FXUS62 KGSP 031540
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1040 AM EST THU DEC 3 2009
.SYNOPSIS...
A DEEP LOW PRESSURE SYSTEM WILL CONTINUE TO MOVE NORTHEAST OVER NEW
ENGLAND ON THURSDAY WHILE A RIDGE OF HIGH PRESSURE BUILDS ACROSS THE
SOUTHEAST AND LINGERS THROUGH FRIDAY. A SURFACE LOW WILL THEN
DEVELOP OVER THE NORTHERN GULF OF MEXICO AND QUICKLY MOVE UP THE
SOUTHEAST COAST. BEHIND THIS SYSTEM...HIGH PRESSURE WILL RETURN OVER
THE REGION AND REMAIN IN PLACE THROUGH EARLY NEXT WEEK.
&&
.NEAR TERM /THROUGH TONIGHT/...
THE PRESSURE GRADIENT AND COLD ADVECTION ARE RELAXING ACROSS THE NRN
NC MTNS THIS MORNING...AND THE WIND ADVISORY WILL BE CANCELLED
EARLY.
WATER VAPOR IMAGERY THIS MORNING SHOWS A POSITIVELY TILTED TROUGH
MOVING INTO THE UPPER MIDWEST. THIS FEATURE IS KEEPING THE STRONG
UPPER JETLET CURRENTLY OVER THE FORECAST AREA STILL IN PLACE...WITH
THE JET AXIS LIKELY SLIPPING BACK TO THE NW TONIGHT. THIS MAY LEAD
TO A SLOW INCREASE IN HIGH LEVEL CLOUDINESS THROUGH THE PERIOD.
LOWER CLOUDS SHOULD LINGER ALONG THE TN BORDER IN THE NC MTN
COUNTIES THROUGH THE DAY...BUT OTHER LOCATIONS WILL GENERALLY
EXPERIENCE A LIGHT DOWNSLOPING FLOW. THERE IS SOME MOISTURE EVIDENT
IN MODEL SOUNDINGS AT THE TOP OF THE MIXED LAYER THIS
AFTERNOON...AND SCATTERED STRATOCUMULUS WILL LIKELY RESULT.
FORECAST TEMPS ARE IN LINE WITH OBSERVATIONS...RUNNING JUST A FEW
DEGREES COOLER THAN THE LATEST LAMP GUIDANCE. A SLIGHT UPWARD TWEAK
IN MAXES WILL BE MADE IN PIEDMONT AREAS SINCE THE HIGH LEVEL CLOUDS
HAVE NOT THICKENED APPRECIABLY YET. WILL LEAVE THE PATCHY DRIZZLE IN
THE NRN MTNS THIS EVENING...FOR NOW...GIVEN THE SPOTTY AND SHALLOW
UPSLOPE MOISTURE EXPECTED THERE.
&&
.SHORT TERM /FRIDAY THROUGH SUNDAY/...
AS OF 415 AM THURSDAY...QUIET CONDITIONS EXPECTED ON FRIDAY AS BROAD
SFC HIGH PRESSURE BUILDS OVER THE FORECAST AREA. MID LEVEL HEIGHTS
WILL YIELD MOIST SOUTHWEST FLOW...SUPPLYING THICK HIGH CLOUDS ACROSS
THE REGION. MOSTLY CLOUDY SKIES AND COOL LLVL THICKNESSES SHOULD
KEEP HIGH TEMPERATURES A FEW DEGREES BELOW NORMAL. I WILL FORECAST
MID 40S WITHIN THE MTN VALLEYS AND L50S EAST.
MID LEVEL TROF AXIS WILL SWING OVER THE MISSISSIPPI RIVER VALLEY
FRIDAY NIGHT. A VERY STRONG JET CORE...PEAK SPEED 190 KTS...WILL
LIKELY MOVE OVER NEW ENGLAND. THE POSITION OF THE JET WILL PLACE
THE CWA UNDER WIDESPREAD DIVERGENCE. THE 3/00Z GFS INDICATES THAT A
STRONG REGION OF H7 OMEGA WILL PUSH OVER THE CWA BETWEEN 6-12Z SAT.
BY SAT MORNING...FORECAST SOUNDINGS SHOW PW AMOUNTS INCREASING TO
.6-.8 OF AN INCH. EXPANSIVE SYNOPTIC LIFT AND AMPLE MOISTURE SHOULD
SUPPORT AT LEAST HIGH COVERAGE OF LOW QPF ACROSS THE FORECAST AREA.
