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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KRAH 070539
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1240 AM EST FRI DEC 6 2013

.SYNOPSIS...A STRONG COLD FRONT WILL MOVE INTO THE AREA TONIGHT FROM
THE NORTH. WINDS WILL SHIFT TO THE NORTH AND THERE WILL BE A SHARP
DROP IN TEMPERATURES BY DAWN. MUCH COLDER TEMPERATURES AND OCCASIONAL
RAIN CAN BE EXPECTED THIS WEEKEND... WITH ICING POSSIBLE IN THE
PIEDMONT TRIAD REGION SUNDAY.

&&

.NEAR TERM /THROUGH SATURDAY/...
AS OF 910 PM FRIDAY...

A STRONG ARCTIC HIGH PRESSURE (1050 MB) WAS BUILDING EAST FROM
MONTANA AND NORTH DAKOTA THIS EVENING. ARCTIC COLD HAD SETTLED AS
FAR SOUTH AS TEXAS AND THE GULF COAST. THE LEADING EDGE OF THE COLD
AIR HAD SURGED INTO SOUTHERN VA AND FAR WESTERN NC AS OF LATE
EVENING. THE FRONT IS EXPECTED TO SLOWLY PUSH SOUTH THROUGH CENTRAL
NC OVERNIGHT. AT 900 PM... THERE WAS A 25 DEGREE DROP JUST BEHIND
THE FRONT NOTED BETWEEN NORTHERN NC AND SOUTHERN VA... WHERE
FARMVILLE REPORTED 46 AND HALIFAX NC 73. THIS TYPE OF TEMP DROP OVER
A 2-3 HOUR PERIOD CAN BE EXPECTED THROUGHOUT CENTRAL NC BETWEEN 08Z-
13Z AS THE FRONT SHIFTS THROUGH. IN ADDITION... RAIN IS EXPECTED TO
DEVELOP JUST BEHIND THE FRONT OVER NORTH-CENTRAL NC OVERNIGHT.
RAINFALL AMOUNTS SHOULD BE LIGHT... BUT WIDESPREAD 0.25 INCH TOTALS
ARE EXPECTED FROM GSO TO RDU NORTHWARD INTO SOUTHERN VA LATER
TONIGHT. WE WILL TRY TO TIME THE FRONT AND HUGE TEMP DROP... ALONG
WITH THE RAIN IN THE HOURLY GRIDS AS BEST AS POSSIBLE.

COLD AIR ADVECTION AND EVEN LOW LEVEL DRY AIR ADVECTION WILL
OVERWHELM OUR REGION FROM THE NNE ON SATURDAY. TEMPERATURES SHOULD
FALL ALL DAY... EVEN WITH A LULL IN THE RAIN FOR MOST OF THE DAY.
TEMPS SHOULD END THE DAY IN THE 30S NORTH AND 40S SOUTH.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
AS OF 335 PM FRIDAY...

A QUICK HITTING ARCTIC HIGH...BUT FAVORABLY STRONG AND LOCATED WHEN
THE PRECIPITATION IS EXPECTED TO OCCUR...WITH AN ASSOCIATED LIKELY
PROBABILITY OF NEARLY ALL-DAY FREEZING RAIN OVER MUCH OF THE NW
PIEDMONT ON SUN...AND A BRIEF PERIOD OF FREEZING RAIN POSSIBLE AS
FAR SOUTHEAST AS A WARRENTON TO RDU TO ALBEMARLE LINE.

A WINTER STORM WATCH HAS BEEN ISSUED FOR FORSYTH...GUILFORD...
ALAMANCE...AND PERSON COUNTIES. A FREEZING RAIN ADVISORY MAY BE
NEEDED FOR A TIER OR TWO OF COUNTIES SOUTHEAST OF THERE AS THE EVENT
DRAWS NEARER.

