FXUS62 KGSP 031744
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1244 PM 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 FRIDAY/...
WATER VAPOR IMAGERY PLACES AN UPPER VORTEX OVER NRN IA EARLY THIS
AFTERNOON. THE TRAILING EDGE OF A STRONG UPPER JET STREAK IS
INDICATED OVER THE WESTERN CAROLINAS. ALTHOUGH IR IMAGERY DOES NOT
FEATURE MUCH CIRRUS DEVELOPMENT YET...THE UPPER JET AXIS IS EXPECTED
TO MIGRATE BACK TO THE NW AS THE TROUGH DEEPENS TO THE WEST. THIS
WILL COMBINE WITH IMPROVING UPPER DIVERGENCE THROUGH THE NEAR TERM
PERIOD TO ALLOW HIGH CLOUDS TO STEADILY LOWER AND THICKEN THROUGH
FRIDAY. BRIEF ISENTROPIC LIFT MAY DEVELOP DURING THE MORNING HOURS
FRIDAY...BUT THIS SHOULD BE FAIRLY TRANSIENT AS 850 MB FLOW VEERS
AND WEAKENS BEFORE BACKING SHARPLY LATE IN THE DAY. WITH LOW LEVELS
QUITE DRY...NO PRECIPITATION IS ANTICIPATED THROUGH THE PERIOD.
TEMPS WILL RANGE FROM THE UPPER 20S TO LOWER 30S IN THE MTNS
TONIGHT...WITH MINS IN THE MID TO UPPER 30S ELSEWHERE. 40S MTNS AND
50S PIEDMONT ARE ANTICIPATED FOR MAXES FRIDAY...EXCEPT FOR CHILLY
30S ALONG THE HIGHER RIDGE TOPS.
&&
.SHORT TERM /FRIDAY NIGHT 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 /18Z THURSDAY THROUGH TUESDAY/...
AT KCLT...A NARROW ZONE OF MOISTURE NEAR THE TOP OF THE MIXED LAYER
WILL PRODUCE SCATTERED STRATOCUMULUS AROUND 3500 FEET THIS AFTN. A
VFR CIG MAY BE BRIEFLY POSSIBLE...BUT COVERAGE SHOULD BE MAINLY
SCATTERED. HIGH LEVEL CLOUDS WILL ALSO SLOWLY THICKEN BACK UP OVER
THE TERMINAL FORECAST AREA. WINDS HAVE ADJUSTED BACK TO THE WNW A
LITTLE QUICKER THAN EXPECTED TODAY AS LEE TROUGHING HAS BEEN WEAK.
FLOW WILL TURN NE OVERNIGHT. MID AND HIGH LEVEL VFR CIGS WILL
THICKEN UP THROUGH THE END OF THE TAF PERIOD.
ELSEWHERE...MAINLY SCATTERED CLOUDS TEETERING ON MVFR LEVEL WILL BE
IN THE VICNITY OF KAVL AS MOIST NW FLOW PERSISTS IN THE FRENCH BROAD
VALLEY. OTHERWISE...SHALLOW MOISTURE AT 3500 TO 4000 FT NEAR THE TOP
OF THE MIXED LAYER WILL LEAD TO FEW TO SCT VFR STRATOCUMULUS. WINDS
WILL BE GENERALLY NW...TURNING NE OVERNIGHT. THE ONLY EXCEPTION WILL
BE AT KAND WHERE BETTER LEE TROUGHING WILL KEEP A WSW DIRECTION
THROUGH THE AFTERNOON HOURS. MID AND HIGH CLOUDS WILL LOWER AND
THICKEN FROM THE TOP DOWN ON FRIDAY WITH GENERALLY LIGHT NE FLOW.
OUTLOOK...MOISTURE AND LIFT WILL INCREASE RAPIDLY FRIDAY NIGHT AND
SATURDAY WITH RESTRICTIONS LIKELY DEVELOPING. SOME SNOW COULD
DEVELOP AND AFFECT KAVL EARLY SATURDAY...WITH LIQUID PRECIPITATION
GENERALLY EXPECTED AT THE OTHER TAF SITES. CONDITIONS SHOULD IMPROVE
QUICKLY SAT NIGHT INTO SUN AS DRY HIGH PRESSURE BUILD IN. VARIABLE
BUT GENERALLY VFR CONDITIONS ARE LIKELY MONDAY THROUGH TUESDAY.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...NONE.
SC...NONE.
&&
$$
SYNOPSIS...JPT
NEAR TERM...HG
SHORT TERM...NED
LONG TERM...JPT
AVIATION...HG
