Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 200605
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
105 AM EST SAT DEC 20 2014

.SYNOPSIS...A FAST MOVING DISTURBANCE WILL TRACK EASTWARD ACROSS THE
REGION TODAY. WEAK HIGH PRESSURE WILL EXTEND ACROSS CENTRAL NORTH
CAROLINA SUNDAY. ANOTHER DISTURBANCE AND ASSOCIATED SURFACE LOW
PRESSURE SYSTEM WILL AFFECT OUR AREA SUNDAY NIGHT INTO MONDAY.

&&

.NEAR TERM /OVERNIGHT/...
AS OF 1010 PM FRIDAY...

A SHORTWAVE TROUGH PROGRESSING THROUGH THE CENTRAL AND LOWER MS
RIVER VALLEY THIS EVENING WILL TRACK E/ENE ACROSS THE TN VALLEY TO
THE SOUTHERN APPALACHIANS BY 12Z SAT...INDUCING WEAK CYCLOGENESIS
ALONG A BAROCLINIC ZONE OFFSHORE THE SOUTHEAST COAST AS HIGH
PRESSURE CONTINUES TO EXTEND SOUTH/SOUTHEASTWARD INTO THE CAROLINAS.

PRECIP: THE BEST UPPER LEVEL FORCING...DPVA ASSOC/W THE APPROACHING
SHORTWAVE...WILL LARGELY REMAIN WEST OF THE AREA THROUGH SUNRISE
SAT...THOUGH WEAK DPVA ATTENDANT SMALL AMPLITUDE DISTURBANCES OVER
THE DEEP SOUTH APPEAR LIKELY AS THEY TRACK ENE INTO THE CAROLINAS
BETWEEN 06-12Z. THOUGH LOWER-LEVEL FORCING IS ABSENT OVER CENTRAL NC
THIS EVENING...WEAK WARM ADVECTION WILL DEVELOP AFTER MIDNIGHT AS 10-
15 KT WESTERLY FLOW AT THE ~850 MB LEVEL BACKS TO THE SOUTHWEST IN
RESPONSE TO THE APPROACHING SHORTWAVE. GIVEN THAT THE APPROACHING
SHORTWAVE WILL REMAIN WEST OF THE APPALACHIANS THROUGH MOST OF THE
NIGHT...WARM ADVECTION WILL BE WEAK (SHORT-LIVED AS WELL)...AND THAT
THE 00Z GSO RAOB SHOWED A THOROUGHLY DRY AIRMASS IN PLACE ACROSS
CENTRAL NC THIS EVENING...IT SEEMS UNLIKELY THAT SUFFICIENT FORCING
AND/OR MOISTURE ADVECTION WILL OCCUR IN ORDER FOR MEASURABLE
PRECIPITATION TO DEVELOP PRIOR TO SUNRISE.

TEMPS: LOWS WILL RANGE FROM THE LOWER 30S IN THE NE COASTAL PLAIN TO
THE MID/UPPER 30S IN THE FAR SW PIEDMONT AS THICKENING MID-LEVEL
CLOUD COVER PROGRESSES INTO THE AREA FROM SW-NE.

THE RELATIVE BEST CHANCE FOR MEASURABLE PRECIP (PRIOR TO 12Z) WILL
BE IN THE WESTERN PIEDMONT. GIVEN THAT MEASURABLE PRECIP WILL
LARGELY DEPEND UPON THE MAGNITUDE/DURATION OF LOW-LEVEL WARM
ADVECTION...THE PRESENCE THEREOF (IN THIS CASE) WOULD IMPLY THERMAL
PROFILES EXCLUSIVELY SUPPORTIVE OF LIQUID. -VINCENT

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY/...
AS OF 300 PM FRIDAY...

