Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 290959 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 459 AM EST MON DEC 29 2014 .SYNOPSIS...
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A FRONTAL BOUNDARY EXTENDS FROM THE TIDEWATER OF VIRGINIA TO THE FLORIDA PANHANDLE. A WAVE OF LOW PRESSURE WAS RIPPLING NORTHEAST ALONG THE FRONT PROVIDING WIDESPREAD RAIN TO MUCH OF THE MID ATLANTIC REGION. THIS FRONTAL SYSTEM WILL SLOWLY MOVE EAST AND OFF THE COAST...COLDER AIR FILTERING INTO THE REGION TONIGHT AND TUESDAY. AN UPPER LEVEL DISTURBANCE IS EXPECTED TO PASS FROM WEST TO EAST ACROSS THE AREA EARLY TUESDAY. THIS FEATURE MAY BRING A LIGHT WINTRY MIX OF PRECIPITATION TO PARTS OF THE AREA...WITH THE POTENTIAL FOR AN INCH OR TWO OF SNOW ACROSS THE VIRGINIA AND WEST VIRGINIA HIGHLANDS.
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&& .NEAR TERM /THROUGH TUESDAY/...
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AS OF 430 AM EST MONDAY... A COLD FRONT SLIPPED THROUGH THE AREA LATE SUNDAY AND IS NOW SLOWLY DRIFTING TOWARD THE MID ATLANTIC COAST. A WAVE OF LOW PRESSURE WAS RIPPLING NORTHEAST ALONG THE FRONT...PROVIDING AREAS MAINLY SOUTH OF INTERSTATE 81 WITH A WIDESPREAD WETTING RAIN...QPF OF A THIRD /0.33/ TO TWO THIRDS /0.66/ OF AN INCH. NORTH OF I-81 THE RAIN WAS NOT SO WIDESPREAD...AND AMOUNTS WILL LIKELY AVERAGE LESS THAN A TENTH OF AN INCH. ONCE THE WAVE PASSES LATER THIS MORNING...RAIN SHOULD END OR TAPER TO DRIZZLE. CLEARING IS NOT ANTICIPATED PER THE DEVELOPMENT OF NORTHEASTERLY LOW LEVEL WINDS WHICH WILL KEEP CLOUDS BANKED AGAINST THE EAST SIDE OF THE APPALACHIANS. TEMPERATURES TODAY WILL BE STEADY OR FALLING...COOLER AIR INFILTRATING THE AREA FROM THE NORTH OR NORTHEAST AND COUNTERING ANY SORT OF DIURNAL TEMPERATURE RISE. WHILE THE MAIN SURFACE FRONT WILL BE WELL TO THE SOUTHEAST OF THE AREA TONIGHT...THE COMBINATION OF A SECONDARY SURGE OF SHALLOW COLDER AIR SPILLING DOWN FROM THE NORTH AND NORTHEAST...ALONG WITH A VERY WEAK UPPER WAVE PASSING OVER FROM THE WEST...WILL CREATE SOME WEAK ISENTROPIC LIFT OVER THE APPALACHIANS OVERNIGHT AND LASTING INTO AT LEAST PART OF TUESDAY. MOISTURE IS SHALLOW AND STRUGGLES TO GET INTO THE ICE CRYSTAL GROWTH ZONE. BOTH THE GFS AND NAM DO INDICATE A 4 TO 8 HOUR WINDOW LATE TONIGHT INTO EARLY TUESDAY WHERE SOME ICE CRYSTAL GROWTH TAKES PLACE...BOTH MODELS SPITTING OUT A TENTH TO TWO TENTHS OF AN INCH OF QPF. SO THE FORECAST CHALLENGE FOR LATE TONIGHT AND EARLY TUESDAY IS WEATHER THE PRECIPITATION THAT DEVELOPS FALLS AS DRIZZLE/FREEZING DRIZZLE...OR AS LIGHT RAIN/LIGHT SNOW. TEMPERATURES TONIGHT AT THE SURFACE ARE FORECAST TO REACH FREEZING ALONG AND NORTH OF ROUTE 460. THIS WOULD BE THE GENERAL REGION WHERE