Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 082009
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
309 PM EST MON FEB 8 2016

.SYNOPSIS...
A BROAD AND MOIST AREA OF LOW PRESSURE WILL SLOWLY CROSS THE REGION
THROUGH WEDNESDAY NIGHT. CANADIAN HIGH PRESSURE WILL THEN BUILD IN
THURSDAY AND REMAIN THROUGH THE WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
AS OF 1245 PM EST MONDAY...CURRENT FCST REMAINS ON TRACK THEREFORE
NO SIG CHANGES WERE MADE WITH THIS BRIEF UPDATE.  DID TWEAK TEMP/DEW
TRENDS A BIT TO BETTER ALIGN WITH OBS.  LIGHT REFLECTIVITY ACROSS
THE NC/SC PIEDMONT REGIONS LOOKS TO BE ENCOUNTERING THE DRY LOW
LEVELS AS LITTLE IF ANY PRECIP IS BEING OBSERVED AT THE SURFACE.
FULL FCST DISCUSSION TO FOLLOW WITHIN AN HOUR OR SO.

PREVIOUS DISCUSSION...WATER VAPOR IMAGERY SHOWS DEEP UPPER TROUGH
WITH EMBEDDED CYCLONE CENTERED OVER THE MIDWEST. THE TROUGH WILL
EXPAND SOUTH AND EAST THROUGH TUESDAY MORNING...BRINGING A COLD
FRONT WITH IT WHICH WILL PUSH THRU OUR CWFA TODAY. LIGHT RADAR
RETURNS ARE SEEN OVER THE OHIO VALLEY IN RESPONSE TO A CHANNELED
POCKET OF VORT AT THE LEADING EDGE OF THE TROUGH. THE VORT MAX WILL
PUSH EASTWARD ACRS THE CWFA LATER TODAY...AND STRONG QG FORCING WILL
ACCOMPANY IT THROUGH THE AREA...AS WILL PRECIPITATION. MOISTURE
DOES NOT APPEAR TO BE MUCH OF A LIMITING FACTOR EVEN EAST OF THE
MTNS. FALLING HEIGHTS WILL ALLOW LAPSE RATES TO INCREASE DURING
THE DAY AS WELL. A BULLSEYE OF CAPE VALUES UP TO NEAR 200 J/KG
IS SHOWN ON BOTH NAM AND GFS...THE GFS HAVING TRENDED TOWARD
THE NAM WHICH HAS BEEN CONSISTENTLY UNSTABLE FOR MONDAY OVER
THE PAST FEW RUNS. THIS INSTABILITY...ESPECIALLY GIVEN THE UPPER
FORCING...LENDS CONFIDENCE TO SHOWERS DEVELOPING AND SPREADING
ACROSS THE AREA TODAY. CONVECTION ALLOWING MODELS ARE SURPRISINGLY
UNRESPONSIVE...THE HI-RES WRF WINDOWS BASICALLY SUPPORTING ONLY
ISOLATED/SCHC POPS DURING THE DAY. I STILL FEEL POPS SIMILAR TO
THOSE FROM THE PREV FCST ARE WARRANTED. INCLUDED A LOW THUNDER
CHANCE OVER WRN NC BASED ON PEAK CAPES.

COLDER AIR FILTERS IN STEADILY AT 850MB THRU THE DAY. BASED ON
FAVORED BLEND /MOSTLY SREF/...TEMPS IN THE HIGHER ELEVATIONS
ARE EXPECTED TO REMAIN NEARLY STEADY...WITH VALLEYS AND THE
PIEDMONT MANAGING TO WARM ENOUGH THAT ANY PRECIP WILL INITIALLY
BE RAIN. STILL LOOKS LIKE SNOW LEVELS WILL FALL AS THE DAY
PROGRESSES. SNOW LEVELS REACH THE VALLEY FLOOR AROUND SUNSET. WITH
THE LEADING SHOT OF UPPER FORCING DEPARTING THIS EVENING...PRECIP
CHANCES TAPER OFF IN THE PIEDMONT...AND WITH TEMPS REMAINING SO WARM
THIS ROUND SHOULD END AS RAIN. HOWEVER...WHILE MODEL QPF RESPONSE
IS AGAIN QUITE LOW...A SECOND VORT LOBE ROTATING AROUND THE UPPER
CYCLONE WILL DIVE THRU THE TENN VALLEY THIS EVENING BEFORE SWINGING
ACROSS WESTERN NC IN THE WEE HOURS TUE. I FELT THIS DEPICTED WAVE
APPEARED STRONG ENOUGH TO BE ABLE TO GENERATE A FEW PIEDMONT SHOWERS
FROM THE STILL-MOIST LOW LEVELS. BY THE TIME THIS MOVES THRU...TEMPS
SHOULD BE COLD ENOUGH FOR SNOW. HOWEVER ACCUMULATION IS NOT LIKELY.

