Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 231154 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 654 AM EST TUE DEC 23 2014 .SYNOPSIS... NARROW RIDGE OF HIGH PRESSURE AND SHALLOW LAYER OF COLD AIR WILL REMAIN WEDGES ALONG THE EAST SIDE OF THE APPALACHIANS TODAY AND TONIGHT. A STRONG COLD FRONT WILL MOVE THROUGH ON WEDNESDAY WITH RAIN AND SOME GUSTY WINDS. HIGH PRESSURE WILL THEN BUILD OVER THE EASTERN UNITED STATES INTO THE WEEKEND. && .NEAR TERM /THROUGH TONIGHT/... AS OF 400 AM EST TUESDAY... SURFACE LOW WAS OFF THE VIRGINIA COAST AND HIGH PRESSURE WAS WEDGED DOWN THE EAST SLOPES OF THE APPALACHIANS. THE WEDGE WILL HOLD TODAY AND TONIGHT. THE 00Z SOUNDING FROM RNK SHOWED DEPTH OF THE WEDGE AROUND 1000 FEET WITH ONLY A SMALL LAYER AROUND 3200FT ELEVATION WITHIN THAT RANGE BELOW FREEZING. MESONET OBSERVATIONS SINCE THEN HAVE COME UP SLOWLY SO THAT ONLY ISOLATED SPOTS WERE BELOW 32 DEGREES AT 2AM. EVEN BENT MOUNTAIN AND NORTHERN RIDGES IN ROCKBRIDGE COUNTY WERE AT LEAST 33 DEGREES BY 3AM. BUFKIT FORECAST HAS THE WEDGE BECOMING EVEN MORE SHALLOW TODAY AND TONIGHT WITH STRONG WARM ADVECTION JUST ABOVE THE SURFACE AND SOUTH TO SOUTHWEST WINDS ABOVE THE INVERSION UP THROUGH 850 MB INCREASING TO 40 TO 50 KNOTS BY LATE TONIGHT. WILL BE RAISING WIND SPEEDS AND GUSTS AT HIGHER ELEVATIONS TONIGHT. TEMPERATURES CREEP UP DESPITE THE WEDGE TODAY WITH THE FAR WESTERN COUNTY WARNING AREA BREAKING OUT OF THE SURFACE BASED COLDER AIR AND INTO THE 50S. LITTLE DROP IS EXPECTED IN TEMPERATURES TONIGHT. SURFACE LOW PRESSURE WILL DEVELOP ALONG A COLD FRONT CROSSING THROUGH EASTERN TEXAS TODAY. THIS LOW WILL DEEPEN AND TRACK INTO THE TENNESSEE VALLEY TONIGHT. THE SHORT WAVE ASSOCIATED WITH THIS LOW WILL SPREAD WIDESPREAD LIGHT TO MODERATE RAIN ACROSS THE FORECAST AREA AFTER 00Z/7PM TONIGHT. PRECIPITATION POTENTIAL PLACEMENT POINTING TO THE SOUTHEAST PART OF THE FORECAST AREA AS HAVING THE POTENTIAL FOR SOME HIGHER RAINFALL AMOUNTS. WILL LEAN TOWARD THE ONE INCH RAINFALL AMOUNTS IN THE SOUTHERN COUNTY WARNING AREA THAT WPC HAS FORECAST THROUGH WEDNESDAY MORNING. