Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 282038 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 338 PM EST SUN DEC 28 2014 .SYNOPSIS...
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A FRONTAL BOUDARY 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. BY MID WEEK...DRY HIGH PRESSURE WILL PUSH INTO THE APPALACHIAN REGION.
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&& .NEAR TERM /THROUGH MONDAY/...
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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.
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&& .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
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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 OROGRAPHY 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.
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&& .LONG TERM /THURSDAY THROUGH SUNDAY/...
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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 FORCAST 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.
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&& .AVIATION /21Z SUNDAY THROUGH FRIDAY/... AS OF 625 AM EST SUNDAY... LIGHT RAIN EXPECTED AT TAF SITES THIS AFTERNOON WITH MAINLY MVFR CIGS/VSBYS ALTHOUGH OCCASIONALLY IFR IN HEAVIER PASSING SHOWERS. COULD SEE A BREAK THIS EVENING BEFORE ANOTHER WEAK WAVE BRINGS RAIN AND LOWER FLT CATEGORIES BACK INTO THE AREA OVERNIGHT. IN BETWEEN THESE WAVES...EXPECT RESTRICTIONS TO REMAIN MARGINAL WITH FOG AND DRIZZLE AND LOW CLOUDS. PRECIPITATION FROM THIS LAST WAVE PULLS OUT FROM WEST TO EAST TOMORROW MORNING BUT WITH ONLY SLOWLY IMPROVING CIGS AS THE MID-LEVEL FLOW REMAINS SOUTHWEST WHILE COLDER AIR AT THE SURFACE SLOWLY PUSHES TO THE SOUTHEAST. 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 AFTERNOON FOR EASTERN SITES TO RETURN TO VFR WHILE LINGERING LOW CLOUDS COULD CAUSE MVFR CIGS TO PERSIST OVER THE WEST UNTIL LATE IN THE DAY. SOME SPOTTY LIGHT RAIN OR WINTRY PRECIP MAY ALSO OCCUR EARLY TUESDAY AS A WEAK TROUGH OF LOW PRESSURE REDEVELOPS OVER THE MOUNTAINS. OTRW HIGH PRESSURE BUILDS EAST WEDNESDAY INTO THURSDAY WITH GOOD FLYING CONDITIONS RETURNING UNDER VFR. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...JH/SK NEAR TERM...PC SHORT TERM...SK LONG TERM...AMS AVIATION...PC/RCS

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