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...
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.

&&

.NEAR TERM /THROUGH MONDAY/...
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
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.

&&

.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 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.

&&

.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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