Preliminary Forecasts
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FXUS02 KWNH 020643
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
142 AM EST FRI DEC 02 2016

VALID 12Z MON DEC 05 2016 - 12Z FRI DEC 09 2016

...OVERVIEW...

MULTI-DAY MEANS SHOW A POSITIVELY TILTED LONGWAVE TROUGH OVER THE
LOWER 48 WITH AMPLIFYING WRN TROUGHING THAT MOVES EWD WITH TIME
BEING THE PRIMARY CONTRIBUTOR.  PACIFIC ENERGY COMING INTO THE
WEST BY DAY 7 FRI WILL LIKELY FEED INTO THE MEAN TROUGH JUST
BEYOND THE END OF THE FCST PERIOD.  AHEAD OF THE DEEP TROUGH
CROSSING THE CONUS WILL BE A SOMEWHAT COMPLEX EVOLUTION OF SFC LOW
PRESSURE AND A TRAILING STRONG COLD FRONT BRINGING THE COLDEST AIR
OF THE SEASON.  FARTHER TO THE E AN UPR LOW THAT BEGINS EJECTING
FROM NRN MEXICO LATE IN THE SHORT RANGE PERIOD.


...MODEL EVALUATION/PREFERENCES...

THE DAYS 3-4 MON-TUE FCST LEANS ABOUT 2/3 TOWARD THE 18Z GFS/GEFS
MEAN VS THE OPERATIONAL 12Z ECMWF.  THIS WEIGHTING IS PRIMARILY
DUE TO PREFS FOR THE EJECTING NRN MEXICO UPR LOW.  THE ECMWF MEAN
STILL SEEMS TOO EAGER/FAST TO PHASE THE FEATURE WITH NRN STREAM
FLOW AND IS THUS NOT INCLUDED IN THE FCST.  AMONG REMAINING SOLNS
THERE IS STILL SOME TIMING SPREAD WITH RECENT HISTORY SUGGESTING A
TILT TOWARD BUT NOT COMPLETELY TO THE SLOWER GFS/GEFS VS FASTER
ECMWF.  THE NEW 00Z UKMET/CMC FALL BTWN RECENT GFS RUNS AND THE
12Z ECMWF THUS CONFIRMING MAINTENANCE OF A COMPROMISE APPROACH
ALOFT.  MOST RECENT GUIDANCE HAS TRENDED TOWARD A WELL DEFINED
SPLIT OF SFC FEATURES TUE-WED... A PRIMARY SYSTEM THAT TRACKS INTO
THE OH VLY/GRTLKS TUE-WED AND A LEADING WRN ATLC WAVE.  A SOMEWHAT
MORE EVEN BLEND MAY BE MORE DESIRABLE OVER THE NRN TIER/SRN CANADA
WHERE SOME GFS RUNS HAVE BEEN A LITTLE DEEPER WITH HGT FALLS ALOFT
AND FARTHER NWD WITH SFC LOW PRES.  THE 00Z GFS APPEARS TO HAVE
TONED DOWN THAT ASPECT OF THE FCST RELATIVE TO PRIOR RUNS.

THEN BY DAYS 5-7 WED-FRI THE FCST SHIFTS WEIGHTING ABOUT 2/3
TOWARD THE 12Z ECMWF MEAN AND TO A SMALL DEGREE 00Z-12Z/DEC 1
ECMWF RUNS VERSUS THE 18Z GEFS MEAN.  THE MOST PROMINENT DIFFS
INVOLVE TIMING OF WRN TROUGH ENERGY AS IT ROUNDS THE TROUGH AND
HEADS INTO THE EAST.  GFS RUNS SINCE ABOUT 24 HRS AGO HAVE STRAYED
TO THE FAST SIDE OF THE SPREAD.  THE 18Z AND NEW 00Z RUNS SHOW A
MODESTLY SLOWER TREND FROM THE 12Z RUN THOUGH.  LATEST GEFS MEANS
ARE NOT QUITE AS EXTREME AS THE OPERATIONAL RUNS.  RECENT 12Z AND
NEW 00Z CMC RUNS ALONG WITH THE 12Z CMC MEAN FAVOR A SLOWER
PROGRESSION MORE SIMILAR TO THE ECMWF MEAN.  ECMWF RUNS HAVE BEEN
A BIT ON THE SLOWER SIDE OF THE SPREAD THUS FAVORING NO MORE THAN
20-30 PCT INCLUSION.  IN VARYING WAYS THE MODELS CONTINUE TO
SIGNAL CONSOLIDATION AND PSBLY SIGNIFICANT DEEPENING OF SFC LOW
PRES SOMEWHERE BTWN THE GRTLKS AND NEW ENGLAND MID-LATE WEEK.