A SOUTHERN STREAM SHORTWAVE IS EXPECTED TO ARRIVE FROM OUR SOUTHWEST
ON SATURDAY MORNING. THE LAST SEVERAL NUMERICAL MODEL RUNS HAVE ALL
TRENDED THIS SYSTEM TO BE DRIER...ESPECIALLY IN OUR NORTHERN
COUNTIES. BOTH THE GLOBAL AND THE HIGH RESOLUTION MODELS DO THIS AND
THEREFORE HAVE TRENDED BACK ON POPS AND QPF FORECASTS FOR SATURDAY
WITH MAINLY CHANCES WITH SOME BRIEF LIKELIES ACROSS THE EXTREME
SOUTHERN TIER DURING THE LATE MORNING/EARLY AFTERNOON. THIS ALSO
SEVERELY HANDICAPS ANY THREAT OF SNOW ACROSS THE TRIAD THAT WAS
POTENTIALLY BEING SHOWN BY PRIOR MODEL RUNS. FIRST...THERE MAY NOT
EVEN BE PRECIPIATION AT ALL IN THIS AREA. IF PRECIPIATION DOES IN
FACT OCCUR HERE...THE BEST OMEGA IS IN THE 700 MB LEVEL AND QUICKLY
DIMINISHES ABOVE THAT. THUS CURRENT MODELS...INCLUDING THE NAM WHICH
WAS THE MOST BULLISH ON THE SITUATION...NEARLY SATURATES IN THE 800-
600 MB LEVEL BUT LESS SO ABOVE THAT INTO THE SNOW GROWTH ZONE.
INCLUDE A WARM BOUNDARY LAYER INTO THE MIX AND CHANCES ARE HIGH THAT
ANY FALLING PRECIP WILL BE LIQUID.

THE CONCLUDING UPSHOT OF THE DAY WILL BE AN OVERCAST DAY WITH
TEMPERATURES IN THE MID 40S WITH BETTER CHANCES OF SOME VERY LIGHT
RAIN IN THE SOUTH. THE SYSTEM WILL MOVE OUT TO THE EAST OVERNIGHT
AND LOW TEMPERATURES ARE EXPECTED TO DROP IN TO THE LOW TO MID 30S
WITH THE COLDEST TEMPS IN THE NORTHWEST PIEDMONT.

SUNDAY WILL FEATURE HIGH PRESSURE BUILDING IN FROM THE NORTH AND
STARTING TO MOVE INTO POSITION TO EVENTUALLY FORM A CAD WEDGE BUT
INITIALLY WILL HELP AID SOME PARTIAL CLEARING EARLY SUNDAY BEFORE
INCREASING CLOUDS FROM THE SOUTH TAKE OVER BY AFTERNOON. IT SHOULD
BE A WARMER DAY THAN SATURDAY WITH TEMPS IN THE UPPER 40S NW TO
LOWER 50S SE. INCREASING MOISTURE FROM APPROACHING SOUTHERN LOW WILL
BRING DEWPOINTS UP NEAR 40 DEGREES BY THE END OF THE AFTERNOON BUT
THE SURFACE HIGH WILL KEEP ANY PRECIPIATION SOUTH OF THE AREA
THROUGH THE AFTERNOON.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
AS OF 205 PM FRIDAY...

