Preliminary Forecasts
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000
FXUS02 KWNH 100641
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
140 AM EST SAT DEC 10 2016

VALID 12Z TUE DEC 13 2016 - 12Z SAT DEC 17 2016

...PATTERN OVERVIEW...

VARIOUS POSSIBILITIES FOR FLOW ALOFT TO THE S/E OF NERN PAC/ALASKA
MEAN RIDGING CONTINUES TO REDUCE CONFIDENCE IN SOME ASPECTS OF THE
FCST.  DIFFS WITH HANDLING OF INITIAL ERN PAC ENERGY ARE ALREADY
PRONOUNCED ON DAY 3 TUE.  THEN THERE IS INCREASING SPREAD FOR
UPSTREAM FLOW... SOME OF WHICH SHOULD FEED INTO ERN PAC/WRN CONUS
TROUGHING AND THE REST CONTINUE ACROSS THE CNTRL/ERN STATES.
RELATIVELY SPEAKING THERE IS MUCH BETTER AGREEMENT FOR THE DEEP
UPR LOW CROSSING NRN ONTARIO AROUND MIDWEEK AND CONTINUING EWD
THEREAFTER.  THE MOST COMMON THEME REGARDLESS OF WHICH SCENARIO
VERIFIES IS THE PERSISTENCE OF MUCH BELOW NORMAL TEMPS ACROSS THE
NRN TIER STATES WITH SOME OF THIS COLD AIR EXTENDING THROUGH THE
REST OF THE PLAINS AND INTO THE EAST FOR A PERIOD OF TIME.  PCPN
COVERAGE/INTENSITY/LOCATION WILL BE DEPENDENT ON THE VARIOUS
UNCERTAIN DETAILS ALOFT.


...GUIDANCE EVALUATION/PREFERENCES...

REVIEWING GUIDANCE THROUGH THE 12Z/18Z CYCLES THERE WERE ALREADY
TWO SEPARATE CLUSTERS FOR ENERGY INITIALLY OFF THE PAC NW COAST AS
OF EARLY TUE.  THE 12Z ECMWF AND MANY OF ITS ENSEMBLE MEMBERS WERE
MORE CLOSED AND FARTHER OFFSHORE THAN THE GFS/GEFS RUNS AND
CMC/UKMET.  GOING FORWARD THESE DIFFS LEAD TO THE CONSENSUS
SHOWING A MORE SUPPRESSED SFC PATTERN OVER THE WEST THAN THE ECMWF
SCENARIO.  WITH TIME THESE DIFFS LEAD TO INCREASINGLY LARGE SPREAD
DOWNSTREAM OVER THE EAST... WITH SOLNS RANGING BTWN HIGH PRES AND
DEVELOPING LOW PRES/FRONTAL SYSTEM.  BEYOND THE INITIAL ERN PAC
FEATURE THERE IS A WIDE SPREAD IN GUIDANCE ON EVOLUTION OF
UPSTREAM ENERGY THAT MAY AMPLIFY OVER THE ERN PAC/WRN U.S. WITH
POTENTIAL FOR SOME OF THE FLOW TO PROGRESS FROM WRN CANADA THROUGH
THE CNTRL-ERN STATES.  TELECONNECTIONS RELATIVE TO UPSTREAM
RIDGING CONTINUE TO SUGGEST THE ECMWF MEAN IS SOMEWHAT TOO
FLAT/EWD WITH ITS WRN TROUGH LATE IN THE PERIOD WHILE OPERATIONAL
MDLS DIFFER IN DETAILS WITH SOME PSBLY ON THE DEEP SIDE.

DESIRE TO MAINTAIN THE BEST CONTINUITY PSBL UNDER THE
CIRCUMSTANCES... WHILE REFLECTING THE MAJORITY CLUSTER OVER THE
ERN PAC EARLY IN THE PERIOD AND LEANING AWAY FROM THE ECMWF MEAN
AND VARIED OPERATIONAL GFS/ECMWF RUNS LATER... LEAD TO A FCST THAT
STARTS WITH MORE 18Z GEFS MEAN THAN OPERATIONAL 12Z/18Z GFS RUNS
EARLY-MID PERIOD AND THE 18Z GEFS MEAN EXCLUSIVELY BY DAYS 6-7
FRI-SAT.  TO A GREATER EXTENT THAN USUAL THERE IS POTENTIAL THAT
SIGNIFICANT ADJUSTMENTS MAY HAVE TO BE MADE TO THE FCST AS AN
ALTERNATE CONSENSUS EMERGES.

