Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 271448
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
948 AM EST SAT DEC 27 2014

.SYNOPSIS...
HIGH PRESSURE ACROSS THE AREA WILL PASS EAST AND OFFSHORE TODAY
IN ADVANCE OF A COLD THAT WILL APPROACH FROM THE WEST THIS
EVENING. WEAK WAVES OF LOW PRESSURE TRACKING ALONG THE BOUNDARY
SUNDAY INTO SUNDAY NIGHT WILL SPREAD AN AREA OF LIGHT TO MODERATE
RAINFALL ACROSS THE REGION FROM EARLY SUNDAY THROUGH MONDAY. COLD
HIGH PRESSURE WILL SLOWLY BUILD EAST INTO THE REGION BEHIND THE
FRONT FOR THE MIDDLE OF NEXT WEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 945 AM EST SATURDAY...

FCST FOR REST OF TODAY LOOKS ON TRACK AS FAR AS MAX TEMPS ARE
CONCERNED WITH MORNING SOUNDINGS SHOWING STRONG INVERSION AND
ABOUT 3 DEG C WARMER IN THE 900 TO 850 MB LAYER COMPARED TO
YESTERDAY MORNING...ABOUT THE LEVEL IT MIXED TO YESTERDAY.
SIMILAR MIXING SUGGESTS AROUND 60 MTNS TO LOW AND MID 60S
PIEDMONT. PLENTY OF HIGH CLOUDS STREAMING BACK TOWARD SOUTHERN
APPALACHIANS THIS MORNING IN ADVANCE OF EMBEDDED WAVES IN
MOISTENING SW FLOW...BUT ALL GUIDANCE SHOWS THESE DISSIPATING
ALONG THE WESTERN FRINGES OF APPALACHIANS FOR MOST OF THE
DAY UNTIL LATE...WITH MOSTLY SUNNY SKIES EAST OF BLUE RIDGE...AND
SCATTERED HIGH CLOUDS COMING AND GOING IN THE WEST. OVERALL
REDUCED SKY COVER A LITTLE BIT MOST LOCATIONS FOR MUCH OF REST OF
THE DAY...WITH A STEADIER INCREASE MAINLY IN THE WEST VERY LATE IN
THE DAY. NO OTHER CHANGES NEEDED AT THIS TIME.

PREV DISCUSSION AS OF 745 AM EST SATURDAY...

MINOR ADJUSTMENTS FOR CURRENT AND NEAR TERM TEMPS TO TRY AND
ACCOUNT FOR EXTREME DIFFERENCES IN OBSERVATIONS THIS MORNING MAINLY
BASED ON ELEVATION...WITH SOME HIGHER RIDGES IN MID 40S AND EVEN
A FEW LOW 50S SEEN IN MESONET OBS...WHILE SOME MTN VALLEYS AND
PARTS OF PIEDMONT WERE IN THE UPPER 20S. SOME OF THESE DIFFERENCES
ALSO DUE TO WHERE SKIES HAVE BEEN CLEAR LONGER. RADAR HAS SHOWN A
FEW VERY LIGHT RETURNS ACROSS PARTS OF THE MOUNTAINS COMING FROM A
MID DECK...BUT LOW LEVELS STILL VERY DRY AND HAVE ONLY SEEN RAIN
BRIEFLY REPORTED AT A COUPLE OBSERVATION SITES. KEEPING THIS OUT
OF FCST GRIDS FROM THIS POINT ON AS TREND ON HAS BEEN FOR THIS TO
DISSIPATE AND THIS SHOULD CONTINUE AS THESE TINY AREAS MOVE EAST
AND OFF THE BLUE RIDGE.

PREV FCST AS OF 130 AM EST SATURDAY...

