Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 241210 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 710 AM EST WED DEC 24 2014 .SYNOPSIS... A NARROW WEDGE OF HIGH PRESSURE REMAINED IN PLACE ALONG AND EAST OF THE APPALACHIANS THIS MORNING. A STRONG COLD FRONT WILL MOVE THROUGH THIS EVENING AND BRING COLDER TEMPERATURES...MORE RAIN AND SOME GUSTY WINDS AS IT PASSES. HIGH PRESSURE WILL THEN BUILD OVER THE EASTERN UNITED STATES INTO THE WEEKEND WITH GENERALLY FAIR WEATHER EXPECTED INTO SATURDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 425 AM EST WEDNESDAY... LONG WAVE TROF LIFTS NORTHEAST OUT OF THE PLAINS AND MISSISSIPPI VALLEY TODAY INTO SOUTHEAST CANADA BY LATE TONIGHT. MODELS BRINGING DRY SLOT INTO THE WESTERN COUNTY WARNING AREA AT SLIGHTLY DIFFERENT SPEEDS LATE THIS AFTERNOON. OUTCOME ENDS UP THE SAME WITH DEEPER MOISTURE WELL TO THE EAST OF THE FORECAST AREA BY THURSDAY MORNING. DRY SLOT WILL BE MOVING ACROSS THE FORECAST AREA THIS EVENING FOLLOWED BY A NARROW AXIS OF HIGHER MOISTURE RIGHT ALONG THE COLD FRONT. AFTER BRIEF DRYING...LOW LEVEL MOISTURE FILLS BACK IN IN THE WESTERN UPSLOPE AREAS BY 06Z/1AM. COLD AIR ALOFT COMES IN LATE TONIGHT SO NOT A LOT OF INSTABILITY EXPECTED. WILL KEEP ISOLATED THUNDER OUTSIDE THE WEDGE THIS AFTERNOON WHERE LIFTED INDEX VALUES ARE LOWER. LATEST SPC SREF HAS FORECAST 0-3KM HELICITY VALUES OF 300 M2/S2 POSSIBLE THIS AFTERNOON. SURFACE COLD FRONT WILL BE JUST REACHING THE WESTERN EDGE OF THE FORECAST AREA AT 00Z/7PM. WINDS BECOME NORTHWEST BRIEFLY BEHIND THE FRONT BUT WITH THE MAIN HIGH ACROSS THE GULF COAST WINDS WILL BACK TO THE WEST AFTER A COUPLE OF HOURS. SURFACE BASED WEDGE WAS VERY SHALLOW AND WILL BREAK JUST BEFORE COLD FRONT COMES THROUGH. UNTIL THEN TEMPERATURE WILL RISE LITTLE WITHIN THE WEDGE...SIMILAR TO PAST COUPLE OF DAYS. WEDGE EROSION WILL FOLLOW TYPICAL PATTERN WITH NORTHEAST COUNTIES IN THE COLDER AIR THE LONGEST. 850 MB TEMPERATURES DROP TONIGHT DOWN TO THE -2 TO -4 RANGE BY MORNING. NAM AND GFS WERE SHOWING STRONG PRESSURE RISES BEHIND THE FRONT...PEAKING IN THE WESTERN COUNTY WARNING AREA AROUND 06Z/1AM AT 7MB/6HR. FORECAST WIND GUSTS WILL BE CLOSE TO...BUT JUST BELOW ADVISORY CRITERIA OVERNIGHT. && .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... AS OF 250 AM EST WEDNESDAY... CHRISTMAS MORNING...AN UPPER LEVEL RIDGE WILL BE BUILDING IN THE MIDDLE MISSISSIPPI RIVER VALLEY. THE PRESSURE GRADIENT BETWEEN THIS RIDGE AND THE DEPARTING TROUGH WILL HELP MAINTAIN GUSTY NORTHWEST WINDS THROUGH THE DAY...ESPECIALLY FROM SUNRISE THROUGH THE EARLY AFTERNOON. BY THE MID TO LATE AFTERNOON...THE CENTER OF THE BUILDING RIDGE WILL HAVE SHIFTED FARTHER TOWARDS THE FORECAST AREA...ALLOWING THE PRESSURE GRADIENT TO SLACKEN AND WINDS CORRESPONDINGLY TO START TO DECREASE. SCATTERED