Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 290223 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 923 PM EST SUN DEC 28 2014 .SYNOPSIS... A FRONTAL BOUNDARY WILL SLOWLY MOVE SOUTHEAST TO NEAR THE CAROLINA COAST BY MONDAY MORNING...WHILE AN UPPER WAVE TRACKS ALONG IT OVERNIGHT. THIS WILL SPREAD ANOTHER WAVE OF LIGHT RAIN ACROSS THE REGION LATE TONIGHT AND INTO THE FIRST HALF OF MONDAY. BEHIND THAT...SHALLOW COLD AIR WILL FILTER IN FROM THE NORTH MONDAY NIGHT AND TUESDAY...WHILE ANOTHER WEAK WAVE SLIDES ACROSS FROM THE WEST. THIS WILL BRING THE POTENTIAL FOR A VERY LIGHT WINTRY MIX TO PARTS OF THE AREA EARLY TUESDAY. && .NEAR TERM /THROUGH MONDAY/... AS OF 852 PM EST SUNDAY... RAINFALL REMAINS AT A RELATIVE MINIMUM THIS EVENING GIVEN MID LEVEL DRY AIR SEEN OFF EVENING SOUNDINGS AND LACK OF LIFT AS IN BETWEEN THE WAVE FROM EARLIER AND THE NEXT JET MAX TO THE SW. THIS DESPITE THE LOW LEVEL COLD FRONT SNAKING ALONG THE BLUE RIDGE WHICH LOOKS TO FINALLY AID ENOUGH CONVERGENCE WITH INCREASING OMEGA ESPCLY AFTER MIDNIGHT TO OVERCOME THE DRYING ALOFT. MOST GUIDANCE CONTINUES TO SUGGEST THAT THE BAROCLINIC ZONE/THETA-E GRADIENT TO THE NW WILL GRADUALLY SHIFT SE AND COMBINE WITH THE RESIDUAL FRONT AND THE NEXT WAVE HEADING UP FROM THE SW LATE. THIS SHOULD FOCUS AN AXIS OF RAINFALL ALONG AND SOUTH OF HIGHWAY 460 PRIOR TO DAYBREAK WITH LIGHTER AMOUNTS TO THE NORTH. THUS HAVE ADJUSTED POPS DOWN TO LOW CHANCE WEST AND LITTLE OUT EAST TO INIT THROUGH ABOUT MIDNIGHT BEFORE RAMPING UP SOUTHERN/EASTERN AREAS TO LIKELYS/CATEGORICAL OVERNIGHT PER LATEST HRRR/RNK WRF AND THE 00Z NAM. TEMPS BASICALLY LOOK ON TRACK WITH ONLY SOME SLIGHT UPWARD TWEAKS EAST-SE WHERE VALUES SLOW TO FALL AND SUPPORTED BY THE LATEST SLIGHTLY WARMER MOS OUTPUT. PREVIOUS VALID DISCUSSION AS OF 315 PM EST SUNDAY... COLD FRONT LOCATED ACROSS THE CENTRAL/SOUTHERN APPALACHIANS EARLY THIS AFTERNOON IS MAKING SLOW PROGRESS AGAINST THE PERSISTENT SOUTHWEST FLOW ALOFT FROM THE STUBBORN RIDGE OFF THE SOUTHEAST ATLANTIC COAST. EXPECTING A LULL IN THE PRECIPITATION PATTERN WHICH HAS SO FAR BEEN VERY UNDERWHELMING WITH RAINFALL AMOUNTS UP TO NOW UNDER A TENTH OF AN INCH /0.10/ OR LESS IN MOST AREAS EXCEPT FOR THE FAR SOUTHWESTERN CWA WHERE A LITTLE OVER A QUARTER-INCH /0.25/ HAS FALLEN SINCE 12Z. THE FRONT WILL CONTINUE TO SLIDE SLOWLY TOWARD THE COAST BY EARLY MONDAY WITH ANOTHER WEAK WAVE FORMING ALONG IT AND BRINGING POSSIBLY ANOTHER ROUND OF RAINFALL IN THE 0.25 TO 0.50 INCH RANGE BY MIDDAY BEFORE TAPERING OFF BY LATE AFTERNOON. OVERNIGHT TEMPS WILL HOLD ABOUT STEADY AS WEAK COLD ADVECTION IS OFFSET BY THE THICK OVERCAST... SHOULD SEE LOWS UPPER 30S NW TO MID-40S SE. THE COLD AIR MAKES VERY SLOW SOUTHWARD PROGRESS ON MONDAY AS WEAK COLD ADVECTION CONTINUES AND CLOUDS LINGER. GUIDANCE FOR HIGHS WELL CLUSTERED IN THE 40S ACROSS THE CWA UNDER MOSTLY CLOUDY SKIES IN WHAT HAS BEEN QUITE A CLOUDY MONTH OF DECEMBER. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... AS OF 320 PM EST SUNDAY... WHILE MAIN SFC FRONT WILL BE WELL TO THE SOUTH OF THE AREA...