SATURDAY NIGHT: COLD AND DRY ADVECTION WILL CONTINUE THROUGH THE
NIGHT...AS 1035-1040 MB ARCTIC HIGH PRESSURE BUILDS FROM THE UPPER
MIDWEST TO THE NORTHERN MIDDLE ATLANTIC AND NORTHEAST AND
SUBSEQUENTLY RIDGES SOUTHWARD EAST OF THE APPALACHIANS - ALL IN A
FAVORABLE POSITION FOR STRONG COLD AIR DAMMING. AT THE SAME
TIME...WARM AND MOIST AIR WILL BE TRANSPORTED NORTH NORTHEASTWARD IN
STRENGTHENING LOW LEVEL FLOW FROM THE GOM...WHICH WILL CONSEQUENTLY
CAUSE ISENTROPIC UPGLIDE TO RAPIDLY INCREASE AND PROMOTE THE
DEVELOPMENT OF RAIN FROM SOUTH TO NORTH...WITH TEMPERATURES FALLING
THROUGH THE 40S AND 30S...OVERNIGHT. BY DAYBREAK...TEMPERATURES
ACROSS THE NORTHERN PIEDMONT WILL BE FLIRTING WITH THE FREEZING
MARK...AS THE SUPPLY OF COLD AND DRY AIR IN THE LOW LEVELS CONTINUES
FROM THE PARENT HIGH...RESULTING IN A TRANSITION TO FREEZING RAIN
WITHIN A COUPLE HOURS EITHER SIDE OF 7 AM. BOTH THE (LIMITED) DEPTH
OF SATURATION AND RAPIDLY WARMING TEMPERATURES ALOFT WILL PRECLUDE
ANY CONCERN FOR FROZEN PRECIPITATION...ASIDE FROM A TYPICAL BRIEF
NUISANCE MIX OF GRAUPEL/"SLEET" NEAR ONSET.

SUNDAY AND SUNDAY NIGHT: ISENTROPIC LIFT...HOWEVER SHALLOW/BELOW THE
EFFICIENT MIXED PHASE PRECIPITATION PRODUCTION ZONE...WILL CONTINUE
THROUGH SUN EVE - STRONGEST AND LONGEST FROM NORTHERN NC TO VA. THIS
WILL SUPPORT THE HIGHEST PRECIPITATION AMOUNTS OVER THE NW PIEDMONT -
AROUND A THIRD TO HALF INCH - AND LESSER AMOUNTS OF AROUND A COUPLE
TENTHS OVER THE EASTERN SANDHILLS AND SOUTHERN COASTAL PLAIN.

THE ARCTIC HIGH WILL BECOME INCREASINGLY ESTABLISHED EAST OF THE
APPALACHIANS DURING THE DAY SUNDAY...SO TEMPERATURES MAY ACTUALLY
FALL A DEGREE OR THREE THROUGH SUNDAY AFTERNOON IN THE HEART OF THE
WEDGE - TO 30 OR 31 DEGREES OVER THE WATCH AREA. ELSEWHERE...
TEMPERATURES WILL CREEP THROUGH THE 30S...TO NEAR 40 DEGREES TOWARD
KFAY TO KCTZ AND KGSB. WITH TEMPERATURES HOLDING AT OR BELOW
FREEZING OVER THE NW PIEDMONT...75% OF THE PRECIPITATION WILL LIKELY
FALL IN THE FORM OF FREEZING RAIN...THEREBY NECESSITATING THE WINTER
STORM WATCH ISSUANCE. THE GREATEST IMPACTS ARE EXPECTED TO OCCUR ON
ELEVATED SURFACES (ESPECIALLY TREES AND POWER LINES).

THE PRECIPITATION WILL TAPER TO DRIZZLE...AND FREEZING DRIZZLE OR
LIGHT FREEZING RAIN OVER THE NW PIEDMONT...AS THE LLJ AND ASSOCIATED
ISENTROPIC LIFT WELL NORTH OF OUR REGION SUN NIGHT. TEMPERATURES
WILL GRADUALLY WARM OVERNIGHT AS THE ARCTIC RIDGE SHIFTS
OFFSHORE...WITH WARMING TO THE MIDDLE 30S NW PIEDMONT TO MIDDLE 40S
SE BY DAYBREAK MON.

