FXUS62 KRAH 251952
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
250 PM EST FRI DEC 25 2009
.SYNOPSIS...
A STORM SYSTEM TO OUR SOUTHWEST WILL MOVE NORTHEAST AND AFFECT THE
AREA THROUGH THIS EVENING...SLOWLY EXITING TO OUR NORTHEAST TONIGHT.
A DRY AREA OF HIGH PRESSURE WILL BUILD INTO CENTRAL NORTH CAROLINA
OVER THE WEEKEND...THEN A SECONDARY COLD FRONT WILL BRING COOLER
TEMPERATURES FOR MONDAY AND TUESDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 130 PM FRIDAY...
18Z SFC ANALYSIS SHOWED 1038 MB HIGH PRESSURE CENTERED EAST OF
HUDSON BAY...WITH A SFC RIDGE AXIS EXTENDING SOUTH THROUGH NEW
ENGLAND...THE MID-ATLANTIC AND INTO THE CAROLINAS. DEEP 986 MB LOW
PRESSURE WAS CENTERED OVER CENTRAL IOWA. A COLD FRONT WAS
APPROACHING THE SOUTHERN APPALACHIANS. A 1006 MB LOW WAS CENTERED
NEAR COLUMBIA SC...ALONG A WARM FRONT EXTENDING ENE THROUGH
NORTHEAST SC AND SOUTHEAST NC. A WEDGE FRONT WAS NOTED OVER PORTIONS
OF WESTERN NC. ALOFT...12Z RAOB ANALYSIS SHOWED AN H85 WARM FRONT
OVER THE SOUTHEAST...ROUGHLY FROM FFC-CAE-MHX. TWO UPPER LOWS WERE
NOTED OVER THE MIDWEST...ONE NEAR THE IL/IA/MO BORDER...THE OTHER
OVER KANSAS...WITH CYCLONIC FLOW ALOFT OVER A LARGE PORTION OF THE
CONUS.
CHRISTMAS DAY OVERVIEW:
THE DEEP SFC LOW OVER THE MIDWEST IS EXPECTED TO CONTINUE
RETROGRADING INTO WESTERN IOWA TODAY AS THE TWO DISTINCT UPPER LOWS
BEGIN TO MERGE OVER IA/MO. A RATHER COMPLEX SURFACE PATTERN WILL
DRIVE THE SENSIBLE WEATHER CONDITIONS ACROSS THE CAROLINAS TODAY.
THE COLD FRONT ASSOC/W THE MIDWEST LOW IS PROGGED TO CONTINUE MOVING
EASTWARD TOWARD THE CAROLINAS THIS AFT/EVE. AT THE SAME TIME...A
WEAK SFC LOW CURRENTLY NEAR COLUMBIA SC IS PROGGED TO MOVE
NORTHEASTWARD THROUGH THE SANDHILLS AND INTO EASTERN NC. THE SFC
WARM FRONT WILL MOVE INTO PORTIONS OF CENTRAL NC...AND THE WEDGE
FRONT CURRENTLY OVER WESTERN NC WILL RETREAT NORTH/NW TOWARD THE
BLUE RIDGE AND SHENANDOAH VALLEY.
TEMPERATURES:
CHALLENGING TEMPERATURE FORECAST...WITH A SURFACE LOW TRAVERSING
EASTERN/SOUTHERN PORTIONS OF THE AREA THIS AFT/EVE...MOVING ALONG OR
IN VICINITY OF A BAROCLINIC ZONE ASSOC/W A RETREATING WEDGE FRONT
AND AN ADVANCING WARM FRONT. WILL CONTINUE TO SHOW A RANGE FROM THE
LOWER/MID 40S (42-47F) IN THE TRIAD TO THE LOWER 60S (60-64F) IN THE
SE COASTAL PLAIN.
PRECIPITATION:
WILL CONTINUE TO SHOW CATEGORICAL (100%) POPS ACROSS THE ENTIRE AREA
THIS AFTERNOON AS STRONG SOUTHERLY/SE LOW LEVEL FLOW (50-60 KNOTS AT
925-850 MB) OVERRUNS THE COOLER AIRMASS IN PLACE ACROSS CENTRAL
NC...FOLLOWED BY GOOD LOW LEVEL CONVERGENCE AS THE WARM FRONT
ADVANCES N/NW INTO PORTIONS OF THE AREA AND THE WEAK SFC LOW
CURRENTLY CENTERED NEAR COLUMBIA SC MOVES NORTHEAST INTO THE
SANDHILLS/SE COASTAL PLAIN AND EVENTUALLY EASTERN NC THIS EVENING.