OVER THE EAST... THE GFS/GEFS MEAN STILL APPEAR TO BE A BIT ON THE
STRONG/SLOW SIDE WITH THE SYSTEM DEPARTING FROM THE CANADIAN
MARITIMES SO THE INITIAL BLEND WAS ADJUSTED PARTIALLY TOWARD OTHER
SOLNS.  THERE IS STILL A LOT OF SPREAD FOR ERN U.S./WRN PAC
WAVINESS IN THE TUE-THU TIME FRAME... BOTH ALONG A FRONT THAT
STALLS OVER THE SOUTHEAST AND THEN LIFTS NEWD AS WELL AS THE
TRAILING ARCTIC FRONT.  A SOLN SKEWED TOWARD THE GEFS MEAN
PROVIDES THE BEST STABILITY FOR THE TIME BEING.  FCST BECOMES EVEN
MORE UNCERTAIN BY FRI-SAT WITH INFLUENCE FROM ERN PAC/WRN U.S.
DIFFS COMING INTO PLAY.  CURRENT MAJORITY FCST KEEPS HIGH PRES
OVER THE EAST BUT WITH THE GEFS GUIDANCE SHOWING AN EXPANSION OF
LGT PCPN OF VARIOUS TYPES... WHICH PROVIDES SOME ACCOUNT FOR PCPN
POTENTIAL SEEN IN THE LESS FAVORED ECMWF GUIDANCE BUT ALSO IN THE
NEW 00Z GFS THAT IS QUITE THE CONTRAST TO THE DRIER 12Z/18Z RUNS.


...WEATHER HIGHLIGHTS/THREATS...

ONGOING SPREAD WITH DETAILS OF ERN PAC/WRN U.S. FLOW LOWERS
CONFIDENCE IN SPECIFICS FOR LOCATION OF BEST PCPN FOCUS AND TIMING
OVER THE WEST.  AT LEAST FOR THE 5-DAY PERIOD THE BEST SIGNALS FOR
HIGHEST RAIN AND INLAND/HIGHER ELEV SNOW TOTALS STILL LIE ACROSS
THE MIDDLE LATITUDES OF THE REGION WITH PARTICULAR FOCUS ON
FAVORED COASTAL/TERRAIN LOCATIONS IN NRN-CNTRL CA AND INTO THE
CNTRL ROCKIES.  ACTIVITY MAY BE ENHANCED BY INCORPORATION OF SOME
LOWER LATITUDE TROPICAL MSTR.  ARCTIC BNDRY DUG/DRAPED THROUGH THE
NORTHWEST AND N-CENTRAL ROCKIES/HIGH PLAINS MAY PROVIDE ANOTHER
FOCUS FOR PCPN WITH SNOWS ENHANCED BY POST-FRONTAL UPSLOPE FLOW.
EXPECT TEMPS TO RANGE UP TO 20-30F BELOW NORMAL FOR COLDER
POST-FRONTAL AREAS OVER NRN PARTS OF THE WEST WITH PERHAPS A FEW
ISOLD DAILY RECORD VALUES.  ABOVE NORMAL TEMPS OVER SRN AREAS
EARLY IN THE PERIOD SHOULD TREND COOLER BY FRI-SAT.

TO THE E OF THE ROCKIES... EXPECT THE NRN PLAINS TO REMAIN 20-30F
BELOW NORMAL.  COLD AIR SURGING SEWD TUE ONWARD MAY BRING
SIMILARLY COLD ANOMALIES INTO THE MIDWEST FOR AT LEAST A COUPLE
DAYS MID-LATE WEEK WITH SIGNIFICANT BUT LESS EXTREME DEPARTURES
FROM NORMAL ELSEWHERE.  THE SRN PLAINS ON TUE AND SOUTHEAST
THROUGH WED WILL BE THE LONE POCKETS OF WARMTH.  AS WITH THE NRN
PARTS OF THE WEST DAILY RECORD VALUES SHOULD BE FAIRLY ISOLD.
COLD FLOW ACROSS THE GRTLKS WILL LIKELY GENERATE PERIODS OF LAKE
EFFECT SNOW FOR MULTIPLE DAYS.  OVER THE REST OF THE EAST THE PCPN
FCST HAS BELOW AVG CONFIDENCE DUE TO ISSUES WITH UPSTREAM FLOW
ALOFT.  ONE PERIOD OF SOMEWHAT HIGHER POTENTIAL WILL BE TUE-WED
WITH THE FRONT STALLING OVER THE SOUTHEAST AND PSBLY WAVY ARCTIC
FRONT APPROACHING FROM THE MIDWEST.  AFTER A BRIEF LULL MSTR MAY
SPREAD ACROSS PARTS OF THE CNTRL-ERN STATES WITH LINGERING COLD
AIR AIDING OVER-RUNNING SNOW/ICE THREAT POTENTIAL.

RAUSCH

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