Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 270143 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 843 PM EST FRI DEC 26 2014 .SYNOPSIS... HIGH PRESSURE WILL REMAIN OVER THE AREA THROUGH TONIGHT. A WEAK COLD FRONT WILL APPROACH FROM THE WEST SATURDAY...WHILE A WAVE OF LOW PRESSURE MOVES TOWARD THE SOUTHERN APPALACHIANS FROM THE GULF COAST AREA BY SUNDAY. ANOTHER WEAK WAVE WILL APPROACH ALONG THIS FRONT SUNDAY NIGHT. THESE LOW PRESSURE WAVES WILL SPREAD AN AREA OF LIGHT TO MODERATE RAINFALL ACROSS THE REGION FROM EARLY SUNDAY THROUGH MONDAY. && .NEAR TERM /THROUGH SATURDAY NIGHT/... AS OF 815 PM EST FRIDAY... EMBEDDED WEAK MID LEVEL WAVE OVER WESTERN KY WILL MOVE NE ACROSS OHIO AFTER MIDNIGHT BEFORE PASSING TO THE NE OF THE REGION EARLY SATURDAY. THIS SHOULD CAUSE THE CURRENT HIGH LEVEL MID DECK TO DIMINISH SOME OVER THE NEXT FEW HOURS...ALLOWING FOR MORE CLEAR BREAKS AFTER MIDNIGHT PER VERY DRY AIR ALOFT OFF EVENING SOUNDINGS. ANY CLEARING WILL ALSO MAKE FOR GOOD RADIATIVE COOLING AGAIN LATER ON WITH THE SURFACE HIGH STILL IN PLACE. HOWEVER WITH DEWPOINTS A BIT HIGHER AND MORE CLOUDS TO INIT...SHOULD NOT GET NEARLY AS COLD AS LAST NIGHT WITH SOME 20S IN THE VALLEYS...AND OUT EAST WHERE CLOSER TO THE SURFACE RIDGE. OTRW GOOD RANGE IN TEMPS FROM THE VALLEYS TO THE RIDGETOPS WHERE SOME SPOTS AT ELEVATION MAY STAY IN THE 40S. THUS TWEAKED LOWS DOWN A NOTCH IN SOME VALLEYS AND UP A BIT ON THE RIDGES. PREVIOUS VALID DISCUSSION AS OF 325 PM EST FRIDAY... HIGH PRESSURE WILL REMAIN IN CONTROL TONIGHT WITH LIGHT WINDS...RESULTING QUICKLY FALLING TEMPS BY THIS EVENING. PATCHY HIGH CLOUDS ARE EXPECTING TO MOVE INTO WESTERN PORTIONS OF THE FCST AREA THIS EVENING...AND AN UPPER WAVE MAY EVEN BRING SOME SCATTERED TO BROKEN CLOUDS MID LAYER CLOUDS IN FROM THE WEST OVERNIGHT AS THEY TRY TO CROSS INTO THE PIEDMONT. THIS IS THE FACTOR THAT WILL MAKE OVERNIGHT TEMPS A LITTLE TRICKY...AND WOULD OTHERWISE GO EVEN COLDER THAN THE CHILLIER MAV GUIDANCE IF THESE WERE TO REMAIN SCATTERED AND THIN. GIVEN THE POSSIBILITY OF A PERIOD OF THICKER CLOUDS...WILL COMPROMISE BETWEEN THE COLDER MAV AND MILDER MET...AT LEAST THATS THE CASE IN THE WEST. STILL MOST TEMPS WILL GET WELL BELOW FREEZING...BUT SOME HIGHER ELEVATION LOCATIONS MAY NOT...SUCH AS BLUEFIELD OR EVEN BOONE. FEEL MORE CONFIDENT THAT THE MID AND HIGH CLOUDS WILL AT LEAST PREVENT RIVER VALLEY FOG FROM FORMING TONIGHT...AND THUS SHOULD BE NO WORRIES OF FREEZING FOG AS THERE WAS LAST NIGHT. MAY STILL NEED TO KEEP AN EYE ON GREENBRIER RIVER VALLEY IF THEY GET SEVERAL HOURS OF CLEAR SKIES TOWARD MORNING. SATURDAY WILL SEE THE SFC HIGH SLOWLY GIVE WAY TO WEAK SW FLOW...AND WEAK WARM ADVECTION...SO ANOTHER MILD DAY ON TAP...LIKELY 3-5 DEG WARMER THAN TODAY. EXPECTING SOME LOW 60S IN EAST OF THE BLUE RIDGE AND EVEN A FEW SPOTS IN THE MTN VALLEYS REACHING 60. WOULD HAVE TO GET INTO MID TO UPPER 60S FOR RECORDS. MORE HIGH CLOUDS MAY BEGIN TO MOVE IN FROM THE WEST AND SOUTHWEST AHEAD OF APPROACHING FRONTAL BOUNDARY AND DEVELOPING WAVE DOWN ALONG GULF COAST...BUT MOST OF THE DAY SHOULD BE SUNNY. MODELS SPREAD LIGHT PRECIP ACROSS THE REGION FROM THE SOUTHWEST OVERNIGHT...WITH SOME DIFFERENCES STILL IN TIMING. GENERAL TREND HAS BEEN