Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220442
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
1142 PM EST SUN DEC 21 2014

.SYNOPSIS...
WARM AND MOIST AIR WILL OVERSPREAD COLD HIGH PRESSURE
AT THE SURFACE TO BRING RAIN...ALONG WITH SOME SLEET AND FREEZING
RAIN...TO THE REGION TONIGHT INTO TOMORROW. CLOUDY CONDITIONS AND
LIGHT PRECIPITATION WILL THEN LINGER THROUGH TUESDAY. A STRONG
COLD FRONT WILL MOVE THROUGH ON WEDNESDAY...BRINGING RAIN AND
GUSTY WINDS TO THE AREA FOR CHRISTMAS EVE. HIGH PRESSURE WILL THEN
BUILD OVER THE EASTERN UNITED STATES WITH FAIR WEATHER AND SEASONABLE
TEMPERATURES THROUGH THE END OF THE WEEK.

&&

.NEAR TERM /THROUGH MONDAY NIGHT/...
AS OF 1100 PM EST SUNDAY...

RADAR TRENDS BEGINNING TO SHOW A NORTHWARD JOG OF THE PRECIP OVER
THE CAROLINAS. LOOK FOR PRECIP TO BEGIN IN OUR NC COUNTIES
BETWEEN MIDNIGHT-2AM...THEN SPREAD INTO VIRGINIA BETWEEN 2-4AM.
DRIZZLE AND FREEZING DRIZZLE MAY BEGIN TO DEVELOP IN THE VCNTY OF
THE BLUE RIDGE B4 THEN AS CIGS LOWER UNDER 1KFT AGL. WILL MAINTAIN
CURRENT WSW CONFIGURATION UNLESS RADAR TRENDS DEEM OTHERWISE.
LOCAL WRF KEEPS THE WESTERN EDGE OF THE PRECIP ALONG AND EAST OF
THE APPALACHIAN DIVIDE. MAY NEED TO EXPAND THE WSW INTO THE MTNS
OF NC...PER TEMPERATURES NEAR FREEZING ALONG THE BLUE RIDGE
PARKWAY. IN BOONE AND WEST JEFFERSON HOWEVER...TEMPS HAVE REMAINED
A FEW DEGREES ABOVE FREEZING AND WILL FORGO NEED FOR ADVISORY
THERE UNLESS TEMPS FALL AND COVERAGE OF PRECIP INCREASES.

UPPER FLOW WILL BE TRANSITIONING FROM ZONAL TO AMPLIFIED AS A
DEEP TROF DEVELOPS OVER THE CENTRAL AND WESTERN US. THIS WILL
BUCKLE THE FLOW TO A SWLY DIRECTION ALOFT...ALLOWING MOISTURE
TRANSPORT TO OCCUR ALONG WITH ISENTROPIC LIFT AND WEAK SHORT WAVE
ENERGY. THIS WILL ALLOW PCPN TO DEVELOP LATE TONIGHT INTO
TOMORROW. MEANWHILE AT THE SURFACE...STRONG HIGH PRESSURE OVER
QUEBEC WILL CONTINUE TO BUILD DOWN THE EASTERN SEABOARD AND
DEVELOP A SOLID COLD AIR DAMMING WEDGE. THUS...AS THE PCPN FALLS
INTO THE COLD AIR AT THE SURFACE THE RAIN WILL FALL AS FREEZING
RAIN AND SLEET...ESP FROM THE NEW RIVER AND ROANOKE VALLEYS NORTH
AND EASTWARD WHERE TEMPERATURES WILL BE FLIRTING WITH THE FREEZING
MARK. WITH ONLY LIGHT QPF EXPECTED THIS SHOULD NOT BE ANYTHING
MORE THAN A NUISANCE EVENT...BUT IT DOES NOT TAKE MUCH FREEZING
RAIN TO CAUSE TROUBLE...SO WILL ISSUE A FREEZING RAIN ADVISORY FOR
THE AFFECTED AREAS. TEMPERATURES WILL BE IN NO HURRY TO WARM UP
TOMORROW AS THE PCPN WANES SO THE ADVISORY WILL GO UNTIL NOONTIME.
THE COLD AIR WEDGE WILL REMAIN IN PLACE THROUGH MONDAY NIGHT AND
EXPECT ENOUGH MOISTURE WILL BE LOCKED IN TO KEEP SOME R-/L-
AROUND.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
AS OF 245 PM EST SUNDAY...

