Preliminary Forecasts
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037
FXUS02 KWNH 110648
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
147 AM EST SUN DEC 11 2016

VALID 12Z WED DEC 14 2016 - 12Z SUN DEC 18 2016

...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES...

FROM THE LARGE SCALE PERSPECTIVE MOST SOLNS SHOW A GRADUAL SWD
DRIFT OF POSITIVE HGT ANOMALIES ASSOC WITH NERN PAC/ALASKA RIDGING
WHILE SRN GULF/CARIBBEAN RIDGING BUILDS INTO THE WRN ATLC.  AS
THIS OCCURS THE ENSEMBLE MEANS IN PARTICULAR AGREE WELL ON AN
EVOLUTION TOWARD A MEAN TROUGH EXTENDING FROM NRN-NERN CANADA
THROUGH THE WRN U.S.. BY NEXT WEEKEND.  SUCH A TROUGH IS
CONSISTENT WITH TELECONNECTION-FAVORED FLOW RELATIVE TO THE TWO
MEAN RIDGES.

SOME INDIVIDUAL MODEL RUNS AND GEFS/ECMWF/CMC ENSEMBLE MEMBERS
STILL DISPLAY CONSIDERABLE DIFFS BUT TRENDS OVER THE PAST 12-24
HRS HAVE BROUGHT GUIDANCE INTO MUCH BETTER AGREEMENT THAN WAS THE
CASE PREVIOUSLY.  THUS WITH GUIDANCE AVBL THROUGH THE 18Z CYCLE
THE LATEST UPDATE ADJUSTS TO A BLEND OF THE 18Z GFS/GEFS MEAN AND
12Z ECMWF/ECMWF FOR DAYS 3-4 WED-THU AND THEN THE ENSEMBLE MEANS
EXCLUSIVELY DAYS 5-7 FRI-SUN AS CONFIDENCE DECREASES IN
OPERATIONAL RUN SPECIFICS.

EVOLUTION OF ENERGY OFF THE WEST COAST AT THE START OF THE PERIOD
WED REMAINS A SOURCE OF UNCERTAINTY FROM THEN ON.  TRENDS OVER THE
PAST DAY APPEAR TO HAVE GONE TOWARD SOME COMPROMISE BTWN EARLIER
GFS/ECMWF SOLNS.  INTERESTINGLY THE 00Z GFS HAS ADJUSTED TO A
SLOWER AND MORE CLOSED SOLN LIKE AN ECMWF RUN FROM A COUPLE DAYS
AGO BUT THE 00Z CMC/UKMET ARE STILL FASTER LIKE THE ECMWF/ECMWF
MEAN.  IT IS LOOKING INCREASINGLY LIKELY THAT AS UPSTREAM NERN
PAC/WRN CANADA FLOW DIGS INTO THE MEAN TROUGH THIS ERN PAC ENERGY
WILL EJECT INTO/THROUGH THE WEST AND GENERATE A SFC SYSTEM THAT
TRACKS FROM THE PLAINS THROUGH THE GRTLKS FROM LATE FRI ONWARD.
FOR THIS PART OF THE FCST THE SIMILAR ENSEMBLE MEANS PROVIDE A
GOOD CONSERVATIVE DEPICTION OF THE SYSTEM IN LIGHT OF
TRACK/TIMING/DEPTH SPREAD AMONG INDIVIDUAL MODELS/ENSEMBLE
MEMBERS.  QUESTIONABLE EVOLUTION OF THE 00Z GFS NEAR THE WEST
COAST EARLIER IN THE PERIOD LEADS TO ONE OF THE STRONGEST SOLNS
FROM THE PLAINS INTO ERN CANADA.

