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Area Forecast Discussion

228
FXUS62 KGSP 261800
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
100 PM EST MON JAN 26 2015

.SYNOPSIS...
LOW PRESSURE WILL WEAKEN AS IT MOVES INTO THE CENTRAL APPALACHIANS
THIS MORNING...WHILE ANOTHER AREA OF LOW PRESSURE WILL DEVELOP AND
QUICKLY INTENSIFY OFF THE MID-ATLANTIC COAST LATER TODAY. DRY HIGH
PRESSURE BUILDS INTO OUR AREA FROM THE WEST THROUGH WEDNESDAY.
A COLD FRONT IS EXPECTED TO CROSS THE REGION LATE THURSDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 1245 PM EST MONDAY...CURRENT FCST REMAINS ON TRACK THEREFORE
NO MAJOR CHANGES NEEDED WITH THIS UPDATE.  DID TWEAK TEMPERATURE AND
DEWPOINT TRENDS ACROSS THE REGION TO REFLECT LATEST OBSERVATIONS.
LATEST REPORTS ALONG WITH WEBCAM DEPICTIONS ACROSS THE REGION REMAIN
CONSISTENT WITH CURRENT ADVISORY PRODUCT AS THE BULK OF ANY SNOW
ACCUMULATION REMAINS ALONG THE HIGHER PEAKS AND RIDGETOPS ABOVE
3500FT.  WILL CONTINUE TO MONITOR CONDITIONS ALONG WITH ANALYSIS OF
LATEST GUIDANCE AS IT PERTAINS TO EXTENSION/EXPANSION OF CURRENT
ADVISORY PRODUCT INTO THE EVENING.

PREVIOUS DISCUSSION...

AT 330 AM EST MONDAY...AND EAST COAST TROUGH WILL SLOWLY PROGRESS
OVER THE EASTERN SEABOARD TODAY AND TONIGHT. AN UPPER LOW WILL MOVE
ACROSS THE SOUTHERN APPALACHIANS TODAY. AN ASSOCIATED SHORTWAVE WILL
CROSS THE WESTERN CAROLINAS AND NE GA THIS MORNING. THE SHORTWAVE
WILL BE FOLLOWED BY SEVERAL SLUGS OF CHANNELED VORTICITY...WITH A
NOTABLE ONE CROSSING THE AREA THIS EVENING...AND ANOTHER EQUALLY
IMPRESSIVE ON ARRIVING TOWARD DAWN.

A COLD FRONT WILL CROSS OUT AREA EARLY TODAY...PRECEDED AND
ACCOMPANIED BY LIGHT PRECIPITATION. THE BEST PRECIPITATION COVERAGE
AND INTENSITY IS EXPECTED TO BE IN NC...CLOSER TO THE TRACK OF THE
UPPER LOW. SNOW LEVELS ARE EXPECTED TO LOWER IN THE NC MOUNTAINS
TOWARD DAYBREAK...REMAINING RATHER LOW AS COLD ADVECTION BEHIND THE
FRONT SETS UP. SOME VALLEY FLOORS WILL REMAIN BELOW SNOW LEVELS.
SNOW MAY EVEN MIX IN WITH RAIN IN A FEW PORTIONS OF THE NORTHERN NC
FOOTHILLS AND PIEDMONT THIS MORNING...BUT THIS SNOW SHOULD BE
BRIEF...AND NOT ACCUMULATE. BY MIDDAY SNOW TOTALS IN THE GREAT SMOKY
MOUNTAINS COULD EXCEED THREE INCHES. THE CURRENT WINTER WEATHER
ADVISORY WILL BE LEFT IN PLACE. HIGH TEMPERATURES TODAY WILL RUN
NEAR NORMAL IN THE LEE OF THE MOUNTAINS...AD AROUND FIVE DEGREES
BELOW NORMAL IN THE MOUNTAINS.