FOR SUN NIGHT THROUGH MON NIGHT: COOL SURFACE HIGH PRESSURE CENTERED
OVER FAR SERN CANADA WILL CONTINUE TO WEDGE SSW THROUGH CENTRAL NC
SUN NIGHT/MON. THE MID LEVEL WAVE NOW OFF THE PACIFIC COAST WILL
TRACK EAST AND DIG THROUGH THE SRN PLAINS SUN... THEN KICK NORTHEAST
THROUGH THE LOWER MISS VALLEY TOWARD THE OH VALLEY/MIDSOUTH/SRN APP
REGION... DAMPENING IN THE PROCESS AS NRN STREAM ENERGY BEGINS TO
DIG AND PHASE WITH A SRN STREAM WAVE OVER THE CENTRAL CONUS. THE
APPROACH OF THE INITIAL WEAKENING WAVE AND RESULTANT HEIGHT FALLS
HELP SATURATE THE MID-UPPER LEVELS FIRST SUN NIGHT... THEN AS THE
700-850 MB WARM FRONTAL ZONE APPROACHES FROM THE SOUTH WITH
STRENGTHENING LOW LEVEL MOIST ISENTROPIC UPGLIDE AROUND 290K-295K...
EXPECT PRECIP TO SPREAD INTO OUR AREA FROM THE SSW AS THE COLUMN
SATURATES. MODEL TIMING AND THEIR RECENT SLOWING TRENDS SUGGEST A
MID EVENING ONSET TIME IN THE SW... SPREADING NNE OVERNIGHT. EXPECT
THE BEST COVERAGE AND HIGHEST AMOUNTS WILL BE ALONG THE MORE STEEPLY
SLOPED PORTION OF THE COLD DOME SURFACE OVER THE PIEDMONT... AND
WILL RAISE POPS HERE TO LIKELY... AND IF TRENDS CONTINUE...
CATEGORICAL POPS MAY BE NEEDED STARTING OVERNIGHT SUN NIGHT. HIGH
COVERAGE (70-80%) OF RAIN SHOULD CONTINUE TO PUSH NORTHEAST THROUGH
CENTRAL NC MON MORNING... AND INTO NE NC IN THE AFTERNOON... WITH
PRECIP THEN TAPERING DOWN TO PATCHY DRIZZLE OR LESS OVER CENTRAL NC
FROM SW TO NE MON AFTERNOON AS THE DRY SLOT ARRIVES FROM THE SW
BEHIND THE RETREATING MID LEVEL WARM FRONTAL ZONE... RESULTING IN
TOP-DOWN DRYING. THIS IS VERY CLOSE TO THE PREVIOUS FORECAST TRENDS.
THE LOW LEVELS WILL REMAIN COOL/MOIST AND STABLE... AS THE DAMMING
WEDGE SHOULD HOLD FIRM... REINFORCED BY NE FLOW AROUND SURFACE LOW
PRESSURE OVER THE OUTER BANKS OR SOUNDS LATE MON. WILL RETAIN
OVERCAST SKIES (LOW CLOUDS PLUS SOME HIGH CLOUDINESS AS THE DIGGING
CENTRAL CONUS TROUGH YIELDS INCREASING SW MID LEVEL FLOW INTO NC)
AND LOW CHANCE POPS MON NIGHT WITH THE POTENTIAL FOR SOME DRIZZLE IN
THIS REGIME. THICKNESSES AND BUFR SOUNDINGS INDICATE ALL-LIQUID
PRECIP WITH THIS EVENT ACROSS CENTRAL NC. THE GFS/ECMWF HINT AT SOME
INLAND PROGRESSION OF THE ASSOCIATED WEDGE WARM FRONT INTO CENTRAL
NC... HOWEVER THIS LOOKS UNLIKELY TO MAKE MUCH HEADWAY GIVEN AN
EXPECTED STRONG COLD DOME... AND WILL NOT INDICATE A DRASTIC WARMUP
IN THE COASTAL PLAIN MON NIGHT... ALTHOUGH TEMPS ARE LIKELY TO BUDGE
LITTLE FROM MON TO MON NIGHT GIVEN THE WARMING ALOFT. LOWS SUN NIGHT
IN THE UPPER 30S TO MID 40S... AND HIGHS MON IN THE LOW 40S TO LOWER
50S (ALTHOUGH THIS DIURNAL RISE IN TEMPS MAY BE TOO LARGE IF WE SEE
STEADY PRECIP INTO THE WEDGE ALL DAY LONG). LOWS MON NIGHT FROM
AROUND 40 NW TO UPPER 40S SE.

FOR TUESDAY THROUGH FRIDAY: STILL LOOKS LIKE A STORMY PATTERN FOR
MID WEEK... ALTHOUGH SOME IMPROVEMENT TO QUIETER WEATHER APPEARS
PROBABLE FOR CHRISTMAS DAY AND FRIDAY.