I WILL FORECAST LIKELY POPS ACROSS THE SAVANNAH RIVER VALLEY WITH
CHCS NORTH. THE PRIMARY FORECAST CHALLENGE WILL REMAIN THE
TEMPERATURE FORECAST FRIDAY NIGHT. AT THIS POINT...I PREFER A BLEND
OF THE 2M TEMPS IN THE NAM AND GFS BLENDED WITH THE SREF. THIS
APPROACH PROVIDES SUB FREEZING TEMPS ACROSS MOST OF THE MTNS WITH
MID 30 VALLEY FLOOR TEMPS. EAST OF THE MTNS...LOW TEMPS ARE FORECAST
TO RANGE FROM THE U30S TO NEAR 40. TOP-DOWN P-TYPE METHOD INDICATES
THAT PRECIP WILL BEGIN EARLY SATURDAY AS PURE SNOW OVER THE MTN
RIDGES WITH A MIX ON SN/RA WITHIN THE VALLEYS...WITH RA ACROSS THE
FOOTHILLS AND PIEDMONT.
ON SATURDAY...ELONGATED SFC LOW PRESSURE WILL ORGANIZE OFF THE
CAROLINA COAST DURING THE DAY. BY 18Z SAT...H7 AND H5 TROF AXIS WILL
MOVE OVER EASTERN TN/NRN AL. IN ADDITION...THE CWA WILL REMAIN UNDER
A BROAD AREA OF JET DIVERGENCE. THE 3/00Z RUN OF THE GFS DEVELOPS A
BAND OF FRONTOGENESIS ACROSS THE NC FOOTHILLS AND MTNS AROUND 18Z
SAT. INTERPOLATION BETWEEN THE 12Z SAT AND 0Z SUN ECMWF INDICATES A
SIMILAR FEATURE CROSSING THE WESTERN CAROLINAS AROUND MID DAY. BOTH
THE GFS AND ECMWF GENERATE AROUND A HALF INCH ACROSS THE CWA FROM
12Z-0Z. HOWEVER...THE 3/06Z RUN OF NAM FAVOR QPF AMOUNTS NEAR 0.10
OF AN INCH. NAM QPF AMOUNTS APPEAR LIMITED BY A LONG BAND OF H850-H7
NEGATIVE EPV ALONG THE COAST...SUPPORTING A THICK BAND OF HEAVY RAIN
WEST OF THE LOW CENTER. GIVEN THE AMOUNT AND DURATION OF THE
SYNOPTIC FORCING...DEEP MOISTURE...AND PERIODS OF ELEVATED NEGATIVE
EPV...I WILL USE A BLEND OF HPC AND GFS QPF. PRECIPITATION SHOULD
RAPIDLY DECREASE WEST TO EAST ACROSS THE CWA WITH THE PASSAGE OF THE
H5 TROF AXIS BETWEEN 21Z SAT - 03Z SUN.
PARTIAL THICKNESSES AND FORECAST TEMPERATURE PROFILES SUPPORT A
MENTION OF WIDESPREAD WINTER PRECIP ACROSS THE MTNS AND ADJACENT NC
FOOTHILLS. IN FACT...IT APPEARS THAT THIS EVENT WILL FEATURE A VERY
MUTED LLVL WARM NOSE UNDER A DEEP LAYER OF HIGH MOISTURE WITHIN
OPTIMAL ICE GROWTH TEMPERATURES. CRITICAL FORECAST IMPACTS WILL REST
PRIMARILY WITH THE SFC TEMPERATURE FORECAST. I BELIEVE THAT A BLEND
OF THE 2M TEMPS FROM THE NAM AND GFS BLENDED TO SREF WILL PROVIDE A
GOOD FORECAST FOR SAT HIGH TEMPERATURES. HIGH ELEVATION TEMPERATURES
WILL REMAIN FREEZING...MID TO U30S WITHIN THE MTN VALLEYS WITH NEAR
40 ACROSS THE PIEDMONT. TOP-DOWN METHOD KEEPS PRECIPITATION AS SNOW
ABOVE 3000 FEET. SNOW WILL LIKELY BECOME MIXED WITH RA WITHIN THE
LOWER MTN VALLEYS AND ADJACENT NC FOOTHILLS. ELSEWHERE ACROSS THE
FOOTHILLS AND PIEDMONT...PRECIPITATION SHOULD FALL PRIMARILY AS RA
WITH PERIODS OF SLEET AND RA/SN MIX.
BASED ON THE P-TYPE AND QPF FORECAST...MTN RIDGES AND PEAKS ARE
FORECAST TO SEE 3-5 INCHES BY SAT EVENING. COOPERATIVE INSTITUTE FOR
PRECIPITATION SYSTEMS WINTER WEATHER ANALOG GUIDANCE INDICATES A 30
PERCENT CHC FOR GREATER THAN 2 INCHES OF SN ACROSS THE NRN NC
MTNS...BASED ON THE TOP 15 ANALOGS. LOWER MTN ELEVATIONS SHOULD SEE
SNOWFALL AMOUNTS FAVOR 1-2 INCHES...MAINLY FALLING LATE IN THE
AFTERNOON AND EARLY EVENING. LITTLE TO NO ACCUMULATION EXPECTED EAST
OF THE MTNS.