WESTERLY UPSLOPE FLOW WILL BE AN ADDITIONAL FACTOR FOR THE TENN
BORDER AREAS. THIS WILL GET UNDERWAY THIS MORNING AND REMAIN
STEADY...BUT THE LOW LEVEL WINDS VEER AND STRENGTHEN IN THE LATE
AFTERNOON AND EVENING. FURTHERMORE THAT SECOND PERIOD OF UPPER
FORCING MAY ENHANCE THE UPSLOPE-FORCED PRECIP RATES EVEN MORE. I
BLENDED IN MORE OF THE CAM/HI-RES GUIDANCE IN ORDER TO BETTER
CAPTURE THE TERRAIN INFLUENCE. NEW SNOW TOTALS MEET WARNING
CRITERIA IN THE UPSLOPE AREAS DURING THE FIRST 24 HOURS OF THE
FCST. THUS...A WINTER STORM WARNING IS BEING RAISED FOR THE TENN
BORDER ZONES. AVERY IS OMITTED SINCE THE CAM GUIDANCE PLACED MORE OF
THE SNOW WEST AND SOUTH OF THE COUNTY. ADVISORY HAS BEEN EXTENDED
THRU 12Z TUE FOR AREAS THAT WERE PREVIOUSLY IN THE ADVISORY...BUT
THAT DO NOT MEET WARNING CRITERIA. THE WARNING COULD BE EXTENDED
TO HIGHLIGHT THE CONTINUING THREAT OF NW FLOW THRU MIDWEEK.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
AS OF 225 PM MONDAY...PROTRACTED NW FLOW SNOW EVENT WILL BE
UNDERWAY AT THE BEGINNING OF THE PERIOD...CONTINUING THROUGH AT
LEAST WED NIGHT BEFORE THE MOISTURE FINALLY BECOMES TOO SHALLOW TO
SUPPORT MUCH IN THE WAY OF SHOWER GENERATION. IN THE INTERIM...THE
MOST IMPRESSIVE ASPECT OF THIS EVENT WILL BE THE THERMODYNAMIC
STRUCTURE IN THE UPSLOPE LAYER...WITH TEMPS AS COLD AS -20 AT THE
TOP OF THE MOIST LAYER THROUGH MUCH OF THE PERIOD. TEMPS THIS COLD
USUALLY SUPPORT EFFICIENT SNOWFALL PRODUCING SHOWERS...WITH OFTEN
OUTLANDISH SNOW:LIQUID RATIOS IN THE NEIGHBORHOOD OF 30:1.
HOWEVER...THE QUESTION IS WHETHER THERE WILL BE ENOUGH LIQUID
PRODUCED TO ACHIEVE WARNING/WATCH CRITERIA SNOWFALL ON APPROPRIATE
TIME AND SPATIAL SCALES.

SOME OF THE OTHER INGREDIENTS THAT ONE TYPICALLY LIKES TO SEE FOR A
HEAVY NW FLOW SNOW EVENT ARE MISSING...INCLUDING THE ORIENTATION OF
THE FLOW. THE MAGNITUDE OF THE H8 FLOW ALMOST NEVER EXCEEDS 25 KTS
THROUGH THE PERIOD...WHILE THE DIRECTION REMAINS MOSTLY WNW.
THUS...THE ACTUAL COMPONENT OF THE FLOW ORTHOGONAL TO THE SOUTHERN
APPALACHIANS IS RELATIVELY WEAK. THE OTHER CONSEQUENCE OF THE
LESS-THAN-IDEAL FLOW ORIENTATION IS THAT LOW LEVEL TRAJECTORIES DO
NOT ORIGINATE FROM THE GREAT LAKES...WHICH IS TYPICALLY NEEDED TO
PRODUCE THE TYPE OF UPSTREAM STEEP LOW LEVEL LAPSE RATES/HIGH FROUDE
NUMBERS NECESSARY TO PRODUCE A LOT OF LIQUID EQUIVALENT. INDEED...
THE SHORT TERM GUIDANCE DEPICTS THE BEST LOW LEVEL LAPSE
RATES/FROUDE NUMBERS UPSTREAM OF THE CENTRAL APPALACHIANS...WHILE
THEY ARE QUITE MODEST FROM CENTRAL KY THROUGH EAST TN. OVERALL...
THIS DOESN/T APPEAR TO BE A SETUP CONDUCIVE TO A WARNING-LEVEL
EVENT. HAVING SAID THAT...THERE ARE LOCATIONS ACROSS THE HIGH
ELEVATIONS OF THE SMOKIES AND OTHER HIGH PEAKS AND RIDGE TOPS ALONG
THE TN BORDER THAT WILL PROBABLY SEE STORM TOTALS OF MORE THAN A
FOOT THROUGHOUT THIS PROLONGED EVENT...BUT THESE AMOUNTS WOULD FALL
ON TIME/SPATIAL SCALES THAT DO NOT WARRANT A WARNING...BUT RATHER AN
ADVISORY.

