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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KRAH 121429
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
ISSUED BY NATIONAL WEATHER SERVICE BLACKSBURG VA
929 AM EST THU DEC 12 2013

.SYNOPSIS...
HIGH PRESSURE WILL BUILD INTO THE REGION TODAY AND
SETTLE OVER THE AREA TONIGHT. A WEAK LOW PRESSURE SYSTEM WILL
DEVELOP IN THE DEEP SOUTH FRIDAY NIGHT...THEN TRACK NORTHEAST ALONG
THE CAROLINA AND MID ATLANTIC COAST ON SATURDAY AND SUNDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
AS OF 930 AM THURSDAY...

TODAY: A DRY COLD FRONT HAS PASSED THROUGH THE AREA THIS
MORNING...WITH COLD ADVECTION AND DECREASING DEW POINTS ADVECTING
IN BEHIND IT FROM THE NORTH AND NORTHWEST. THIS WILL RESULT IN
BELOW NORMAL TEMPERATURES...DESPITE SUNNY SKIES. IT APPEARS THAT
ANY THREAT OF CIRRUS FROM THE WEAK UPPER LEVEL SHORT WAVE OR ANY
OROGRAPHIC EFFECTS IS MINIMAL...THUS WILL MAINTAIN CLEAR SKIES IN
THE FORECAST THROUGH THE DAY. THUS... GIVEN AFTERNOON LOW LEVEL
THICKNESS VALUES EXPECTED TO BE AROUND 45 METERS BELOW NORMAL WILL
GO WITH AFTERNOON HIGH TEMPS TODAY AROUND 40 NORTH TO GENERALLY
MID 40S SOUTH. MADE NO CHANGES TO HIGHS FROM OVERNIGHT FORECAST AS
MORNING TEMPS APPEAR TO BE ON TRACK. LOWERED WINDS SPEEDS SLIGHTLY
BASED ON LATEST OBSERVATIONS THROUGH MIDDAY...BUT STILL EXPECTING
SUFFICIENT MIXING TO CAUSE SOME GUSTS IN THE 15 TO 20 MPH RANGE AT
TIMES EARLY THIS AFTERNOON.

TONIGHT: SURFACE HIGH PRESSURE WILL SETTLE OVER THE REGION EARLY
FRIDAY MORNING... ALLOWING WINDS TO BECOME CALM AT MOST LOCATIONS.
GIVEN THIS AND A DRY AIRMASS WITH TEENS DEW POINTS AT BEST ALONG
WITH CLEAR SKIES EXPECTED WE WILL SEE TEMPS FALL QUITE NICELY...
WITH LOWS RANGING FROM AROUND 20 IN THE USUAL RURAL COLD AREAS TO
THE MID 20S IN THE URBAN AREAS AND FAR SOUTHEASTERN REACHES OF THE
CWA.

FRIDAY: SURFACE HIGH PRESSURE WILL SHIFT EASTWARD AND OFFSHORE ON
FRIDAY ALLOWING FOR A SOUTHERLY RETURN FLOW TO DEVELOP. THE NEXT
SYSTEM WILL TO AFFECT US WILL BE MOVING INTO THE SOUTHERN PLAINS ON
FRIDAY AFTERNOON. HOWEVER... MID AND HIGH LEVEL CLOUD COVER
ASSOCIATED WITH THIS SYSTEM WILL BEGIN TO ADVECT INTO OUR AREA LATE
IN THE DAY... THOUGH ANY AFFECTS ON TEMPS SHOULD BE SMALL TO NON-
EXISTENT. GIVEN THIS... WE SHOULD HAVE MOSTLY SUNNY/SUNNY SKIES FOR
A GOOD PART OF THE DAY. THUS... WILL CONTINUE TO SHOW HIGH TEMPS
REBOUNDING INTO THE UPPER 40S TO LOWER 50S... AS AFTERNOON LOW LEVEL
THICKNESS VALUES ARE EXPECTED TO RISE SOME 30 METERS FROM TODAYS
VALUES.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
AS OF 320 AM THURSDAY...

FRIDAY NIGHT THROUGH SATURDAY NIGHT... FORECAST PRETTY MUCH ON TRACK
WITH JUST A FEW MINOR TWEAKS NECESSARY...BASED ON LATEST
OBSERVATIONAL DATA AND MODEL GUIDANCE.

