FXUS62 KGSP 020148
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
848 PM EST TUE DEC 1 2009
.SYNOPSIS...
HIGH PRESSURE WILL CONTINUE PUSHING OFFSHORE TODAY IN ADVANCE OF A
DEVELOPING LOW PRESSURE SYSTEM. THIS LOW WILL MOVE NORTHEAST ACROSS
THE SOUTHERN APPALACHIANS WEDNESDAY...SPREADING A CONSIDERABLE
AMOUNT OF MOISTURE OVER THE FORECAST AREA THROUGH THURSDAY. A
COOLER AREA OF HIGH PRESSURE WILL THEN BUILD ACROSS THE REGION
FRIDAY.
&&
.NEAR TERM /THROUGH WEDNESDAY/...
THE LEADING EDGE OF THE PCPN OVER GA/AL LINES UP WELL WITH THE H7
FRONTOGENESIS SEEN ON THE GFS AND NAM. USING LINEAR INTERPOLATION
WITH THE EXISTING ARE OF PCPN...AND THE MODEL FIELDS...IT APPEARS
THAT THE PCPN WILL ENTER THE SOUTHWEST PART OF THE FA A COUPLE
HOURS FASTER THAN PROGGED. THEREFORE I/VE UPPED THE POPS...WHICH
LINES UP WELL WITH FFC AND CAE. EVERYTHING ELSE LOOKS GOOD FOR THE
FIRST PERIOD RIGHT NOW AND FEW OTHER CHANGES HAVE BEEN MADE.
THE PREVIOUS DISCUSSION FOLLOWS...
COMPLICATED FCST WITH SEVERAL DIFFERENT VARIABLES DURING THIS TIME
FRAME. MDLS ARE IN GOOD AGREEMENT THAT PRECIP WILL BEGIN AFTER
MIDNIGHT TONIGHT AS SFC LOW PRESSURE MOVES TOWARD THE AREA FROM THE
GULF OF MEXICO. THIS SYSTEM IS VERY DYNAMIC WITH UPPER DIVERGENCE
INCREASING AHEAD OF SHORT WAVES EJECTING TOWARD THE AREA IN THE SWLY
FLOW AHEAD OF AN APPROACHING UPPER LEVEL LOW. THE GFS REMAINS
FASTEST WITH ITS TIMING. HOWEVER...EVEN WITH THE SLOWER NAM...PRECIP
SHOULD BE ONGOING BY 12Z ACROSS MOST OF THE AREA ALONG AND WEST OF
I-26. ALTHOUGH COOL...LOWS WILL BE ABOVE NORMAL. THAT SAID...FCST
SOUNDINGS OVER THE MTNS SUGGEST THAT A LITTLE SLEET COULD MIX IN
WITH THE RAIN AS IT BEGINS IN THE VALLEYS AND PORTIONS OF THE COLUMN
WET BULB BELOW FREEZING. THIS IS A TYPICAL SCENARIO WITH PRECIP
ONSET DURING THE WINTER AND NO ACCUMULATIONS ARE EXPECTED.
MDLS BEGIN TO DIVERGE WED...ALTHOUGH THEY ARE IN GENERAL AGREEMENT.
STRONG SYNOPTIC SCALE FORCING CONTINUES AS SHORT WAVE ENERGY AND
UPPER DIVERGENCE MOVE OVER THE AREA. ISENTROPIC LIFT INCREASES AS
WELL AS SLY LOW LEVEL FLOW INCREASES. HOWEVER...THE GFS IS MUCH
FASTER THAN ANY OTHER MDL. THE SREF AND GEM REGIONAL ARE NOT AS SLOW
AS THE NAM...BUT SUPPORT A SLOWER SOLUTION. THERE WILL ALSO BE SOME
NON-CLASSICAL DAMMING ACROSS THE AREA AS WEAK RIDGING REMAINS
ENTRENCHED ALONG THE BLUE RIDGE. THE GFS KEEPS THE COASTAL WARM
FRONT WELL TO OUR EAST AND BRINGS THE SCOURING COLD FRONT THRU IN
THE AFTERNOON. THE NAM IS MUCH SLOWER WITH THE SCOURING COLD FRONT.