WE`LL START THIS PERIOD WITH THE PARENT SURFACE HIGH OVER THE
CANADIAN MARITIMES SHIFTING EASTWARD YET STILL EXTENDING SE DOWN
THROUGH WRN/CENTRAL NC... WHILE THE COASTAL/WEDGE FRONTAL ZONE SITS
OVER COASTAL NC EXTENDING BACK THROUGH NRN GA TO A SURFACE LOW OVER
THE MIDWEST. MID LEVEL LOW PRESSURE CONTINUES TO DEEPEN OVER THE
CENTRAL CONUS TUE... THEN DRIFT NE AS ITS LONGWAVE TROUGH PIVOTS
EASTWARD INTO THE EASTERN STATES WED. THE GFS/ECMWF BRING THE WEDGE
FRONT WELL INLAND TUE EVENING/NIGHT... AS THE PRIMARY SURFACE LOW
MOVES OVER LAKE MICHIGAN AND A TRIPLE POINT LOW PUSHES INTO SW NC
(MORE PROMINENT ON THE ECMWF THAN THE GFS). TRYING TO TRACK THE
PROGRESSION OF WEDGE FRONTS ASSOCIATED WITH LATTER-STAGE DAMMING
EVENTS IS ALWAYS DIFFICULT... GIVEN THAT THE PARENT HIGH WILL BE
PUSHING OVER AND JUST OFF THE MARITIMES BUT MAINTAINING A CENTRAL
STRENGTH NEAR 1030 MB. WILL FOLLOW THE CLIMATOLOGICAL LEANINGS OF
SUCH EVENTS AND KEEP A COOL STABLE REGIME IN THE NW PIEDMONT... AS
IS TYPICAL WITH THESE EVENTS. WE SHOULD SEE THE RELATIVE LULL IN
PRECIP EXTEND INTO TUE... CONTINUED CLOUDY SKIES AND A CHANCE OF
LIGHT RAIN AS THE COLUMN REMAINS FAIRLY MOIST BUT WITH MARGINAL/WEAK
FORCING FEATURES. STRENGTHENING AND DEEPENING MOISTURE INFLUX AT
MULTIPLE LEVELS WILL OCCUR TUE NIGHT INTO WED... INCLUDING
STRENGTHENING LOW LEVEL MOIST ISENTROPIC UPGLIDE AND INCREASING
UPPER DIVERGENCE AS MID-UPPER LEVEL WINDS INCREASE JUST TO OUR SW.
WILL STICK WITH POPS INCREASING TO LIKELY FOR NOW. THE GFS/ECMWF
STILL INDICATE THE OCCLUDED/COLD FRONT SWEEPING ENE THROUGH THE
REGION SOMETIME WED (WITH THE ECMWF SLIGHTLY FASTER THAN THE GFS)...
AND DEPENDING ON THE LOCATION/TRACK OF VORTICITY CENTERS AND THE
TRIPLE POINT LOW... WE MAY STILL SEE A THREAT OF A FEW STRONG STORMS
LATE TUE NIGHT INTO WED IF WE ARE ABLE TO ACHIEVE ANY INSTABILITY...
AND WE WILL CONTINUE TO MONITOR FOR THIS POTENTIAL AS THIS TIME
FRAME APPROACHES. IMPROVING CONDITIONS ARE EXPECTED THE EVENING OF
CHRISTMAS EVE AS THE COLUMN DRIES OUT... ALTHOUGH WE SHOULD KEEP
SOME CLOUDS WITH THE TROUGH AXIS SWINGING THROUGH THE REGION. TEMPS
TUE/WED ARE TRICKY AND DON`T HAVE A LOT OF CONFIDENCE... AS A MINOR
SWING ONE WAY OR THE OTHER WITH THE WEDGE FRONT OR LOW TRACKS WILL
GREATLY ALTER TEMPS. WITH EXPECTATIONS OF SOME DAMMING AIR MASS
HOLDING IN THE NW CWA... EXPECT HIGHS TUE FROM 49 NW TO 61 SE.
MINIMAL DROP TUE NIGHT WITH STRENGTHENING LOW LEVEL WINDS AND
POSSIBLE ERODING WEDGE AIR MASS OVER MOST OF THE CWA... AND WILL GO
WITH LOWS OF 44-52. HIGHS 52 NW TO 61 SE WED WITH ALL BUT POSSIBLY
THE NW CWA IN THE WARM SECTOR. TEMPS SHOULD THEN FALL OFF WED
EVENING/NIGHT WITH RAPIDLY CLEARING AND COOL AIR ADVECTION... TO
LOWS OF 33-38. CHRISTMAS DAY AND FRI APPEAR FAIR WITH A WARMING
TREND AS SOUTHERN-STREAM HIGH PRESSURE BUILDS OVER THE GULF AND
SOUTHEAST STATES WITH DECREASING AMPLITUDE OF THE MID LEVEL FLOW.
HIGHS IN THE LOW 50S CHRISTMAS DAY (NEAR NORMAL) AND MID TO UPPER
50S FRI. LOWS IN THE 30S. -GIH

