A MAJOR HOLIDAY STORM BREWING
Posted by JAC on 12/21/2009, 10:00 am
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
210 PM EST SUN DEC 20 2009

VALID 12Z WED DEC 23 2009 - 12Z SUN DEC 27 2009

...A MAJOR HOLIDAY STORM BREWING...

LATEST MODEL/ENSEMBLE SOLUTIONS OFFER REASONABLY GOOD LARGER SCALE
AGREEMENT WITH THE FORECAST SCENARIO DURING THE MEDIUM RANGE
PERIOD...BUT CONTINUE TO OFFER MORE IMPORTANT SMALLER SCALE
SYSTEM/STREAM INTERACTION DIFFERENCES.  IN THIS PATTERN...GUIDANCE
EXPECTS FLOW WITHIN A MODERATELY AMPLIFIED PACIFIC TROF TO
SEPARATE TO SOME DEGREE LATE IN THE PERIOD AND APPROACH THE WEST
COAST DURING NEXT WEEKEND.  FARTHER EWD...A NARROW RIDGE WILL
BUILD INTO WRN NOAM FROM THE ERN PAC AND AN AMPLIFIED WEST-CENTRAL
NOAM TROF WILL PROGRESS GRADUALLY EWD WITH TIME.  THIS AMPLIFIED
TROF WILL WORK INTO A LEAD AMPLIFIED ERN/SERN US RIDGE...BUT THE
SLOW PROGRESSION AND LINGERING HIGH LATITUDE BLOCKING SHOULD ALLOW
FOR A COOLING MID-UPPER LEVEL TROF TO HOLD ON OVER THE NERN US
INTO WED/THU.  THERE ARE TYPICAL DETAIL DIFFS WITHIN THE AGREED
UPON MEAN FLOW...WITH A NEARLY EVEN BLEND OF THE 00Z ECMWF/00Z
GEFS MEAN/00Z ECMWF ENSEMBLE MEAN OFFERING A REASONABLE STARTING
POINT TO RESOLVE CURRENT FCST UNCERTAINTIES.  HOWEVER...FINAL HPC
PROGS WERE MANUALLY ADJUSTED TO ACCOUNT FOR POTENTIAL TO HOLD ONTO
A WEDGE OF COOLING LOW-LEVEL HIGHER PRESSURES MIDWEEK INTO
CHRISTMAS EVE/CHRISTMAS ACROSS THE MID-ATLANTIC AND NERN US IN
CONSIDERATION OF SHORT RANGE MODEL PREFERENCS AS PER THE PMDHMD
DISCUSSION FROM HPC.  THIS COULD SUPPORT ICING IN THE LEAD PCPN
SHIELD AHEAD OF THE MAIN UPSTREAM STORM ACROSS THE CENTRAL US.

OVER RECENT DAYS SOLUTIONS HAVE DISPLAYED SOME SPREAD AND
CONTINUITY CHANGES WITH THE DOMINANT SYSTEM FORECAST OVER THE
PLAINS AS OF DAY 3 WED...WITH THE QUESTION BEING HOW MUCH SHORT
RANGE GREENLAND/ERN CANADA RIDGING MIGHT SUPPRESS THE SYSTEM AS IT
TRACKS ACROSS THE ERN HALF OF THE CONUS.  THIS HAS HIGH POTENTIAL
TO BECOME A MAJOR HOLIDAY WINTER STORM WITH A LARGE SWATH OF
WRAP-BACK HEAVY SNOWS...WINDS...AND WIDESPREAD LEAD HEAVY PCPN AS
PER DYNAMICS ALOFT...LOW LEVEL CONVERGENCE...AND FUELING GULF
MOISTURE.  00Z MODELS/ENSEMBLES CONTINUE THEIR IMPROVED CLUSTERING
FROM THEIR 12Z RUNS IN DEPICTING SUFFICIENT PHASING OF ERN
CANADA/ERN CONUS RIDGES TO PROMOTE A PRIMARY SURFACE LOW TRACK
INTO THE GREAT LAKES WITH SOME DEGREE OF COASTAL WAVE DEVELOPMENT.
THE PAST THREE ECMWF RUNS HAVE BEEN ON THE SLOWER SIDE OF THE
SOLN ENVELOPE INCLUDING THE 00Z ECMWF ENSEMBLE MEAN...BUT
OPERATIONAL RUN CONTINUITY MERITED ABOUT 1/3 WEIGHTING WITH A
COMPROMISE OF THE 00Z GEFS MEAN AND FARTHER NWWD 00Z ECMWF MEAN.
FINAL HPC PROGS WERE THEN MANUALLY ADJUSTED TO SHOW A BIT LESS
PROGRESSIVE TRANSLATION THAN HPC PRELIM GUIDANCE IN CONSIDERATION
OF 12 UTC GUIDANCE TRENDS. IN THIS PATTERN...SOME ASPECTS OF THE
00 UTC GFS SEEM QUESTIONABLE AS THE MODEL SHOWS MORE SEWD
AMPLITUDE THAN OTHER GUIDANCE WITH THE SUPPORTING MID LVL TROF
THRU THE LATTER HALF OF THE SHORT RANGE PERIOD AND THRU WED.  THE
00Z GFS ALSO DOES NOT COMPARE WELL TO THE GUIDANCE SPREAD WITH THE
SWWD AMPLITUDE OF FLOW THAT IS FCST TO AMPLIFY INTO THE
ROCKIES/PLAINS BY FRI-SAT AND THEN CONTINUE EWD. THE 06/12 UTC GFS
RUNS DO NOT MAINTAIN GOOD CONTINUITY WITH THESE SYSTEMS IN
COMPARISON TO ITS 00 UTC RUN.

THERE IS WIDE SPREAD AMONG THE 00Z MODELS WITH DETAILS OF EVOLVING
ERN PAC SPLIT FLOW APPROACHING THE WEST COAST NEXT WEEKEND.  AT
THE VERY LEAST THE 00Z CANADIAN THAT SHOWS NEARLY ZONAL FLOW
ENTERING THE WEST COMPARES POORLY TO ENSEMBLE CONSENSUS INCLUDING
THE 12Z CANADIAN MEAN.  THE 00Z GFS IS A LITTLE SLOWER/SWD OF THE
GEFS/ECMWF ENSEMBLE MEANS WHILE THE 00Z ECMWF DOES NOT PULL THE
CORE OF ITS SYSTEM AS FAR SWWD AS THE GEFS/ECMWF MEANS.  THE
ECMWF/GEFS MEAN/ECMWF MEAN BLEND PREFERRED FOR THE CNTRL/ERN CONUS
SYSTEM PROVIDES A SUITABLE LEANING TOWARD A SOMEWHAT MORE
PRONOUNCED SRN STREAM ERN PAC TROF AS INDICATED IN MULTI-DAY
MEANS.

RAUSCH/SCHICHTEL
125
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A MAJOR HOLIDAY STORM BREWING - JAC, 12/21/2009, 10:00 am
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