Thunder Pig

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
406 AM EST FRI JAN 21 2011

.LONG TERM /MONDAY THROUGH THURSDAY/...
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AS OF 400 AM FRIDAY...SIGNIFICANT CHANGES HAVE BEEN MADE TO THE
MEDIUM RANGE FORECAST. THE 00Z RUNS OF THE OPERATIONAL
GFS...ECMWF..AND GEM HAVE CHANGED DRASTICALLY FROM THEIR PREVIOUS
RUNS AND NOW SHOW A LOW PRESSURE SYSTEM DEVELOPING OVER THE NE GULF
AND TRACKING UP THE SOUTHEAST COASTLINE. AMAZINGLY...THE THREE
MODELS SHOW GREAT CONSISTENCY FOR DAY 5/6 IN RAPIDLY DEEPENING THE
CYCLONE AS IT MOVES NORTHEASTWARD. PER HPC GUIDANCE A BLEND OF THE
GFS AND ECMWF IS PREFERRED...AND THE CURRENT FORECAST GENERALLY
FOLLOWS THIS BLEND FOR THE DETAILS OF THE FORECAST.

VIGOROUS SHORTWAVE ENERGY IS EXPECTED TO DIVE SOUTHWARD ACROSS THE
PLAINS ON MONDAY...AND EVENTUALLY DEVELOP A CLOSED UPPER LOW OVER
THE MS VALLEY ON TUESDAY. THE MODELS THEN TRACK THIS CLOSED LOW
FEATURE ACROSS THE DEEP SOUTH AND TN VALLEY TO THE CAROLINAS BY
WEDNESDAY MORNING. THE GFS IS FURTHER SOUTH WITH THIS FEATURE THAN
THE ECMWF THOUGH THEY ARE QUITE SIMILAR WITH THE STRENGTH OF THE MID
AND UPPER LEVEL IMPULSE. BOTH MODELS DEPICT CYCLOGENESIS BEGINNING
OVER THE NE GULF OF MEXICO ON TUESDAY...WITH THE SYSTEM UNDERGOING
RAPID DEEPENING OVER THE NEXT 24 HOURS. IN FACT THE CYCLONE WOULD
DEEPEN FROM AROUND 1012MB TUESDAY MORNING...TO AROUND 986MB
WEDNESDAY AFTERNOON AS IT MOVES INTO THE VA TIDEWATER REGION. AS IT
DOES SO...COPIOUS AMOUNTS OF DEEP LAYER MOISTURE SPREAD ACROSS THE
FA WITH PLENTIFUL FORCING FOR ASCENT. AT THIS POINT THE TIMING OF
PRECIP AND EXACT TRACK OF THE LOW ARE STILL UP IN THE AIR. OVERALL
THE GFS IS SLOWER WITH THE PROGRESSION OF THE SYSTEM...AND IS MORE
CLOSELY ALIGNED WITH THE COASTLINE FOR ITS LOW TRACK. I TEND TO
PREFER THIS COAST HUGGING TRACK COMPARED TO THE MORE INLAND TRACK OF
THE ECMWF. RIGHT NOW IT APPEARS THE TIME PERIOD OF HEAVIEST PRECIP
IS MOST LIKELY TUESDAY NIGHT THROUGH WEDNESDAY MORNING. AS SUCH THE
UPDATED FORECAST EXTENDS POPS THROUGH MUCH OF THE DAY ON WEDNESDAY
WITH THE HIGHEST POP PERIOD BETWEEN 00-12Z WEDNESDAY. AS THE LOW
DEEPENS AND PUSHES OUT TO THE NORTHEAST ON WEDNESDAY NIGHT AND
THURSDAY...UPSLOPE NW FLOW WILL OCCUR ON THE BACKSIDE LEADING TO
SNOW SHOWERS ACROSS THE MOUNTAINS.

CONCERNING PTYPE...ALL THREE MODELS SUPPORT SOME DEGREE OF FROZEN OR
FREEZING PRECIP. THE GFS AND ECMWF WOULD TEND TO FAVOR MOSTLY...IF
NOT ALL SNOW...THOUGH NOMOGRAMS SHOW AREAS OUTSIDE OF THE MOUNTAINS
BEGINNING AS LIQUID AND THEN TRANSITIONING AS COLD AIR IS USHERED IN
AROUND THE RAPIDLY DEEPENING CYCLONE. GIVEN THE SIGNIFICANT AMOUNTS
OF PRECIP GENERATED BY THE MODELS...THIS COULD BE A SITUATION WHERE
HIGH PRECIP RATES ULTIMATELY DETERMINE THE EXACT PTYPE AND ITS
PLACEMENT. AS THEY CURRENTLY SHOW...MODELS WOULD SUPPORT AREAS OF
INTENSE PRECIP WITH DYNAMIC COOLING LIKELY PRODUCING AREAS OF HEAVY
WET SNOW ACROSS THE FA. THIS FORECAST PACKAGE DOES NOT PRESENT A
RIGOROUS EXAMINATION OF PTYPES AND MAINLY SERVES TO UPDATE THE POP
TREND DURING THE PERIOD. STAY TUNED TO SUBSEQUENT FORECAST UPDATES
AS RUN TO RUN MODEL TRENDS WILL SHED MUCH MORE LIGHT IN THE COMING
DAYS.
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