SURFACE HIGH OVER COASTAL CAROLINA FRIDAY EVENING WILL CONTINUE TO
SCOOT EASTWARD...ALLOWING A SLY RETURN FLOW TO BECOME ESTABLISHED.
THIS FLOW WILL PULL WARM MOIST AIR OVER THE RESIDUAL COOL DRY AIR
NEAR THE SURFACE. THIS MECHANISM SHOULD THICKEN AND LOWER CLOUD
COVER AND POSSIBLY LEAD TO THE DEVELOPMENT OF PATCHY LIGHT DRIZZLE
ACROSS THE WESTERN PIEDMONT AFTER MIDNIGHT. WHILE PARTIAL
THICKNESSES SUGGEST ALL LIQUID. SFC WET BULB TEMPS HOVER
NEAR/SLIGHTLY COOLER THAN FREEZING (ESPECIALLY THE 00Z NAM) SUGGEST
THE POSSIBILITY OF THE PRECIP TRANSITIONING TO A BRIEF PERIOD OF
NUISANCE FREEZING DRIZZLE. ONCE THE PRECIP INTENSITY INCREASES AFTER
DAYBREAK...FREEZING DRIZZLE THREAT WILL END. FORECAST CONFIDENCE
BELOW AVERAGE AS MODEL TREND HAS BEEN DELAYING PRECIP ARRIVAL...
WITH MOST GUIDANCE NOW SUGGESTING MEASURABLE PRECIP HOLDING OFF
UNTIL AFTER DAYBREAK. THUS WILL REFRAIN FROM MENTIONING FREEZING
PRECIP IN THE FORECAST AT THIS TIME. MIN TEMPS 30-34 ACROSS THE
NORTH AND MID 30S SOUTH.

WATER VAPOR IMAGERY THIS MORNING DEPICTS AN EXTENDED PLUME OF HIGH
MOISTURE CONTENT EXTENDING FROM SOUTH OF HAWAII...CROSSING WESTERN
MEXICO THEN INCHING NWD TOWARD NM AND TEXAS. AN UPPER LEVEL LOW SEEN
ON THE IMAGERY OFF THE SOUTHERN CA COAST WILL MOVE INLAND AND CROSS
THE SOUTHERN ROCKIES INTO THE SOUTHERN PLAINS BY BY FRIDAY NIGHT.
THIS SYSTEM MAY BE ABLE TO BACK THE MID-UPPER LEVEL FLOW ENOUGH TO
TAP THIS PLUME AND ADVECT MOISTURE INTO THE GULF COAST STATES AND SE
U.S. AS THIS LOW APPROACHES OUR REGION SATURDAY AND SATURDAY
NIGHT...THIS ATTENDANT MOISTURE ALONG WITH AVAILABLE LIFT FROM A
STRONG JET CROSSING THE APPALACHIANS INTO THE NORTHERN MID ATLANTIC
SHOULD CAUSE WIDESPREAD RAIN ACROSS CENTRAL...ESPECIALLY LATE
SATURDAY MORNING THROUGH AFTERNOON. WILL CONTINUE CATEGORICAL POPS
AND HAVE INCREASED PRECIP AMOUNTS FROM EARLIER FORECAST TO REFLECT
POSSIBILITY OF TAPING THE PACIFIC MOISTURE PLUME. HALF TO ONE INCH
RAIN AMOUNTS PROBABLE DURING THE DAYLIGHT HOURS SATURDAY.

THE RESIDUAL COOL DRY AIR DEPOSITED BY THE DEPARTING SFC HIGH
COUPLED WITH INCREASING WARM/MOIST AIR ALOFT WILL LIKELY CAUSE A
COLD AIR DAMMING EVENT TO OCCUR SATURDAY (INSITU IN CLASSIFICATION
DUE TO LACK OF A CONNECTION/PRESENCE OF PARENT HIGH). MODEL GUIDANCE
HAS BEEN VERY CONSISTENT WITH THIS SIGNAL. PREVIOUS FORECAST HAD A
GOOD HANDLE ON TEMP FORECAST WITH LOW-MID 40S NW TO NEAR 60-LOWER
60S FAR SE. MODEL GUIDANCE SEEMS TO HAVE CENTERED ON THE IDEA OF THE
WEDGE BOUNDARY REMAINING JUST EAST-SOUTH OF THE RDU AREA. STILL
TEMPS ACROSS THIS BOUNDARY WILL VARY AS MUCH AS 10-15 DEGREES IN A
RELATIVELY SHORT DISTANCE.

KINEMATICS STILL IN PLACE TO GENERATE A FEW T-STORMS IN THE WARM
SECTOR LATE SATURDAY INTO SATURDAY EVENING AS A WEAK SFC LOW MOVES
ALONG THE WEDGE BOUNDARY. HOWEVER THE NEAR SURFACE INSTABILITY IS
WEAK TO NON-EXISTENT. WHILE SOME ROTATION WILL PROBABLY BE OBSERVED
WITH THE STRONGER RETURNS...UNLIKELY THAT THIS AFFECT WILL BE
REALIZED NEAR THE SURFACE SO WILL REFRAIN FROM MENTIONING STRONG
STORMS IN THE HWO AT THIS TIME.

