Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000
FXUS61 KRNK 240943
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
443 AM EST WED DEC 24 2014

.SYNOPSIS...
A NARROW WEDGE OF HIGH PRESSURE REMAINED IN PLACE ALONG AND EAST
OF THE APPALACHIANS THIS MORNING. A STRONG COLD FRONT WILL MOVE
THROUGH THIS EVENING AND BRING COLDER TEMPERATURES...MORE RAIN
AND SOME GUSTY WINDS AS IT PASSES. HIGH PRESSURE WILL THEN BUILD
OVER THE EASTERN UNITED STATES INTO THE WEEKEND WITH GENERALLY
FAIR WEATHER EXPECTED INTO SATURDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 425 AM EST WEDNESDAY...

LONG WAVE TROF LIFTS NORTHEAST OUT OF THE PLAINS AND MISSISSIPPI
VALLEY TODAY INTO SOUTHEAST CANADA BY LATE TONIGHT. MODELS BRINGING
DRY SLOT INTO THE WESTERN COUNTY WARNING AREA AT SLIGHTLY DIFFERENT
SPEEDS LATE THIS AFTERNOON. OUTCOME ENDS UP THE SAME WITH DEEPER
MOISTURE WELL TO THE EAST OF THE FORECAST AREA BY THURSDAY MORNING.
DRY SLOT WILL BE MOVING ACROSS THE FORECAST AREA THIS EVENING
FOLLOWED BY A NARROW AXIS OF HIGHER MOISTURE RIGHT ALONG THE COLD
FRONT. AFTER BRIEF DRYING...LOW LEVEL MOISTURE FILLS BACK IN IN THE
WESTERN UPSLOPE AREAS BY 06Z/1AM.

COLD AIR ALOFT COMES IN LATE TONIGHT SO NOT A LOT OF INSTABILITY
EXPECTED. WILL KEEP ISOLATED THUNDER OUTSIDE THE WEDGE THIS AFTERNOON
WHERE LIFTED INDEX VALUES ARE LOWER. LATEST SPC SREF HAS FORECAST
0-3KM HELICITY VALUES OF 300 M2/S2 POSSIBLE THIS AFTERNOON.

SURFACE COLD FRONT WILL BE JUST REACHING THE WESTERN EDGE OF THE
FORECAST AREA AT 00Z/7PM. WINDS BECOME NORTHWEST BRIEFLY BEHIND THE
FRONT BUT WITH THE MAIN HIGH ACROSS THE GULF COAST WINDS WILL BACK
TO THE WEST AFTER A COUPLE OF HOURS. SURFACE BASED WEDGE WAS VERY
SHALLOW AND WILL BREAK JUST BEFORE COLD FRONT COMES THROUGH. UNTIL
THEN TEMPERATURE WILL RISE LITTLE WITHIN THE WEDGE...SIMILAR TO PAST
COUPLE OF DAYS. WEDGE EROSION WILL FOLLOW TYPICAL PATTERN WITH
NORTHEAST COUNTIES IN THE COLDER AIR THE LONGEST. 850 MB
TEMPERATURES DROP TONIGHT DOWN TO THE -2 TO -4 RANGE BY MORNING. NAM
AND GFS WERE SHOWING STRONG PRESSURE RISES BEHIND THE
FRONT...PEAKING IN THE WESTERN COUNTY WARNING AREA AROUND 06Z/1AM AT
7MB/6HR. FORECAST WIND GUSTS WILL BE CLOSE TO...BUT JUST BELOW
ADVISORY CRITERIA OVERNIGHT.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/...
AS OF 250 AM EST WEDNESDAY...

CHRISTMAS MORNING...AN UPPER LEVEL RIDGE WILL BE BUILDING IN THE
MIDDLE MISSISSIPPI RIVER VALLEY. THE PRESSURE GRADIENT BETWEEN THIS
RIDGE AND THE DEPARTING TROUGH WILL HELP MAINTAIN GUSTY NORTHWEST
WINDS THROUGH THE DAY...ESPECIALLY FROM SUNRISE THROUGH THE EARLY
AFTERNOON. BY THE MID TO LATE AFTERNOON...THE CENTER OF THE BUILDING
RIDGE WILL HAVE SHIFTED FARTHER TOWARDS THE FORECAST AREA...ALLOWING
THE PRESSURE GRADIENT TO SLACKEN AND WINDS CORRESPONDINGLY TO START
TO DECREASE. SCATTERED UPSLOPE RAIN/SNOW SHOWERS ARE EXPECTED ALONG
THOSE PREFERRED WESTERN SLOPES OF SOUTHEAST WEST VIRGINIA...SOUTH
INTO THE NORTHERN MOUNTAINS OF NORTH CAROLINA. IF YOU HAVE BEEN
HOPING FOR A WHITE CHRISTMAS IN THIS PART OF THE COUNTRY...YOU MAY
FIND A FINE CARPET OF SNOW A FEW TENTHS OF AN INCH DEEP ACROSS THE
HIGHER TERRAIN OF GREENBRIER COUNTY WEST VIRGINIA...OR FROM NEAR
MOUNT ROGERS IN WESTERN GRAYSON COUNTY VA...SOUTH ALONG THE HIGHER
WESTERN PEAKS AND RIDGES OF ASHE AND WATAUGA COUNTIES IN NORTH
CAROLINA.