WEAK MID LEVEL WAVE RESPONSIBLE FOR THE EXTENSIVE HIGH LEVEL MID DECK
ACROSS THE REGION EARLY THIS MORNING WILL SHEAR BY TO THE NE LEAVING
WEAK 5H NVA IN PLACE DURING THE DAY. THIS ALONG WITH HEATING SHOULD
TEND TO DIMINISH GOING CLOUD COVER BY MIDDAY INTO THE AFTERNOON AND
SUPPORTED BY MOST LATEST MODEL SOLUTIONS. HOWEVER GIVEN THE GRADUAL
INCREASE IN SW FLOW ALOFT INCLUDING MOISTURE TRANSPORT OFF LATEST
TRAJECTORY FORECASTS...MAY STILL SEE SOME PERIODS OF MID/HIGH CLOUDS...
SO PARTLY TO MOSTLY SUNNY OVERALL AFTER EARLY CLOUDS FADE SOME. OTRW
SURFACE HIGH PRESSURE WILL SLIP TO THE EAST AND OFFSHORE LATE IN THE
DAY ALLOWING WEAK SW RETURN FLOW TO TAKE SHAPE. THIS LOOKS TO PUSH 85H
TEMPS TO ABOVE +10C THIS AFTERNOON BUT GIVEN LIGHT MIXING DOESNT APPEAR
ALL OF THIS WARMING WILL BE REALIZED AT THE SURFACE. THUS TRENDED TEMPS
UP A NOTCH BUT STILL CLOSER TO THE COOLER MET MOS PER CLOUDS AND WEAK
LOW LEVEL ADVECTION. IF MORE INSOLATION DOES TAKE SHAPE EARLY THEN NOT
OUT OF THE QUESTION TO SEE SOME MID 60S EAST AND 60-65 WEST.

NEXT COLD FRONT APPROACHES FROM THE WEST THIS EVENING BUT SLOWS UNDER
THE SW FLOW ALOFT. LIGHT PRECIP AHEAD OF THIS BOUNDARY WILL MAKE A RUN
TOWARD THE CWA ESPCLY AFTER MIDNIGHT AS AN INITIAL WEAK WAVE ALOFT
SLIDES UP FROM THE SW. HOWEVER THE BOUNDARY LAYER LOOKS TO REMAIN QUITE
DRY EVEN OVER THE WEST UNTIL LATE PER FORECAST SOUNDINGS WITH MOST
GUIDANCE NOW HOLDING MOST -RA JUST WEST OF THE CWA UNTIL ALMOST
DAYBREAK. THIS TYPICALLY IS TOO SLOW ESPCLY GIVEN THE STRONG WEST/SW
JET ALOFT AND MODELS TENDENCY TO HOLD ONTO DRY AIR TOO LONG. THUS ONLY
SLOWED POPS UP MOSTLY EARLY...WITH A SURGE TO LIKELY/CAT POPS WESTERN
THIRD...AND CHANCE COVERAGE FOR SPOTTY -RA ELSW BETWEEN 09-12Z. PRECIP
THOUGH LOOKS PRETTY LIGHT WITH BEST LIFT TO THE SW AT THIS POINT.
THEREFORE QPF MOSTLY BELOW A TENTH OF AN INCH OVERNIGHT WEST WITH
LITTLE NE SECTIONS...AND ALL LIQUID GIVEN STEADYING/RISING TEMPS IN THE
40S AFTER EARLY FALLS...AHEAD OF THE THICKENING CLOUD CANOPY ESPCLY
AFTER MIDNIGHT.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
AS OF 210 AM EST SATURDAY...

A COLD FRONT WILL STALL ACROSS THE TENNESSEE AND OHIO VALLEYS SUNDAY
MORNING. A COUPLE OF WAVES WILL TRACK ALONG THIS FRONT SUNDAY AND
MONDAY. THE FIRST WAVE WILL PASS OVER THE AREA SUNDAY MORNING WITH
PERIODS OF LIGHT TO MODERATE RAIN FALLING OVER THE AREA. ONCE THIS
WAVE TRACKS NORTH...THE FRONT WILL SLIP OVER THE MOUNTAINS. THE NEXT
WAVE WILL TRAVEL OVER THE SOUTHERN APPALACHIANS AND ACROSS THE
CAROLINAS MONDAY MORNING WITH ANOTHER ROUND OF LIGHT TO MODERATE
RAIN. THIS WAVE WILL ALLOW THE SURFACE FRONT TO SINK SOUTH OF THE
AREA MONDAY NIGHT INTO TUESDAY. THE DEW POINT FRONT REMAINS OVER THE
OHIO VALLEY SUNDAY AND MONDAY THEN FINALLY CATCHES UP TO THE FRONT
ON TUESDAY...AS AN UPPER LEVEL TROUGH AND SHORT WAVE TRACKS OVER THE
OHIO VALLEY AND TO THE MID ATLANTIC.