OTHERWISE...IT WILL BE QUITE COLD THROUGH THE PERIOD...WITH TEMPS
AVERAGING A GOOD 15 DEGREES BELOW CLIMO. WIND CHILL VALUES WILL
REACH ADVISORY CRITERIA ACROSS THE HIGH ELEVATIONS OF WESTERN NC TUE
NIGHT...AND ACROSS ALL OF THE MOUNTAINS WED NIGHT. IN
FACT...LOCATIONS ABOVE 3500 FT MAY SEE WARNING CRITERIA WIND
CHILL OF -15 OR LESS WED NIGHT.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
AS OF 215 PM EST MONDAY...A BROAD EASTERN CONUS TROUGH WILL BE IN
PLACE AT THE START OF THE PERIOD ON FRI. GENERAL AGREEMENT REMAINS
AMONG THE MODEL SOLUTIONS IN THE LARGE SCALE PATTERN...BUT WITH
IMPORTANT DIFFERENCES IN THE SPECIFICS AND THE ASSOCIATED SENSIBLE
WEATHER. THERE IS CONFIDENCE THAT A MOSTLY CHANNELED LOBE OF
VORTICITY WILL MOVE SWIFTLY THROUGH THE BASE OF THE TROUGH OVER THE
FORECAST AREA ON FRI. HOWEVER..MOISTURE AVAILABILITY IN THE PROFILES
REMAINS UNCERTAIN...WITH THE GFS EXHIBITING LESS LOW LEVEL MOISTURE
AND SLIGHTLY WARMER PROFILES...WHILE THE ECMWF HAS SLIGHTLY DEEPER
MOISTURE...COLDER TEMPERATURES...AND BETTER UPGLIDE AHEAD OF THE
WAVEY  ENSEMBLE MEANS ARE IN BETWEEN AND A CONSENSUS APPROACH WILL
BE FOLLOWED WITH MAINLY MOUNTAIN PRECIP CHANCES...SLIGHT CHANCES OUT
OVER THE EASTERN PIEDMONT...AND SOME DEGREE OF A SHIFTING NW TO SE
RAIN/SNOW LINE FRIDAY TO FRIDAY NIGHT. ANY SNOW ACCUMULATIONS AT
THIS POINT SHOULD BE CONFINED TO THE NC MOUNTAINS.

ALL SOLUTIONS THEN EXHIBIT ROBUST NORTHWEST FLOW THAT PERSISTS ON
THE BACK SIDE OF THE FRIDAY NIGHT THROUGH SATURDAY...WITH UPSLOPE
MOISTURE STEADILY PARING BACK TOWARD THE TN LINE AND THEN THE
NORTHERN MOUNTAINS BEFORE ENDING. HEIGHTS WILL BUILD FROM THE WEST
SATURDAY NIGHT THROUGH SUNDAY...WITH LOW LEVEL MOISTURE CONTINUING
TO DRY UP ACROSS THE UPSLOPE AREAS. A LINGERING NW FLOW PRESSURE
GRADIENT COMBINED WITH VERY COLD AIR FROM THE ARCTIC HIGH SETTLING
IN COULD WELL LEAD TO WIND CHILL ISSUES SATURDAY NIGHT.