STORM TOTAL RAINFALL AMOUNTS OF 0.75-1.00" ARE EXPECTED OVER THE
AREA TODAY...WITH LOCALLY HIGHER AMOUNTS IN ASSOC/W ANY CONVECTIVE
ELEMENTS. W/REGARD TO THE THREAT FOR FLOODING...MOST OR ALL OF THE
SNOW THAT FELL ON DEC 18-19 HAS ALREADY MELTED IN THE NW PIEDMONT...
AND 1-HR/3-HR FLASH FLOOD GUIDANCE VALUES ARE WELL ABOVE WHAT WOULD
BE EXPECTED QPF-WISE TODAY. ALTHOUGH A FEW AREA CREEKS WILL NEED TO
BE MONITORED...THE THREAT FOR RIVER FLOODING WILL BE FAIRLY
LOCALIZED AND ON THE MARGINAL SIDE.
SEVERE THREAT:
THE STORM PREDICTION CENTER CONTINUES TO MAINTAIN A SLIGHT RISK OF
SEVERE THUNDERSTORMS OVER EASTERN SC AND FAR SOUTHEAST NC TODAY.
HOWEVER...ANY SIGNIFICANT UPPER LEVEL FEATURES ARE WELL REMOVED FROM
THE CAROLINAS...AND UPPER SUPPORT WILL LIKELY BE LIMITED TO
DIVERGENCE ALOFT ASSOC/W THE EXIT REGION OF A 100-125 KT SOUTHERN
STREAM JET OVER THE DEEP SOUTH AND SOUTHEAST. DEEP LAYER SHEAR IS
MORE THAN ADEQUATE FOR ORGANIZED STORMS...AND WITH A 50-60 KNOT
SOUTHEASTERLY LOW LEVEL JET...LOW-LEVEL SHEAR/HELICITY ARE THROUGH
THE ROOF...PARTICULARLY IN VICINITY OF THE SURFACE LOW TRIPLE POINT
EXPECTED TO MOVE NORTHEAST THROUGH THE EASTERN SANDHILLS AND FAR SE
COASTAL PLAIN THIS AFT/EVE. AS IS TYPICAL FOR THIS TIME OF YEAR...
INSTABILITY WILL BE VERY MARGINAL OR NON-EXISTENT...OWING TO WEAK
MID-LEVEL LAPSE RATES AND PERVASIVE CLOUD COVER/LIMITED INSOLATION.
DOUBT THAT ANY CONVECTION IN THE WFO RALEIGH AREA OF RESPONSIBILITY
WILL BE ROOTED IN THE BOUNDARY LAYER...HOWEVER...SFC-BASED
CONVECTION CANNOT BE ENTIRELY RULED OUT...PRIMARILY IN CUMBERLAND/
SAMPSON/WAYNE COUNTIES...IN VICINITY OF THE SFC LOW TRIPLE POINT
WHERE A VERY BRIEF/NARROW MARITIME-TROPICAL WARM SECTOR IS POSSIBLE
LATER THIS AFTERNOON. UPSTREAM OBS CURRENTLY SHOW DEWPOINTS IN THE
MID/UPPER 60S IN A VERY NARROW ZONE FROM CHARLESTON SC NORTH/NW
TOWARD COLUMBIA...WHERE A TORNADO WATCH REMAINS IN EFFECT. PRIMARY
THREAT FROM ANY CONVECTION ROOTED IN THE BOUNDARY LAYER THIS
AFTERNOON WOULD BE AN ISOLATED TORNADO OR DAMAGING WINDS...ALTHOUGH
GUT FEELING IS THAT THIS THREAT WILL REMAIN SOUTH OF THE AREA...
PRIMARILY ALONG THE CAROLINA COAST. -VINCENT
TONIGHT:
PRECIPITATION IS EXPECTED TO BE EXITING THE COASTAL PLAIN BY
MIDNIGHT TONIGHT (03-06Z) AS DRIER MID/UPPER LEVEL AIR FILTERS INTO
THE REGION FROM THE SOUTHWEST AND LOW LEVEL FLOW VEERS AROUND TO THE
WEST. FCST SOUNDINGS SHOW LINGERING STRATUS AND MOISTURE POOLING
ALONG A SECONDARY LOW LEVEL (925 MB) TROUGH ACROSS THE AREA
TONIGHT...PARTICULARLY IN THE EAST. LOWS ARE EXPECTED TO RANGE FROM
THE LOWER/MID 30S (NW) TO MID 40S (EAST)...OR 34-46F. -VINCENT
&&
.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
AS OF 250 PM FRIDAY...
