVENING.

NEXT WAVE THEN ARRIVES ALONG BOUNDARY BY EVENING...AND THIS ZONE
OF STEADIER RAIN APPEARS MOST LIKELY TO TAKE A TRACK A LITTLE MORE
TO SOUTH OF FIRST ONE...WITH FOOTHILLS TO PIEDMONT GETTING MOST OF
THE MODERATE RAINFALL. OTHER THAN GFS...SEEING SOME TRENDS OVER
LOWERING AMOUNTS WITH THIS...WHICH IS PARTLY REFLECTIVE OF
WEAKENING UPPER WAVE BEING STRETCHED WITH TIME AS WELL AS SOME
MOISTURE STEALING FROM CONVECTION WELL TO SOUTH. NOT LOOKING AT
NEARLY ENOUGH RAINFALL FOR ANY PROBLEMS. SEE THE HYDRO SECTION
BELOW FOR MORE DETAILS ON THAT.

TEMPS THROUGH THIS PERIOD WILL BE MILD...ALTHOUGH SOME COOLER AIR
WILL FILTER INTO WESTERN APPALACHIAN SLOPES BY EARLY MONDAY
MORNING...BUT NOT COLD ENOUGH FOR ANY SNOW...EVEN AT HIGHEST
ELEVATIONS.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
AS OF 300 PM EST SATURDAY...

THE LAST WAVE IN THE SERIES ALONG A SLOW-MOVING CLOD FRONT SHOULD BE
EXITING TOWARD THE ATLANTIC COAST FROM EARLY MONDAY THROUGH THE DAY
WITH POPS SLOWLY DIMINISHING. COLD AIR ADVECTION BEHIND THIS SYSTEM
SLOW TO DEVELOP DURING THE DAY SO HIGHS SHOULD BE ABLE TO BOUNCE
BACK SOME FROM MORNING LOWS AND WILL GENERALLY RANGE FROM UPPER 30S
NW TO UPPER 40S SE UNDER STILL MOSTLY CLOUDY SKIES. LINGERING RAIN
SHOWERS ON THE BACK END OF THIS SYSTEM WILL ENCOUNTER THE INCREASING
SOUTHWARD SPILL OF COLDER AIR AND MAY CAUSE A CHANGE OVER TO SNOW
SHOWERS IN THE HIGHER TERRAIN OF THE NORTHWEST CWA BY EVENING MONDAY
AND HAVE INTRODUCED VERY LIGHT SNOW ACCUMULATIONS..LESS THAN 1 INCH
IN THOSE COLDER AREAS...MAINLY GREENBRIER AND BATH COUNTIES. COLDER
AIR EMANATING FROM THE MASSIVE HIGH OVER THE WESTERN AND CENTRAL
CONUS WILL OOZE EASTWARD AND SOUTHWARD OVERNIGHT MONDAY AND THROUGH
THE DAY TUESDAY AS H85 TEMPS FALL BELOW 0C ACROSS THE ENTIRE CWA BY
TUESDAY EVENING AND CONTINUE TO SLIP INTO EARLY WEDNESDAY. HIGHS
TUESDAY WILL BE CHILLY..RANGING FROM L/M 30S WEST TO MID-40S
SE...ACTUALLY A FEW DEGREES BELOW LATE DECEMBER CLIMO. WEDNESDAY
MORNING LOWS FALL BACK CLOSE TO ABOUT NORMAL FOR THIS TIME OF
YEAR...NEAR 20F NW TO UPPER 20S SE.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
AS OF 430 PM EST SATURDAY...

LONG RANGE MODELS...AT LEAST FOR WEDNESDAY AND THURSDAY ARE IN
AGREEMENT WITH FAIR CONTINUITY AS WE TRANSITION INTO THE NEW
YEAR. BROAD UPPER LEVEL TROUGH IS PROGGED TO EXTEND FROM EASTERN
CANADA...SOUTHWEST THROUGH THE CENTRAL CONUS INTO A CUTOFF AREA OF
LOW PRESSURE OVER SOUTHERN CALIFORNIA. A HIGH AMPLITUDE RIDGE WILL
BE POSITIONED OFF THE PAC NW COAST. THIS PATTERN FAVORS COLD
TEMPERATURES PERSISTING OVER THE NORTHERN CONTINENTAL UNITED
STATES (CONUS) AND SOUTHWARD OVER THE PLAINS...AND MILDER AIR
CONFINED TO THE EXTREME SOUTHEAST CONUS...I.E. FLORIDA. FOR THE
MID ATLANTIC WE WILL LIKELY END UP IN THE TRANSITION ZONE...OR
PRETTY CLOSE TO NORMAL TO FINISH OUT THE WEEK. WORST CASE...WE MAY
END UP 5 DEGREES OR SO BELOW NORMAL FOR NEW YEARS DAY...BUT
NOTHING EXTREME.

