Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 082025
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
325 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 /THROUGH TUESDAY/...
AS OF 245 PM EST MONDAY...A BROAD UPPER LONGWAVE TROF CONTINUES TO
HIGHLIGHT THE PATTERN ALOFT...AND ALSO DRIVE THE WX AT THE SURFACE
WHERE A COLD FRONT CONTINUES TO SLIDE OUT OF THE NC FOOTHILLS
INTO/THROUGH THE NC/SC PIEDMONT AS WELL AS NORTHEAST GA.  LIGHT
SHOWERS ASSOCIATED WITH SAID FRONT ARE HAVING A ROUGH GO AT IT WITH
SURFACE TD DEPRESSIONS UPWARDS OF 25-30 DEGREES THUS PROHIBITING
MUCH OF SAID PRECIP FROM REACHING THE SURFACE.  THAT SAID...SURFACE
TEMPS WILL BEGIN TO COOL BEHIND THE FRONT ALLOWING FOR THAT GAP
TO NARROW AND THUS MOISTENING OF THE BOUNDARY LAYER WILL OCCUR.
LIKEWISE...A SERIES OF H5 VORTICITY MAXIMAS WILL PASS THROUGH
THE TROF BASE OVER THE NEXT 4-6 HOURS...WITH THE FIRST OF WHICH
DOING SO AT PRESENT TIME.  THIS REGION OF UPPER SUPPORT CONTINUES
TO YIELD ENHANCED RA/SN SHOWERS OVER THE NC MOUNTAINS...WITH RA
SHOWERS BREAKING CONTAINMENT INTO THE FOOTHILLS AND UPPER PIEDMONT
LOCALES.  THUS...POPS THROUGH THE REMAINDER OF THE AFTERNOON WILL
REMAIN AT CHANCE LEVELS OVER THE LOWER TERRAIN...INCREASING WITH
ELEVATION TOWARDS THE TN LINE WHERE CATEROGICALS ARE FAVORED.
POPS WILL THEN TAPER EVERYWHERE OUTSIDE OF THE MTNS INTO TUESDAY
MORNING AS THE FLOW VEERS GRADUALLY AND THE FIRST OF SAID H5 VORTS
EXITS TO THE EAST.  POPS ALONG THE TN LINE WILL REMAIN ELEVATED AT
CATEGORICAL LEVELS AS FLOW BECOMES MORE ORTHOGONAL TO THE TERRAIN
THUS SETTING UP INCREASED UPSLOPE POTENTIAL...LASTING THROUGH THE
REMAINDER OF THE PERIOD.

AS FOR PRODUCTS...WILL CONTINUE TO WITH CURRENT WINTER STORM WARNING
ACROSS THE ALREADY WARNED LOCALES...EXTEND THE CURRENT ADVISORY
PRODUCT IN TIME THROUGH THE DAY TOMORROW...ALL THE WHILE EXPANDING
THE ADVISORY A BIT FURTHER SOUTH TO INCLUDE RABUN COUNTY GA GIVEN
LATEST HIGH ELEVATION SNOW TOTALS MEETING ADVISORY CRITERIA.
NEW SNOW TOTALS TOP OUT IN THE 2-4 INCH RANGE ALONG THE TN LINE
WITH A 1-3 INCHES POSSIBLE EASTWARD ALONG THE ESCARPMENT.

&&

.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 CCRITERIA 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 /21Z 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.  COMPLET 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
SHORT TERM...JDL
LONG TERM...HG
AVIATION...CDG


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