THE FREEZING DRIZZLE OR LIGHT SNOW WOULD OCCUR. SOUTH OF 460...THE BOUNDARY LAYER WOULD MOST LIKELY REMAIN WARM ENOUGH TO SUPPORT DRIZZLE/LIGHT RAIN. SINCE BOTH THE GFS AND NAM ARE IN GENERAL AGREEMENT THAT A PERIOD OF LIGHT PRECIP WILL OCCUR...WILL ERR ON THE SIDE OF CAUTION AND FORECAST AT LEAST A TENTH OF AN INCH OF QPF. FOR AREAS WHERE TEMPERATURES ARE COLD ENOUGH...MAINLY NORTH OF HIGHWAY 460...THIS QPF WILL FALL AS LIGHT SNOW OR A MIXTURE OF RAIN AND SNOW. THE HIGHER ELEVATIONS...HIGHLANDS OF VA/WV...WOULD HAVE THE BEST CHANCE FOR SEEING SOMETHING THAT WOULD ACCUMULATE...OUR GRIDDED FORECAST REFLECTING 1-2 INCHES OF SNOW FOR ELEVATIONS ABOVE 2500 FEET. SINCE ACCUMULATIONS ARE NOT EXPECTED TO EXCEED 2 INCHES...NO HEADLINES PLANNED ATTM. THE ISENTROPIC LIFT IS FORECAST TO WEAKEN BY TUESDAY AFTERNOON/EVENING...ANY PRECIPITATION COMING TO AN END. SIMILAR TO MONDAY...TEMPERATURES TUESDAY WILL EXPERIENCE LITTLE CHANGE REMAINING NEARLY STEADY THROUGHOUT THE DAY.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
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AS OF 400 AM EST MONDAY... A SHORT WAVE WILL TRACK ACROSS THE OHIO VALLEY TO THE MID ATLANTIC COAST TUESDAY EVENING. ONCE THIS WAVE PASSES EAST...DRIER COLDER AIR WILL FILTER INTO THE REGION OVERNIGHT. CONSIDERING THE MILD WEATHER THE PAST COUPLE OF WEEKS...THIS COLD AIR WILL SEEM LIKE AN ARCTIC BLAST TO SOME...BUT WE WILL ONLY BE DROPPING TEMPERATURES BACK TO SEASONAL LEVELS. THE COLD SURFACE HIGH WILL RESIDE BACK OVER THE PLAINS TUESDAY NIGHT AND SLOWLY MOVE EAST IN ZONAL FLOW THROUGH THE REST OF THE WEEK. NO PRECIPITATION IS EXPECTED DURING THE PERIOD.
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&& .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... AS OF 300 PM EST SUNDAY... PERIOD BEGINS WITH SPLIT FLOW AND MODEST TROF OVER THE NORTHEAST UNITED STATES WITH BROAD WESTERLY FLOW OVER THE FORECAST AREA AND A CUT OFF LOW IN THE SOUTHWEST UNITED STATES. PATTERN AMPLIFIES BY FRIDAY WITH DIGGING TROF IN THE CENTRAL UNITED STATES THAT GENERATES A SURFACE LOW IN THE LEE OF THE ROCKIES. MODELS SIMILAR IN THE EVOLUTION OF THE LOW THROUGH FRIDAY AND BOTH THE ECMWF AND GFS TRACK THE LOW FROM THE MISSISSIPPI VALLEY INTO THE GREAT LAKES. THE ECMWF IS FASTER THAN THE GFS WITH THE LOW AND COLD FRONT. WPC STAYED CLOSE TO THE TRACK OF THE ECMWF BUT AT A SLOWER SPEED. SOME IN-SITU WEDGING MAY BE IN PLACE ON FRIDAY AND FRIDAY NIGHT. SURFACE AND 850 WARM FRONT LIFT NORTH OF THE FORECAST AREA BY SATURDAY. NO CLEAR TRENDS FOR PRECIPITATION TYPE ALTHOUGH WARMING ALOFT SUPPORTS FREEZING RAIN/SLEET...DEPENDING ON HOW COLD TEMPERATURES AREA AT THE SURFACE. BEST MOISTURE AND LIFT WILL BE SATURDAY AND SATURDAY NIGHT JUST