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
AS OF 105 AM SATURDAY...

AN UPPER LEVEL DISTURBANCE ADVANCING EAST ACROSS THE
DEEP SOUTH WILL PASS SOUTH OF CENTRAL NC LATER THIS MORNING THROUGH
EARLY AFTERNOON. THIS WEATHER SYSTEM WILL INCREASE CLOUDINESS ALONG
WITH PATCHES OF LIGHT RAIN OVER OUR REGION BY DAYBREAK. EXPECT
CEILINGS TO LOWER TO THE 4000-6000FT LAYER WITH SCATTERED PATCHES OF
STRATOCU WITH CEILINGS 1500-2500FT POSSIBLE BY LATE MORNING THROUGH
THE AFTERNOON...MAINLY SOUTH OF HIGHWAY 64. THIS THREAT FOR LOWER
CEILINGS WILL AFFECT THE KFAY TERMINAL. SURFACE WINDS WILL BECOME
NELY WITH SUSTAINED WINDS AOB 10KTS.

WEAK HIGH PRESSURE WILL BUILD INTO CENTRAL NC FROM NORTH TONIGHT AND
SUNDAY...MAINTAINING MOSTLY VFR CONDITIONS.

A STRONGER UPPER LEVEL DISTURBANCE AND ASSOCIATED SURFACE LOW
PRESSURE SYSTEM WILL BRING A HIGH PROBABILITY OF ADVERSE AVIATION
CONDITIONS TO CENTRAL NC SUNDAY NIGHT INTO MONDAY. WIDESPREAD
MVFR/IFR CEILINGS AND AREAS OF RAIN ARE EXPECTED. THESE CONDITIONS
WILL SPREAD NORTH INTO SOUTHERN SECTIONS OF CENTRAL NC SHORTLY AFTER
SUNSET SUNDAY THEN CONTINUE NORTHWARD SUNDAY NIGHT.

A DECENT TO HIGH THREAT FOR MVFR CEILINGS WILL LINGER MONDAY NIGHT
INTO TUESDAY AS THE LOW LEVEL AIR MASS REMAINS MOIST. A STRONG COLD
FRONT WILL APPROACH TUESDAY NIGHT AND CROSS THE REGION WEDNESDAY.
GUSTY SLY WINDS ARE EXPECTED AHEAD OF THE FRONT TUESDAY NIGHT ALONG
WITH HEAVY SHOWERS AND POSSIBLY A THUNDERSTORM OR TWO. THE FRONT
WILL CROSS THE AREA WEDNESDAY WITH BLUSTERY WESTERLY WINDS EXPECTED
IN ITS WAKE.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

SYNOPSIS...WSS
NEAR TERM...VINCENT
SHORT TERM...ELLIS
LONG TERM...HARTFIELD
AVIATION...WSS



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