UPSLOPE RAIN/SNOW SHOWERS ARE EXPECTED ALONG THOSE PREFERRED WESTERN SLOPES OF SOUTHEAST WEST VIRGINIA...SOUTH INTO THE NORTHERN MOUNTAINS OF NORTH CAROLINA. IF YOU HAVE BEEN HOPING FOR A WHITE CHRISTMAS IN THIS PART OF THE COUNTRY...YOU MAY FIND A FINE CARPET OF SNOW A FEW TENTHS OF AN INCH DEEP ACROSS THE HIGHER TERRAIN OF GREENBRIER COUNTY WEST VIRGINIA...OR FROM NEAR MOUNT ROGERS IN WESTERN GRAYSON COUNTY VA...SOUTH ALONG THE HIGHER WESTERN PEAKS AND RIDGES OF ASHE AND WATAUGA COUNTIES IN NORTH CAROLINA. HIGH PRESSURE WILL BUILD OVER AND THEN EAST OF THE REGION THURSDAY NIGHT INTO FRIDAY. IT WILL BE CENTERED JUST OFF THE NORTH CAROLINA COAST FRIDAY NIGHT. IT INFLUENCE WILL ALLOW FOR A PRECIPITATION FREE FORECAST DURING THIS PORTION OF THE FORECAST. THIS PROGRESSION WILL ALSO ALLOW FOR SURFACE AND LOW LEVEL WINDS TO BECOME SOUTHWEST AND ALLOW FOR WARM AIR ADVECTION ACROSS THE AREA. BY SATURDAY INTO SATURDAY NIGHT...OUR NEXT POTENTIAL WEATHER MAKER WILL BE DEVELOPING NEAR THE MISSISSIPPI DELTA. CONCURRENTLY...THE NEXT UPPER STREAM TROUGH WILL BE MOVING THROUGH THE GREAT LAKES REGION. AS THESE FEATURES MAKE MORE PROGRESS EASTWARD...A TREND TOWARDS INCREASING CLOUD COVER IS EXPECTED ON SATURDAY...WITH LIGHT RAIN GRADUALLY OVERSPREADING THE AREA SATURDAY NIGHT...WITH THE GREATEST COVERAGE OVER THE NORTHERN MOUNTAINS OF NORTH CAROLINA. WHILE THERE WILL BE A SLIGHT DIP IN TEMPERATURES THURSDAY NIGHT WITH THE PRESENCE OF THE SURFACE HIGH OVERHEAD...BY FAR THIS PERIOD OF OUR FORECAST WILL EXPERIENCE TEMPERATURES WELL ABOVE NORMAL. HIGH TEMPERATURES WILL AVERAGE 10 TO 15 DEGREES ABOVE NORMAL...AND LOWS WILL AVERAGE 5 TO 10 DEGREES ABOVE NORMAL. && .LONG TERM /SUNDAY THROUGH TUESDAY/... AS OF 400 PM EST TUESDAY... CONTINUE TO ENTERTAIN A MODEL BLENDED FORECAST... CONSENSUS/COMPROMISE/MEAN...WHATEVER YOU WANT TO CALL IT FOR THE WEEKEND AND FIRST PART OF NEXT WEEK. THERE ARE BIG DIFFERENCES IN THE MODELS DUE TO UPPER LEVEL TIMING/PHASING OF SHORT WAVE ENERGY WITHIN THE MID CONUS TROUGH WHICH WILL BE WORKING ITS WAY EASTWARD FROM THE ROCKIES DURING THE PERIOD. THE ECMWF HAS THE GREATEST AMPLIFICATION WITH SHORTWAVE ENERGY PASSING THROUGH THE TROF RESULTING IN A SLOWER BUT STRONGER PROGRESSION OF SURFACE FEATURES LATER IN THE WEEKEND...WHEREAS THE GFS ENSEMBLES ARE FASTER AND LESS AMPLIFIED. THE GFS PUSHES THE SURFACE FRONT THROUGH THE FORECAST AREA LATE SATURDAY FOLLOWED BY A WEAK WAVE RIPPLING ALONG THE FRONT LATE SUNDAY. A SOLUTION THAT WOULD LEAVE THE FORECAST AREA DRY UNTIL THE ARRIVAL OF PRECIP LATE IN THE WEEKEND...POSSIBLY MIXED WITH SNOW SINCE WE WOULD BE ON THE COLD SIDE OF THE BAROCLINIC ZONE. THE ECMWF ON THE OTHER HAND...STALLS THE FRONT TO OUR WEST ON SATURDAY WITH LOW PRESSURE DEVELOPING TO OUR WEST AND MOVING NORTHEAST ALONG THE WESTERN SLOPES OF THE