Preliminary Forecasts
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FXUS02 KWNH 200645
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
144 AM EST MON FEB 20 2017

VALID 12Z THU FEB 23 2017 - 12Z MON FEB 27 2017

...OVERVIEW...

A STRONG MEAN RIDGE CONTINUES TO BE A PROMINENT FEATURE THROUGH
THE FCST PERIOD... SUPPORTING A DOWNSTREAM MEAN TROUGH FROM N OF
HUDSON BAY THROUGH THE WRN U.S. AND BROAD WSWLY FLOW/ABOVE NORMAL
HGTS OVER THE ERN STATES.  THIS PATTERN SHOULD PROMOTE BELOW
NORMAL TEMPS OVER THE WEST AND AT TIMES EXTENDING INTO PARTS OF
THE PLAINS ALONG WITH ABOVE NORMAL TEMPS OVER THE EAST.
MODELS/MEANS AGREE WELL ON THE LARGE SCALE FLOW BUT SIGNIFICANT
DIFFS PERSIST WITH SHRTWV ENERGY DROPPING SWD NEAR THE WEST COAST
AS WELL AS FOR THE SIGNIFICANT LEADING STORM SYSTEM EMERGING FROM
THE ROCKIES/PLAINS AND TRACKING NEWD.


...MODEL EVALUATION/PREFERENCES...

BASED ON GUIDANCE THROUGH THE 18Z CYCLE THE OVERALL PREFERENCE
ACROSS THE ERN PAC/WRN U.S. WOULD BE BEST DESCRIBED AS A SLIGHTLY
WEAKER VERSION OF THE 18Z GEFS MEAN.  THE NERN PAC RIDGE APPEARS
FAIRLY LIKELY TO EXIST AS A CLOSED HIGH FOR AT LEAST PORTIONS OF
THE PERIOD AND CLOSED HIGHS TEND TO PRESENT CONSIDERABLE FCST
DIFFICULTIES DOWNSTREAM.  THUS IT IS NOT SURPRISING TO SEE
PERSISTENCE OF WIDE SPREAD IN LATEST ENSEMBLE SPAGHETTI PLOTS.
12Z ECMWF-BASED GUIDANCE APPEARED TOO EAGER TO RETROGRADE AN
INITIAL COMPACT SHRTWV TO THE S OF THE UPR HIGH THU-FRI... WHICH
MAY PUT ITS SOLN AT A DISADVANTAGE FOR THE REST OF THE PERIOD.  OF
GREATER INTEREST ALONG THE WEST COAST AND EVENTUALLY DOWNSTREAM IS
ENERGY LIKELY TO BE NEAR THE CNTRL/N-CNTRL WEST COAST BY EARLY DAY
5 SAT.  GFS/CMC RUNS HAVE CONSISTENTLY BEEN THE DEEPEST/MOST
CLOSED WITH THIS ENERGY WHILE THE ECMWF/ECMWF MEAN HAVE TENDED TO
BE ON THE WEAKER END OF THE SPECTRUM.  MULTI-RUN TRENDS FROM THE
GEFS MEAN AND ECMWF/ECMWF MEAN ARE SUPPORTIVE OF LOWER HGTS ALONG
THE WEST COAST BUT TELECONNECTION-FAVORED FLOW STILL SUGGESTS
TONING DOWN THE GFS/CMC SCENARIO SOMEWHAT.  AFTER SAT THE 12Z/18Z
GFS EVOLUTION RESULTED IN A SWRN U.S. TROUGH AMPLITUDE AT THE
EXTREME END OF THE ENSEMBLE SPREAD AND THEN A PLAINS CLOSED LOW
WHICH SEEMS DIFFICULT TO MAINTAIN IN STRONG SWLY MEAN FLOW.  THE
12Z CMC LOOKED MORE REASONABLE IN THAT RESPECT... QUICKLY
DAMPENING OUT THE SHRTWV.  PREFERRED EVOLUTION MAINTAINS SOME
DEFINITION OF THE SHRTWV OVER THE SOUTHWEST DAY 6 SUN AND THEN A
QUICKLY WEAKENING SHRTWV LIFTING THROUGH THE PLAINS WITH ASSOC
WEAK SFC LOW PRESSURE.  THE NEW 00Z GFS MAY STILL BE A LITTLE
STRONG BUT HAS TRENDED IN A FAVORABLE DIRECTION.  DEEP/VERY
OFFSHORE UKMET RUNS APPEAR TO BE A VERY LOW PROBABILITY SCENARIO.
THERE IS DECENT AGREEMENT THAT ANOTHER BUNDLE OF SHRTWV ENERGY
SHOULD REACH THE NORTHWEST BY DAY 7 MON.