MONDAY: THOUGH THE CENTER OF THE SURFACE HIGH WILL BE WELL OFFSHORE
THE NE COAST BY MON...THE RESIDUAL WEDGE AIR MASS MAY NOT FULLY BE
ERADICATED BEFORE A COLD/OCCLUDED FRONT PASSES FROM THE WEST MON
EVE. WITH LOW OVERCAST AND EVEN A LITTLE DRIZZLE LINGERING THROUGH
THE DAY...TEMPERATURES WILL LIKELY REACH HIGHS IN THE UPPER 40S MON
EVENING...WITH AFTERNOON-EVENING HIGHS IN THE 50S...TO LOWER 70S IN
THE COASTAL PLAIN...AS THE WEDGE FRONT RETREATS INLAND. SCATTERED
SHOWERS AND EVEN A FEW STORMS WILL BE POSSIBLE INVOF THE WEDGE FRONT
- OF THE ELEVATED VARIETY OVER THE NW AND SURFACE-BASED WITH AN
ATTENDANT THREAT OF A STRONG STORM OVER THE COASTAL PLAIN.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AS OF 232 PM FRIDAY...

MONDAY NIGHT THROUGH TUESDAY NIGHT...CENTRAL NORTH CAROLINA LIES IN
THE SURFACE FRONTAL ZONE THAT ONLY VERY SLOWLY MOVES SOUTHEAST OVER
TIME DURING THIS PART OF THE LONG-TERM PERIOD IN DEEP SOUTHWEST FLOW
ALOFT. AT LEAST MODEST LIFT IS FORECAST BY THE GFS...AND MOISTURE ON
THE 12Z GFS AND THE 00Z ECMWF IS SUFFICIENT FOR WHAT SHOULD BE AT
LEAST NUMEROUS AREAS OF RAIN MONDAY NIGHT THROUGH MUCH OF THE DAY
TUESDAY. WET-BULB TEMPERATURES START TO APPROACH FREEZING IN THE
NORTHWEST PIEDMONT LATE IN THE DAY TUESDAY...AT THE TIME WHEN DEEP
MOISTURE BEGINS TO WANE AS DRIER AIR AND SUBSIDENCE START TO WORK
THEIR WAY INTO CENTRAL NORTH CAROLINA. THE LATEST GFS BUFR SOUNDING
FOR KGSO NEAR DUSK TUESDAY SHOWS MOISTURE REMAINING...WITH RAPID
DRYING DURING THE EARLY EVENING WHICH SPREADS EAST ACROSS THE AREA.
WILL RAISE CHANCES OF RAIN INTO THE LOW LIKELY CATEGORY FOR NOW FROM
MONDAY NIGHT INTO TUESDAY...GRADUALLY DIMINISHING CHANCES FROM WEST
TO EAST TUESDAY AFTERNOON INTO EARLY TUESDAY EVENING. IT SEEMS
PLAUSIBLE THAT MUCH IF NOT ALL OF CENTRAL NORTH CAROLINA WILL
EVENTUALLY SEE CATEGORICAL CHANCES FOR RAIN MONDAY NIGHT INTO
TUESDAY.

WEDNESDAY THROUGH FRIDAY...THE UPPER FLOW FLATTENS EARLY IN THIS
PART OF THE LONG-TERM PERIOD BEFORE BACKING TO THE SOUTHWEST TOWARD
THE END OF THE WEEK. THIS IS IN RESPONSE TO RISING HEIGHTS OVER THE
SOUTHEAST STATES AS UPPER LOW PRESSURE DEEPENS OVER THE SOUTHWEST
CONUS. THE MEAN AIR MASS ON BOTH THE 12Z GFS AND 00Z ECMWF IS DRY
WEDNESDAY THROUGH MUCH OF THE DAY FRIDAY...BEFORE THE MID-LEVEL
RIDGE AXIS MOVES EAST MAKING FOR A FAIRLY DEEP SOUTHWEST FLOW ALOFT
OVER CENTRAL NORTH CAROLINA...WHILE THE SURFACE RIDGE AXIS MOVES
EAST AS WELL HELPING TO BEGIN SOME LEVEL OF MOISTURE RETURN AT THE
LOW LEVELS TOWARD THE AREA. CLEARING WEDNESDAY FOLLOWED BY AN
INCREASE IN CLOUDS AT THE END OF THE WEEK. WILL KEEP THE FORECAST
FOR THE DAY FRIDAY PROPER DRY FOR NOW...ALTHOUGH ONE ALTERNATIVE IS
THAT MOISTURE RETURN IS FAST ENOUGH FOR THE SOUTHWEST PIEDMONT IN
PARTICULAR TO RECEIVE SOME AREAS OF RAIN LATE IN THE DAY.

