Preliminary Forecasts
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000
FXUS02 KWNH 230645
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
145 AM EST THU FEB 23 2017

VALID 12Z SUN FEB 26 2017 - 12Z THU MAR 02 2017

...OVERVIEW...

GUIDANCE AGREES WITH THE OVERALL LARGE SCALE EVOLUTION DURING THE
PERIOD.  EXPECT AN UPR TROUGH TO DEEPEN/SHARPEN OVER THE WEST
COAST/INTERIOR WEST AS AN UPR HIGH STRENGTHENS OVER THE FL
PENINSULA/BAHAMAS/CUBA.  THE UPR TROUGH WILL EJECT NEWD AS
CYCLONIC FLOW BEGINS TO TAKE THE PLACE OF RIDGING OFF THE SRN
COAST OF ALASKA.  HOWEVER AHEAD OF THE CONSENSUS MEAN EVOLUTION...
ERN PAC/WEST COAST EVOLUTION DURING THE WEEKEND IS STILL CAUSING
GREATER THAN DESIRED UNCERTAINTY FOR SOME FCST SPECIFICS ACROSS
THE LOWER 48.


...MODEL EVALUATION/PREFERENCES...

EVEN THOUGH THE ERN PAC/WRN U.S. EVOLUTION DURING THE WEEKEND HAS
REACHED INTO THE SHORT RANGE TIME FRAME THERE IS STILL MEANINGFUL
SPREAD... SPECIFICALLY WITH ENERGY FCST TO BE NEAR THE N-CNTRL
WEST COAST AS OF 12Z SAT.  THE 12Z-18Z CYCLES OF THE GFS/GEFS
MEANS SWITCHED TO THE PROGRESSIVE EJECTION SCENARIO OF THE PAST
THREE ECMWF RUNS VERSUS A SLOWER/OFFSHORE CLOSED LOW SEEN IN SOME
PRIOR RUNS AND STILL MAINTAINED BY UKMET/CMC RUNS.  THERE IS AT
LEAST A TREND IN THE UKMET/CMC TOWARD LESS INTERACTION WITH THE
SYSTEM THAT IS FARTHER OFFSHORE.  A NEARLY EVEN DISTRIBUTION AMONG
ENSEMBLE MEMBERS HAD FAVORED CONTINUITY OVER THE PAST 1-2 DAYS BUT
THE 12Z ENSEMBLE CYCLE MADE A MORE PRONOUNCED SHIFT TOWARD THE
PROGRESSIVE GFS/ECMWF IDEA.  THE SLOWER SOLN WAS IN ONLY A
MINORITY OF 12Z GEFS/ECMWF MEMBERS THOUGH A LITTLE MORE COMMON IN
12Z CMC MEMBERS.  GIVEN TYPICAL VERIFICATION OF RESPECTIVE MODELS
AND SEEMINGLY MORE PLAUSIBLE EVOLUTION OF THE PROGRESSIVE
CLUSTER... PREFERRED TO ADJUST THE FCST TOWARD THE GFS/GEFS AND
ECMWF/ECMWF MEAN IDEA THAT HAD BEEN THE CONSENSUS 3-4 DAYS AGO.  A
COMPROMISE FOR STRENGTH OF WEAK DOWNSTREAM CNTRL-ERN U.S. WAVINESS
SEEMS REASONABLE AS STRONG SWLY FLOW ALOFT MAY MAKE IT DIFFICULT
TO SUPPORT A FEATURE QUITE AS STRONG AS IN GFS RUNS.

IN SOME RESPECTS GUIDANCE BEGINS TO LINE UP BETTER AS ENERGY
REACHING NEAR THE PAC NW AS OF EARLY SUN CONTINUES TO AMPLIFY INTO
A LARGER SCALE TROUGH WHICH THEN EJECTS NEWD.  DETAILS ON
TRACK/TIMING/STRENGTH OF LOW PRESSURE LIKELY TO TRACK INTO THE
GRTLKS AROUND MIDWEEK WILL TAKE A WHILE TO GET RESOLVED BUT SO FAR
THE ENSEMBLE MEANS HAVE AT LEAST BEEN FAIRLY STABLE.  RECENT ECMWF
RUNS HAVE TENDED TO BE CLOSER TO THE MEANS WHILE GFS RUNS HAVE
TRENDED SLOWER SUCH THAT AN AVG OF THE 12Z/18Z RUNS COMPARES
BETTER THAN IN PRIOR CYCLES.