AS A RESULT...IT BRINGS THE COASTAL WARM FRONT TO AROUND THE I-85
CORRIDOR AND BRINGS A TRIPLE POINT LOW ALONG IT DURING THE
AFTERNOON. THE SREF AND GEM REGIONAL ARE CLOSER TO THE NAM TIMING
AND FRONTAL PLACEMENT. HOWEVER...THEY DO KEEP THE COASTAL WARM FRONT
SOUTH IF I-85. GIVEN THE GUIDANCE CONSENSUS HAVE TRENDED AWAY FROM
THE GFS SOLUTION.
THIS BRINGS A COUPLE OF HAZARDS INTO PLAY FOR WEDNESDAY. HEAVY RAIN
FROM THE BLUE RIDGE EAST IS LIKELY WED AND WED EVENING. GOING WITH
HPC QPF GUIDANCE...WE HAVE ANYWHERE FROM 2 TO 4 INCHES ACROSS THAT
AREA. 1 TO 2 ACROSS WEST OF THE BLUE RIDGE. THIS AMOUNT OF PRECIP IN
THAT SHORT OF A TIME WILL LIKELY PRODUCE FLASH FLOODING ACROSS THE
AREA. WILL ISSUE A FLASH FLOOD WATCH FOR WED AND WED EVENING. LOW
LEVEL WINDS WILL INCREASE DRAMATICALLY WED...WITH H85 WINDS
INCREASING TO AROUND 60 KTS. WITH THE WEAK DAMMING IN PLACE...THE
MTNS WILL BE THE MOST LIKELY TO SEE ANY STRONG WINDS. EVEN WITH
VALUES THIS HIGH...WIDESPREAD DAMAGE IS RARE IN THESE SLY WIND WWA
CASES. WILL ISSUE AND ADVISORY FOR ALL THE MTNS...BUT MENTION WINDS
MOST LIKELY IN THE HIGHER ELEVATIONS. SEVERE THUNDERSTORMS ARE ALSO
POSSIBLE SOUTH OF I-85 WED AFTERNOON AND WED EVENING. WITH THE
STRONG LOW LEVEL WINDS...WARM FRONTAL BOUNDARY AND POSSIBLE TRIPLE
POINT LOW...WIND SHEAR WILL BE VERY HIGH. SOUTH OF THE WARM FRONTAL
BOUNDARY...DEW POINTS WILL BE HIGH FOR THIS TIME OF YEAR. THIS WILL
LEAD TO CAPE VALUES OF A COUPLE OF HUNDRED J/KG. THIS COULD BE ENUF
INSTABILITY TO PRODUCE THUNDERSTORMS WHICH COULD THEN TAP THAT WIND
SHEAR. SEVERE POTENTIAL WILL BE HIGHEST ALONG AND SOUTH OF THE WARM
FRONT...AND THE EXACT LOCATION OF THIS BOUNDARY IS STILL IN QUESTION.
&&
.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
AS OF 230 PM EST TUESDAY...A WEAKENING UPPER LOW WILL BE LOCATED
OVER EAST TN WEDNESDAY EVENING...LIFTING NORTHEAST BETWEEN A MEAN
TROUGH OVER THE CENTER OF THE CONTINENT AND A RIDGE OVER THE WESTERN
ATLANTIC. BY THURSDAY MORNING THE REMNANTS OF THIS LOW ARE
DOWNSTREAM OVER NEW ENGLAND. THE SOUTHERN APPALACHIANS REMAIN UNDER
SOUTHWEST FLOW ALOFT THROUGH FRIDAY AS A STRONG UPPER LEVEL SYSTEM
DIVES INTO THE BASE OF THE MEAN TROUGH.
