FXUS62 KRAH 251656
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1156 AM EST FRI DEC 25 2009
.SYNOPSIS...
A STORM SYSTEM TO OUR SOUTHWEST WILL MOVE NORTHEAST AND AFFECT THE
REGION THROUGH THIS EVENING...SLOWLY EXITING TO OUR NORTHEAST
SATURDAY MORNING. A DRY HIGH PRESSURE AREA WILL BUILD IN OVER THE
WEEKEND...THEN A SECONDARY COLD FRONT WILL BRING COOLER TEMPERATURES
FOR MONDAY THROUGH TUESDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 1045 AM FRIDAY...
15Z SFC ANALYSIS SHOWED 1040 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 985 MB LOW
PRESSURE CENTERED OVER IOWA. AN OCCLUDED FRONT WAS APPROACHING THE
SOUTHERN APPALACHIANS. A 1006 MB LOW WAS CENTERED NEAR AUGUSTA
GA...ALONG A WARM FRONT EXTENDING ENE THROUGH EASTERN SC AND FAR
SOUTHEAST NC. A WEDGE FRONT WAS NOTED OVER PORTIONS OF CENTRAL/
WESTERN NC AND UPSTATE SC. 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 OCCLUDED FRONT ASSOC/W THE MIDWEST LOW IS PROGGED TO CONTINUE
MOVING EASTWARD TOWARD THE CAROLINAS DURING THE DAY. AT THE SAME
TIME...A WEAK SFC LOW THAT HAS DEVELOPED ALONG A WARM FRONT OVER
GA/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 CENTRAL/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 AFTERNOON...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
TODAY AS STRONG SOUTHERLY/SE LOW LEVEL FLOW (50-60 KNOTS AT 925-850
MB) OVERRUNS THE WEDGE CURRENTLY IN PLACE ACROSS CENTRAL NC...
FOLLOWED BY GOOD LOW LEVEL CONVERGENCE AS THE WARM FRONT ADVANCES
N/NW INTO PORTIONS OF CENTRAL NC AND THE WEAK SFC LOW CURRENTLY
CENTERED NEAR AUGUSTA GA MOVES NORTHEAST INTO THE SANDHILLS/SE
COASTAL PLAIN AND EVENTUALLY EASTERN NC THIS EVENING. A WIDESPREAD
0.75" TO 1.00" OF RAIN IS EXPECTED OVER THE AREA TODAY...WITH A
POTENTIAL FOR 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. AS A RESULT...HAVE DECIDED TO
NOT ISSUE A FLOOD WATCH...WITH THE BEST POTENTIAL FOR FLOODING
FURTHER WEST IN THE FOOTHILLS/MOUNTAINS WHERE STRONG LOW LEVEL
UPSLOPE FLOW WILL LIKELY ENHANCE PRECIPITATION RATES IN AN AREA
WHERE SNOW IS STILL MELTING.
SEVERE THREAT:
THE STORM PREDICTION CENTER CONTINUES TO MAINTAIN A SLIGHT RISK OF
SEVERE THUNDERSTORMS OVER EASTERN SC AND FAR SOUTHEAST NC TODAY. ANY
SIGNIFICANT UPPER LEVEL FEATURES ARE WELL REMOVED FROM THE CAROLINAS
TODAY...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 TODAY...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 AFTERNOON. 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 AFT/EVE. UPSTREAM OBS CURRENTLY SHOW DEWPOINTS IN THE
MID/UPPER 60S IN A VERY NARROW ZONE FROM CHARLESTON SC WEST/NW
TOWARD AUGUSTA GA...AND SOUTH TO JACKSONVILLE FL...WHERE TORNADO
WATCHES ARE CURRENTLY IN EFFECT. PRIMARY THREAT FROM ANY CONVECTION
ROOTED IN THE BOUNDARY LAYER THIS AFTERNOON WOULD BE ISOLATED
TORNADOES AND DAMAGING WINDS. -VINCENT
TONIGHT:
PRECIP SHOULD BE EXITING THE COASTAL PLAIN BY MIDNIGHT TONIGHT AS
THE DRY PUNCH WITH THIS SYSTEM OVERWHELMS THE AREA AND WINDS TURN TO
WESTERLY. A LARGER THAN NORMAL RANGE OF OVERNIGHT LOWS...AS MODEL
SOUNDINGS SHOW LINGERING STRATUS AND MOISTURE POOLING ALONG A
SECONDARY 925MB TROUGH IN THE EAST..WITH BETTER CLEARING IN THE
WEST. LOWS 34-46. -BLS
&&
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
AS OF 245 AM FRIDAY...
FOR SATURDAY/SATURDAY NIGHT: MODEL CONSENSUS PLACES THE SURFACE LOW
OVER SOUTHEAST VA AT 12Z SATURDAY... WITH WEAK AND MODIFIED HIGH
PRESSURE BUILDING IN FROM THE SOUTHWEST. FORECAST SOUNDINGS SHOW A
SUBSTANTIAL LOW LEVEL INVERSION DEVELOPING WHICH SERVES TO TRAP
QUITE A BIT OF LOW LEVEL MOISTURE THROUGH THE MORNING. EXPECT AREAS
OF STRATUS TO SLOWLY LIFT AND MIX WITH WEAK SURFACE WINDS AND THE
STABLE LOW LEVELS INHIBITING VERTICAL AND HORIZONTAL DISPERSION. AND
GIVEN THE FAST FLOW IN THE MID-UPPER LEVELS AND MODEL INDICATIONS OF
AMPLE MOISTURE ALOFT STREAMING FROM OLD MEXICO ACROSS THE SOUTHEAST
WITH MOIST UPGLIDE AT 320K-330K... WE SHOULD ALSO SEE A STEADY
STREAM OF HIGH CLOUDS THROUGH THE DAY. ALL IN ALL... SKIES WILL BE
MAINLY PARTLY SUNNY. 1000-850 MB THICKNESSES HOLD ABOVE NORMAL
THROUGH SATURDAY WITH ONLY VERY WEAK SURFACE COLD AIR ADVECTION...