TRENDS OVER THE NERN PAC/WRN U.S. LATE IN THE PERIOD HAVE BEEN
TOWARD SLOWER SEWD PROGRESSION OF THE CYCLONIC FLOW THAT SETTLES
OFF THE SRN ALASKA COAST.  ON THE OTHER HAND THE 12Z ECMWF WAS
FAIRLY EXTREME RELATIVE TO THE ENSEMBLE SPREAD IN HOLDING THE
OVERALL TROUGH NWWD WHILE BRINGING A STRONG RIDGE TO THE WEST
COAST... THOUGH THE NEW 00Z GFS HAS TRENDED IN THAT DIRECTION.

BASED ON CONSIDERATIONS EARLY IN THE PERIOD AND THEN A DESIRE TO
MAINTAIN A MODEL/ENSEMBLE MEAN COMPROMISE FOR TRAITS OF THE SYSTEM
CROSSING THE GRTLKS AROUND WED LED TO A STARTING BLEND CONSISTING
OF VARIOUS WEIGHTS OF THE 12Z-18Z GFS/18Z GEFS MEAN AND 12Z
ECMWF/ECMWF MEAN.


...WEATHER HIGHLIGHTS/THREATS...

DETAIL UNCERTAINTIES CONTINUE TO TEMPER CONFIDENCE IN SPECIFICS
FOR PRECISE COVERAGE AND INTENSITY OF COASTAL/LOW ELEV RAIN AND
INLAND/HIGHER ELEV SNOW FROM THE WEST COAST INTO THE ROCKIES.
LATEST TRENDS SEEM TO BE REDUCING HVY PRECIP POTENTIAL OVER PARTS
OF CALIFORNIA.  EXACT TIMING/LATITUDE OF AN EJECTING OFFSHORE ERN
PAC FEATURE WILL DETERMINE HOW MUCH MSTR REACHES FAVORED TERRAIN
FROM SRN CA THROUGH AZ AND INTO THE S-CNTRL ROCKIES.  CNTRL-SRN
AREAS SHOULD SEE A DRYING TREND BY THE LATTER HALF OF THE PERIOD
WHILE PERIODS OF PRECIP MAY LINGER OVER THE NORTHWEST/NRN ROCKIES.
 OVER THE ERN HALF OF THE COUNTRY EXPECT TWO PRIMARY WAVES OF
PRECIP... THE FIRST WITH A LEADING WEAK FEATURE AND THEN A SECOND
WITH LOW PRES THAT IS MOST LIKELY TO TRACK THROUGH THE GRTLKS
AROUND WED.  COMBINED PRECIP TOTALS SHOULD BE HIGHEST WITHIN AN
AREA FROM THE OH VLY TO JUST S OF THE TN VLY.  WINTRY WEATHER IS
PSBL OVER NRN AREAS WITH RAIN FARTHER S.  THE WEST SHOULD SEE
BELOW NORMAL TEMPS FOR MOST OF THE PERIOD THOUGH WITH SOME
MODERATION LIKELY TO OCCUR TOWARD NEXT WED-THU AS UPR TROUGHING
DEPARTS.  MEANWHILE TEMPS WILL RETURN TO ABOVE/MUCH ABOVE LEVELS
OVER THE ERN HALF OF THE COUNTRY AHEAD OF PLAINS-GRTLKS LOW
PRESSURE AND TRAILING COLD FRONT.  EXPECT GREATEST ANOMALIES OF
PLUS 20-30F FOR MORNING LOWS ON WED.  AS WITH PRECEDING SYSTEMS
THERE WILL BE A COOLING TREND AFTER FRONTAL PASSAGE BUT LIKELY
WITH TEMPS NOT GETTING MUCH COLDER THAN WITHIN A FEW DEGS OF
NORMAL.

RAUSCH

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