FXUS62 KGSP 241950
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
250 PM EST THU DEC 24 2009
.SYNOPSIS...
HIGH PRESSURE WILL WEAKEN AND MOVE EAST OF THE AREA THROUGH THE
OVERNIGHT...BEFORE A STRONG COLD FRONT PUSHES ACROSS THE CAROLINAS
AND NORTHEAST GEORGIA CHRISTMAS DAY. HIGH PRESSURE WILL BUILD BACK
OVER THE REGION SATURDAY AND SLOWLY MOVE EAST THROUGH NEXT TUESDAY.
&&
.NEAR TERM /THROUGH FRIDAY/...
DRAMATIC INCREASE IN LLVL WAA FLOW/FRONTOGENESIS...FOLLOWED BY A SLUG
OF DEEPER FORCING...INDICATED BY AN INCREASE IN MID/UPPER LVL
Q-VECTOR CONVERGENCE...WILL RESULT IN PRECIPITATION BECOMING
WIDESPREAD REGIONWIDE TONIGHT. CATEGORICAL PCPN CVRG CHRISTMAS
MORNING TO DWINDLE BY THE AFTERNOON AS LATEST OP MODEL SOLUTIONS...
ALTHOUGH SOME TIMING DIFFERENCES REMAIN...PROG DRY SLOT TO
OVERSPREAD THE REGION FROM THE SW.
STILL EXPECTING DAMMING HIPRES TO DEVELOP AND REMAIN ORIENTED IN
FAVORABLE POSITION TO ADVECT SUB-FREEZING SFC WET BULB TEMPS SWWD
ALONG THE NC LEE SLOPES TONIGHT. ALTHOUGH THE SREF P-TYPE PLUME
DIAGRAMS OVER THE PAST FOUR RUNS HAVE BEEN TRENDING TOWARD
PROGRESSIVELY LESS ICE ACCRETION FOR ASHEVILLE/HICKORY/BOONE...
WARNINGS AND ADVISORIES ARE ALREADY IN PLACE THRU NOON TOMORROW AND
WILL SIMPLY NUDGE ACCUMS DOWNWARD TOWARD...FCSTG A MARGINAL WARNING
ACCRETION FOR THE LEE SLOPES BEFORE WARM RAIN PROCESSES AND OVERALL
DIMINISHMENT OF PCPN WINS OUT.
PLUME OF UNSEASONABLY HIGH PWAT VALUES...1.00 TO 1.25 INCHES...
REMAIN PROGGED TO OVERSPREAD THE CWFA LATER TONIGHT. AIDED BY
THE AFOREMENTIONED DEEP FORCING AND STOUT UPSLOPE FLOW...A PERIOD OF
HEAVY RAIN IS PROBABLE. THE CONSENSUS RAINFALL TOTALS ARE 2 TO 2.5
INCHES ALONG AND JUST EAST OF THE BLUE RIDGE. HIGH RUNOFF WILL
CERTAINLY OCCUR...PERHAPS EVEN AT AN EXCESSIVE LVL...ESPECIALLY
WHERE MTN SNOWMELT OCCURS. GIVEN SPEED OF SYSTEM AND THE CHANCE FOR
SOME INTERRUPTION OF MOISTURE TRANSPORT DUE TO ANTICIPATED DEEP
CONVECTION TO OUR S/SE...WILL HOLD OFF ON AN AREAL FLOOD WATCH
BUT MENTION HEAVY RAIN IN THE FCST. IT IS A LITTLE MORE PROBABLE
THAT AREAS ALONG THE UPPER FRENCH BROAD WILL EXPERIENCE LARGER
STEM FLOOD FLOWS AND A FLOOD WATCH HAS ALREADY BEEN ISSUED FOR THIS
AREA.
THE TIMING AND MAGNITUDE OF STOUT 65-70 KT SSELY 85H JET REMAINS ON
TRACK...TO LIFT NE ACRS THE CWFA TONIGHT/FRIDAY MORNING. CURRENT
PLACEMENT OF MTN WIND ADVISORY STILL LOOKS GOOD ALTHOUGH HAVE
EXPANDED THREAT EASTWARD INTO THE UPSTATE AND GEORGIA PIEDMONT
WHERE GUSTS TO 35 MPH HAVE THE POSSIBILITY OF REACHING THE SFC.
WHILE WERE TALKING ABOUT WIND CONCERNS...THERE IS ALSO THE
POSSIBILITY OF LATE NIGHT/EARLY MORNING GRAVITY WAVE DEVELOPMENT
ACRS THE PIEDMONT...AS WITHIN STABLE AIR JUST N/NW OF WEDGE FRONT AS
NOSE OF UPPER JET STREAK IS PROGGED TO ROTATE IN FROM THE
SW...ENCROACHING UPON INFLECTION POINT OF MID-LVL TROUGH.