SATURDAY...MODEL VERY CONSISTENT IN PLACING SURFACE LOW IN
VICINITY OF SE VA WITH A RELATIVELY WEAK SURFACE RIDGE BUILDING INTO
FROM THE SW. LOW LEVEL SUBSIDENCE INVERSION MAY INITIALLY TRAP ANY
LOW LEVEL MOISTURE...RESULTING IN SCATTERED/BROKEN STRATUS EARLY.
HOWEVER MIXING OF DRIER AIR FROM LOFT SHOULD HELP TO ERODE/DISSIPATE
THIS STRATUS LAYER BY MID OR LATE MORNING. UPSTAIRS...SW FLOW WILL
CONTINUE TO TRANSPORT PACIFIC MOISTURE ACROSS THE REGION CAUSING A
DECENT VEIL OF CIRRUS. OPAQUENESS OF THIS CIRRUS SHIELD WILL PLAY A
ROLE IN DETERMINING MAX TEMPS SATURDAY. LOW LEVEL THICKNESSES IN THE
MID 1302S WITH FULL SUN SUGGEST MAX TEMPS IN THE MID/UPPER 50S. PER
MODEL SOUNDINGS...CIRRUS LIKELY TO BE RATHER THICK SO WILL ACCOUNT
FOR OVERCAST SKIES FOR MOST OF THE DAY. EXPECT MAX TEMPS IN THE
LOWER 50S NW TO THE UPPER 50S SE. IF STRATUS CLOUDS HANG AROUND
LONGER THAN EXPECTED...MAY SEE MAX TEMPS CLOSER TO THE COOLER NAM
MOS GUIDANCE.
SATURDAY NIGHT...SURFACE RIDGE LIES SW-NE ACROSS REGION WHILE
MOISTURE ALOFT THINS OUT AFTER 06Z. CLEARING SKIES AND NEAR CALM
WIND REGIME AT THE SURFACE SHOULD ALLOW AIR MASS NEAR THE SURFACE TO
COOL EFFICIENTLY. LEANED MIN TEMPS CLOSER TO THE COOLER MOS GUIDANCE
OF NEAR 30 NORTH TO LOWER 30S SOUTH.
SUNDAY...SURFACE RIDGE DRIFTS EAST. WITH DEEP WLY FLOW IN THE
LOW/MID LEVELS...LEE SIDE SURFACE TROUGH SHOULD DEVELOP BY
AFTERNOON. RESULTANT SW SURFACE FLOW SHOULD AID IN TEMP RECOVERY.
THOUGH LOW LEVEL THICKNESSES ABOUT 7-10M LOWER THAN
SATURDAY...WARMING FROM SW FLOW SHOULD RESULT IN MAX TEMPS
COMPARABLE TO SATURDAY.
SUNDAY NIGHT INTO MONDAY...MID/UPPER LEVEL TROUGH CROSSES REGION
SUNDAY NIGHT AS UPPER LOW MOVES ACROSS THE GREAT LAKES. ATTENDANT
SURFACE FRONT WILL CROSS REGION LATE SUNDAY NIGHT/EARLY MONDAY. NW
FLOW BEHIND THIS FEATURE WILL USHER END A NEW ROUND OF CHILLY DRY
AIR. THICKNESS MONDAY COOL 30-35M FROM THOSE OF SUNDAY. EXPECT LOW
LEVEL CAA ALL DAY AS 850MB THERMAL TROUGH POSITIONED TO OUR NW. THIS
WILL TRANSLATE TO MAX TEMPS IN THE MID 40S.
SURFACE RIDGE BUILDS INTO FORECAST AREA FROM THE WEST. MAINLY CLEAR
SKIES AND SUBSIDING WIND FLOW WILL CAUSE TEMPS TO DROP INTO THE
MID/UPPER 20S. IF SURFACE WINDS DECOUPLE AROUND SUNSET...POTENTIAL
FOR NORMALLY COLDER SITES TO COOL TO NEAR 20 DEGREES.
&&
.LONG TERM /TUESDAY THROUGH FRIDAY/...
AS OF 230 PM EST FRIDAY...
UPPER AIR PATTERN INITIALLY CHARACTERIZED BY A SPLIT FLOW REGIME IN
THE WEST WITH THE STREAMS CONVERGING OFF THE MID ATLANTIC COAST.