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 MONDAY NIGHT AND LASTING INTO AT LEAST PART OF TUESDAY. MOISTURE IS SHALLOW AND STRUGGLES TO GET INTO THE ICE CRYSTAL GROWTH ZONE...SO SOME DRIZZLE IS POSSIBLE...BUT ALSO SEE POTENTIAL FOR SOME MOISTURE LAYER ALOFT AND THUS A SEEDER-FEEDER PROCESS...AND SO FOR NOW WILL ASSUME MOST PRECIP WILL START AS ICE CRYSTALS ALOFT. THE TWO NEXT QUESTIONS ARE HOW MUCH PRECIP...AND WHAT WILL THE LOW LEVEL THERMAL PROFILE LOOK LIKE ACROSS THE AREA. LOOKING UPSTREAM TO THE NORTHWEST AT THIS VERY COLD AND DRY AIRMASS CURRENTLY OVER THE UPPER MIDWEST...THINK THAT THE NAM AND OTHER HIGHER RESOLUTION MODELS ARE GOING TO HANDLE THE EVOLUTION OF THE SHALLOW COLD AIR BETTER THAN GFS AND EVEN ECMWF. GIVEN THIS...THE NAM ALSO PROBABLY HAS A BETTER HANDLE ON THE SHALLOW LIFT OVER THIS COLD AIR...INCLUDING A COMPONENT AIDED BY OROGRAPHICS WITH MORE OF AN 850 SOUTHERLY COMPONENT FOR AWHILE MON NIGHT. THUS NAM IS DEVELOPING MORE QPF...AND EVEN THOUGH WILL LEAN IN THIS DIRECTION...WILL NOT GO QUITE AS WET AS THE ONE THIRD INCH OR MORE OF LIQUID EQUIV...BUT NOT NEARLY AS DRY AS THE GFS EITHER. THE COMBINATION OF NAM THERMAL PROFILE...AND EVEN GOING SLIGHTLY COLDER THAN THE NAM GUIDANCE WOULD SUGGEST AT THE SURFACE GIVEN MODEL BIASES IN THESE COLD AIR DAMMING SITUATIONS...ALONG WITH THE BLEND OF THE MODEL QPF...RESULTS IN SOME LIGHT SNOW/SLEET MIX ACROSS THE NORTHERN PORTIONS OF THE AREA...CHANGING TO MOSTLY SNOW EXTREME NORTH TOWARD TUES MORNING. HAVE SOME MIX AS FAR SOUTH AS NEW RIVER VALLEY...BUT QPF SO LIGHT AND GROUND INITIALLY WARM THAT DO NOT SEE A THREAT NOW FOR STICKING EXCEPT FOR HIGHER ELEVATIONS. WITH LIQUID EQUIV QPF BETWEEN 0.10 AND 0.20...AND THE MIX OF SNOW AND SLEET...EVEN THE HIGHEST ELEVATIONS SHOULD SEE NO MORE THAN AN INCH OF SNOW/SLEET. BUT A LIGHT COATING ON SOME ROADS BY EARLY TUESDAY MAINLY NORTH OF I-64 AS WELL AS ALONG BLUE RIDGE PARKWAY NORTH OF ROANOKE IS CERTAINLY POSSIBLE. THIS MINOR EVENT STILL NEEDS TO BE MONITORED CLOSELY FOR TRENDS...MAINLY IN QPF POTENTIAL...BUT REALLY WOULD BE SURPRISED IF WE END UP WITH ADVISORY THRESHOLDS ANYWHERE. LATER TUESDAY BEHIND THE AFOREMENTIONED UPPER WAVE...AND AS SFC LOW ALONG THE COAST DEEPENS A BIT...NAM MAY BE OVERDONE WITH THIS PARTICULAR TREND THOUGH...FLOW TURNS MORE WESTERLY AND SOME BRIEF UPSLOPE SNOW SHOWERS WILL BE POSSIBLE ON FAR WESTERN SLOPES...ESPECIALLY IN WEST VIRGINIA...LATE TUESDAY THROUGH POSSIBLE EARLY WED MORNING...BEFORE COLD BUT DRY HIGH PRESSURE EXERTS ITS INFLUENCE ON ENTIRE MID ATLANTIC REGION BY WED AND WED NIGHT. EXPECT SOME OF THE COLDEST MINIMUM TEMPS WE HAVE SEEN ALL OF DECEMBER RIGHT AS THE YEAR COMES TO AN END...WITH LOWS TUESDAY NIGHT IN THE 20S AND BY WED NIGHT IN THE TEENS. I LIKE THE COLDER ECWMF MOS-BASED GUIDANCE VS THE GFS MEX WITH THIS AIRMASS IN PLACE. && .LONG TERM /THURSDAY 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 /02Z MONDAY THROUGH FRIDAY/... AS OF 555 PM EST SUNDAY... MUCH OF THE AREA HAS SLIPPED INTO A PRECIP LULL EARLY THIS EVENING BEHIND THE EXITING INITIAL WAVE FROM EARLIER AND THE NEXT STRONGER UPSTREAM IMPULSE TO THE SW. HOWEVER WITH THE SURFACE FRONT NOW JUST WEST OF THE BLUE RIDGE...SEEING WESTERLY UPSLOPE FLOW KEEP LIFR/IFR CIGS/VSBYS IN AT KBLF AND VARYING MVFR TO VFR CONDITIONS AT KLWB. ALSO CONVERGENCE NEAR THE FRONT HELPING TO SPREAD MVFR CIGS CLOSER TO THE BLUE RIDGE WITH KBCB LIKELY TO MAINTAIN THIS LEVEL OF CIGS INTO THE EVENING. OTRW MAINLY VFR EAST OF THE MOUNTAINS WHERE PRECIP HAS BEEN QUITE SPOTTY. MAIN CONCERN WITH HOW FAST CONDITIONS WILL DETERIORATE AGAIN OVERNIGHT AS THE NEXT WAVE APPROACHES AND COVERAGE OF RAINFALL QUICKLY INCREASES ESPCLY AFTER MIDNIGHT. MOST GUIDANCE DROPPING ALL SITES INTO MVFR IF NOT ALREADY BY 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...JH/PC SHORT TERM...SK LONG TERM...AMS AVIATION...JH/PC/RCS

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