SUNDAY MORNING...THE COLDEST TEMPERATURES SO FAR THIS SEASON ARE
EXPECTED DURING THE PRE DAWN HOURS. THE COMBINATION OF THIN SKY
COVER...FRESH SNOW PACK...COLD LLVL THICKNESSES SHOULD RESULT IN MIN
TEMPS NEAR 20 DEGREES ACROSS THE MTNS WITH U20S EAST. PRECIP TIMING
AND P-TYPE FORECASTS MAY LEAD TO DAMP ROADS AND BRIDGES SAT NIGHT.
RAPIDLY FALLING TEMPS MAY FREEZE MOISTURE...LEADING TO SIGNIFICANT
AREAS OF BLACK ICE. TEMPS ARE FORECAST TO RISE INTO THE 40S ON
SUNDAY. CONSIDERABLE MELTING WILL LIKELY OCCUR THROUGH THE
DAY...RESULTING IN ANOTHER POSSIBLE BLACK ICE SITUATION MON
MORNING. DETAILS OF THE WINTER STORM AND BLACK ICE WILL BE
HIGHLIGHTED IN THE HWO.
&&
.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
AS OF 145 AM EST THURSDAY...THE MEDIUM RANGE FCST PICKS UP AT 00Z ON
MONDAY WITH A DOME OF HIGH PRESSURE SITUATED OVER THE EASTERN CONUS
AND AN UPPER LVL TROUGH DEEPENING OVER THE DAKOTAS. MODELS HAVE THIS
TROUGH MOVING WELL TO OUR NORTH EARLY IN THE PERIOD AND THEN
GENERATING A BROADER H5 TROUGH TOWARDS THE END OF THE MEDIUM RANGE.
THE LATEST RUN OF THE GFS40 GENERATES A MORE AMPLIFIED H5 TROUGH
WITH STRONGER MIDLVL FLOW OVER THE FCST AREA ON DAY 7. I CONTINUED
TO LEAN TOWARDS THE OLDER 12Z HPC GUIDANCE WHICH TAKES A BLEND OF
THE GFS AND ECMWF. THE NEWER 00Z MOS GUIDANCE HAS ESSENTIALLY THE
SAME TREND ALTHOUGH IT IS A BIT MORE OPTIMISTIC WRT POPS LATE TUES
THRU WED. IT ALSO INCREASES TEMPS A FEW DEGREES ON DAYS 6 AND 7. I
DID NOT MAKE ANY MAJOR CHANGES TO THE FCST GRIDS...BUT I DID BUMP UP
POPS LATE TUES INTO WED TO HIGH END CHANCE TO BETTER REFLECT THE
NEWER GUIDANCE. TEMPERATURES SHOULD BE NEAR MID WINTER CLIMATOLOGY
THROUGH TUESDAY WITH COOL HIGH PRESSURE TO THE NORTH. CLOUD COVER
WILL MODERATE LOW TEMPERATURES TUESDAY NIGHT. MAXIMUM TEMPERATURES
WEDNESDAY CONTINUE TO BE A CHALLENGE...AS THE TIMING OF WARM FRONTAL
PASSAGE WILL DICTATE HOW MUCH WARMING OCCURS BEFORE THE COLD FRONT
ARRIVES FROM THE WEST.
&&
.AVIATION /16Z THURSDAY THROUGH MONDAY/...
AT KCLT AND ELSEWHERE...SATELLITE IMAGERY SHOWS LOW CLOUDS
RETREATING FROM THE PERIPHERY OF THE WRN CAROLINAS LATE THIS
MORNING. DOWNSLOPE FLOW WILL CONTINUE TO KEEP CLOUDS AT BAY EAST OF
THE MTNS...WHILE MAINLY VFR CIGS REMAIN POSSIBLE AT KAVL. SCATTERED
STRATOCUMULUS COULD DEVELOP THROUGHOUT THIS AFTERNOON GIVEN SHALLOW
MOISTURE AT THE TOP OF THE MIXED LAYER. WINDS WILL GENERALLY BE
SOUTHWEST DURING THE DAYTIME HOURS...WITH GUSTS ENDING. WIND SHOULD
BECOME LIGHT N OR NE AFTER MIDNIGHT.
OUTLOOK...VFR EXPECTED FRIDAY...BUT BRIEF MVFR POSSIBLE AT KAVL
AROUND SUNRISE. ANOTHER PERIOD OF MVFR CONDITIONS POSSIBLE SATURDAY
AS AN UPPER DISTURBANCE MOVES THROUGH...WHICH COULD RESULT IN SOME
LIGHT SNOW OVER THE MTNS. HI PRES BUILDS OVER AREA SUN THROUGH TUE
WITH VFR CONDITIONS.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...NONE.
SC...NONE.
&&
$$
SYNOPSIS...JPT
NEAR TERM...HG
SHORT TERM...NED
LONG TERM...JPT
AVIATION...HG/PM