AFTER THURSDAY THE MODELS DIVERGE...WHICH IS TO BE EXPECTED SINCE
THEY DO NOT HANDLE CUTOFF LOWS VERY WELL...AND ESPECIALLY THOSE
THAT EJECT FROM THE DESERT SOUTHWEST. WITHIN THE EVOLVING MEAN
PATTERN THE PRIMARY FEATURE OF INTEREST WILL BE THE UPPER LOW
WHICH ALL GUIDANCE CLOSES OFF OVER/NEAR CA/AZ INITIALLY BUT THEN
DIVERGE ON WHEN THE LOW OPENS UP AND MOVES EAST DURING THE FRIDAY
SATURDAY TIME FRAME. THERE IS A FAIR AMOUNT OF MODEL/ENSEMBLE
SPREAD REGARDING THE TIMING AND TO A SOMEWHAT LESSER DEGREE TRACK
OF THE UPPER LOW. THE OPERATIONAL RUN OF THE GFS IS 12-24 HOURS
FASTER THAN THE ECMWF...WHICH MAKES A DIFFERENCE IN WHETHER IT
PRECIPITATES HERE ON FRIDAY VS SATURDAY. THIS HIGH SPREAD/LOW
CONFIDENCE FORECAST FAVORS AN ENSEMBLE MEAN APPROACH FOR BOTH
FRIDAY AND SATURDAY...THE FORECAST REFLECTING A BLEND...WITH CHC
PRECIP BOTH FRIDAY AND SATURDAY...THE GREATEST POPS FAVORING
FRIDAY NIGHT WHERE THE BLENDED SOLUTIONS OVERLAP. TIMING AND
TRACK OF THE SYSTEM WILL ALSO DETERMINE P-TYPE. TEMPERATURES LOOK
TO BE BORDERLINE CRITICAL FRIDAY...SUGGESTING AT LEAST SOME
WINTRY MIX IF PRECIP ARRIVES FRIDAY MORNING. IF ONSET HOLDS OFF
UNTIL LATER FRIDAY...THE P-TYPE WILL BE MOSTLY RAIN.

&&

.AVIATION /04Z SUNDAY THROUGH THURSDAY/...
AS OF 1145 PM EST SATURDAY...

VFR CONTINUES TO PREVAIL TONIGHT WITH SOMEWHAT LOWER CIGS IN SPOTS
OVER THE WEST WHERE HAVE SEEN A FEW SPRINKLES. STILL EXPECTING
OVERALL CIGS TO REMAIN IN THE 4-6K FT RANGE UNDER AREAS OF MID
DECK OVERNIGHT SO APPEARS WILL STAY MAINLY VFR AT ALL LOCATIONS
FOR AT LEAST A FEW MORE HOURS.

DEEPER MOISTURE AHEAD OF THE UPSTREAM COLD FRONT AND ASSOCIATED
INITIAL WAVE WILL PUSH NE INTO THE SE WEST VA SITES AROUND
DAYBREAK SUNDAY WITH MORE IN THE WAY OF WIDESPREAD MVFR CIGS WEST
OF THE BLUE RIDGE SUNDAY MORNING. THIS WILL ALSO BE FOLLOWED BY
PATCHY LIGHT RAIN WHICH SHOULD ARRIVE AT KBLF/KLWB JUST PRIOR TO
SUNRISE IF NOT A BIT SOONER...BUT MAINLY LIGHT...THUS OVERALL VFR
VSBYS IN ANY PRECIP EARLY SUNDAY. FIRST WAVE OF LIGHT RAIN WILL
ALSO SCOOT ACROSS SUNDAY MORNING WITH MAINLY VFR CIGS AND VSBYS IN
THE SPOTTY COVERAGE FROM THE BLUE RIDGE EAST...AND MVFR OVER THE
WEST ESPCLY KBLF/KLWB. APPEARS THIS WILL MAINLY GO TO MOISTEN THE
COLUMN BEFORE ANOTHER ROUND OF STEADIER RAIN ARRIVES OVER THE FAR
WEST/NW IN THE AFTERNOON. THIS IS WHEN IT APPEARS THAT CIGS/VSBYS
WILL DETERIORATE QUICKER PENDING HOW FAST THINGS BECOME SATURATED
IN THE WAKE OF THE INITIAL RAINFALL. THIS LOOKS TO HAPPEN IN THE
MID/LATE AFTERNOON TIME FRAME ESPCLY SE WEST VA SITES WHERE LIKELY
WITHIN STEADIER RAINFALL...AND BLUE RIDGE LOCATIONS GIVEN SOMEWHAT
TRAPPED LOW LEVEL MOISTURE. STILL WONT GO AS LOW AS THE LATEST NAM
IN SHOWING WIDESPREAD LIFR/IFR AT ALL SITES BY LATE IN THE DAY OR
EARLY EVENING. HOWEVER TRENDING INTO SOLID IFR WEST...MIX OF
MVFR/OCNL IFR BLUE RIDGE INCLUDING KBCB/KROA...AND MAINLY MVFR AT
KDAN/KLYH PENDING DEGREE OF PRECIP THEY SEE SUNDAY MORNING.