BY MIDDAY LOW LEVEL WINDS VEER TO NW BEHIND THE FRONT AND SET UP A
TRUE NW FLOW SNOW EVENT IN THE NC MOUNTAINS. SNOW LEVEL WILL LOWER TO
VALLEY FLOORS TONIGHT...BUT THE SNOW ZONE WILL RETREAT CLOSER TO THE
TN BORDER. WITH THE UPPER LOW DEPARTING...AND MOISTURE OVERNIGHT
RATHER SHALLOW...ADDITIONAL ACCUMULATIONS ARE EXPECTED REMAIN UNDER
TWO INCHES ALONG THE TN BORDER. LOW TEMPERATURES TONIGHT WILL RUN A
FEW DEGREES BELOW NORMAL AS POST FRONTAL WINDS KEEP THE LOWER
ATMOSPHERE MIXED...PARTIALLY OFFSETTING COLD ADVECTION.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
AS OF 300 AM MONDAY...NORTHWEST FLOW SNOW POTENTIAL WILL LINGER INTO
TUESDAY. THE HIGHEST POPS (SOLID CHANCE) WILL RESIDE NEAR THE
SMOKIES...AS SHORT TERM GUIDANCE DEPICTS ONLY VERY SHALLOW UPSLOPE
MOISTURE FROM ROUGHLY THE FRENCH BROAD VALLEY NORTHEAST. ADDITIONAL
ACCUMS OF AROUND AN INCH ARE POSSIBLE...MAINLY ACROSS THE HIGHER
ELEVATIONS...BEFORE THE SNOW TAPERS OFF WITH DEPLETING MOISTURE BY
TUE EVENING.

AFTER A CHILLER-THAN-NORMAL TUE...THE REMAINDER OF THE SHORT TERM
PERIOD WILL BE DRY WITH NEAR CLIMO TEMPS...AS A SHIRT WAVE RIDGE
PROGRESSES THROUGH AND FLATTENS THE EASTERN LONG WAVE TROUGH.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
AS OF 310 AM MONDAY...A CLIPPER (OR DUO OF CLIPPERS) WILL MOVE
RAPIDLY FROM THE UPPER MISS VALLEY TO THE MID-ATLANTIC IN THE EARLY
PART OF THE MEDIUM RANGE. ALTHOUGH BOTH THE GFS AND THE ECMWF
FEATURE TWO WAVES...THE EC SHEARS THE FIRST ONE OUT...WHILE THE GFS
MAINTAINS A RATHER STRONG LEAD WAVE. THIS IS PROBABLY WHY IT SPREADS
LIGHT PRECIP INTO THE FORECAST AREA SEVERAL HOURS EARLIER THAN THE
ECMWF. GRADUALLY INCREASING POPS WILL BE ADVERTISED THROUGHOUT
THURSDAY...PEAKING AT 20-40 PERCENT ACROSS THE NORTHWEST HALF OF THE
AREA BY THE END OF THE DAY. WITH THIS TIMING...THE PRIMARY P-TYPE
WOULD BE RAIN...ALTHOUGH SOME SNOW WILL BE POSSIBLE AT THE HIGH
ELEVATIONS. AS COLD ADVECTION OVERSPREADS THE AREA...A BRIEF PERIOD
OF SNOW COULD FALL AT THE VALLEY FLOORS THU NIGHT BEFORE THE PRECIP
COMES TO AN END (AS MORE OF A TYPICAL NW FLOW EVENT).

