Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 270718
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
218 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 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 LEERY 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 /07Z SATURDAY THROUGH WEDNESDAY/...
AS OF 1201 AM EST SATURDAY...

HIGH PRESSURE WILL COVER THE REGION OVERNIGHT THROUGH SATURDAY
WITH VFR CONDITIONS AND LIGHT WINDS. PERIODS OF BROKEN MID/HIGH
CLOUDS WILL CROSS THE AREA THROUGH DAWN AS A WEAK MID LEVEL WAVE
PASSES TO THE NORTH. THIS SHOULD ACT TO LIMIT MUCH IN THE WAY OF
FOG IN THE RIVER VALLEYS SUCH AS AT KLWB. HOWEVER AGAIN GIVEN
FAVORABLE FOG STABILITY VALUES AND POTENTIAL FOR THE VALLEYS TO
RADIATE FASTER LATE PENDING DEGREE OF CLOUDS...INCLUDED A BRIEF
TEMPO GROUP FOR MVFR TYPE FOG/STRATUS AT KLWB TOWARD DAWN. IF
SKIES SHOULD HAPPEN TO CLEAR EVEN MORE BEFORE DAYBREAK THEN MORE
FOG COULD FORM NEAR THE RIVER AGAIN...RESULTING IN VERY LOCALIZED
IFR CONDITIONS LATE. CANT TOTALLY RULE OUT SOME ISOLATED PATCHES
OF FOG AS WELL AT KBCB/KLYH BUT WONT INCLUDE FOR NOW GIVEN CLOUDS
AND DRYNESS.

ANOTHER COLD FRONT WILL APPROACH THE REGION LATE SATURDAY 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 AT LEAST MIDNIGHT SAT NIGHT. LOWER CIGS AND
LIGHT RAIN WILL BE MAKING SOME INROADS ACROSS THE WESTERN SITES
LATE SAT NIGHT BUT MOSTLY BEYOND THE VALID PERIOD OF THE TAF
FORECAST FOR NOW.

EXTENDED DISCUSSION...

SATURDAY NIGHT 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
SHORT TERM...RCS
LONG TERM...PM
AVIATION...AMS/JH/SK
HYDROLOGY...PC


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