ALSO ON FRIDAY MORNING...AS WEDGE BNDRY RETREATS INTO THE PIEDMONT
AND TRIPLE POINT LOW MOVES NE ALONG IT...PLUME OF NEAR IF NOT
SFC BASED INSTABILITY WILL BEND NWWD INTO THE FAR SE CWFA. WITHIN
THE EXPECTED STRONG/DEEP SHEAR ENVIRONMENT...SHALLOW CONVECTION IS
POSSIBLE...PERHAPS RESULTING IN LOCALIZED DAMAGING WIND GUSTS.
&&
.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
AS OF 200 PM THU...AN INTENSE UPPER LOW WILL PARK OVER THE WESTERN
GREAT LAKES STATES FRI NGHT AND SAT. OUR AREA WILL BE UNDER A STRONG
SOUTHWESTERLY UPPER LEVEL FLOW...THOUGH THE AIRMASS WILL BE QUITE
DRY OVERALL. THE NAM SOUNDINGS AND TIME/HEIGHTS STILL HAVE AMPLE
LLVL HUMIDITY OVER THE REGION FRI NIGHT...ESPECIALLY OVER THE NC
FOOTHILLS AND PIEDMONT. HYDROLAPSES ARE FAVORABLE FOR FOG
FORMATION...WHICH MAKES SENSE AS THE DEEP LAYER FLOW ONLY HAS A VERY
WEAK DOWNSLOPE COMPONENT...AND WE SHOULD HOLD ONE TO A DEEP MOIST
LAYER UP IN THOSE ZONES. I THINK WE/LL SEE FOG IN THESE AREAS AND
I/VE ADDED IT TO THE GRIDS. A BIT OF A GAP WIND FLOW DOWN THE FRENCH
BROAD VALLEY SHOULD KEEP THE FOG FROM FORMING THERE OVERNIGHT.
BY SUNDAY THE UPPER LOW WILL WILL BEGIN TO BE PICKED BACK UP BY THE
WESTERLIES...DIVING BRIEFLY TO THE SOUTH BEFORE LIFTING OFF TO THE
NORTH. AS THIS HAPPENS...SOME UPPER DIVERGENCE DEVELOPS OVER THE SRN
PART OF THE FA ON THE GFS AS A SHORT WAVE SHOOTS ACROSS THE REGION.
THE ECMWF SUPPRESSES THIS SRN STREAM ENERGY AND IS DRY OVER THE
REGION. THE NAM IS ALSO DRY...AND THE SREF POPS ARE ONLY ON THE
ORDER OF 15 TO 20 PERCENT. I HAVE KEPT SLIGHT CHANCE POPS ALONG THE
TN LINE...BUT EVEN THAT LOOKS A LITTLE HIGH. THICKNESSES WOULD
SUPPORT A RAIN/SNOW MIX.
&&
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
AS OF 230 PM THURSDAY...LOOKS LIKE WE ARE STILL UP FOR A BRIEF NW
FLOW SNOW SHOWER EVENT SUNDAY NIGHT AND MONDAY MORNING. THE MODELS
ARE IN A BIT BETTER AGREEMENT IN THAT THEY SWING THE UPPER TROF AXIS
ACROSS THE REGION DURING THIS TIME FRAME. THIS WILL FINALLY BRING
THE FLOW AT 850MB AROUND TO THE NW SUNDAY NIGHT AND PUSH THE
REMAINING LOW LEVEL MOISTURE UP THE WEST SIDE OF THE MTNS. THE
MOISTURE IS NOT PARTICULARLY DEEP BUT THE AIR MASS WILL BE COLD
ENOUGH TO SUPPORT SNOW SHOWERS. KEPT A CHANCE POP ALONG THE TN
BORDER OVERNIGHT AND INTO THE MORNING OVER THE NORTHERN MTNS...AND
THIS SHOULD END IN THE AFTERNOON. RIGHT NOW IT LOOKS LIKE A MINIMAL
EVENT. TEMPS WERE GENERALLY KEPT NEAR A GUIDANCE BLEND.
MONDAY NIGHT AND TUESDAY ARE THE QUIET PERIODS AS HIGH PRESSURE
MOVES ACROSS THE REGION...WHILE THE NEXT SRN STREAM SYSTEM TAKES
SHAPE OVER THE WRN GULF. THE MODEL TREND HAS BEEN TO DELAY THE ONSET
OF MOISTURE RETURN...SO OPTED TO KEEP TUESDAY NIGHT DRY.
