Out of Control --> Shooting Deep Shortwaves & Cold Air South
Posted by JAC on 12/17/2009, 8:53 am
http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/12/index_500_mu_loop.shtml


PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
818 AM EST THU DEC 17 2009

VALID 12Z MON DEC 21 2009 - 12Z THU DEC 24 2009

A REX-BLOCK EXTENDING FROM GREENLAND TO ONTARIO/QUEBEC AND A LARGE
AMPLIFYING UPPER TROUGH OVER THE NORTHERN PACIFIC WILL SUPPORT AN
UNUSUALLY HIGH-AMPLITUDE SPLIT-FLOW REGIME OVER THE CONUS THROUGH
THE MEDIUM RANGE PERIOD. ONE SOURCE OF SHORTWAVE TROUGH
DEVELOPMENT WILL OCCUR WITHIN AND AROUND THE LOW SEGMENT OF THE
REX-BLOCK...WITH NORTHWESTERLY FLOW ALOFT LIKELY ALLOWING THESE
SYSTEMS TO PERPETUATE COLD AND SOMEWHAT UNSETTLED CONDITIONS
ACROSS THE DAKOTAS/GREAT LAKES/OHIO VALLEY/NORTHEAST.
MEANWHILE...AN UPPER LOW NEARING THE PACIFIC NORTHWEST DAY 3
SHOULD REMAIN CUT OFF FROM THE MAIN FLOW AS IT DRIFTS OVER THE
NORTHWEST BRINGING WET CONDITIONS TO THAT REGION DAYS 3-5 BEFORE
SHIFTING EASTWARD AHEAD OF THE APPROACHING PACIFIC UPPER TROUGH.
ALTHOUGH TIMING/PLACEMENT/INTENSITY DIFFERENCES ARE QUITE
LARGE...SEVERAL DETERMINISTIC SOLUTIONS ALONG WITH MANY GEFS
MEMBERS SHOW CYCLOGENESIS COMMENCING IN THE LEE OF THE SOUTHERN
ROCKIES DAYS 5/6...ESPECIALLY AS THE UPPER TROUGH NEARS THE GULF
COAST REGION BY DAY 7...WITH ALL MODELS HINTING TOWARD ANOTHER
SIGNIFICANT WINTER STORM DEVELOPING. GENERALLY PREFER THE DETAILS
OF THE ECMWF/GFS ENS MEANS WITH THIS SYSTEM...ESPECIALLY EARLY IN
THE PERIOD ACROSS THE NORTHWEST WHERE THE GFS LIES NEAR THE FAST
EDGE OF ALL THE GUIDANCE.

FINALLY...DEEPENING LOW PRESSURE OFF THE MID-ATLANTIC COAST AT THE
START OF DAY 3 SHOULD BE ACCELERATING NORTHEASTWARD AFTER LEAVING
BEHIND A SIGNIFICANT MID ATLC REGION SNOW EVENT. INTENSIFYING LOW
MOVING ALONG THE GULFSTREAM EDGE LIKELY TO BE A SERIOUS MARINE
THREAT. SEE OPC WARNINGS.

UPDATED MORNING MEDIUM RANGE PRESSURES/FRONTS WILL BEGIN WITH THE
OPERATIONAL 00Z ECMWF DAYS 3-5 BEFORE SHIFTING TO A 70/30 BLEND OF
THE ECMWF ENSEMBLE MEAN AND GFS ENSEMBLE MEAN DAYS 6/7. AT 6-7
DAYS OUT THESE MEANS TONE DOWN THE SIGNIFICANT MS VALLEY
CYCLOGENESIS SEEN BY OP MODEL RUNS WITH LARGE DIFFERENCES LATE
PERIOD.  
JAMES/ROSENSTEIN
83
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Major Snowstorm( Saturday) Mid-Atlantic??? - Doorman, 12/15/2009, 6:59 pm
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