UPPER TROUGH AMPLIFYING OVER THE CENTRAL U.S. WILL SUPPORT
DEVELOPMENT OF A SURFACE LOW EARLY TUESDAY OVER THE LOWER TN VALLEY
WHICH WILL LIFT NORTH DURING THE DAY. DEEP LAYER SOUTHERLY FLOW
AHEAD OF THE TROUGH AND ASSOCIATED COLD FRONT WILL PULL ABUNDANT
MOISTURE NORTHWARD AS WELL WITH PWATS SURGING FROM LESS THAN 0.50
INCHES MONDAY UP CLOSE TO 1.25 INCHES ACROSS THE CENTRAL CWA BY
EARLY WEDNESDAY WHICH IS CLOSE TO 99TH PERCENTILE FOR THE DATE PER
KRNK PWAT CLIMATOLOGY.  ALL THICKNESS PARAMETERS AND THERMAL
PROFILES SUPPORT A PURE RAIN EVENT WITH POPS INCREASING FROM WEST TO
EAST THROUGH THE DAY BUT ESPECIALLY LATE TUESDAY NIGHT AS PERSISTENT
LOW LEVEL MOISTURE TRANSPORT AND FAVORABLE RIGHT ENTRANCE REGION
UPPER JET SUPPORT...UP TO 140 KT AT 250 MB. HIGHS TUESDAY HOLD IN
THE 40S UNDER THE WEDGE AND DO NOT FALL MUCH OVERNIGHT WITH THICK
OVERCAST AND SOUTHERLY FLOW. EXPECT MODERATE TO AT TIMES BRIEFLY
HEAVY RAINFALL IN THE PERIOD FROM LATE TUESDAY THROUGH MUCH OF
WEDNESDAY...CHRISTMAS EVE...AS A WARM FRONT LIFTS NORTH ACROSS THE
AREA AND SURFACE LOW TRACKS STEADILY NORTH TOWARD THE GREAT LAKES.
SEE HYDRO SECTION FOR MORE ON EXPECTED RAINFALL AND RELATED ANY
IMPACTS.

THE SYSTEM WILL HAVE PLENTY OF DEEP LAYER SHEAR BUT VERY LOW CAPES
AS WOULD BE EXPECTED IN LATE DECEMBER WITH COMPLETE OVERCAST. SPC
NOT IMPRESSED AT THIS POINT WITH THE POTENTIAL FOR A HIGH-SHEAR LOW-
CAPE EVENT BUT WILL HAVE TO BE MONITORED AS THE SYSTEM EVOLVES AS
SIMILAR SYSTEMS HAVE SURPRISED IN THE PAST AND CIPS ANALOGS OF 15
MOST SIMILAR EVENTS SHOWS AT LEAST LOW END PROBABILITY OF SEVERE
IN THE FAR SOUTHEAST CWA. MODELS HAVE COLD FRONT CROSSING THE
MOUNTAINS BETWEEN ABOUT 03-09Z EARLY CHRISTMAS MORNING AND EXIT
THE EASTERN CWA BY IN 12-18Z TIME FRAME. ALL THE HEAVY
PRECIPITATION WILL HAVE EXITED TO THE NORTHEAST BUT STRONG WEST-
NORTHWEST WINDS WILL SURGE IN BEHIND THE FRONT AND BEGIN UPSLOPE
SNOW SHOWERS IN THE USUAL AREAS BY AROUND 09Z ON CHRISTMAS MORNING
AS H85 TEMPS PLUMMET FROM AROUND +10C AT 00Z TO -5C BY 12Z ON THE
25TH.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
AS OF 245 PM EST SUNDAY...