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... AS OF 250 AM EST TUESDAY... ON WEDNESDAY THE PARENT UPPER TROUGH EXTENDS FROM MINNESOTA SOUTH INTO EASTERN TEXAS. AN ASSOCIATED SURFACE LOW WILL BE PROGRESSING NORTHEAST ON THE EASTERN SIDE OF THE UPPER TROUGH THROUGH THE TENNESSEE AND OHIO VALLEYS AT DAYBREAK...AND THEN BE POSITIONED NEAR MICHIGAN AT SUNSET. MODELS VARY ON THE SPECIFIC TIMING AND PATH OF THE SURFACE LOW...AND LIKEWISE THE ASSOCIATED COLD FRONTAL PASSAGE THROUGH THE FORECAST AREA. A CONSENSUS SOLUTION KEEPS STRONG ISENTROPIC LIFT AND WIDESPREAD MODERATE RAIN ACROSS THE AREA DURING THE MORNING...WITH THE COLD FRONTAL PASSAGE OCCURRING DURING THE AFTERNOON. WHILE NOT A SIGNIFICANT AMOUNT...THERE STILL CONTINUES TO BE IN THE MODELS A HINT OF SOME INSTABILITY ACROSS THE FAR SOUTHEAST PART OF THE REGION DURING WEDNESDAY AFTERNOON. OUR EARLIER FORECAST OFFERED THE POTENTIAL AND THE STORM PREDICTION CENTER DAY 3 OUTLOOK HAS GENERAL THUNDER FORECAST FOR AN AREA ABOUT HALF THE SIZE AS OUR CURRENT FORECAST REFLECTS. WILL LEAVE OUR ISOLATED THUNDERSTORM FORECAST AS IS FOR CONSISTENCY AND GOOD INTEROFFICE COLLABORATION. WEDNESDAY NIGHT...THE COLD FRONT MAKES ITS WAY EASTWARD...LEAVING IN ITS WAKE A GUSTY NORTHWEST FLOW. THIS WIND TRAJECTORY WILL HELP YIELD A DOWNSLOPING AFFECT EAST OF THE CREST OF THE BLUE RIDGE. OTHER THAN SOME EARLY EVENING LINGERING SHOWERS IN THE FAR EASTERN PARTS OF THE FORECAST AREA...THE TREND WILL BE FOR DECREASING CLOUD COVER. IN THE WEST...SCATTERED UPSLOPE RAIN/SNOW SHOWERS ARE EXPECTED. THE GUSTY NORTHWEST WINDS WILL CONTINUE INTO CHRISTMAS MORNING...AND PERHAPS STRENGTHEN A BIT. SOME OF THE HIGHER ELEVATIONS...PERHAPS AT OR ABOVE 3500 FEET MSL...MIGHT SEE WIND GUSTS APPROACH ADVISORY LEVELS. THE SCATTERED RAIN/SNOW SHOWERS IN THE WEST WILL GRADUALLY DISSIPATE AS A GREATER AMOUNT OF DRIER HIGH PRESSURE WORKS ITS WAY INTO THE REGION. PLUS...BY THE LATE AFTERNOON...85H WINDS WILL START WEAKENING AND BEGIN TO BACK MORE WESTERLY...A LESS FAVORABLE ORIENTATION FOR UPSLOPE RAIN/SNOW SHOWERS. THURSDAY NIGHT THROUGH FRIDAY NIGHT...THE SURFACE HIGH BUILDS OVER...AND THEN EAST OF THE REGION INTO THE WESTERN ATLANTIC. THIS TRANSITION WILL ALLOW FOR A CONTINUATION OF A PRECIPITATION FREE FORECAST...AND A TREND TOWARD MILDER CONDITIONS AS 85H SOUTHWEST FLOW INCREASING...AND AN UPPER LEVEL RIDGE BUILDS OVER THE REGION. HIGH TEMPERATURES DURING THIS PERIOD OF THE FORECAST WILL DECREASE WEDNESDAY INTO THURSDAY BEHIND THE COLD FRONT...AND THEN INCREASE THURSDAY INTO FRIDAY AS SOUTHWEST FLOW INCREASES ACROSS THE REGION. LOW TEMPERATURES WILL THE MILDEST WEDNESDAY NIGHT BEHIND THE COLD FRONT...BUT NOT YET AT THEIR COLDEST. THE EVEN COLDER READINGS WILL COME THURSDAY NIGHT AS WINDS BECOME CALM OR LIGHT AND VARIABLE AS THE CENTER OF