Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 272150 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 450 PM EST SAT DEC 27 2014 .SYNOPSIS... A WEAK COLD FRONT WILL APPROACH FROM THE WEST TONIGHT AND SLOWLY MAKES IT WAYS 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 TRANSISTION 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 /21Z SATURDAY THROUGH THURSDAY/... AS OF 1245 PM EST SATURDAY... MAINLY THIN HIGH CLOUDS FOR REMAINDER OF TODAY IN ADVANCE OF WEAK COLD FRONT AND SHORT WAVE ALONG FRONT MOVING UP FROM THE SOUTHWEST. HIGH CLOUDS MAY THICKEN UP LATE THIS AFTERNOON BUT LIKELY NOT BEGIN TO LOWER TO MID DECK UNTIL AFTER 06Z BUT STILL VFR. GUIDANCE TRENDING A LITTLE SLOWER ON ARRIVAL TIME...AND RAIN LOOKS TO BE VERY LIGHT INITIALLY. MAY BE A FEW HRS OF VERY LIGHT RAIN AND STILL VFR OR MVFR CONDITIONS. THEN MID MORNING FAR WEST AND LATE MORNING ELSEWHERE COULD SEE STEADIER RAIN BRING CIGS DOWN TO IFR...MOST LIKELY AT KBLF. NOT AS CONFIDENT AT KLWB AND KBCB AS MODELS SUGGESTING THE BAND OF MODERATE RAIN MAY STAY ON FAR WESTERN FRINGES. FAIRLY CONFIDENT WILL NOT SEE EVEN MVFR CONDITIONS AT KROA AND KLYH BEFORE 18Z...EVEN THOUGH LAMP GUIDANCE DOES BRING THEM DOWN TO IFR. WITHOUT A GOOD COLD AIR DAMMING SIGNAL...WILL TAKE STEADIER RAIN FOR THAT TO HAPPEN...AND THAT WILL PROBABLY HOLD OFF UNTIL AFTERNOON...IF AT ALL...IN THOSE LOCATIONS. KDAN PROBABLY AS WELL...BUT LESS CONFIDENT THERE. FOR NOW LEFT THESE LOWER ELEVATION AIRPORTS AT VFR IN LIGHT RAIN THROUGH 18Z...BUT WILL NEED TO SEE WHAT THE AREA OF PRECIP LOOKS LIKE AS IT APPROACHES LATE TONIGHT AND MAY HAVE TO ADJUST DOWN. EXTENDED DISCUSSION... SUNDAY 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 AT LEAST MVFR CONDITIONS. IFR CONDITIONS MAY BE INTERMITTENT BUT WITH SECOND WAVE APPROACHING LATE SUNDAY NIGHT COULD HAVE BETTER CHANCE FOR IFR AT EASTERN AIRPORTS. THE FRONT MAY LINGER JUST TO THE SOUTH OF THE AREA EARLY IN THE 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 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...RCS/SK HYDROLOGY...PC

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