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Friday, January 21, 2011

Potential Major Winterstorm for WNC Mountains Next Week

I've been tracking a potential storm for our area for the middle of next week, and GSP has felt it necessary to start mentioning it, so I'll pass along the information and let you know to be wary in making travel plans for next week. Some of the computer models have been spitting out some insane snowfall amounts, and I'm waiting until Sunday so we can get a better handle on it.




This one still has the opportunity to bust and not present as all snow, but I felt it necessary to share the info so far...


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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Source: GSP Office of the National Weather Service AFD


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