FXUS62 KGSP 120617
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
117 AM EST SAT DEC 12 2009
.SYNOPSIS...
HIGH PRESSURE WILL REMAIN OVER THE REGION THROUGH TOMORROW MORNING.
AN AREA OF LOW PRESSURE WILL THEN MOVE UP THE CAROLINA COAST ON
SUNDAY. HIGH PRESSURE BRIEFLY BUILDS BACK OVER THE AREA MONDAY
BEFORE ANOTHER COLD FRONT APPROACHES THE REGION FROM THE WEST ON
TUESDAY. BROAD HIGH PRESSURE THEN BUILDS BACK IN FOR THE REMAINDER
OF NEXT WEEK.
&&
.NEAR TERM /THROUGH TODAY/...
CONFLUENT UPPER FLOW WITH 170 KT 300 MB JET OVER THE MID ATLANTIC
TONIGHT...AS SHORTWAVE ENERGY PUSHES INTO THE SOUTHWEST STATES. IR
SATELLITE IMAGERY SHOWS EXTENSIVE STREAM OF HIGH CLOUDS FROM THE
EAST PACIFIC TO THE CAROLINAS IN STRONG WSW FLOW. THE GFS AND NAM
FCST SNDGS INDICATE SOME THINNING OF CLOUDS OVERNIGHT...AND THIS IS
IN AGREEMENT WITH THE 12Z REG GEM CLOUD COVER FCST. AT 00Z...SFC
HIGH PRES WAS ANALYZED TO BE CENTERED ROUGHLY OVER THE COASTAL PLAIN
OF THE CAROLINAS...AND METARS INDICATE CALM WINDS ACRS THE ENTIRE
AREA. WITH ALL THAT SAID...MIN TEMPS MAY BE ABOUT A CATEGORY COLDER
THAN CURRENTLY FCST. HOWEVER...THE 850 MB RIDGE AXIS IS PROGGED TO
SHIFT EAST OF THE AREA OVERNIGHT...WITH SOME RETURN FLOW COMMENCING
ACRS THE FA BY DAYBREAK SATURDAY. THIS MAY KEEP TEMPS FROM TANKING
AS MUCH AS WOULD BE OTHERWISE EXPECTED. SO I WILL NOT CHANGE THE MIN
TEMPS FOR TNGT. FCST IS IN GOOD SHAPE...AND UPDATE WILL BE SENT TO
FRESHEN WORDING.
PREVIOUS DISCUSSION...
A LOW AMPLITUDE UPPER RIDGE WILL AMPLIFY OVER THE GULF STATES
TONIGHT...AND PROGRESS TO THE SOUTHERN APPALACHIANS ON SATURDAY. A
SURFACE RIDGE WILL CROSS THE MOUNTAINS TONIGHT...AND TAKE UP
RESIDENCE ALONG THE EASTERN SEABOARD SATURDAY...WITH ITS CENTER JUST
OFF THE VA COAST. CIRRUS WILL CONTINUE TO STREAM NORTHEAST ACROSS
OUR AREA...HOLDING MINIMUM TEMPERATURES TO TO WARM END OF GUIDANCE
DESPITE DRY AIR AND LIGHT WINDS...BUT STILL A CATEGORY OR TWO
BELOW NORMAL.
ISENTROPIC UPGLIDE WILL SPREAD MOISTURE OVER THE SOUTHERN PERIPHERY
OF THIS RIDGE ON SATURDAY...BUT MODEL SOUNDINGS AND TIME HEIGHTS DO
NOT MOISTEN THE LOW AND MID LEVELS UNTIL AFTERNOON. POPS WILL BE
LOWERED SATURDAY MORNING...REMOVING THE MENTION OF PRECIPITATION.
EVEN EARLY SATURDAY AFTERNOON HAS ONLY A SHALLOW LAYER OF LOW LEVEL
MOISTURE...AND POPS WILL BE LOWERED TO CHANCE IN THE SOUTH AND
SLIGHT CHANCE ALONG THE SC/NC BORDER. TEMPERATURES WILL HAVE AN
EASIER TIME WARMING IN THE NORTH THAN SOUTH...AS CLOUD COVER WILL BE
HIGHER AND THINNER THERE EARLY ON...BUT MAXIMUMS WILL STILL BE BELOW
NORMAL.
BY LATE AFTERNOON THE LOW LEVELS MOISTEN...AND MODEL GUIDANCE
SUPPORTS LIKELY POPS IN THE SOUTH...WITH CHANCE POPS BETWEEN
INTERSTATES 25 AND 40...AND SLIGHT CHANCE POPS NORTH OF INTERSTATE
40. A PRONOUNCED LOW LEVEL WARM NOSE IS APPARENT IN MODEL
SOUNDINGS...AND WITH SURFACE TEMPERATURES ABOVE FREEZING...NO
FREEZING RAIN IS EXPECTED. THE WARM NOE IS GREAT ENOUGH TO PRECLUDE
SLEET SOUTH OF HIGHWAY 74 AND WEST OF INTERSTATE 26...INCLUDING KCLT
PROPER. THE AREA FROM KAVL EAST TO KJQF AND NORTH MAY SEE SLEET
MIXED WITH RAIN...BUT THAT FAR NORTH NO SIGNIFICANT ACCUMULATION OF
PRECIPITATION IS EXPECTED.