FOR THE STORM EMERGING FROM THE ROCKIES/HIGH PLAINS THE ONGOING
SPREAD IN SFC LOW TRACK/TIMING RESULTS FROM COMBINED UNCERTAINTIES
WITH ERN PAC/WEST COAST DETAILS AS WELL AS THE EVOLUTION OF SRN
CANADA ENERGY FOR WHICH SIGNIFICANT DISCREPANCIES ARISE ALREADY BY
12Z THU.  ESPECIALLY AFTER 12Z FRI THE 12Z ECMWF QUICKLY STRAYED
TO THE FAST/NRN FRINGE OF THE FULL ENSEMBLE SPREAD.  LATEST GFS
RUNS ARE CLOSEST TO ESTABLISHED CONSENSUS... ALBEIT PERHAPS A
LITTLE SWD OVER THE PLAINS/MIDWEST... WHILE CANADIAN ENSEMBLES
EVENTUALLY LEAN TO THE SLOWER SIDE.  LIKELY DUE IN PART TO THE
DIFFERENT ERN PAC EVOLUTION THE PAST TWO UKMET RUNS SHEAR OUT THE
LEADING ENERGY THAT OTHER SOLNS USE TO SUPPORT THE SFC LOW AND
THEN USES TRAILING ENERGY TO STRENGTHEN THE SYSTEM.  CMC RUNS ARE
A LITTLE NWWD OF THE MAJORITY CLUSTER.  PREFERRED MULTI-GUIDANCE
BLEND MINIMIZES CONTINUITY CHANGE FROM PRIOR CYCLE.

AHEAD OF THIS SYSTEM GFS RUNS HAVE CONTINUED TO BE ON THE FAST
SIDE WITH A SERN CANADA WAVE THAT WILL BRING A COLD FRONT THROUGH
THE NERN STATES.  THE 00Z GFS HAS ADJUSTED FAVORABLY SLOWER BUT IS
STILL IN THE FASTER HALF OF THE SPREAD.

WITH THE OLD 00Z/19 ECMWF COMPARING SOMEWHAT BETTER TO PREFS
ACROSS THE LOWER 48 THROUGH THE PERIOD... IT WAS INCLUDED AS A
BLEND COMPONENT INTO DAY 7 MON.  THE 12Z ECMWF COULD BE USED AS
PART OF A COMPROMISE ONLY THROUGH 12Z FRI WHILE A SMALL WEIGHTING
OF THE 18Z GFS WAS INCLUDED INTO DAY 6 SUN.  THE 18Z GEFS/12Z
ECMWF MEANS PROVIDED THE REMAINING GUIDANCE FOR THE FCST.


...WEATHER HIGHLIGHTS/THREATS...

EXPECT THE STORM SYSTEM TRACKING OUT OF THE ROCKIES/PLAINS THU
ONWARD TO GENERATE A VARIETY OF SIGNIFICANT WEATHER INCLUDING
STRONG WINDS... A BAND OF POTENTIALLY HVY SNOW TO THE NW OF THE
STORM TRACK... AND LOCALLY HVY RNFL PLUS PSBLY STRONG TO SEVERE
CONVECTION IN THE WARM SECTOR MAINLY TO THE E OF THE MS RIVER.
YDAYS SPC OUTLOOK INDICATED BEST SEVERE POTENTIAL OVER/NEAR
INDIANA IN THE FRI-FRI NIGHT PERIOD.  CHECK LATEST SPC PRODUCTS
FOR UPDATED DETAILS.  COLD SECTOR TEMPS DO NOT LOOK PARTICULARLY
LOW SO SOME SNOW MAY FALL AT TEMPS CLOSE TO FREEZING.  MEAN TROUGH
ALOFT OVER THE WEST WILL SUPPORT AREAS OF MOSTLY LGT-MDT SNOW AND
COASTAL/LOW ELEV RAIN.  THERE IS STILL A FAIRLY HIGH LEVEL OF
UNCERTAINTY OVER POTENTIAL FOR SOME NARROWLY FOCUSED ACTIVITY OVER
CA FOR A DAY OR SO CENTERED ON SAT THOUGH LATEST TRENDS SEEM
HEADED FOR INTERMEDIATE TOTALS VS SOME RELATIVELY DRY ECMWF RUNS
AND WET GFS RUNS.  MOST LIKELY THOUGH STILL MODERATELY UNCERTAIN
EVOLUTION OF THE SHRTWV ENERGY SUPPORTING THIS PCPN WOULD HAVE IT
EJECT THROUGH THE SOUTHWEST AND INTO THE PLAINS SUN-MON LEADING TO
ANOTHER AREA OF CNTRL-ERN U.S. PCPN IN THAT TIME FRAME.  DEPENDING
ON SYSTEM STRENGTH THE NRN PART OF THE PCPN SHIELD COULD CONTAIN
SOME WINTRY WEATHER.  MEANWHILE THE FL PENINSULA MAY SEE AREAS OF
RAIN WITH AN UPR LVL SYSTEM CROSSING THE AREA THU AND PERIODS OF
LOW LVL ATLC FLOW.

VERY WARM TEMPS OVER MUCH OF THE EAST WILL EXTEND THROUGH LATE
WEEK WITH A SIZABLE AREA OF PLUS 15-30F ANOMALIES AND POTENTIAL
FOR A DECENT NUMBER OF DAILY RECORD HIGHS AND WARM LOWS.  WHILE
THE COLD FRONT TRAILING FROM THE PLAINS-GRTLKS STORM WILL BRING A
SIGNIFICANT COOLING TREND... TEMPS ARE LIKELY TO REMAIN NEAR TO
ABOVE NORMAL WITH AN UPWARD TREND ALREADY COMMENCING BY SUN-MON.
AT THE SAME TIME TEMPS OVER THE WEST SHOULD BE PERSISTENTLY ON THE
COOL SIDE WITH SOME LOCATIONS 10-15F BELOW NORMAL FOR HIGHS ON ONE
OR MORE DAYS.  AT TIMES SOME OF THIS COOL AIR WILL EXTEND INTO
PARTS OF THE NRN-CNTRL PLAINS.

RAUSCH

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