THE SURFACE HIGH CROSSES THE AREA. DESPITE THESE VARIOUS TRENDS...BOTH FORECAST HIGH AND LOW TEMPERATURES ARE EXPECTED TO BE ABOVE NORMAL FOR THIS TIME OF YEAR. && .LONG TERM /SATURDAY THROUGH MONDAY/... AS OF 225 PM EST MONDAY... MODELS IN GENERAL AGREEMENT THROUGH EARLY SATURDAY...SUGGESTING THIS FRONT WILL BE RELATIVELY DRY AS IT APPROACHES THROUGH SATURDAY GIVEN MOISTURE FROM GULF INITIALLY BLOCKED...BUT DIFFERENCES START TO SHOW UP AFTER THAT. ECMWF AND CANADIAN CONSISTENTLY HOLDING ONTO AN UPPER WAVE BEGINNING TO DIG INTO THE SOUTHERN PLAINS LATER SATURDAY...WHICH IN TURNS SLOWS THE ARRIVAL OF FRONTAL SYSTEM WITH A SFC WAVE DEVELOPING ALONG THE WESTERN GULF COAST. GFS AND MOST OF ITS ENSEMBLE MEMBERS ARE MORE PROGRESSIVE OVERALL...BRINGING A WEAK FRONT THROUGH BY LATE SAT...WITH A WEAK SFC WAVE REMAINING DOWN IN GULF AND CROSSING NRN FLORIDA INTO SUNDAY. THIS SITUATION HAS SOME SIMILARITIES TO THE POTENTIAL EVENT FROM THIS PAST WEEKEND WHICH NEVER AMPLIFIED...AND THUS THOSE EARLY ECWMF FORECASTS ENDED UP BEING OVERDONE. CURRENTLY...THE ECWMF FROM 00Z AND NEW RUN FROM 12Z THIS MORNING WOULD BRING A SFC LOW UP ON THE WEST SIDE OF APPALACHIANS WITH MUCH MORE PRECIP BY SUNDAY...LIKELY MOSTLY RAIN...WITH UPSLOPE MTN SNOW ON BACK SIDE BY MONDAY. GFS IS MAINLY DRY THROUGH THIS PERIOD WITH SFC HIGH PRESSURE DOMINATING AGAIN SUNDAY INTO MONDAY. CONSENSUS AT THIS TIME IS TO COMPROMISE SOMEWHAT WITH A WEAK SFC WAVE PASSING JUST TO SOUTH AND GIVEN THE HIGH DEGREE OF UNCERTAINTY...KEEPING POPS IN A LOW CHC CATEGORY FOR NOW. WOULD TEND TO WANT TO BELIEVE THE FLATTER MOVE PROGRESSIVE GFS BASED ON RECENT HISTORY...BUT WITH BETTER HISTORICAL TRACK RECORD OF ECMWF AND ITS INTERNAL CONSISTENCY IN LAST FEW RUNS...CERTAINLY CAN`T RULE OUT THE IDEA OF STRONGER WAVE. FCST GRIDS REFLECT THIS COMPROMISE WITH INITIAL WEAK FRONTAL PASSAGE BY SAT NIGHT AND SOME INCREASE IN POPS BY SUNDAY IF AT LEAST A WEAK WAVE DEVELOPS. TEMPS GENERALLY TOO WARM FOR ANY SNOW EXCEPT POSSIBLY AT HIGHEST ELEVATIONS...BUT ALL THIS WOULD BE SUBJECT TO THE ULTIMATE TRACK OF ANY DEVELOPING SFC LOW. CERTAINLY WORTH KEEPING A CLOSE EYE ON POTENTIAL DEVELOPMENTS FOR NEXT WKND. