FXUS62 KGSP 231956
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
256 PM EST WED DEC 23 2009
.SYNOPSIS...
HIGH PRESSURE WILL REMAIN OVER THE REGION THROUGH THURSDAY...BEFORE
A STRONG COLD FRONT PUSHES ACROSS THE REGION ON CHRISTMAS DAY. DRY
HIGH PRESSURE WILL THEN RETURN DURING THE WEEKEND AND REMAIN THROUGH
THE EARLY PART OF NEXT WEEK.
&&
.NEAR TERM /THROUGH THURSDAY/...
AS SHORTWAVE AND DEVELOPING SFC LOW OVER THE SW US TRACK E...S FLOW
AHEAD OF THE SYSTEM WILL BRING INCREASING MOISTURE OVER THE REGION.
CLOUDS WILL CONTINUE TO INCREASE ACROSS THE FCST AREA OVERNIGHT. AT
THE SAME TIME SFC HIGH PRESSURE WILL BUILD SE DOWN THE E SIDE OF THE
APPALACHIANS BRINGING COOL...DRY AIR IN FROM THE NE. TEMPERATURES
OVERNIGHT WILL COOL INTO THE 30S...WITH SOME MID 20S OVER THE MTNS.
AS THE SYSTEM CONTINUES TO APPROACH THU...LOW LVL MOISTURE WILL
SURGE N FROM THE GULF...AND FROM THE ATLANTIC...AND BEGIN TO
SATURATE THE LOWER LEVELS OF THE ATMOSPHERE. ISENTROPIC LIFT AND
UPSLOPE FLOW BEGIN TO INCREASE AS WELL...WHILE DRYER AIR CONTINUES
TO GET PULLED DOWN FROM THE NE. MODELS ARE NOT IN GOOD AGREEMENT
TIMING THE ONSET OF PRECIP WITH THE GFS FASTER THAN THE NAM AND
ECMWF. IN THIS CASE WE USED A MODEL BLEND WHICH KEEPS THE AREA DRY
THRU THE MRNG HOURS...THEN BEGINS TO BRING CHC POPS IN FROM THE SW
BY MID-LATE AFTN. CONSIDERING THE TIME OF DAY...KEPT THE PRECIP AS
ALL RAIN ATTM...EVEN WHERE THERE IS A SLIGHT CHC OF PRECIP UP INTO
THE NC N MTNS AND PART OF THE NC FOOTHILLS. ESP SINCE HIGH TEMPS ARE
EXPECTED TO REACH THE L-M40S...WITH A FEW U40S TO NEAR 50 ACROSS THE
S ZONES.
&&
.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
AS OF 245 PM WEDNESDAY...STRENGTHENING WAA PATTERN REMAINS ON TRACK
TO OVERSPREAD THE REGION THURSDAY NIGHT ACCOMPANIED BY PROGRESSIVELY
DEEPER FORCING AND MOISTURE. WILL PLAN ON CONTINUING TO RAMP UP TO
100 POP CWFAWIDE AS THE NIGHT WEARS ON. THERE EXISTS SUBTLE
DIFFERENCES BETWEEN 12 UTC MODEL QPF...WITH THE 15 UTC SREF A
GOOD COMPROMISE...PROGGING AN AXIS OF HEAVY PCPN UP THE FTHLS.
SFC HIPRES REMAINS PROGGED TO ORIENT ITSELF INTO A FAVORABLE
POSITION TO DRIVE AND MAINTAIN SUB 32 DEG F WETBULB TEMPERATURES
SWWD ALONG THE NC ESCARPMENT AS PCPN BECOMES HEAVIER AND WIDESPREAD
OVERNIGHT.
CONSEQUENTLY...WILL ADVERTISE EXPANDING FREEZING RAIN
AREA...EVENTUALLY MAKING IN TO THE UPPER FRENCH BROAD VALLEY AND
PERHAPS EAST ALONG I-40. NAM BUFKIT SNDGS ADVERTISE DEVELOPING
COLD NOSE AT KTNB...LEADING TO A POSSIBILITY OF SOME SLEET AS WELL.
SREF PLUME P-TYPE DIAGRAMS HAVE BEEN CONSISTENT THE PAST SVRL RUNS
IN FCSTG MEAN VALUES OF ICE ACCRETION IN THE ADVISORY LVL AT BOTH
ASHEVILLE AND HICKORY AND WARNING LVLS AT BOONE.
BASED ON THIS...WILL HOIST WINTER STORM WATCH FOR THE NORTHERN NC
MTNS/FTHLS FOR A DAMAGING ICE ACCUMULATION THURSDAY NIGHT INTO
FRIDAY MORNING. THERE WILL LIKELY BE A TIER OF WINTER WX ADVISORIES
BOUNDING THIS AREA...WHICH LATER SHIFTS MAY ISSUE AS WE GET CLOSER
TO THE EVENT.
60 KT SSE LLVL JET IS ALSO PROGGED TO TRANSLATE ACRS THE MTNS
THURSDAY NIGHT INTO FRIDAY MORNING...LEADING TO THE POTENTIAL
FOR STRONG WIND GUSTS ACRS HIGHER ELEVATIONS...PERHAPS TOPPLING
SOME TREES WITHIN THE WET SOILS.
