Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000
FXUS61 KRNK 291230
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
730 AM EST MON DEC 29 2014

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TUESDAY/...
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.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
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.

&&

.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 /12Z MONDAY THROUGH FRIDAY/...
AS OF 700 AM EST MONDAY...

LARGE AREA OF RAIN THIS MORNING OVER VA/NC WILL MOVE EAST AND OFF
THE COAST THIS AFTERNOON. LINGERING MOISTURE WILL REMAIN
HOWEVER...IN THE FORM OF LOW LEVEL STRATUS WHICH WILL START OFF
SCT-BKN EARLY THIS MORNING AND BECOME BKN-OVC BETWEEN 500-1500FT
AGL ONCE THE RAIN ENDS LATER THIS MORNING. LOW CIGS ARE THEN
EXPECTED TO PERSIST THROUGH THE REMAINDER OF THE 24 HOUR TAF
PERIOD.

WINDS TODAY WILL PRIMARILY BE OUT OF THE NORTHEAST...TRENDING MORE
EASTERLY TONIGHT. THE EASTERLY WINDS WILL HELP MAINTAIN THE LOW
CIGS INTO THE DAY TUESDAY. AN UPPER LEVEL DISTURBANCE WILL CROSS
THE AREA TUESDAY MORNING. THIS FEATURE WILL LIKELY RESULT IN A
PERIOD OF DRIZZLE OR LIGHT SNOW LATE TONIGHT AND EARLY TUESDAY
MORNING. 1-2 INCHES OF ACCUMULATION IS POSSIBLE ACROSS THE VA/WV
HIGHLANDS...JUST NORTH OF ROANOKE...AND INCLUDING LEWISBURG WV AND
VICINITY. SOUTH AND EAST OF ROANOKE...TEMPERATURES ARE EXPECTED TO
REMAIN WARM ENOUGH THAT ONLY DRIZZLE OR LIGHT RAIN REACHES THE
SURFACE.

EXTENDED DISCUSSION...

A LARGE AREA OF HIGH PRESSURE WILL START TO NOSE INTO THE AREA
FROM THE WEST TUESDAY INTO WEDNESDAY. THIS WILL RESULT IN GRADUAL
CLEARING...CLOUD BASES LIFTING INTO THE MVFR CATEGORY TUESDAY
AFTERNOON AND TUESDAY NIGHT...BEFORE DISSIPATING WEDNESDAY AND
RETURN OF VFR. VFR WILL THEN PREVAIL THROUGH AT LEAST THURSDAY
BEFORE TRENDING DOWN AGAIN FOR THE WEEKEND PER RETURN OF MOISTURE
FROM THE SOUTHWEST. THE NEXT LOW PRESSURE SYSTEM TO AFFECT THE
AREA WILL COME FROM THE SOUTHWEST...LOW PRESSURE DEVELOPING ALONG
THE TEXAS GULF COAST FRIDAY...THEN MOVING NORTHEAST ACROSS THE MID
ATLANTIC REGION FOR THE WEEKEND. THIS SYSTEM LOOKS LIKE IT WILL
PRODUCE A MULTI-DAY PERIOD OF SUB-VFR CONDITIONS...BEGINNING AS
EARLY AS FRIDAY AFTERNOON...AND LINGERING THROUGH THE WEEKEND.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...NONE.
NC...NONE.
WV...NONE.

&&

$$
SYNOPSIS...JH/SK
NEAR TERM...PM
SHORT TERM...RCS
LONG TERM...AMS
AVIATION...PM


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