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... AS OF 650 AM EST TUESDAY... COLD AIR WEDGE WILL REMAIN IN PLACE TODAY AND TONIGHT ALTHOUGH THE COVERAGE OF THE SURFACE BASED COLD AIR WILL GRADUALLY ERODE AND BECOME VERY SHALLOW BY WEDNESDAY MORNING. ANTICIPATE WIDESPREAD IFR ALONG AND EAST OF THE BLUE RIDGE. VISIBILITY WILL VARY FROM MVFR WITH PERIODS OF LIGHT RAIN TO IFR WITH DRIZZLE AND FOG. MEDIUM CONFIDENCE OF SOME MODEST IMPROVEMENT DURING THE DAY. KBLF ON THE WESTERN EDGE OF THE COLD AIR DAMMING MAY OCCASIONAL LIFT TO VFR CEILINGS BUT KDAN/KROA/KLWB/KBCB/KLYH WILL REMAIN IFR OR MVFR. WIDESPREAD LIGHT TO MODERATE RAIN WILL OVERSPREAD THE REGION FROM SOUTH TO NORTH AFTER 00Z/7PM. HIGH CONFIDENCE THAT CONDITIONS WILL LOWER TO LIFR AT MOST LOCATIONS AS THE RAIN BEGINS. HAVE ADDED LOW LEVEL WIND SHEAR TO THE TAFS...JET INCREASES FROM THE SOUTHWEST OVERNIGHT AND BUFKIT INDICATED SPEEDS AROUND 40 KNOTS AFTER MIDNIGHT. EXTENDED DISCUSSION... WEDNESDAY AFTERNOON AND EVENING A STRONG COLD FRONT WILL SWEEP OVER THE MID ATLANTIC...AND ERODE THE WEDGE. POTENTIAL FOR LOW LEVEL WIND SHEAR AHEAD OF THIS FRONT...WITH A POTENT LOW LEVEL JET. RAIN AND MVFR CONDITIONS WILL ACCOMPANY THE FRONTAL PASSAGE AS WELL AS GUSTY SURFACE WINDS. BEHIND THE COLD FRONT WEDNESDAY NIGHT AND THURSDAY...SURFACE WINDS SHOULD TURN TO THE NORTHWEST...AND BECOME GUSTY BY CHRISTMAS DAY. MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS SHOULD PERSIST IN THE MOUNTAINS...WHILE VFR CONDITIONS WILL RETURN IN THE PIEDMONT. THURSDAY NIGHT AND FRIDAY HIGH PRESSURE SHOULD BUILD OVER THE SOUTHEAST...WHICH WILL PROVIDE VFR WEATHER WITH LIGHTER WINDS. && .HYDROLOGY... AS OF 415 AM EST TUESDAY... A SIGNIFICANT WETTING RAIN IS LIKELY TONIGHT INTO WEDNESDAY EVENING. FORECAST STORM TOTAL QPF...IS AROUND 1.00 ACROSS THE ENTIRE CWA...WITH LOCAL AMOUNTS NEAR THE BLUE RIDGE OF OVER 2.00 INCHES POSSIBLE. THIS WILL ACTUALLY BE WELCOME FROM A HYDROLOGIC STANDPOINT AS MUCH OF THE PIEDMONT HAS BEEN DRIER THAN NORMAL OVER SEVERAL MONTHS AND DECEMBER HAS HAD BELOW NORMAL PRECIPITATION IN MOST OF THE CWA. SHOULD NOT SEE ANY EXCESS RUNOFF ISSUES..I.E. FLOODING...ALTHOUGH SOME DITCHES WILL LIKELY FILL UP AND MAYBE A FEW PONDING ISSUES WHERE HEAVIER RAINFALL RATES. RIVER FORECASTS FROM THE NORTH AMERICAN AND GEFS ENSEMBLES PORTRAY BASIN-AVERAGE QPF GENERALLY IN THE 0.75 TO 1.25 INCH RANGE WITH SOME OUTLIERS AS USUAL BUT ALL SOLUTIONS AT THIS POINT PRODUCE ONLY MINOR TO MODEST WITHIN-BANK RISES ON MAINSTEM RIVERS BY LATER IN THE WEEK. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...AMS NEAR TERM...AMS SHORT TERM...DS LONG TERM...SK AVIATION...AMS/PM HYDROLOGY...AMS/PC/SK

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