TEMPERATURES WILL REMAIN WARM THROUGH THE PERIOD TO KEEP
PRECIPITATION AS RAIN. RAIN MAY CHANGE OVER TO SNOW ACROSS THE
HIGHER ELEVATIONS EARLY TUESDAY MORNING AS COLDER AIR GETS DRAWN
INTO THE REGION BEHIND THE FRONT. PWATS SUNDAY ARE AROUND AN INCH
/1.00/ AND THREE-QUARTERS TO AN INCH /0.75-1.00/ ON MONDAY. WITH
EACH PASSING WAVE...MODERATE RAIN IS POSSIBLE WITH A 48 HOUR TOTALS
OF THREE-QUARTERS TO ONE HALF INCHES /0.75-1.50/. SINCE THERE
APPEARS TO BE A 18-24 HOURS GAP BETWEEN WAVES...NO WIDESPREAD
FLOODING IS EXPECTED. ALSO POSSIBLE IN BETWEEN WAVES WILL BE FOG AND
DRIZZLE...MORESO SUNDAY NIGHT INTO MONDAY AS LOW LEVEL FLOW TURNS
EASTERLY.

TEMPERATURES SUNDAY WILL BE AROUND 10F WARMER THAN NORMAL WITH
WIDESPREAD 50S ACROSS THE REGION. COOLER AIR AND RAIN WILL LOWER
HIGHS MONDAY BUT STILL RUN WARMER THAN NORMAL WITH UPPER 40S NORTH
OF HWY 460 TO THE LOWER TO MID 50S SOUTH.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
AS OF 330 PM EST FRIDAY...

LONG RANGE MODELS...AT LEAST AT THE LARGER SCALE...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 WILL FOCUS VERY COLD
AIR INTO THE ROCKIES AND WESTERN PLAINS...TEMPERATURES 15 TO 25
DEGREES BELOW NORMAL FROM WYOMING TO WEST TEXAS.  HERE IN THE EAST
IT WILL TREND COLDER...BUT ONLY TO THE TUNE OF 5 DEGREES OR SO BELOW
NORMAL...NOTHING EXTREME. THE MILDER AIR WILL GET SUPPRESSED INTO
THE FAR SOUTHEAST CONUS...I.E. FLORIDA.

ITS GOOD TO SEE ALL THE MODELS AGREEING FOR A CHANGE...YET HAVE TO
BE A LITTLE LEARY OF MODEL SKILL AS THEY GENERALLY DO NOT HANDLE
ENERGY THAT MAY GET EJECTED FROM CUTOFF LOWS OVER THE SOUTHWESTERN
CONUS.  ATTM...MODELS KEEP OUR FORECAST AREA DRY FROM TUESDAY
THROUGH THURSDAY...ANY UPPER LEVEL ENERGY THAT TRIES TO EJECT OUT OF
THE UPPER LEVEL TROUGH BECOMING SHEARED IN THE FAST WESTERLY FLOW
ALOFT.  CERTAINLY CAN`T RULE OUT SOME VERY LIGHT PRECIP FROM ONE OF
THE WAVES DURING THE WEEK...BUT CHANCES FOR MEASURABLE LOOKS PRETTY
MINISCULE...AND WILL LEAVE POPS OUT OF THE FORECAST ATTM IN FAVOR OF
A MAINLY DRY FORECAST.

BY THE END OF THE WEEK...AND PROBABLY MORE SO FOR NEXT WEEKEND...
THERE SHOULD BE ENOUGH OF A CHANGE IN THE UPPER LEVEL FLOW TO ALLOW
FOR DEEPER MOISTURE TO RETURN TO THE FORECAST AREA.  THE UPPER LOW
WHICH IS FORECAST TO BE NEARLY STATIONARY OVER THE SOUTHWESTERN
CONUS FOR MUCH OF NEXT WEEK IS PROGGED TO BE KICKED EAST BY THE
ARRIVAL OF A STRONG PAC NW SHORTWAVE.  TIMING OF THE EJECTING UPPER
LOW WILL BE CRUCIAL IN DETERMINING TIMING AND EXTENT OF DEEPER
MOISTURE THAT RETURNS TO OUR FORECAST AREA.  THIS MAY BE AS EARLY AS
FRIDAY AND AS LATE AS SATURDAY.  TIMING AND TRACK OF THE SYSTEM WILL
ALSO DETERMINE P-TYPE.  TEMPERATURES LOOK TO BE BORDERLINE
CRITICAL...SUGGESTING AT LEAST SOME WINTRY MIX.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
AS OF 6201 AM EST SATURDAY...