FRIDAY AND FRIDAY NIGHT SHOULD BE DRY AND COOL AS THE EASTERN TROUGH
REASSERTS ITSELF. HOWEVER...THE BOTTOM DROPS OUT OF FORECAST
CONFIDENCE OVER THE WEEKEND...AS THERE ARE SIGNIFICANT DISCREPANCIES
AMONG THE GLOBAL MODELS REGARDING THE TIMING OF NORTHERN STREAM
SHORT WAVE FEATURES...AND THE EVOLUTION OF A TROUGH/UPPER LOW ACROSS
THE FOUR CORNERS REGION. THE RESULT IS A DIFFERENCE IN TIMING OF
AROUND 36 HOURS IN TERMS OF PRECIP CHANCES ASSOCIATED WITH
DEVELOPING BAROCLINIC ZONE ACROSS THE SOUTHEAST STATES. TOKEN SLIGHT
CHANCE POPS WERE INHERITED FROM SAT NIGHT THROUGH SUNDAY...MAINLY
ACROSS THE WESTERN HALF OF THE AREA...AND CONSIDERING THE HIGH LEVEL
OF UNCERTAINTY...OPTED TO LEAVE THIS ALONE FOR NOW.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
AT KCLT...VFR CONDITIONS WILL PREVAIL THROUGH THE PERIOD.  NORTHWEST
WINDS WILL INCREASE THIS AFTERNOON WITH GUSTS SUBSIDING INTO THE
EARLY EVENING AS THE PRESSURE GRADIENT AND MIXING RELAX.  LOW VFR
CIGS WILL PERSIST THROUGH MIDDAY BEFORE INCREASING SLIGHTLY INTO THE
6-8KFT RANGE AROUND 21Z.  MID LEVEL SCT DECK IS FCST THROUGH MUCH OF
THE OVERNIGHT AHEAD OF AN APPROACHING SHORTWAVE IMPULSE AND ITS
ASSOCIATED MOISTURE.  THUS...THE MID LEVEL SCT DECK IS PROGGED TO
THICKEN WITH CIGS IN THE 10KFT RANGE POSSIBLE AROUND DAYBREAK
TUESDAY LASTING THROUGH MID MORNING BEFORE SCT OUT ONCE AGAIN.
WINDS WILL REMAIN NORTHERLY THROUGH THE REMAINDER OF THE PERIOD WITH
SPEEDS GENERALLY LESS THAN 5-7KTS.

ELSEWHERE...VFR CIGS AND GUSTY WINDS WILL HIGHLIGHT THIS TAF
CYCLE...ESPECIALLY AT KAVL WHERE VALLEY CHANNELING COULD YIELD GUSTS
IN EXCESS OF 25KTS.  ALSO AT KAVL...LOW VFR...AND POSSIBLE HIGH
LEVEL MVFR CIGS WILL PREVAIL THROUGH MUCH OF THE TAF CYCLE DUE TO
UPSTREAM NORTHWEST FLOW SNOW SHOWERS ALONG THE TN LINE.  THUS...TAF
FEATURES LOW VFR DETERIORATING TO MVFR JUST AFTER MIDNIGHT.  WOULD
NOTE THAT CONFIDENCE IN THE MVFR CIGS REMAINS RATHER LOW.
OTHERWISE...GUSTY WEST TO NORTHWEST WINDS WILL PREVAIL THROUGH
SUNSET AT ALL OTHER TAF SITES BENEATH SCT MID LEVEL CIGS.  A SECOND
ROUND OF MID LEVEL CIG FORMATION IS PROGGED AS UPSTREAM SHORTWAVE
IMPULSE AND ASSOCIATED CHANNELED VORTICITY SLIDE INTO THE REGION
JUST BEFORE DAYBREAK.  THUS TAFS FEATURE BKN CIGS AROUND 8-10KFT
BEFORE SCT AFTER SUNRISE.

OUTLOOK...A SERIES OF CLIPPER SYSTEMS WILL BRING PERIODIC CHANCES
FOR SNOW SHOWERS...MAINLY TO THE MOUNTAINS THROUGH THE WEEK. MOST OF
THESE SNOW SHOWERS WILL REMAIN WEST AND NORTH OF KAVL...BUT BRIEF
PERIODS OF RESTRICTIONS AND SNOW SHOWERS CANNOT BE RULED OUT THERE.
OTHERWISE...MAINLY VFR CONDITIONS ARE EXPECTED.

CONFIDENCE TABLE...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     HIGH 100%     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 SCHEDULED TAF ISSUANCE FLIGHT RULE CATEGORY. COMPLETE HOURLY
EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLES AND ENSEMBLE FORECASTS
ARE AVAILABLE AT THE FOLLOWING LINK: (MUST BE LOWER CASE)

WWW.WEATHER.GOV/GSP/AVIATION_TABLES

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...WINTER WEATHER ADVISORY UNTIL 4 PM EST THIS AFTERNOON FOR
     NCZ051-052-058-059.
SC...NONE.

&&

$$
SYNOPSIS...JDL
NEAR TERM...CDG/JAT
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...CDG



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