ON WEDNESDAY EVENING A DRY SLOT WILL NORTHEAST FROM GA INTO THE
CAROLINAS BEHIND A DEPARTING COLD FRONT. THE NAM IS SLOWER TO DRY
THINGS OUT...AND THIS MORE CONSERVATIVE APPROACH WAS USED IN THE
FORECAST. MEANWHILE...MOIST NORTHWEST FLOW WILL BRING SCATTERED
SHOWERS TO THE NC MOUNTAINS BORDERING TN. ALTHOUGH MODEL 850 MB
TEMPERATURES DIP JUST BELOW ZERO...MODEL SOUNDINGS IN THAT AREA ARE
ABOVE FREEZING AT THE SURFACE...AND LOW LEVEL MOISTURE DOES NOT
EXTEND HIGH ENOUGH TO BE AT ICE NUCLEATION TEMPERATURES. FOR THESE
REASONS...SNOW HAS BEEN REMOVED FORM THE FORECAST FOR WEDNESDAY
NIGHT AND THURSDAY. THE LOW LEVEL WINDS ARE REALLY ONLY ORTHOGONAL
TO THE RIDGES AFTER 06Z THURSDAY...AND THEN WEAKEN AFTER 18Z
THURSDAY...LIMITING THE WINDOW OF BETTER SHOWER ACTIVITY. BY
THURSDAY NIGHT LOW LEVEL FLOW BECOMES PARALLEL TO THE RIDGES...AND
UPSLOPE FLOW SHOWERS SHOULD ABATE.
TEMPERATURES WILL BE MILD WEDNESDAY NIGHT BEFORE COLD ADVECTION SETS
IN...AND NEAR NORMAL THURSDAY AND THURSDAY NIGHT...AS COLD ADVECTION
IS OFFSET BY DOWNSLOPE FLOW AND SLIGHTLY RISING HEIGHTS AS THE
TROUGH DEEPENS TO THE WEST. TEMPERATURES FALL BELOW NORMAL ON FRIDAY
AS HEIGHTS BEGIN TO FALL AND DOWNSLOPE FLOW ENDS.
&&
.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
AS OF 145 PM TUESDAY...AT 00 UTC SATURDAY...THE OP NAM/GFS AND SREF
ALL DEPICT PCPN SHIELD BLOSSOMING FROM THE NW GULF OF MEXICO WITHIN
THE FAST FLOW ALONG FRONT SIDE OF L/WV AXIS. PERFECT PROGGING THESE
SOLUTIONS WOULD SUPPORT PCPN STREAKING DOWNSTREAM INTO SATURDAY
AHEAD OF LIFTING TROUGH AXIS. THERE IS NO SUPPORT FOR THIS SOLUTION
FROM THE 12 UTC GGEM...WHOSE QPF IS RESTRICTED TO THE LLVL BAROCLINIC
ZONE WELL TO OUR SOUTH AND SE. WILL AWAIT THE 12 UTC ECMWF BEFORE
COMMITTING TO A NEW POP...BUT INITIAL THOUGHTS ARE TO AT LEAST NUDGE
POP UPWARD FRI NITE AND SATURDAY...AND WILL PLAN ON USING GFS/ECMWF
THICKNESS VALUES TO ACHIEVE THE DETAILS OF THE FCST P-TYPE...LIKELY
RESULTING IN A PERIOD OF MTN SNOW AND A RAIN/SNOW MIX ELSEWHERE.
EVEN THE SLOWER 00 UTC ECMWF SOLUTION IS PROGGING A DRY FCST FOR
SUNDAY AND WILL CONTINUE TO FAVOR THIS...REMAINING CONSISTENT WITH
THE GOING FCST. WITH THE FAST...QUASI-ZONAL FLOW DEVELOPING FOR
EARLY NEXT WEEK...CONFIDENCE IN ANY SPECIFIC SOLUTION IS LOW...AS
SUBTLE IMPULSES WILL HAVE THE ABILITY TO PRODUCE A PERIOD OR TWO OF
UNSETTLED WX. FOR THE TIME BEING...WILL PLAY PERSISTENCE...KEEPING
THE MONDAY FCST DRY AND COOL...WITH NOTHING MORE THAN A TOKEN SLIGHT
CHANCE ON TUESDAY.