HIGH PRESSURE WILL BUILD OVER AND THEN EAST OF THE REGION THURSDAY
NIGHT INTO FRIDAY. IT WILL BE CENTERED JUST OFF THE NORTH CAROLINA
COAST FRIDAY NIGHT. IT INFLUENCE WILL ALLOW FOR A PRECIPITATION FREE
FORECAST DURING THIS PORTION OF THE FORECAST. THIS PROGRESSION WILL
ALSO ALLOW FOR SURFACE AND LOW LEVEL WINDS TO BECOME SOUTHWEST AND
ALLOW FOR WARM AIR ADVECTION ACROSS THE AREA.

BY SATURDAY INTO SATURDAY NIGHT...OUR NEXT POTENTIAL WEATHER MAKER
WILL BE DEVELOPING NEAR THE MISSISSIPPI DELTA. CONCURRENTLY...THE
NEXT UPPER STREAM TROUGH WILL BE MOVING THROUGH THE GREAT LAKES
REGION. AS THESE FEATURES MAKE MORE PROGRESS EASTWARD...A TREND
TOWARDS INCREASING CLOUD COVER IS EXPECTED ON SATURDAY...WITH LIGHT
RAIN GRADUALLY OVERSPREADING THE AREA SATURDAY NIGHT...WITH THE
GREATEST COVERAGE OVER THE NORTHERN MOUNTAINS OF NORTH CAROLINA.

WHILE THERE WILL BE A SLIGHT DIP IN TEMPERATURES THURSDAY NIGHT WITH
THE PRESENCE OF THE SURFACE HIGH OVERHEAD...BY FAR THIS PERIOD OF
OUR FORECAST WILL EXPERIENCE TEMPERATURES WELL ABOVE NORMAL. HIGH
TEMPERATURES WILL AVERAGE 10 TO 15 DEGREES ABOVE NORMAL...AND LOWS
WILL AVERAGE 5 TO 10 DEGREES ABOVE NORMAL.

&&

.LONG TERM /SUNDAY THROUGH TUESDAY/...
AS OF 400 PM EST TUESDAY...

CONTINUE TO ENTERTAIN A MODEL BLENDED FORECAST...
CONSENSUS/COMPROMISE/MEAN...WHATEVER YOU WANT TO CALL IT FOR THE
WEEKEND AND FIRST PART OF NEXT WEEK. THERE ARE BIG DIFFERENCES IN
THE MODELS DUE TO UPPER LEVEL TIMING/PHASING OF SHORT WAVE ENERGY
WITHIN THE MID CONUS TROUGH WHICH WILL BE WORKING ITS WAY EASTWARD
FROM THE ROCKIES DURING THE PERIOD.

THE ECMWF HAS THE GREATEST AMPLIFICATION WITH SHORTWAVE ENERGY
PASSING THROUGH THE TROF RESULTING IN A SLOWER BUT STRONGER
PROGRESSION OF SURFACE FEATURES LATER IN THE WEEKEND...WHEREAS THE
GFS ENSEMBLES ARE FASTER AND LESS AMPLIFIED. THE GFS PUSHES THE
SURFACE FRONT THROUGH THE FORECAST AREA LATE SATURDAY FOLLOWED BY
A WEAK WAVE RIPPLING ALONG THE FRONT LATE SUNDAY. A SOLUTION THAT
WOULD LEAVE THE FORECAST AREA DRY UNTIL THE ARRIVAL OF PRECIP LATE
IN THE WEEKEND...POSSIBLY MIXED WITH SNOW SINCE WE WOULD BE ON THE
COLD SIDE OF THE BAROCLINIC ZONE.