A MINOR SLOW DOWN OF ARRIVAL...AND WITH INITIALLY DRY AIR IN PLACE THIS CERTAINLY SEEMS REASONABLE. INITIALLY VIRGA MAY MANAGE TO GET SOME SPRINKLES DOWN TO SOME HIGHER ELEVATIONS OVER NW NC AND SE WV BY AROUND MIDNIGHT...BUT BEST CHANCES OF LIGHT MEASURABLE RAINFALL LOOK TO MOVE IN WELL AFTER THAT. FROM ROANOKE NORTHEASTWARD...PROBABLY WILL NOT SEE ANY LIGHT RAIN UNTIL DAYBREAK. NO ISSUES WITH ANY LOW LEVEL OR SFC TEMPS AT OR BELOW FREEZING AS PRECIP MOVES IN...SO WE ARE CONFIDENTLY LOOKING AT RAIN ONLY WITH THIS EVENT. IN FACT...MOST PLACES WILL REMAIN IN THE 40S FOR OVERNIGHT LOWS GIVEN WARM ADVECTION AND CLOUD COVER. THROUGH SATURDAY NIGHT ALL THE COLD AIR WILL REMAIN LOCKED OVER THE MIDWEST U.S. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... AS OF 300 PM EST FRIDAY... MOST OF THIS FORECAST PERIOD WILL BE DEALING WITH UPCOMING RAINFALL EVENT WHICH SHOULD BE GETTING UNDERWAY EARLY SUNDAY FROM SOUTHWEST TO NORTHEAST AS DEEP LAYER SOUTHWESTERLY FLOW AROUND SOUTH ATLANTIC RIDGE WILL PROVIDE A MOIST GULF OF MEXICO INFLOW WITH PWATS STEADILY INCREASING TO SLIGHTLY IN EXCESS OF 1.0 INCH AT KRNK ON BOTH NAM AND GFS BUFKIT PROFILES BY MIDDAY SUNDAY...WHICH IS ROUGHLY 2 STD. DEVIATIONS ABOVE LATE DECEMBER CLIMO...SO PLENTY OF MOISTURE AVAILABLE. SEVERAL WEAK WAVE WILL TRACK ALONG THE FRONTAL BOUNDARY SUNDAY INTO EARLY MONDAY BUT WITH MODEST UPPER LEVEL SUPPORT. WILL CONTINUE WITH LIKELY TO CATEGORICAL POPS MUCH OF SUNDAY WITH TEMPS AGAIN FAIRLY MILD...FROM UPPER 40S NW TO UPPER 50S SE. SURFACE TEMPS REMAIN WELL ABOVE FREEZING SUNDAY NIGHT EXCEPT POSSIBLY AT HIGHEST NORTHWEST ELEVATIONS AND THICKNESS PARAMETERS CONTINUE TO INDICATE A PURE LIQUID EVENT ALTHOUGH A FEW HOURS OF MIXED R-S CANNOT BE RULED OUT IN FARTHEST NW GREENBRIER. LIKELY OR HIGH CHANCE POPS WILL DIMINISH SLOWLY AS THE FINAL WEAK SURFACE WAVE TRACKS OFF TO THE NORTHEAST BY EARLY TO MIDDAY MONDAY. SEE HYDROLOGY SECTION FOR QPF/HYDRO IMPACTS. COLD AIR ADVECTION MONDAY NOT VERY IMPRESSIVE AS H85 TEMPS FALL TO AROUND 0C IN THE CENTRAL CWA BY LATE MONDAY NOT TO 0C IN THE SOUTHEAST UNTIL EARLY TUESDAY MORNING AND THAT IS ON THE COLDER ECMWF SOLUTION...NAM/GFS ARE SOMEWHAT WARMER. STILL A FEW SNOW SHOWERS POSSIBLE IN THE NW MOUNTAINS AS TEMPS CLIMB ONLY INTO LOW 40S FAR NW BUT STILL REACH LOW 50S IN THE SOUTHEAST UNDER DIMINISHING CLOUD COVER. && .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 /01Z SATURDAY THROUGH WEDNESDAY/... AS OF 550 PM EST FRIDAY... HIGH PRESSURE WILL COVER THE REGION TONIGHT THROUGH SATURDAY WITH VFR CONDITIONS AND LIGHT WINDS. PERIODS OF BROKEN MID/HIGH CLOUDS WILL CROSS THE AREA TONIGHT AND SHOULD LIMIT TOO 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 OR WORSE LATE. CANT TOTALLY RULE OUT SOME ISOLATED PATCHES OF FOG AS WELL AT KBCB/KLYH BUT WONT INCLUDE FOR NOW. ANOTHER COLD FRONT WILL APPROACH THE REGION LATE SATURDAY WITH VFR CLOUDS FILLING IN ACROSS THE REGION AND CEILINGS STEADILY LOWERING OVERNIGHT. 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 SATURDAY 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...AMS/SK NEAR TERM...JH/SK SHORT TERM...PC LONG TERM...PM AVIATION...AMS/JH/SK HYDROLOGY...PC

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