FRONT WILL SLOWLY PUSH EAST ACROSS THE AREA SUNDAY NIGHT AS
ANOTHER AREA OF LOW PRESSURE TRACKS UP THE BOUNDARY BRINGING MORE
WIDESPREAD RAINFALL TO SOUTHERN/EASTERN SECTIONS INTO EARLY
MONDAY. THIS LOOKS TO BE THE BEST TIME TO SEE WIDESPREAD IFR CIGS
AND MVFR/IFR VSBYS AT ALL LOCATIONS EXCLUDING PERHAPS KLWB THAT
MAY STAY WITH ONLY MVFR CONDITIONS JUST NORTH OF THE MAIN PRECIP
SHIELD. OTRW EXPECTING PLENTY OF LOW CLOUDS...-RA AS WELL AS
DRIZZLE/FOG TO COVER MUCH OF THE REGION SUNDAY NIGHT...WITH ONLY
SLOW IMPROVEMENT INTO MONDAY IN THE WAKE OF THE FRONT GIVEN LOW
LEVEL NE FLOW.

EXTENDED DISCUSSION...

THE FRONT DRIFTS OFF TO THE SE MONDAY NIGHT INTO TUESDAY WHILE
LEAVING PLENTY OF RESIDUAL LOW LEVEL MOISTURE IN ITS WAKE. HIGH
CONFIDENCE IN STAYING SUB-VFR MONDAY NIGHT WITH MORE FOG/DRIZZLE
AROUND OVERNIGHT. SLOW DRYING TO TAKE SHAPE ON TUESDAY AS A LARGE
AREA OF HIGH PRESSURE STARTS TO NOSE IN FROM THE WEST. HOWEVER MAY
TAKE UNTIL AFTERNOON FOR EASTERN SITES TO RETURN TO VFR WHILE
LINGERING LOW CLOUDS COULD CAUSE MVFR CIGS TO PERSIST OVER THE
WEST UNTIL LATE IN THE DAY. SOME SPOTTY LIGHT RAIN OR WINTRY
PRECIP MAY ALSO OCCUR EARLY TUESDAY AS A WEAK TROUGH OF LOW
PRESSURE REDEVELOPS OVER THE MOUNTAINS. OTRW HIGH PRESSURE BUILDS
EAST WEDNESDAY INTO THURSDAY WITH GOOD FLYING CONDITIONS RETURNING
UNDER VFR.
&&

.HYDROLOGY...
AS OF 300 PM EST SATURDAY...

RIVER MODEL RUNS FROM THE VARIOUS RFCS FROM THIS MORNING INCORPORATE
THE UPCOMING WET PERIOD WITH BASIN AVERAGE QPF VALUE ACROSS THE
HYDROLOGIC SERVICE AREA BASINS RANGING FROM ABOUT 0.75 INCHES IN THE
UPPER JAMES TO AS MUCH AS 1.20 INCHES OVER THE LOWER DAN. THE
RESULTING RIVER FORECASTS BRING STAGE UP SEVERAL FEET...BUT STILL
WELL WITHIN BANKFULL...SO NO FLOODING ISSUES EXPECTED. SOILS ARE
FAIRLY WET BUT RAINFALL RATES ARE EXPECTED TO BE MODEST SO FLASH
FLOODING NOT AN ISSUE EITHER.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...NONE.
NC...NONE.
WV...NONE.

&&

$$
SYNOPSIS...JH/SK
NEAR TERM...JH/SK
SHORT TERM...PC
LONG TERM...PM
AVIATION...JH/RCS/SK
HYDROLOGY...PC


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