Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 271750 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 1250 PM 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 1155 AM EST SATURDAY... TEMPS IN SOME SPOTS HAVE QUICKLY CLIMBED TO OR EVEN ABOVE FORECAST HIGHS...ESPECIALLY NW NC MOUNTAINS..SO WENT AHEAD AND BUMPED UP HIGHS MAINLY ACROSS FAR SOUTHWEST AND ALONG FOOTHILLS. THE HIGHER ELEVATIONS WILL NOT RISE A GREAT DEAL MORE...PARTLY DUE TO SOME INCREASING CLOUD COVER ARRIVING IN THE FAR SW SOON. BUT SINCE TEMPS IN A FEW AREAS WILL STILL GET A BIT BEYOND PREV FCST MAX TEMPS...NEED FOR UPDATE. PREV DISCUSSION 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 /18Z SATURDAY THROUGH THURSDAY/...
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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.
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&& .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...RCS/SK HYDROLOGY...PC

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