&&
.SHORT TERM /TONIGHT THROUGH MONDAY/...
AS OF 245 PM FRIDAY...ISENTROPIC LIFT STRENGTHENS RAPIDLY SAT NITE
AS LOW AMPLITUDE SHORT WAVES MOVE INTO THE AREA. THIS COMBINED WITH
SURGING LOW LEVEL MOISTURE WILL SPREAD PRECIP ACROSS THE AREA FROM
SOUTHWEST TO NORTHEAST DURING THE EVENING. WIDESPREAD PRECIP
CONTINUES AFTER MIDNIGHT AS THE LIFT MOVES ACROSS THE AREA. THERE IS
GOOD MDL AGREEMENT ON THIS PART OF THE FCST. THERE IS ALSO GOOD
AGREEMENT THAT THE CENTER OF THE COLD DRY SFC HIGH IN PLACE OVER THE
AREA IS TRANSIENT MOVING OFF SHORE OVERNIGHT. HOWEVER...RIDGING
REMAINS IN PLACE ACROSS THE AREA AS A SFC LOW MOVES EASTWARD TO OUR
SOUTH. TEMPS SHOULD COOL INITIALLY DUE TO WET-BULB PROCESS AS THE
PRECIP MOVES IN...THEN SLOW WARMING BEGINS...ESPECIALLY OVER THE WRN
MTNS AS WARM NOSE DEVELOPS. THE NAM/SREF REMAIN WARMER THAN THE GFS
THRU THE PERIOD...BUT USING A MDL BLEND CREATES TEMPS THRU THE
COLUMN AND AT THE SFC A LITTLE WARMER THAN THE MDL BLEND FROM THU.
THIS WILL SHRINK THE AREA WHERE SIGNIFICANT ICE ACCUMULATIONS ARE
POSSIBLE. HOWEVER...IT DOES NOT LESSEN THE CHANCE THAT ADV LEVEL
ACCRETIONS WILL DEVELOP. TOP DOWN P-TYPE METHOD SHOWS SLEET WILL MIX
IN WITH THE RAIN AS IT MOVES IN...THEN RAIN AND FREEZING RAIN
DEVELOP AS THE WARM NOSE STRENGTHENS. MOST LIKELY LOCATIONS FOR ANY
WINTRY MIX WILL BE NC...WITH THE NRN MNTNS AND NRN FOOTHILLS SOUTH
TO THE BLUE RIDGE AREAS OF BUNCOMBE AND HENDERSON COUNTIES THE MOST
LIKELY LOCATIONS FOR ANY ICING OR SLEET ACCUMS. WILL HOLD OFF ON ANY
ADV FOR NOW HOPING THAT THE MIDNIGHT SHIFT WILL HAVE BETTER
AGREEMENT ON ANY LOCATIONS OR AMOUNTS.
PRECIP SLOWLY TAPERS OFF FROM WEST TO EAST DURING THE DAY SUNDAY AS
ISENTROPIC LIFT MOVES EAST. THERE COULD BE SOME LINGERING RAIN SUN
AFTERNOON AS WEAK LIFR CONTINUES OVER THE INCREASINGLY SHALLOW COLD
DOME. NE GA AND THE UPSTATE COULD END UP WITH 1 TO 1.5 INCHES OF
RAIN BY SUNDAY MORNING. THERE IS STILL MUCH UNCERTAINTY ABOUT THE
AMOUNTS...BUT A LOW END FLOOD THREAT CANNOT BE RULED OUT.
GFS IS FASTER THAN ANY MDL ON MOISTURE RETURN MONDAY AFTERNOON AHEAD
OF THE NEXT WX SYSTEM. WILL KEEP FCST DRY FOR NOW. HOWEVER...TEMPS
WILL WARM ABOVE NORMAL ON SWLY FLOW AND INCREASING THICKNESSES.
&&
.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AS OF 230 PM FRI...A POTENT NRN STREAM WAVE WILL MOVE INTO THE GREAT
LAKES MON NIGHT...WHILE A SRN STREAM WAVE TRANSITS TX. THE ECM
APPEARS TO BE AN OUTLIER IN CLOSING OFF THE SRN STREAM SYSTEM OVER
TX EARLY TUE. CONSENSUS IS TO KEEP BOTH WAVES FAIRLY
PROGRESSIVE...ALLOWING A DEEPENING TROUGH TO DEVELOP ACROSS THE ERN
CONUS TUE INTO WED. THIS SHOULD MAKE FOR A FAIRLY QUICK SHOT OF
PRECIPITATION MON NIGHT AND TUE...WITH DRYING FROM THE W TUE
EVENING. TEMPERATURE PROFILES ARE ONLY INTERESTING AT THE TAIL END
OF THE EVENT TUE EVENING...WITH SNOW LEVELS FALLING ACROSS THE MTNS.