SOUTHERN STREAM APPEARS TO BE MORE DOMINATE WITH A WELL DEFINED S/W
CROSSING THE SOUTHERN ROCKIES TUESDAY...LIFTING NE THROUGH THE
SOUTHERN PLAINS AND MID MS VALLEY TUESDAY NIGHT INTO WEDNESDAY WHEN
IT BEGINS TO SHEAR OUT IN INCREASING CONFLUENCE ALOFT. S/W RIDGE
PRECEDING THIS FEATURE WILL BE IMMEDIATELY WEST OF CENTRAL
NC...PROVIDING US WITH DRY AND SEASONABLY COOL CONDITIONS TUESDAY.
WEDNESDAY...SHEARING OF THE MID LEVEL S/W TO OUR NORTH SHOULD INDUCE
SLY FLOW IN THE MID LAYERS WITH SOME DEGREE OF MOISTURE ADVECTION.
AT THE SURFACE...RIDGE PROJECTED TO EXTEND ACROSS REGION FROM A
PARENT HIGH (APPROX 1030MB) NEAR OR OFFSHORE OF THE NORTHERN MID
ATLANTIC. THIS SET-UP SUGGEST POTENTIAL FOR A HYBRID DAMMING EVENT.
PRECIP AMOUNTS SHOULD BE LIGHT. TOO EARLY TO DETERMINE P-TYPE THOUGH
PARTIAL THICKNESSES SUGGEST POTENTIAL FOR WINTRY MIX IN THE FAR N-NW
SECTIONS. WILL INTRODUCE A SLIGHT CHANCE INTO THE FORECAST WED
AFTERNOON WITH SLIGHTLY BETTER CHANCES LEADING INTO THU.
MEDIUM RANGE MODELS DIVERGE QUITE A BIT BY FRI. 12Z GFS DEPICTS TWO
SEPARATE STREAMS WITH FAIRLY POTENT S/W IN THE SOUTHERN STREAM
CROSSING THE LOWER MS VALLEY. IN CONTRAST...ECMWF DEPICTS A CLOSED
DEEP LOW IN THE NORTHERN STREAM IN VICINITY OF THE UPPER MS VALLEY
WITH A WEAKER SOUTHERN STREAM S/W FARTHER WEST OVER WEST TX. SINCE
CENTRAL NC PROJECTED TO REMAIN IN SW FLOW ALOFT BY BOTH
MODELS...POTENTIAL FOR PERTURBATIONS IN FLOW ALOFT TO CROSS THE
FORECAST AREA RESULTING IN VARYING DEGREES OF CLOUDINESS AND A
SLIGHT CHANCE OF PRECIP. DUE TO CLOUDS AND PRECIP
POSSIBILITIES...MAX TEMPS PROJECTED TO REMAIN BELOW NORMAL...DEGREE
OF WHICH WILL BE DETERMINED BY THICKNESS OF CLOUDS AND COVERAGE OF
PRECIP.
&&
.AVIATION /18Z FRIDAY THROUGH TUESDAY/...
AS OF 1200 PM FRIDAY...
24-HR TAF PERIOD:
AVIATION CONDITIONS WILL CONTINUE TO DETERIORATE ACROSS THE AREA
THROUGH 00Z THIS EVENING AS A LARGE AREA OF LIGHT/MODERATE RAIN
ASSOC/W A LOW PRESSURE SYSTEM TO THE SOUTH MOVES INTO CENTRAL NC.
IFR/LIFR CONDITIONS WILL LIKELY PREDOMINATE ONCE RAINFALL BEGINS IN
EARNEST AT ANY GIVEN AREA. IFR/LIFR CONDITIONS SHOULD PERSIST WELL
AFTER RAINFALL HAS ENDED...POTENTIALLY AS LONG AS 12-15Z SATURDAY
MORNING...DUE TO THE EXPECTED SLOW PROGRESS OF THE DRIER SURFACE
AIRMASS WORKING INTO THE AREA FROM THE WEST. SOUTHEASTERLY LOW LEVEL
WIND SHEAR WILL IMPACT ALL TAF SITES THIS AFT/EVE...ENDING IN THE
21-03Z TIME FRAME FROM WEST-EAST THIS EVENING AS LOW LEVEL FLOW
WEAKENS AND VEERS TO THE WEST.
SATURDAY THROUGH MONDAY:
VFR CONDITIONS ARE EXPECTED TO RETURN BY LATE SATURDAY MORNING AND
PERSIST THROUGH MONDAY AS HIGH PRESSURE BUILDS TOWARD THE CAROLINAS
FROM THE SOUTHWEST. -VINCENT
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...VINCENT
NEAR TERM...VINCENT
SHORT TERM...WSS
LONG TERM...WSS
AVIATION...VINCENT