DEEPER LIFT/MOISTURE DEPARTS BY FRIDAY AFTERNOON AND WIDESPREAD
MORNING PCPN WILL DWINDLE AS THE DAY WEARS ON. IMPINGING WEDGE FRONT
FROM THE SE LATE THU/EARLY FRIDAY ALONG WITH POSSIBLE TRIPLE POINT
LOW...DOES RAISE A SLIGHT EYEBROW TO DEEP CONVECTIVE CHANCES...BUT
WITH THE NAM/S FCST OF POSITIVE VALUES OF 0-3 KM MUCAPE CLOSER TO
THE COAST...WITH KEEP ALL RAINFALL STRATIFORM.
LOCALLY AS MUCH AS 2 INCHES OF LIQUID EQIV PCPN IS PSBL...LEADING TO
RUNOFF CONCERNS...ESPECIALLY WHERE SNOWMELT WILL AID IN FLOWS.
AT THIS POINT...GIVEN DIFFERENCES IN THE PLACEMENT OF HEAVIEST
QPF AXIS AMONG THE MODELS...AND HEAVY RAIN POTENTIAL HOLDING
OFF TO LATER THIRD PERIOD...WILL CONTINUE TO JUST ADDRESS THREAT IN
HWO. EVEN THOUGH BROAD CYCLONIC FLOW WILL LINGER ACRS THE EASTERN
CONUS FOR SATURDAY...DEEP DRYING OVERSPREADING THE REGION FRIDAY
NIGHT AND LINGERING INTO SATURDAY TO PROVIDE GENERALLY FAIR AND
SEASONABLE CONDITIONS.
&&
.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
AS OF 230 PM WEDNESDAY...THE MEDIUM RANGE LOOKS QUIET COMPARED TO
WHAT IT HAS BEEN LATELY. WHILE THERE IS SOME SIGNIFICANT DIFFERENCES
AS TO HOW THE GFS AND ECMWF ARE HANDLING THE DEEP UPPER LOW OVER THE
MIDWEST BEGINNING SATURDAY NIGHT...IN THE END IT DOES NOT HAVE TOO
MUCH IMPACT ON OUR WEATHER. EVEN IF THE MORE SRN TRACK OF THE UPPER
LOW SHOWN BY THE ECMWF COMES TO PASS...THE FLOW AT 850MB IS STILL
TOO WESTERLY TO GENERATE MUCH IN THE WAY OF UPSLOPE SNOW SHOWER
ACTIVITY...THAT IS UNTIL SUNDAY NIGHT AND MONDAY MORNING. THIS IS
GENERALLY ACCEPTED AND A SLIGHT CHANCE POP ALONG THE TN BORDER IS
APPROPRIATE FOR THIS FAR OUT IN THE FUTURE. THE MOISTURE SHOULD PULL
OUT ON MONDAY WITH HIGH PRESSURE BUILDING IN FROM THE WEST FOR THE
EARLY PART OF NEXT WEEK. TEMPS WERE NUDGED TOWARD A BLEND OF THE
MOSGUIDE AND HPCGUIDE.
BOTH MODELS SHOW THE DEVELOPMENT OF A SRN STREAM LOW PRESSURE SYSTEM
DURING THE MIDDLE PART OF NEXT WEEK...WITH THE GFS ABOUT 24 HOURS
FASTER BUT WEAKER THAN THE ECMWF. CONFIDENCE COULD NOT BE MUCH LOWER
THAN IT IS. WENT WITH HPC PRECIP CHANCES...WHICH ARE DOWN IN THE
SLIGHT CHANCE RANGE. PARTIAL THICKNESSES SUGGEST IT WILL BE A
RAIN/SNOW DEAL SO THAT IS WHAT WAS PUT IN THE GRIDS...BUT NOT TOO
MUCH STOCK SHOULD BE PUT ON THIS EVENT JUST YET BECAUSE A LOT WILL
CHANGE BETWEEN NOW AND THEN.
&&
.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
AT KCLT AND ELSEWHERE...EXPECT GRADUALLY LOWERING CIRRUS CIG
OVERNIGHT. EARLY MRNG MVFR FOG PSBL...ESP KAVL...BUT CLOUD COVER
SHOULD LIMIT RADIATIONAL COOLING. WINDS WILL BE LIGHT E TO SE THRU
EARLY THIS EVE...THEN TURNING NE AS HIGH PRESSURE RIDGES DOWN FROM
THE NE. LOW CLOUD CIGS SHOULD BEGIN TO MOVE IN FROM THE S-SW AFTER
DAYBREAK. KEPT THEM VFR UP TO 18Z FOR NOW BUT HINTED AT PSBL LOWER
CIGS WITH SCT MVFR LAYER. IN ANY CASE THEY WILL CONTINUE TO LOWER
THRU THE DAY.
OUTLOOK...A STORM WILL APPROACH FROM THE WEST LATE THURSDAY
PRODUCING WIDESPREAD MVFR/IFR RESTRICTIONS. A BRIEF PERIOD OF WINTRY
WEATHER WILL BE POSSIBLE THURSDAY NIGHT INTO FRI BETWEEN KAVL AND
KHKY...MAINLY AT THE ONSET OF PRECIP. CONDITIONS SHOULD IMPROVE FRI
NIGHT-SAT AS DRY AIR COMES IN FROM THE W.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...WIND ADVISORY FROM 6 PM THURSDAY TO NOON EST FRIDAY FOR GAZ010-
017.
NC...WINTER STORM WATCH FROM THURSDAY EVENING THROUGH FRIDAY
MORNING FOR NCZ033-049-050-501>506.
WIND ADVISORY FROM 6 PM THURSDAY TO NOON EST FRIDAY FOR NCZ051-
058-062.
SC...NONE.
&&
$$
SYNOPSIS...SBK
NEAR TERM...RB
SHORT TERM...CSH
LONG TERM...PM
AVIATION...RB