BLUE RIDGE SUNDAY INTO MONDAY. THIS SOLUTION IS WARMER AND MUCH WETTER. DIFFERENCES REALLY SHOW UP IN THE SOUNDING PROFILES BY 00Z MONDAY (7PM SUNDAY) WITH THE ECMWF INDICATING 85H TEMPS OF +10 DEG C VS 0 DEG C ON THE GFS. ALL THINGS CONSIDERED WOULD TEND TO WANT TO BELIEVE THE FLATTER MORE 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 A COMPROMISE WITH INITIAL WEAK FRONTAL PASSAGE BY SAT NIGHT AND SOME INCREASE IN POPS BY SUNDAY IF AT LEAST A WEAK WAVE DEVELOPS. WITH A BLENDED SOLUTION...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 WEDNESDAY THROUGH SUNDAY/... AS OF 700 AM EST WEDNESDAY... SEQUENCE OF EVENTS DURING THIS TAF FORECAST PERIOD WILL EVOLVE SOMETHING LIKE THIS...WEDGE IN PLACE THIS MORNING WITH IFR TO LIFR CEILINGS...LOW LEVEL WIND SHEAR ABOVE THE SHALLOW LAYER OF COOLER AIR...AND WIDESPREAD LIGHT TO MODERATE RAIN. THEN THE DRY SLOT COMES THROUGH THE AREA...POTENTIALLY LIFTING CEILINGS BRIEFLY TO VFR...ESPECIALLY AT KLWB AND KBLF. NEXT THE COLD FRONT WITH A LINE OF SHOWERS AND GUSTY WIND COMES THROUGH THIS EVENING. ISOLATED THUNDERSTORMS ARE POSSIBLE AT KBLF/KLWB AND KDAN. BEHIND THE FRONT PRECIPITATION ENDS...CEILINGS IMPROVE TO VFR EAST OF THE BLUE RIDGE AND WILL REMAIN MVFR IN THE MOUNTAINS. WINDS WILL BE GUSTY OUT OF THE WEST...WITH GUSTS UP TO 30 KNOTS LIKELY. TIMING IS THE MOST QUESTIONABLE PART OF THE FORECAST. MODELS STILL SHOWED DIFFERENCES IN WHEN THE WEDGE BREAKS TODAY AND WHEN THE COLD FRONT PASSES THROUGH. TAFS REFLECT THE MOST LIKELY TIMING OF THESE EVENTS WITH MEDIUM CONFIDENCE. EXTENDED DISCUSSION... THURSDAY...MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS SHOULD PERSIST IN THE MOUNTAINS...WITH VFR CONDITIONS IN THE PIEDMONT. THURSDAY NIGHT AND FRIDAY HIGH PRESSURE SHOULD BUILD OVER THE SOUTHEAST...WHICH WILL PROVIDE VFR WEATHER WITH LIGHTER WINDS. ANOTHER COLD FRONT WITH ASSOCIATED LOW PRESSURE WILL APPROACH THE REGION SATURDAY AND BE OVER OR JUST WEST OF THE AREA SUNDAY. GIVEN THE SLOWER TRENDS...APPEARS WEATHER MAY STAY VFR FOR SATURDAY WITH SUB-VFR CONDITIONS RETURNING WITH CLOUDS/PRECIP ON SUNDAY FOR NOW. && .HYDROLOGY... AS OF 335 PM EST TUESDAY... A SIGNIFICANT WETTING RAIN IS EXPECTED INTO THIS EVENING. FORECAST STORM TOTAL RAINFALL IS FROM AROUND 1.00 TO 1.50 INCHES MOST LOCATIONS...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 GIVEN THE EXPECTED RAINFALL SHOW ONLY A SMALL CHANCE THAT SOME RIVERS SUCH AS THE DAN FROM DANVILLE TO SOUTH BOSTON MAY EXPERIENCE MINOR FLOODING LATE IN THE WEEK. THERE IS A VERY GOOD CHANCE HOWEVER THAT THE RAINFALL WILL RESULT IN MODEST RISES ON SOME RIVERS. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...AMS NEAR TERM...AMS SHORT TERM...DS LONG TERM...PM AVIATION...AMS/JH HYDROLOGY...AMS/PC/SK

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