&&
.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
AT KCLT AND ELSEWHERE...LOW PRESSURE DEVELOPING ALONG THE GULF COAST
WILL SPREAD RAIN ACROSS THE REGION LATE TONIGHT AND EARLY WED. AS
THE RAIN SETS IN...CIGS WILL FALL INTO THE IFR RANGE BETWEEN 12 AND
14 UTC ON WED. COLD AIR DAMMING WILL HELP HOLD THE CIGS AT THIS
LEVEL FOR SEVERAL HOURS...BEFORE THE WINDS VEER TO THE SOUTH...JUST
AFTER THE VALID PERIOD OF THE TAFS. THE RAIN WILL FALL HEAVILY AT
TIMES LATE TONIGHT AND WED. A NARROW LINE OF TSTMS MAY MOVE ACROSS
THE REGION LATE TOMORROW AFTERNOON OR DURING THE EVENING...THOUGH I
HAVEN/T ADDED ANY THUNDER TO THE TAFS AT THIS TIME.
WINDS WILL GENERALLY BE OUT OF THE NE TONIGHT. AS THE LOW APPROACHES
THE WINDS WILL BECOME SUSTAINED OVER 10 KTS AT MOST SITES AND GUSTY
AT TIMES. LOW LEVEL WIND SHEAR IS LIKELY AT MOST SITES STARTING
AROUND SUNRISE TOMORROW AS THE WINDS AROUND 2KFT INCREASE TO 45 TO
50 KTS FROM THE SE. SITES WHERE THE COLD AIR DAMMING KEEPS THE SFC
WINDS OUT OF THE NE WILL SEE THE MOST SIGNIFICANT WIND
SHEAR...THOUGH THE STRONG WINDS BLOWING ACROSS THE FRENCH BROAD
VALLEY WILL CAUSE PROBLEMS AT KAVL AS WELL.
OUTLOOK...CONDITIONS SHOULD BEGIN TO IMPROVE WEDNESDAY NIGHT AS
WINDS TURN OUT OF THE SOUTHWEST AND EVENTUALLY THE WEST...SCOURING
OUT THE LOW CLOUDS. VFR EXPECTED FRIDAY. ANOTHER PERIOD OF MVFR
CONDITIONS IS POSSIBLE ON SATURDAY.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...FLASH FLOOD WATCH FROM 6 AM EST WEDNESDAY THROUGH WEDNESDAY
EVENING FOR GAZ010-017-018-026-028-029.
WIND ADVISORY FROM 6 AM WEDNESDAY TO MIDNIGHT EST WEDNESDAY
NIGHT FOR GAZ010-017.
NC...FLASH FLOOD WATCH FROM 6 AM EST WEDNESDAY THROUGH WEDNESDAY
EVENING FOR NCZ035>037-056-057-062>065-068>072-082-
501>510.
WIND ADVISORY FROM 6 AM WEDNESDAY TO MIDNIGHT EST WEDNESDAY
NIGHT FOR NCZ033-048>053-058-059-062>065-501-503-505-507-
509.
SC...FLASH FLOOD WATCH FROM 6 AM EST WEDNESDAY THROUGH WEDNESDAY
EVENING FOR SCZ001>014-019.
WIND ADVISORY FROM 6 AM WEDNESDAY TO MIDNIGHT EST WEDNESDAY
NIGHT FOR SCZ001>003.
&&
$$
SYNOPSIS...SBK
NEAR TERM...MCAVOY/RWH
SHORT TERM...JAT
LONG TERM...CSH
AVIATION...MCAVOY