THERE IS SOME AGREEMENT AMONG THE ECM/GFS/ENSEMBLES ON ANOTHER
RAPIDLY MOVING WAVE DIVING THROUGH THE PLAINS SUNDAY NIGHT AND
REACHING THE SOUTHERN APPALACHIANS ON MONDAY. THIS COULD SET US UP
FOR YET ANOTHER ROUND OF RAIN/SNOW SHOWERS...WITH MIXED PTYPES
DEFINITELY POSSIBLE GIVEN THE RECENT ARCTIC AIR. GIVEN THE
APPEARANCE OF THE INCOMING 12Z ECMWF...STAY TUNED ON THIS ONE.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
AT KCLT AND ALL SITES OUTSIDE THE MTNS...VFR THROUGH THE PERIOD WITH
THE POSSIBLE EXCEPTION OF A BRIEF WINDOW OF MVFR VISB ASSOCIATED
WITH LIGHT PREFRONTAL SHRA.  INITIALIZED TAFS WITH VCSH AND MID
LEVEL VFR CIGS WITH A 4HR TEMPO FOR SHRA AS UPPER VORT MAX SLIDES
THROUGH THE REGION AHEAD OF THE SURFACE FRONT WHICH WILL YIELD
LIGHT PRECIP AROUND THE AREA.  CIGS SHOULD REMAIN AT LOW VFR
LEVELS THROUGH THE AFTERNOON BEFORE INCREASING TO MID/HIGH VFR
AS THE COLD FRONT SWEEPS EAST.  IN RESPONSE...WINDS WILL VEER NW
AND REMAIN ELEVATED THROUGH THE NIGHT WITH PERHAPS AN OCCASIONAL
GUST AS COLD ADVECTION PREVAILS...WITH INCREASED GUSTING LIKELY
ON TUESDAY MORNING AMIDST SOLIDLY VFR SKIES.

AT KAVL...TRENDS SIMILAR TO THAT OF THE ABOVE DISCUSSION...HOWEVER
WITH FASTER TIMING AND LOWERED FLIGHT CATEGORIES.  INITIALIZED TAF
WITH GUSTING NW WINDS AS THE FRONT HAS CLEARED THE AIRFIELD...WITH
LOW VFR STRATUS PREVAILING.  COLD ADVECTION IS UNDERWAY THEREFORE
A TEMPO FOR SHRASN CONTINUES THROUGH THE AFTERNOON AS GUIDANCE
FAVORS POSSIBLE PRECIP YIELDED FROM SECOND SHORTWAVE PASSING BY.
OTHERWISE...THINK ANY FURTHER SN ACCUMS WILL REMAIN TIED TO THE
TN LINE THUS ALLOW FOR SN MENTION TO LAPSE BY 00Z HOWEVER WITH
PERSISTENT LOW VFR CIGS AND ELEVATED NW FLOW THROUGH THE PERIOD.

OUTLOOK...SEVERAL STRONG UPPER DISTURBANCES EMBEDDED IN A DEEP
UPPER TROUGH WILL SWING ACROSS THE AREA THRU WEDNESDAY. MOISTURE
AND COLD AIR WILL CONTINUE TO FLOW INTO THE AREA DURING THIS
TIME PERIOD. RESTRICTIONS WILL BE POSSIBLE...PARTICULARLY IN THE
MOUNTAINS WHERE THERE ARE HIGH CHANCES OF ACCUMULATING SNOW.

CONFIDENCE TABLE...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     HIGH  98%     HIGH 100%
KHKY       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%

THE PERCENTAGE REFLECTS THE NUMBER OF GUIDANCE MEMBERS AGREEING
WITH THE SCHEDULE TAF ISSUANCE FLIGHT RULE CATEGORY. COMPLETE HOURLY
EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLES AND ENSEMBLE
FORECASTS ARE AVAILABLE AT THE FOLLOWING LINK:

WWW.WEATHER.GOV/GSP/AVIATION

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...WINTER WEATHER ADVISORY UNTIL 7 PM EST TUESDAY FOR GAZ010.
NC...WINTER STORM WARNING UNTIL 7 AM EST TUESDAY FOR NCZ048>052-058-
     059.
     WIND CHILL ADVISORY FROM 7 PM TUESDAY TO NOON EST WEDNESDAY
     FOR NCZ033-048>052-058-059-062>064.
     WINTER WEATHER ADVISORY UNTIL 7 PM EST TUESDAY FOR NCZ033-053-
     062-063.
SC...NONE.

&&

$$
SYNOPSIS...SBK
NEAR TERM...CDG/WIMBERLEY
SHORT TERM...JDL
LONG TERM...HG
AVIATION...CDG



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