PRECIP INTENSITY AND COVERAGE WILL TAPER OFF FROM THE SW SATURDAY
NIGHT AS SFC WAVE AND ATTENDANT BOUNDARY MOVES E-NE ACROSS THE
COASTAL PLAIN. WILL DEPICT DECREASING POPS WEST-TO-EAST SATURDAY
NIGHT ALONG WITH PRECIP AMOUNTS AVERAGING LESS THAN A QUARTER OF AN
INCH.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
AS OF 320 AM THURSDAY...

SUNDAY...MODEL GUIDANCE A BIT SLOWER WITH THE PASSAGE OF THE MID
LEVEL TROUGH. THE LATEST ECMWF HAS COME MORE IN LINE WITH THE MORE
PROGRESSIVE GFS BUT EVEN THE GFS SUGGESTS A SECONDARY MID LEVEL
VORTICITY MAX AND WEAK SFC WAVE LIFTING NE ACROSS EASTERN NC SUNDAY
MORNING. THUS WILL CONTINUE CHANCE-LOW END LIKELY POPS ACROSS THE
REGION SUNDAY MORNING WITH THE GREATEST THREAT FOR MEASURABLE RAIN
EAST OF HIGHWAY 1...THOUGH AMOUNTS EXPECTED TO BE LESS THAN A COUPLE
TENTHS.

EXPECT DRIER CONDITIONS TO PREVAIL SUNDAY AFTERNOON THOUGH CLOUDS
WILL STICK AROUND FOR A GOOD PART OF THE DAY...LIKELY NOT DECREASING
UNTIL MID-LATE AFTERNOON WITH THE PASSAGE OF THE 700MB TROUGH. MAX
TEMPS SUNDAY MID-UPPER 40S NW-MID 50S SE.

MONDAY THROUGH WEDNESDAY... RELATIVELY TRANQUIL WEATHER PATTERN
EXPECTED THIS PERIOD AS BROAD UPPER TROUGH WILL PERSIST ACROSS THE
NATION WITH THE NORTHERN STREAM BEING MORE ENERGIZED THAN THE
SOUTHERN BRANCH. MAIN STORM TRACK WILL BE DISPLACED FROM THE
CAROLINAS...EXTENDING FROM THE NORTHERN PLAINS-GREAT LAKES-NEW
ENGLAND. THIS TRANSLATES TO DRY AND SEASONABLY COOL CONDITIONS FOR
CENTRAL NC. AN AMPLIFYING S/W IN THE NORTHERN STREAM MAY INCREASE
CLOUD COVERAGE WEDNESDAY INTO WEDNESDAY NIGHT WITH A THREAT FOR RAIN
LATE.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
AS OF 6 AM THURSDAY...

24-HR TAF PERIOD: HIGH CONFIDENCE IN VFR CONDITIONS FOR THE 24 HOUR
TAF PERIOD.

HIGH PRESSURE WILL BUILD INTO THE AREA TODAY... THEN SETTLE OVER THE
AREA TONIGHT. THIS WILL RESULT IN VFR CONDITIONS WITH MOSTLY
SUNNY/CLEAR SKIES. EXPECTED WIND SPEEDS WILL INCREASE AFTER SUNRISE
WITH MIXING (13-14Z)... WITH SUSTAINED NORTHWESTERLY TO NORTH-
NORTHWESTERLY WINDS IN THE 10 TO 15 MPH RANGE... WITH SOME GUSTS OF
UP TO 20 MPH. WINDS WILL GRADUALLY DECREASE DURING THE AFTERNOON..
WITH WINDS EVENTUALLY BECOMING LIGHT AND VARIABLE TO CALM BY SUNSET
WITH THE LOSS OF DIURNAL MIXING ALONG WITH THE WEAKENING PRESSURE
GRADIENT AS HIGH PRESSURE WILL SETTLE OVER THE REGION BY EARLY
FRIDAY MORNING.

LOOKING AHEAD: VFR/DRY CONDITIONS ARE EXPECTED TO PREVAIL THROUGH
FRIDAY AS HIGH PRESSURE CONTINUES TO CONTROL THE WEATHER ACROSS THE
REGION. WIDESPREAD ADVERSE AVIATION CONDITIONS ARE EXPECTED OVER THE
WEEKEND (SAT/SUN) IN ASSOCIATION WITH A GOMEX LOW PRESSURE SYSTEM
TRACKING NORTHEAST ALONG THE CAROLINA/MID-ATLANTIC COAST.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
NONE.
-- End Changed Discussion --

&&

$$
SYNOPSIS...BSD/VINCENT
NEAR TERM...BSD/XXX
SHORT TERM...WSS
LONG TERM...WSS
AVIATION...BSD/VINCENT