THE ECMWF ON THE OTHER HAND...STALLS THE FRONT TO OUR WEST ON
SATURDAY WITH LOW PRESSURE DEVELOPING TO OUR WEST AND MOVING
NORTHEAST ALONG THE WESTERN SLOPES OF THE BLUE RIDGE SUNDAY INTO
MONDAY. THIS SOLUTION IS WARMER AND MUCH WETTER. DIFFERENCES
REALLY SHOW UP IN THE SOUNDING PROFILES BY 00Z MONDAY (7PM SUNDAY)
WITH THE ECMWF INDICATING 85H TEMPS OF +10 DEG C VS 0 DEG C ON THE
GFS.

ALL THINGS CONSIDERED WOULD TEND TO WANT TO BELIEVE THE FLATTER MORE
PROGRESSIVE GFS BASED ON RECENT HISTORY...BUT WITH BETTER HISTORICAL
TRACK RECORD OF ECMWF AND ITS INTERNAL CONSISTENCY IN LAST FEW
RUNS...CERTAINLY CAN`T RULE OUT THE IDEA OF STRONGER WAVE. FCST
GRIDS REFLECT A COMPROMISE WITH INITIAL WEAK FRONTAL PASSAGE BY SAT
NIGHT AND SOME INCREASE IN POPS BY SUNDAY IF AT LEAST A WEAK WAVE
DEVELOPS. WITH A BLENDED SOLUTION...TEMPS GENERALLY TOO WARM FOR ANY
SNOW EXCEPT POSSIBLY AT HIGHEST ELEVATIONS...BUT ALL THIS WOULD BE
SUBJECT TO THE ULTIMATE TRACK OF ANY DEVELOPING SFC LOW. CERTAINLY
WORTH KEEPING A CLOSE EYE ON POTENTIAL DEVELOPMENTS FOR NEXT WKND.

&&

.AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
AS OF 1120 PM EST TUESDAY...

WEDGE SCENARIO CONTINUES TONIGHT WITH IFR/LIFR CIGS ALONG AND
EAST OF THE BLUE RIDGE WITH MAINLY MVFR/IFR ACROSS THE WEST ON
THE PERIPHERY OF THE COOL POOL. VSBYS ALSO MVFR/IFR IN FOG/-RA/-DZ
AS WELL ESPCLY FROM KBCB EAST WITH POOR FLYING CONDITIONS LIKELY
TO PERSIST OVERNIGHT UNDER PATCHY RAIN/DRIZZLE DURING THE NEXT
FEW HOURS...AND LIKELY SOME HEAVIER RAIN BY DAYBREAK WEDNESDAY PER
LATEST RADAR LOOPS.

CIGS AND VSBYS MAY VARY OVER THE NEXT COUPLE OF HOURS GIVEN
CURRENT COVERAGE OF RAINFALL AND MORE IN THE WAY OF FOG/DRIZZLE
UNDER STRATUS. HOWEVER ONCE THE NEXT ROUND OF MORE STEADIER
RAINFALL SETS IN LATE...AND LOWER CIGS BECOME MORE
UNIFORM...WOULD EXPECT MORE OF A STEADY STATE WITH IFR/LIFR CLOUD
BASES AND MVFR TO IFR VSBYS IN PERIODS OF RAIN AND PERSISTENT
DENSE FOG THAT MAY LOWER VSBYS TO LIFR AT TIMES ESPCLY WEST WHERE
MOUNTAIN OBSCURATIONS WILL PREVAIL OVERNIGHT. THIS SITUATION
LIKELY TO CONTINUE THROUGH MUCH OF WEDNESDAY WITH SOME EROSION OF
THE WEDGE POSSIBLE OVER THE SE AND FAR WEST WHERE COULD BREAK INTO
MVFR OR BETTER DURING THE AFTERNOON. HOWEVER THIS LOOKS UNCERTAIN
GIVEN SO MUCH RAIN AROUND...WITH ANY IMPROVEMENT ALOFT ABOVE THE
COOL POOL PER ARRIVAL OF A LATE DAY DRY SLOT FROM THE SW. IF
FASTER MODELS DO WORK OUT THEN SPOTS SUCH AS KBLF/KLWB AS WELL AS
KBCB AND KDAN COULD BREAK OUT SOONER.