ON CHRISTMAS DAY...A COLD FRONT WILL MOVE TO THE COAST WHILE AN
UPPER LEVEL TROUGH PIVOTS OVER THE REGION. PRESSURE RISES...A TIGHT
GRADIENT AND FALLING TEMPERATURES WILL MAKE FOR A BLUSTERY DAY
ACROSS THE FORECAST AREA. THE ONLY AREA THAT WILL SEE SNOW FALLING
ON CHRISTMAS WILL BE WESTERN SLOPES OF SE WV AND SW VA...POSSIBLE
FLURRIES TO THE BLUE RIDGE. THESE BLUSTERY CONDITIONS WILL NOT LAST
LONG AS HIGH PRESSURE WILL EDGE IN ALLOWING WINDS TO SUBSIDE AND
ZONAL FLOW AND LOW LEVEL SOUTHWESTERLY FLOW TO USHER IN WARMER AIR
ON FRIDAY.

ACCORDING TO THE 12Z GFS...THE NEXT FRONT IS SCHEDULED TO ARRIVE
SATURDAY. THIS FRONT WILL NOT HAVE MUCH MOISTURE AS DRY HIGH
PRESSURE TRACKS ACROSS THE SOUTHERN STATES...BLOCKING GULF MOISTURE
FROM RETURNING INTO THE REGION. ANY SHOWERS WITH THIS FRONT SATURDAY
WILL BE ALONG WESTERN SLOPES AS WINDS BECOME WESTERLY RATHER
QUICKLY. TEMPERATURES AND BOUNDARY LAYER WARM ENOUGH TO KEEP ANY
PRECIPITATION AS RAIN. BY THE TIME THE COLD AIR ARRIVES...MOISTURE
IS GONE. ALSO WITH A MOSTLY DRY FRONT...BREEZY AND GUSTY CONDITIONS
EXPECTED SATURDAY.

THE 12Z ECM STALLS THE FRONT ACROSS THE SOUTHERN APPALACHIANS
SATURDAY. A DEEP UPPER LEVEL TROUGH DIGGING INTO TEXAS WILL DEVELOP
A SURFACE LOW NEAR THE MOUTH OF THE MISSISSIPPI SATURDAY...TRACKING
ACROSS THE CAROLINAS ON SUNDAY. THE ECM SCENARIO WILL BRING RAIN
WHEREAS THE GFS IS DRY. WILL LEAN THIS WEEKEND`S FORECAST TOWARDS
WPC AND KEEP POPS LOW IN THE 20-30 PERCENT RANGE.

WITH EITHER MODEL...BEHIND THE FRONT SATURDAY OR SUNDAY...ANOTHER
AREA OF HIGH PRESSURE WILL TRACK ALONG THE GULF STATES...KEEPING THE
REGION DRY INTO EARLY NEXT WEEK.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
AS OF 1130 PM EST SUNDAY...

HIGH PRESSURE WILL REMAIN WEDGED AGAINST THE EASTERN SIDE OF THE
APPALACHIAN MOUNTAINS WHILE AN AREA OF LOW PRESSURE ADVANCES
NORTHEAST ALONG THE SOUTHEAST ATLANTIC COAST. THIS AREA OF LOW
PRESSURE WILL CAST MOISTURE INTO THE MID ATLANTIC REGION
OVERNIGHT. MVFR CIGS WILL ADVANCE NORTHWARD FROM THE
CAROLINAS...OVERSPREADING VIRGINIA AND WEST VIRGINIA DURING THE
OVERNIGHT. ONSET OF PRECIPITATION IS EXPECTED TO BEGIN BETWEEN
2-5AM TIME FRAME.