CONFIDENCE IS LOW ABOUT THE EVOLUTION OF THE NEXT SYSTEM...ALTHO AT
LEAST THE MODELS AGREE THAT SOMETHING WILL TRY TO DEVELOP IN THE SRN
STREAM DURING THE MIDDLE PART OF THE WEEK. THE FORECAST GENERALLY
FOLLOWS CLOSER TO THE ECMWF AND HPC LOW POSITION BECAUSE OF BETTER
CONTINUITY. THE FORECAST WILL SHOW A CHANCE OF PRECIPITATION
SPREADING ACROSS THE REGION WEDNESDAY AND CONTINUING WEDNESDAY NIGHT
AND THURSDAY. PARTIAL THICKNESSES ARE SUCH THAT WE WOULD SEE A MIX
OF PRECIP TYPES. HOWEVER...I WOULD CAUTION YOU TO NOT PUT TOO MUCH
STOCK INTO ANY POTENTIAL WINTER WEATHER EVENT BECAUSE SIGNIFICANT
CHANGES ARE LIKELY. JUST FILE IT AWAY FOR NOW THAT WE COULD HAVE A
PROBLEM AROUND NEW YEARS EVE.
&&
.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
AT KCLT...EXPECT A CONTINUED DETERIORATION OF CIGS TO LIFR THROUGH
THE TAF PERIOD. A STRONG COLD FRONT WILL ADVECT ATL MOISTURE AHEAD
OF IT AND BRING IN MVFR STRATOCU BY SUNSET THIS EVENING. THESE
CONDITIONS WILL GENERALLY PREVAIL WITH NE/LY WINDS BECOMING GUSTY
AFT MIDNIGHT. AFT 08Z...RAIN WILL SPREAD OVER THE AREA AND LOWER
CIGS TO IFR AND LIFR THROUGH THE REMAINDER OF THE TAF PERIOD.
VSBYS WILL LINGER IN THE MVFR RANGE AS THE LOWER ATMOS REMAINS
SCOURED WITH PRECIP. A WIND SHIFT TO THE NW IS EXPECTED LATE IN
THE PERIOD...AROUND 17Z AS THE SFC FRONT CROSSES THE TERMINAL.
ELSEWHERE...THE GENERAL TREND WILL BE LOWERING CIGS TO MVFR BY 00Z
THEN TO IFR/LIFR BY 07Z PERSISTING THROUGH THE REMAINDER OF THE TAF
PERIOD. NE/LY WINDS WILL BECOME GUSTY AND EVIDENCE IS HAD IN THE
SYNOPTIC PATTERN THAT A GRAVITY WAVE MAY DEVELOP BTW 08Z AND
SUNRISE...WHICH COULD BRING STRONGER NW/LY SFC GUSTS TO 45
KTS...INCREASED LLVL WIND SHEAR AND SIGNIFICANT LLVL TURB THROUGH
DAYBREAK. WINDS WILL BACK TO THE NW AND WEAKEN AFT 13/14Z OUTSIDE
OF KAVL WHERE SE/LY WINDS WILL PERSIST...YET WEAKEN AFT 15Z.
OUTLOOK...VFR CONDITIONS EXPECTED SAT THROUGH TUE. A WEAK COLD FRONT
WILL APPROACH THE AREA WED AND POSSIBLY LOWER CIGS TO MVFR ACROSS
MOST TAF SITES THROUGHOUT THE DAY. HIGH PRESSURE AND DRIER AIR WILL
MOVE IN QUICKLY WED NIGHT RETURNING FLIGHT CATS TO VFR FOR THU.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...LAKE WIND ADVISORY FROM 6 PM THIS EVENING TO NOON EST FRIDAY
FOR GAZ018-026-028-029.
WIND ADVISORY FROM 6 PM THIS EVENING TO NOON EST FRIDAY FOR
GAZ010-017.
NC...ICE STORM WARNING FROM 6 PM THIS EVENING TO NOON EST FRIDAY
FOR NCZ033-035-049-050-501>506.
WIND ADVISORY FROM 6 PM THIS EVENING TO NOON EST FRIDAY FOR
NCZ048-051-052-058-059-062-063.
FREEZING RAIN ADVISORY FROM 6 PM THIS EVENING TO NOON EST
FRIDAY FOR NCZ036-053-056-064-065-068-507>510.
SC...LAKE WIND ADVISORY FROM 6 PM THIS EVENING TO NOON EST FRIDAY
FOR SCZ001>014-019.
&&
$$
SYNOPSIS...SBK
NEAR TERM...CSH
SHORT TERM...MCAVOY
LONG TERM...PM
AVIATION...SBK