AHEAD OF THE COLD FRONT. AT THIS TIME EXPECTING THE FRONT TO COME THROUGH THE FORECAST AREA EARLY SUNDAY. THIS TIMING IS LIKELY TO CHANGE AS THE SYSTEM GOES THROUGH A FEW MORE FORECAST CYCLES IN THE MODELS. && .AVIATION /09Z MONDAY THROUGH FRIDAY/... AS OF 1125 PM EST SUNDAY... RAIN SLOWLY SPREADING BACK EAST ACROSS THE MOUNTAINS ATTM AND SHOULD PUSH A BAND EAST OVER MOST OF THE TAF SITES AFTER MIDNIGHT. THIS ALONG WITH UPSLOPE FLOW WILL KEEP KBLF IN LIFR CIGS OVERNIGHT WITH PERIODIC IFR TO LIFR VSBYS IN FOG/RAIN. OTRW MAIN CONCERN WITH HOW FAST CONDITIONS WILL DETERIORATE AGAIN OVERNIGHT AS THE THE NEXT WAVE CROSSES THROUGH EARLY MONDAY. MOST GUIDANCE DROPPING ALL SITES INTO MVFR IF NOT ALREADY JUST AFTER MIDNIGHT AND THEN INTO WIDESPREAD IFR/LIFR CIGS AND MVFR TO IFR VSBYS THROUGH DAYBREAK MONDAY IN RAIN/FOG/DRIZZLE AND STRATUS. HOWEVER GIVEN CURRENT SLOWER TRENDS MAY HOLD CONDITIONS UP A BIT LONGER BEFORE BASICALLY TANKING CIGS/VSBYS AT ALL LOCATIONS WITHIN THE RAIN SHIELD BY EARLY MONDAY. WAVE SHIFTS EAST MONDAY ALLOWING THE FRONT TO SINK SE AND AWAY FROM THE AREA IN THE AFTERNOON. THIS SHOULD ACT TO TAPER STEADY RAIN BACK TO MAINLY DRIZZLE OR JUST FOG IN THE AFTERNOON BUT GIVEN MORE EASTERLY FLOW APPEARS VERY LOW CIGS WILL PERSIST WITH MOSTLY IFR OR WORSE THROUGH MUCH OF THE DAY. KLWB BEING ON THE NORTHERN FRINGE OF THE PRECIP COULD SEE JUST ENOUGH DRYING TO PROVIDE A FEW HOURS OF MVFR BUT IFFY. EXTENDED DISCUSSION... THE FRONT DRIFTS OFF TO THE SE MONDAY NIGHT INTO TUESDAY WHILE LEAVING PLENTY OF RESIDUAL LOW LEVEL MOISTURE IN ITS WAKE. HIGH CONFIDENCE IN STAYING SUB-VFR MONDAY NIGHT WITH MORE FOG/DRIZZLE AROUND OVERNIGHT. SLOW DRYING TO TAKE SHAPE ON TUESDAY AS A LARGE AREA OF HIGH PRESSURE STARTS TO NOSE IN FROM THE WEST. HOWEVER MAY TAKE UNTIL LATE AFTERNOON IF AT ALL FOR EASTERN SITES TO RETURN TO VFR WHILE LINGERING LOW CLOUDS COULD CAUSE MVFR CIGS TO PERSIST OVER THE WEST UNTIL TUESDAY EVENING. SOME SPOTTY LIGHT RAIN OR WINTRY PRECIP MAY ALSO OCCUR MONDAY NIGHT INTO TUESDAY AS A WEAK TROUGH OF LOW PRESSURE REDEVELOPS OVER THE MOUNTAINS. THIS LOOKS MOST LIKELY ALONG THE KLWB-KBCB CORRIDOR INCLUDING PARTS OF THE BLUE RIDGE AT THIS POINT. OTRW HIGH PRESSURE BUILDS EAST WEDNESDAY INTO THURSDAY WITH GOOD FLYING CONDITIONS RETURNING UNDER VFR. THE NEXT LOW PRESSURE SYSTEM TO THE SOUTH WILL BRING A RETURN OF CLOUDS AND LIGHT PRECIP DURING FRIDAY AS MOISTURE OVERRUNS THE COOL AIR IN PLACE. THIS LOOKS TO BRING SUB- VFR CONDITIONS BACK TO MOST OF THE TAF SITES BY FRIDAY AFTERNOON IF NOT SOONER. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...JH/SK NEAR TERM...PM SHORT TERM...RCS LONG TERM...AMS AVIATION...JH/PC/RCS

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