LIGHT RAIN/DRIZZLE IS ADVANCING NORTHWARD THROUGH THE CAROLINAS.
WHILE THERE IS HIGH CONFIDENCE THAT THE MAIN PRECIPITATION TYPE
WILL BE RAIN...A PERIOD OF FREEZING RAIN AND SLEET IS LIKELY NORTH
OF HIGHWAY 460...ALONG AND NORTH OF A BLF- BCB-ROA-LYH LINE.
MODELS ARE FORECASTING ABOUT A TENTH OF AN INCH OF QPF...SO ITS
NOT OUT OF THE QUESTION TO HAVE UP TO A TENTH OF AN INCH OF ICE
ACCRETION ON ELEVATED SURFACES MONDAY MORNING. A FREEZING RAIN
ADVISORY IS IN EFFECT FROM 5AM TO NOON MONDAY TO ACCOUNT FOR THE
FREEZING RAIN. SOME OF THE FORECAST MODELS...ESP THE NAM...SUGGEST
A PERIOD OF IFR MAY ACCOMPANY THE PRECIPITATION MONDAY MORNING...
SO THERE IS A CHC THAT CIGS COULD DROP TO BETWEEN 400-800 FT AGL
WITH VSBYS BETWN 1-3SM.

THE PRECIPITATION IS EXPECTED TO COME TO AN END MONDAY AFTERNOON.
LINGERING MOISTURE...HOWEVER...WILL RESULT IN A PERSISTENT LOW
CEILING...MAINLY MVFR...IN ADDITION TO SOME PATCHY DRIZZLE.

EXTENDED DISCUSSION...

A LOW PRESSURE SYSTEM ALONG THE NORTH CAROLINA COAST WILL HEAD
OFFSHORE BY MONDAY NIGHT...BUT MVFR CEILINGS AND DRIZZLE MAY
LINGER INTO TUESDAY DUE TO THE WEDGE OF HIGH PRESSURE EAST OF THE
APPALACHIAN MOUNTAINS. BY WEDNESDAY...A STRONG COLD FRONT WILL
SWEEP OVER THE MID ATLANTIC...AND ERODE THE WEDGE. MODEL SOUNDINGS
CONTINUE TO SHOW THE POTENTIAL FOR LOW LEVEL WIND SHEAR AHEAD OF
THIS FRONT...WITH A POTENT LOW LEVEL JET DEPICTED. RAIN AND MVFR
CONDITIONS WILL ACCOMPANY THE FRONTAL PASSAGE.

BEHIND THE COLD FRONT...SURFACE WINDS SHOULD TURN FROM THE
SOUTHEAST TO THE NORTHWEST...AND BECOME RATHER GUSTY BY CHRISTMAS
DAY. MEANWHILE...MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS
SHOULD PERSIST IN THE MOUNTAINS...WHILE VFR CONDITIONS WILL RETURN
IN THE PIEDMONT. HIGH PRESSURE SHOULD BUILD OVER THE SOUTHEAST BY
THURSDAY NIGHT...WHICH WILL PROVIDE GOOD FLYING WEATHER WITH
LIGHTER WINDS ON FRIDAY.

&&

.HYDROLOGY...
AS OF 245 PM EST SUNDAY...

BIGGER STORY MAY BE THE RAINFALL WITH UPCOMING EVENT AS FORECAST
STORM TOTAL QPF IS IN THE 1.00 TO 1.50 INCH RANGE ACROSS THE ENTIRE
CWA. 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. RIVER FORECASTS FROM NORTH AMERICAN AND GEFS
ENSEMBLES PORTRAY BASIN- AVERAGE QPF GENERALLY IN THE 0.75 TO 1.25
INCH RANGE WITH SOME OUTLIERS AS USUAL BUT ALL SOLUTIONS AT THIS
POINT PRODUCE ONLY MINOR WITHIN-BANK RISES ON MAINSTEM RIVERS BY
LATER IN THE WEEK.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...FREEZING RAIN ADVISORY FROM 5 AM TO NOON EST MONDAY FOR VAZ011-
     013-014-016>020-022>024-034-035-045>047.
NC...NONE.
WV...FREEZING RAIN ADVISORY FROM 5 AM TO NOON EST MONDAY FOR WVZ044-
     507-508.

&&

$$
SYNOPSIS...MBS
NEAR TERM...MBS/PM
SHORT TERM...PC
LONG TERM...RCS
AVIATION...PM/PW
HYDROLOGY...PC


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