HOWEVER...ANY NW FLOW MOISTURE WILL BE SHALLOW AND BRIEF...SO LITTLE
OR NO SNOW ACCUMULATIONS ARE PRESENTLY EXPECTED IN THE NC MTNS.
WINDS COULD BE MORE OF AN ISSUE TUE NIGHT THROUGH EARLY WED AS THE
GRADIENT INCREASES IN NW FLOW. HAVE ADDED SOME HIGH MTN GUSTS OF 30
TO 40 MPH FOR NOW. AS THE NRN STREAM WAVE DEEPENS INTO A STRONG
VORTEX OVER NOVA SCOTIA INTO THU...PERSISTANT NW FLOW WILL REMAIN
OVER THE FORECAST AREA THROUGH FRI. EMBEDDED SHORTWAVES MAY MOVE
THROUGH THE REGION...BUT MOISTURE WILL BE TOO LIMITED FOR ANY POP
MENTION. EXPECT COOL CONDITIONS THROUGHOUT WED THROUGH FRI...WITH
MOSTLY SUNNY SKIES EVERYWHERE BUT THE TN BORDER IN NW FLOW.
&&
.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
AT KCLT...WINDS WILL REMAIN LIGHT AND VARIABLE TO CALM OVERNIGHT AS
THE SFC HIGH CENTERS OVER THE REGION. BKN TO OVC UPPER LEVEL CLOUDS
WILL DROP THROUGHOUT THE DAY AND VFR FLIGHT CATS WILL GIVE WAY TO
LOWER CIGS IN THE MVFR RANGE BY 21Z AND CONTINUE DETERIORATING
THROUGH THE EVENING AND EARLY MORNING OF THE 13TH TO IFR. A VERY
DRY WEDGE IN PLACE WILL BECOME SATURATED AS MOISTURE IS LIFTED
OVERHEAD AND EVAPORATION WILL COOL SFC LAYERED TEMPS ALLOWING A
COUPLE HOURS OF -FRZA/IP MIXED WITH -RA AROUND 21Z. NO SIGNIFICANT
ACCUMULATIONS ARE EXPECTED AS THE LOWER ATMOS WILL WARM QUICKLY AFT
23Z WITH A CHANGEOVER TO ALL -RA THROUGH THE END OF THE PERIOD.
WINDS WILL REMAIN OUT OF THE NE AND GENERALLY UNDER 10 KTS...WITH
OCCASIONAL GUSTS FROM THE SAME DIRECTION BTW 15-18 KTS AFT 00Z.
ELSEWHERE...EXPECT NE/LY WINDS SHORTLY AFT SUNRISE PERSISTING AND
STRENGTHNENING A BIT THROUGH THE TAF PERIOD...EXCEPT AT KAVL WHERE
SE/LY DOWN VALLEY WINDS WILL DEVELOP. WINDS SPEEDS UNDER 10 KTS ARE
LIKELY...HOWEVER OCCASIONAL GUSTS TO NEAR 18 KTS ARE POSSIBLE AFT
THE PRECIPITATION SETS IN AROUND 21Z. THE SOUTH CAROLINA TAF SITES
WILL EXPERIENCE BRIEF PERIODS OF -FZRA/PL/RA AT THE ONSET OF
PRECIP...LASTING ONLY A COUPLE HOURS BEFORE CHANGING OVER TO ALL -RA
BY 00Z. AT KAVL AND KKHY...THE PRECIP TYPE WILL START OUT WITH A
GENERAL MIX OF -FZRA/PL/RA...BUT THEN CHANGING OVER THE -FZRA AS THE
DOMINATE PRECIP TYPE...WHICH WILL PERSIST THROUGH THE END OF THE TAF
PERIOD. SOME ACCRETION OF ICE IS EXPECTED ON THE GROUND AND ELEVATED
SURFACES. CIGS AND VSBYS WILL WORSEN AS THE EVENING PROGRESSES.
CIGS ARE EXPECTED IN THE MVFR RANGE AFTER 22Z OR SO...WITH A DROP TO
IFR EXPECTED AT ALL TAF SITES AFT 03Z. VSBYS SHOULD DROP TO MVFR
AFT 03Z...HOWEVER KAVL COULD REACH IFR VSBY AFT THAT SAME TIME.
OUTLOOK...WIDESPREAD RAIN AND LOW CLOUDS WILL LIKELY RESULT IN
FLIGHT RESTRICTIONS INTO SUNDAY MORNING. VFR CONDITIONS RETURN BY
EARLY MONDAY BEFORE ANOTHER GULF COAST SYSTEM AFFECTS THE AREA
DURING MID-WEEK.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...NONE.
SC...NONE.
&&
$$
SYNOPSIS...JPT
NEAR TERM...ARK/JAT
SHORT TERM...RWH
LONG TERM...HG
AVIATION...SBK