FOR THE LARGER SCALE TROUGH MOVING INTO THE WEST BEHIND LEAD
EJECTING ENERGY... MULTIPLE GFS RUNS HAVE BEEN ON THE
DEEP/AMPLIFIED SIDE OF THE SPREAD AFTER THU.  THE 12Z PARALLEL GFS
ADDED SUPPORT FOR A LESS AMPLIFIED TROUGH.  WHILE NOT COMPARING
WELL TO CONSENSUS EARLY IN THE PERIOD... THE 00Z GFS DOES
FAVORABLY TREND LESS AMPLIFIED WITH THE LATE PERIOD TROUGH.
AMPLITUDE ALOFT APPEARS TO YIELD GREATEST SENSITIVITY FOR THE PCPN
FCST OVER THE SOUTHWEST LATE THIS WEEK.  TOWARD THE END OF THE
PERIOD GFS/ECMWF RUNS ARE BRINGING SHRTWV ENERGY THROUGH THE NERN
PAC RIDGE MORE AGGRESSIVELY THAN IN THE MEANS.


...WEATHER HIGHLIGHTS/THREATS...

DURING WED-THU EXPECT HEAVIEST PCPN TO BE IN WHAT MAY BE A NARROW
BUT VERY FOCUSED BAND CNTRD OVER NRN-CNTRL CA... ESPECIALLY THE
SIERRA NEVADA RANGE... WITH SOME OF THE MSTR EXTENDING EWD.
INCORPORATION OF LOW LATITUDE MSTR MAY PROVIDE ADDITIONAL
ENHANCEMENT OF RAIN AND INLAND/HIGHER ELEV SNOW.  MSTR WILL
PROGRESS SEWD WITH TIME AS INITIAL ERN PAC ENERGY EJECTS INLAND
AND TRAILING UPR TROUGH AMPLIFIES LEADING TO A DRYING TREND OVER
MUCH OF THE WEST BY NEXT WEEKEND.  THERE IS SOME UNCERTAINTY OF
PCPN AMTS IN THE SOUTHWEST BUT CONSENSUS IS SHOWING LIGHTER AMTS
THAN SEEN IN THE 12Z/18Z GFS.  AS LEADING ENERGY EMERGES FROM THE
WEST... A POTENTIAL PLAINS THROUGH GRTLKS SYSTEM WILL LIKELY
SPREAD A VARIETY OF PCPN TYPES ACROSS THE CNTRL-ERN STATES FROM
LATE WEEK INTO THE WEEKEND.  CURRENTLY THE NRN PLAINS AND UPR
MIDWEST APPEAR TO HAVE BEST POTENTIAL FOR PCPN TO BE MOSTLY SNOW.
FARTHER EWD PCPN MAY START AS SNOW OR A WINTRY MIX WITH AN
EVENTUAL CHANGEOVER TO RAIN AS WARM AIR PUSHES NWD.  WELL AHEAD OF
THIS SYSTEM THERE WILL BE SOME LGT TO LOCALLY MDT PCPN WITH AN
ARCTIC FRONT PUSHING OFF THE EAST COAST EARLY IN THE PERIOD...
FOLLOWED BY PERIODS OF LAKE EFFECT SNOW.

THE MOST EXTREME COLD ANOMALIES SHOULD PERSIST OVER THE NRN PLAINS
WITH READINGS 15-30F BELOW NORMAL MOST DAYS.  FRONT DEPARTING FROM
THE EAST COAST EARLY IN THE PERIOD WILL BRING SOME OF THIS COLD
AIR INTO THE EAST FOR 2-3 DAYS WITH TEMPS 10-25F BELOW NORMAL THAT
WILL QUALIFY AS THE COLDEST OF THE SEASON SO FAR.  LATE PERIOD
SYSTEM TRACKING THROUGH THE GRTLKS MAY PROMOTE ANOTHER SEWD SURGE
OF COLD AIR INTO THE MIDWEST BY NEXT SUN WHILE PULLING ABOVE
NORMAL TEMPS INTO THE SOUTHEAST.  NRN PARTS OF THE WEST WILL BE
CONSISTENTLY WELL BELOW NORMAL BUT WITH SOMEWHAT LESS EXTREME
ANOMALIES THAN OVER THE NRN PLAINS.  NEAR TO ABOVE NORMAL TEMPS
OVER CNTRL-SRN PARTS OF THE WEST ON WED-THU WILL TREND STEADILY
LOWER TO BELOW NORMAL LEVELS AS HGTS ALOFT DECREASE.

RAUSCH

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