PARTIAL THICKNESSES FROM BOTH THE 12Z GFS AND 00Z ECMWF ARE WELL
INTO THE LIQUID RANGE FOR FRIDAY EVENING. SURFACE WET-BULB
TEMPERATURES ARE ALSO AT OR ABOVE FREEZING...WHILE SHOWING A
TENDENCY TO WARM OVER THE LAST RUN ON THE GFS...TRENDING TOWARD THE
ALREADY WARMER ECMWF. WILL FORECAST FOR NOW JUST A SMALL CHANCE OF
RAIN AT THE VERY END OF THIS PERIOD AS THE AIR MASS MOISTENS AND
WARMS IN THE DEEPENING SOUTHWEST FLOW ALOFT.

WITH MOISTURE IN PLACE TO START...AND CHILLY HIGH PRESSURE TO
FOLLOW...WILL LEAN TOWARD TEMPERATURES AT OR BELOW THE MEX GUIDANCE
AND GENERALLY CLOSER TO THE ECMWF MEAN AND HPC GUIDANCE
TEMPERATURES. HIGHS SHOULD MOSTLY BE IN THE 40S...WARMER IN THE
SANDHILLS AND SOUTHERN COASTAL PLAIN TUESDAY. MANY 20S FOR OVERNIGHT
LOWS MID-WEEK...MOSTLY 30S FRIDAY NIGHT AND A WIDE RANGE FROM UPPER
30S TO NEAR 50 MONDAY NIGHT.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
AS OF 1240 AM SATURDAY...

A STRONG COLD FRONT WILL DROP SOUTH-SOUTHEAST ACROSS THE AREA
BETWEEN 06 TO 15Z. IFR TO MVFR CEILINGS WITH RAIN WILL IMPACT TAFS
FROM NW TO SE. IN THE WAKE OF THE FRONT...LOW-LEVEL DRY COLD AIR
ADVECTION FROM THE WEST SHOULD ALLOW CEILINGS TO LIFT TO VFR ACROSS
THE WESTERN AND CENTRAL PIEDMONT DURING THE AFTERNOON...HOWEVER MVFR
CEILINGS SHOULD LINGER AT EASTERN TAF SITES(KRWI AND KFAY).

PRECIP IS FORECAST TO RE-DEVELOP FROM SOUTH TO NORTH ACROSS THE AREA
LATE THIS EVENING AND INTO THE OVERNIGHT HOURS AND WILL PERSIST
THROUGH MUCH OF THE DAY ON SUNDAY. THERE IS A GOOD CHANCE THAT KINT
AND KGSO WILL SEE A PERIOD OF FREEZING RAIN SUNDAY AS THE
TEMPERATURES FALL TO NEAR OR SLIGHTLY BELOW FREEZING. SOME LIGHT
FREEZING RAIN IS ALSO POSSIBLE AT KRDU SUNDAY MORNING BEFORE
CHANGING OVER TO A COLD RAIN. OTHERWISE... LIFR CIGS AND VSBYS ARE
EXPECTED SUNDAY INTO MONDAY.

SUB-VFR CONDITIONS ARE EXPECTED TO PREVAIL THROUGH TUESDAY WITH
PERIODS OF LIGHT RAIN AGAIN MONDAY INTO TUESDAY.
-- End Changed Discussion --

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM LATE SATURDAY NIGHT THROUGH SUNDAY
EVENING FOR NCZ007-021>023.

&&

$$
SYNOPSIS...PWB/MWS
NEAR TERM...PWB/SEC
SHORT TERM...MWS
LONG TERM...DJF
AVIATION...CBL