Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 272335 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 635 PM EST SAT DEC 27 2014 .SYNOPSIS... A WEAK COLD FRONT WILL APPROACH FROM THE WEST TONIGHT AND SLOWLY MAKES IT WAY THROUGH THE APPALACHIANS ON SUNDAY. WEAK WAVES OF LOW PRESSURE TRACKING ALONG THE BOUNDARY SUNDAY INTO EARLY MONDAY MORNING WILL SPREAD LIGHT TO MODERATE RAINFALL ACROSS THE REGION FROM EARLY SUNDAY 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 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 /00Z SUNDAY THROUGH THURSDAY/... AS OF 558 PM EST SATURDAY... SOMEWHAT LOWER VFR CIGS ARE SLOWLY MAKING THEIR WAY IN FROM THE WEST EARLY THIS EVENING AND SHOULD CONTINUE EAST TO THE BLUE RIDGE SHORTLY AND OUT EAST BEFORE MIDNIGHT. HOWEVER MUCH OF THIS IN THE 4-6K FT RANGE WITH SOME POCKETS OF MID DECK SO APPEARS WILL STAY VFR AT ALL LOCATIONS THIS EVENING. DEEPER MOISTURE AHEAD OF THE UPSTREAM COLD FRONT AND ASSOCIATED INITIAL WAVE WILL PUSH NE INTO THE SE WEST VA SITES AFTER MIDNIGHT WITH WIDESPREAD MVFR CIGS WEST OF THE BLUE RIDGE BY DAYBREAK SUNDAY. 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...SO OVERALL VFR VSBYS IN ANY PRECIP OVERNIGHT. 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. WONT GO AS LOW AS THE LATEST NAM IN SHOWING WIDESPREAD LIFR/IFR AT ALL SITES BY LATE IN THE DAY. 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 A FINAL 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. 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...SK SHORT TERM...PC LONG TERM...PM AVIATION...JH/RCS/SK HYDROLOGY...PC

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