FOR THE MOST PART DIFFS ALONG THE WEST COAST LATE IN THE PERIOD
ARE FAIRLY SUBTLE RELATIVE TO TYPICAL DAYS 6-7 PREDICTABILITY.
THERE IS SOME TIMING SPREAD WITH THE ARRIVAL OF MSTR WITH GFS RUNS
PSBLY A BIT FAST.  TOWARD FRI RECENT ECMWF RUNS HAVE BEEN BRINGING
THE NERN PAC SFC LOW TOWARD THE PAC NW COAST WHICH CONTRASTS TO A
MUCH FARTHER NWD CONSENSUS... FAVORING AT MOST A LOW WEIGHTING OF
OPERATIONAL ECMWF INPUT AT THAT TIME.


...WEATHER HIGHLIGHTS/THREATS...

AMPLIFYING TROUGH OVER THE WEST DURING THE FIRST PART OF THE WEEK
WILL SPREAD AREAS OF PCPN ACROSS PORTIONS OF THE REGION.  SNOW
LVLS WILL BE ON THE LOW SIDE.  AFTER A BREAK HEAVIER PCPN SHOULD
REACH THE NRN-CNTRL WEST COAST DURING THE LATTER HALF OF THE WEEK
WITH SOME OF THIS MSTR REACHING THE NRN ROCKIES.  LINGERING COLD
AIR MAY SUPPORT SOME POTENTIAL OF WINTRY PCPN AT FAIRLY LOW ELEVS.
 AS MID LVL ENERGY/SFC LOW PRES HEAD INTO THE PLAINS/GRTLKS ONE OR
MORE AREAS OF SNOW ARE PSBL IN THE COLD SECTOR.  FARTHER SEWD HVY
RNFL IS PSBL OVER THE SOUTHEAST/SRN MID ATLC IN ASSOC WITH THE
SHORT RANGE UPR LOW EJECTING FROM NRN MEXICO... WITH SOMEWHAT
LIGHTER ACTIVITY ELSEWHERE.  CONSULT LATEST SPC OUTLOOKS FOR
FURTHER DETAILS ON SEVERE WEATHER POTENTIAL THAT MAY EXIST OVER
SRN AREAS.  COMBINATION OF STRONG HIGH PRES BUILDING SEWD FROM
CANADA AND THEN POTENTIALLY STRONG GRTLKS-NORTHEAST LOW PRES LATER
IN THE PERIOD MAY GENERATE STRONG WINDS.  COLD CYCLONIC FLOW UNDER
THE CORE OF THE UPR TROUGH WILL LIKELY SUPPORT AREAS OF LAKE
EFFECT SNOW LATE NEXT WEEK.

FCST REMAINS ON TRACK FOR WELL BELOW NORMAL TEMPS ACROSS MUCH OF
THE WRN-CNTRL STATES... REACHING ERN AREAS LATE IN THE PERIOD.
EXPECT ONE OR MORE DAYS WITH HIGHS 20-30F BELOW NORMAL ALONG THE
ERN SLOPES OF THE ROCKIES INTO THE NRN-CNTRL PLAINS TUE-THU.
TOWARD THE LATTER HALF OF THE WEEK TEMPS SHOULD TREND WARMER OVER
THE WEST WHILE MINUS 10-20F ANOMALIES FOR HIGHS REACH THE ERN
STATES.  MIN TEMPS COULD DECLINE TO OR BELOW FREEZING ALONG/NEAR
PORTIONS OF THE GULF COAST BY NEXT FRI.

RAUSCH

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