HIGH PRESSURE WILL COVER THE REGION INTO THIS EVENING WITH VFR
CONDITIONS AND LIGHT WINDS. PERIODS OF BROKEN MID/HIGH CLOUDS WILL
CROSS THE AREA THIS MORNING AS A WEAK MID LEVEL WAVE PASSES TO
THE NORTH. THIS WAVE AND ASSOCIATED MOISTURE IS MOVING INTO VERY
DRY AIR BUT CAN NOT RULE OUT A SPRINKLE HERE AND THERE THROUGH
THE MORNING.

A COLD FRONT WILL APPROACH THE REGION LATE TONIGHT WITH MID DECK
CANOPY FILLING IN ACROSS THE REGION...AND CEILINGS STEADILY
LOWERING FROM WEST TO EAST LATE IN THE DAY INTO THE OVERNIGHT.
HOWEVER APPEARS MOST SITES SHOULD KEEP A LAYER OF DRY AIR IN THE
LOWER LEVELS WHICH WILL KEEP ALL LOCATIONS VFR AND MAINLY DRY
THROUGH THE TAF PERIOD. LOWER CIGS AND LIGHT RAIN WILL BE MAKING
SOME INROADS ACROSS THE WESTERN SITES SUNDAY MORNING BUT MOSTLY
BEYOND THE VALID PERIOD OF THE TAF FORECAST FOR NOW.

EXTENDED DISCUSSION...

SUNDAY THROUGH MONDAY...THE FRONT WILL STALL NEAR THE WEST SIDE
OF THE APPALACHIANS AS MULTIPLE WEAK WAVES OF LOW PRESSURE ARE
EXPECTED TO RIDE UP THIS BOUNDARY...RESULTING IN A PROLONGED
PERIOD OF RAIN AND IFR TO MVFR CONDITIONS. IFR CONDITIONS ARE NOT
LIKELY TO DEVELOP UNTIL EARLY MORNING SUNDAY IN THE WEST...AND
PERHAPS NOT UNTIL SUNDAY EVENING IN THE EAST.

THE FRONT MAY LINGER JUST TO THE SOUTH OF THE AREA EARLY NEXT
WEEK...BUT AT THIS TIME CONFIDENCE IS FAIRLY HIGH THAT AIRPORTS
WILL REMAIN DRY WITH VFR CONDITIONS PREVAILING AS STRONG HIGH
PRESSURE SLOWLY BUILDS IN FROM THE WEST. SOME MVFR CEILINGS COULD
LINGER AT KDAN MONDAY NIGHT INTO TUESDAY BEFORE ALL LOCATIONS
RETURN TO VFR ON WEDNESDAY.

&&

.HYDROLOGY...
AS OF 300 PM EST FRIDAY...

DETERMINISTIC MODELS AND VARIOUS ASSOCIATED ENSEMBLE PACKAGES
CONTINUE TO DIFFER SOMEWHAT ON EXACT TIMING OF THE RAINFALL AND
TOTAL QPF BUT GENERAL CONSENSUS IS ANOTHER MODERATE RAINFALL EVENT
IS ON TAP WITH FINAL QPF RANGING FROM 0.75 TO 1.00 INCH BY THE
TIME IT WRAPS UP DURING THE DAY MONDAY. NAEFS AND SREF ENSEMBLES
SEEM TO BE CLUSTERING WELL IN THE 0.75 INCH BASIN AVERAGE RANGE
FOR MOST OF OUR RIVERS BASIN. THESE AMOUNTS ARE FAIRLY SIMILAR TO
THE RECENT DEC. 23-24 EVENT. ENSEMBLE RIVER FORECASTS USING THE
ABOVE RANGE OF QPFS ABOVE DO NOT INDICATE ANY FLOOD THREAT BUT DO
ALLOW FOR MINOR RISES OF SEVERAL FEET ON SOME RIVERS. RATES OF
RAINFALL WILL BE MODEST AND NOT EXPECTED TO POSE MUCH OF AN ISSUE
WITH FASTER RUNOFF...DESPITE FAIRLY WET SOILS. 6-HOUR FLASH FLOOD
GUIDANCE VALUES RANGE FROM ABOUT 1.7 TO OVER 3 INCHES SO SHOULD
NOT APPROACH EVEN BANKFULL ON THE SMALLER STREAMS.

&&

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

&&

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



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