OTHER CONCERN WITH SURFACE WINDS ACROSS THE RIDGES AFFECTING
LOCATIONS SUCH AS KBLF WHERE WILL LIKELY OSCILLATE IN AND OUT OF
THE WEDGE INTO WEDNESDAY...AND THE STRONG JET ALOFT ESPCLY WED
MORNING ACROSS THE ENTIRE AREA. WINDS AT MOST TAF SITES SHOULD
GENERALLY BE LIGHT E/SELY IN THE COLD AIR WEDGE...THOUGH KBLF WILL
SEE SOME GUSTS DEVELOPING AS WE GET INTO LATE TONIGHT AND WED
MORNING. HOWEVER...THE LOW LEVEL JET ABOVE THE WEDGE WILL INCREASE
OVERNIGHT AND BE IN THE 45-60 KT RANGE FROM WED MORNING INTO THE
AFTERNOON. THIS WILL NECESSITATE THE CONTINUED INCLUSION OF OF
LLWS IN ALL TAFS OVERNIGHT...AND EVEN MORE SO ON WED WITH THE
EXCEPTION OF KBLF WHERE WILL AGAIN MIX DOWN GUSTS THROUGH THE
COLUMN MORE READILY...THUS LIMITING THE DEGREE OF SHEAR.

COLD FRONT TO THE WEST SHOULD FINALLY ARRIVE FROM WEST TO EAST WED
EVENING PRECEDED BY PERHAPS ANOTHER LINE OF SHOWERS THAT WILL
SWEEP RAPIDLY EAST BY MIDNIGHT. CONTINUED POTENTIAL OF LLWS JUST
AHEAD OF THE FRONT ESPCLY WITHIN THE LINGERING COLD POOL WED AND
MORE SURFACE BASED WINDS WITH ANY SHOWERS INTO WED EVENING.

EXTENDED DISCUSSION...

BEHIND THE COLD FRONT WEDNESDAY NIGHT AND THURSDAY...SURFACE
WINDS SHOULD TURN TO THE WEST...AND BECOME GUSTY BY CHRISTMAS
DAY. MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS SHOULD PERSIST IN
THE MOUNTAINS...WHILE VFR CONDITIONS WILL RETURN IN THE PIEDMONT.

THURSDAY NIGHT AND FRIDAY HIGH PRESSURE SHOULD BUILD OVER THE
SOUTHEAST...WHICH WILL PROVIDE VFR WEATHER WITH LIGHTER WINDS.

ANOTHER COLD FRONT WITH ASSOCIATED LOW PRESSURE WILL APPROACH THE
REGION SATURDAY AND BE OVER OR JUST WEST OF THE AREA SUNDAY. GIVEN
THE SLOWER TRENDS...APPEARS MAY STAY VFR FOR SATURDAY WITH MORE IN
THE WAY OF SUB-VFR CONDITIONS RETURNING WITH CLOUDS/PRECIP ON
SUNDAY FOR NOW.

&&

.HYDROLOGY...
AS OF 335 PM EST TUESDAY...

A SIGNIFICANT WETTING RAIN IS EXPECTED INTO THIS EVENING.
FORECAST STORM TOTAL RAINFALL IS FROM AROUND 1.00 TO 1.50 INCHES
MOST LOCATIONS...WITH LOCAL AMOUNTS NEAR THE BLUE RIDGE OF OVER
2.00 INCHES POSSIBLE.

THIS WILL ACTUALLY BE WELCOME FROM A HYDROLOGIC STANDPOINT AS
MUCH OF THE PIEDMONT HAS BEEN DRIER THAN NORMAL OVER SEVERAL
MONTHS AND DECEMBER HAS HAD BELOW NORMAL PRECIPITATION IN MOST OF
THE CWA. SHOULD NOT SEE ANY EXCESS RUNOFF ISSUES..I.E.
FLOODING...ALTHOUGH SOME DITCHES WILL LIKELY FILL UP AND MAYBE A
FEW PONDING ISSUES WHERE HEAVIER RAINFALL RATES.

RIVER FORECASTS FROM GIVEN THE EXPECTED RAINFALL SHOW ONLY A SMALL
CHANCE THAT SOME RIVERS SUCH AS THE DAN FROM DANVILLE TO SOUTH
BOSTON MAY EXPERIENCE MINOR FLOODING LATE IN THE WEEK. THERE IS A
VERY GOOD CHANCE HOWEVER THAT THE RAINFALL WILL RESULT IN MODEST
RISES ON SOME RIVERS.

&&

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

&&

$$
SYNOPSIS...AMS
NEAR TERM...AMS
SHORT TERM...DS
LONG TERM...PM
AVIATION...AMS/JH/MBS
HYDROLOGY...AMS/PC/SK



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