BUT MUCH OF THE WARMTH IN THIS LAYER IS ABOVE 950 MB. HAVE TRIMMED
BACK HIGHS A BIT TO 51-60. WITH THE 320-330K UPGLIDE DEPARTING
SATURDAY EVENING... EXPECT SKIES TO BECOME MOSTLY CLEAR... ALTHOUGH
THE LIGHT WINDS AND ANY RESIDUAL GROUND MOISTURE COULD LEAD TO
PATCHY FOG DEVELOPMENT LATE SATURDAY NIGHT. LOWS 29-35.
FOR SUNDAY/SUNDAY NIGHT: THE ANOMALOUSLY STRONG VORTEX THAT WILL
HAVE BEEN SITTING AND SPINNING OVER THE MIDWEST FOR TWO FULL DAYS
FINALLY EJECTS EASTWARD ACROSS NRN IN/OH TO PA/NY BY DAYBREAK
MONDAY. THE WEAK SURFACE HIGH COVERS THE SOUTHEAST AND RIDGES UP
INTO NC THROUGH SUNDAY... PRODUCING CONTINUED DRY WEATHER...
ALTHOUGH A WEAK MID LEVEL VORTICITY LOBE SWEEPING THROUGH THE AREA
SHOULD BRING INCREASING MID CLOUDS. LOW LEVEL THICKNESSES DIP
NOMINALLY AND STILL ANTICIPATE SLIGHTLY ABOVE NORMAL HIGHS OF 51-56.
THE WRF/GFS/ECMWF/CANADIAN ARE IN GOOD AGREEMENT ON SECONDARY COLD
FRONTAL PASSAGE SUNDAY NIGHT... MOST LIKELY AROUND OR SHORTLY AFTER
MIDNIGHT... AND ALL ALSO DEPICTS AN UPPER JET CORE ACCELERATING OVER
OR JUST SOUTHEAST OF NC WITH A QUICK SHOT OF LIGHT PRECIP STREAKING
ACROSS SOUTHEAST NC NEAR THE OFFSHORE BAROCLINIC ZONE. THE WRF WITH
ITS GREATER SATURATION THROUGH A DEEPER LAYER AS COMPARED TO THE GFS
SUGGESTS ANY PRECIP MIGHT BE A LIGHT RAIN/SNOW MIX... ALTHOUGH THE
SATURATION JUST BARELY GETS INTO THE CRYSTAL GROWTH ZONE ALOFT. SUCH
RAPID VERTICAL MOTION IS CERTAIN POSSIBLE WITH THE LIKELIHOOD OF
STRONG AGEOSTROPHIC RESPONSE TO THE ACCELERATING JET... BUT SINCE
THE GFS/WRF CONCENTRATE THIS LIFT IN THE MID AND UPPER LEVELS (EVEN
THE WRF HAS A RATHER DRY SUBCLOUD LAYER) WITH THE DEGREE OF MOISTURE
RETURN QUESTIONABLE... WILL LEAVE THE FORECAST DRY FOR NOW BUT HAVE
BEEFED UP CLOUDINESS. LOWS 26-33. -GIH
&&
.LONG TERM /MONDAY THROUGH THURSDAY/...
AS OF 300 AM FRIDAY...
MONDAY THROUGH WEDNESDAY: ARCTIC HIGH PRESSURE BUILDS INTO AND OVER
THE REGION THROUGH MIDWEEK... WITH NORTHWESTERLY MID LEVEL FLOW
FLATTENING TO WESTERLY. WE MAY SEE OROGRAPHIC ENHANCEMENT OF MID AND
HIGH CLOUDS TUESDAY NIGHT INTO WEDNESDAY... OTHERWISE WITH A DRY AND
STABLE COLUMN AND NO MECHANISMS TO GENERATE LIFT... EXPECT DRY
WEATHER AND TEMPS ABOUT A CATEGORY BELOW NORMAL.
WEDNESDAY NIGHT/THURSDAY: IT APPEARS THAT WE TRANSITION BACK TO A
SOUTHERN STREAM DOMINANT PATTERN WITH BROAD TROUGHING COMING INTO
THE FAR SOUTHWEST STATES AND NORTHERN MEXICO INTO THE SOUTHERN
PLAINS. THE 00Z/25 ECMWF SHOWS THIS LEADING TO SURFACE CYCLOGENESIS
OVER THE NORTHWEST GULF AND CENTRAL GULF COAST WITH INCREASING
OVERRUNNING OF THE SOUTH-SOUTHWEST 850 MB FLOW ATOP THE WEDGING
SURFACE HIGH CENTERED TO OUR NORTH. THE LATEST GFS IS NOT TOO
DISSIMILAR... SHOWING A SECOND LOW DEVELOPING OFF THE CAROLINA COAST
THURSDAY WITH A 160 KT JET JUST TO OUR NORTH. WILL INCLUDE JUST
SLIGHT CHANCE POPS THURSDAY GIVEN THE GOOD MODEL AGREEMENT. EXPECT
SOME MODERATION OF TEMPS BACK TOWARD NEAR NORMAL VALUES... ALTHOUGH
ANY APPRECIABLE PRECIP INTO THE SURFACE STABLE AIR MAY MEAN BELOW
NORMAL HIGHS THURSDAY OVER THE PIEDMONT. -GIH
&&
.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/SMITH
SHORT TERM...HARTFIELD
LONG TERM...HARTFIELD
AVIATION...VINCENT
