Preliminary Forecasts
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FXUS02 KWNH 300647
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 AM EDT SUN APR 30 2017

VALID 12Z WED MAY 03 2017 - 12Z SUN MAY 07 2017

...PATTERN OVERVIEW AND GUIDANCE EVALUATION...

LATEST GUIDANCE CONTINUES TO SHOW AN EVOLUTION TOWARD AN AMPLIFIED
BLOCKY PATTERN BY THE WEEKEND, CONSISTING OF EAST AND WEST COAST
TROUGHS WITH A STRONG RIDGE OVER THE CENTRAL U.S..  OPERATIONAL
MODEL RUNS FOR DETAILS WITHIN THE ERN MEAN TROUGH HAVE BEEN
REMARKABLY ERRATIC AND DIVERGENT.  INDIVIDUAL ENSEMBLE MEMBERS
HAVE BEEN SIMILARLY DIVERSE BUT THE RESULTING MEANS CONTINUE TO
PROVIDE A MORE CONSISTENT FCST.  MEANWHILE EARLY-PERIOD GFS TRENDS
OVER THE NRN PAC IN THE PAST DAY HAVE YIELDED BETTER CLUSTERING
FOR THE WEST COAST TROUGH ALBEIT WITH TYPICAL EMBEDDED DETAIL
UNCERTAINTIES REMAINING.

THE FCST FOR AMPLIFYING CNTRL-ERN U.S. TROUGH ENERGY REMAINS VERY
SENSITIVE IN TERMS OF THE DEGREE OF FLOW SEPARATION THAT OCCURS BY
LATE IN THE WEEK AND THEN TO WHAT EXTENT FLOW AROUND THE CANADIAN
EXTENSION OF THE CNTRL U.S. RIDGE ULTIMATELY INTERACTS WITH THE
SYSTEM ASSOC WITH THE INITIAL BUNDLE OF ENERGY.  12Z GFS/ECMWF
RUNS THAT SEPARATED INITIAL ENERGY ENOUGH TO LEAVE BEHIND A SLOW
MOVING SRN TIER SYSTEM WERE IN THE SLOWEST 10-15 PCT OF ENSEMBLE
MEMBERS.  THE 12Z UKMET WAS SIMILAR IN PRINCIPLE.  THE 18Z GFS
REVERTED BACK TO A MORE PHASED/CONSOLIDATED SYSTEM, CLOSER TO WHAT
THE 00Z/29 ECMWF RUN SHOWED AND WHAT MOST ENSEMBLE MEAN RUNS HAVE
BEEN ADVERTISING IN RECENT DAYS.  MODEL/ENSEMBLE SPREAD AND POOR
CONTINUITY IN OPERATIONAL RUNS KEEP CONFIDENCE LOWER THAN DESIRED,
AND MAINTAIN PREFS FOR AN ENSEMBLE MEAN APPROACH THAT MAINTAINS
BETTER CONTINUITY THAN OTHER SOLNS.  THE ONE TREND OF NOTE IN THE
LATEST MEANS AS OF THE 12Z/18Z CYCLE WAS TOWARD SLIGHTLY SLOWER
PROGRESSION, WHICH MAY BE A REASONABLE ADJUSTMENT GIVEN THE
PATTERN AMPLITUDE.  NOT SURPRISINGLY THE NEW 00Z MODEL RUNS THUS
FAR OFFER SOME FURTHER OPTIONS.  THE 00Z GFS HAS A SRN CLOSED LOW
BUT ONE THAT IS FASTER THAN THE 12Z RUN AND EVENTUALLY PHASES WITH
NRN STREAM FLOW OVER THE NORTHEAST.  THE UKMET HAS JUMPED TO THE
CONSOLIDATED CLUSTER WHILE THE CMC ALSO HAS THE CORE OF ITS SYSTEM
ALOFT N OF THE GFS BY MID PERIOD.

MEANWHILE SLOWER TRENDS IN THE GFS FOR SHORTER TERM NRN PAC FLOW
OVER THE LAST 24 HRS HAVE BROUGHT ITS RUNS INTO BETTER ALIGNMENT
WITH CONSENSUS FOR THE GENERAL AXIS AND AMPLITUDE OF THE UPR
TROUGH AMPLIFYING NEAR THE WEST COAST.  FOR AT LEAST THE PAST DAY
ECMWF/ECMWF MEAN/CMC RUNS HAVE BEEN ADVERTISING POTENTIAL FOR AN
EMBEDDED LOW TO CLOSE OFF OVER OR NEAR CA BY DAY 7 SUN AND 12Z/18Z
GFS RUNS CLOSED OFF A LOW AS WELL, WHILE CORRESPONDING GEFS MEANS
WERE A LITTLE LATER TO CLOSE OFF A WEAKER LOW.  OVERALL A
MODEL/ENSEMBLE MEAN BLEND PROVIDED A FAIR REPRESENTATION OF BEST
CLUSTERING/TRENDS THROUGH THE 12Z/18Z CYCLES.  INCOMING 00Z RUNS
CAST A LITTLE MORE DOUBT ON EXACTLY WHEN/WHERE AN UPR LOW MAY
CLOSE OFF.  WITH THE LEADING RIDGE STILL TRENDING STRONGER IN THE
MEANS AS WAS THE CASE YDAY, LEADING HGT FALLS ALOFT/SFC FRONT
PUSHING INTO THE WEST HAVE SLOWED DOWN SOMEWHAT.

BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FCST STARTED WITH A
BLEND OF 12Z NAEFS/ECMWF MEANS AND TO A LESSER EXTENT 18Z GFS/GEFS
MEAN THROUGH THE PERIOD WITH MINOR EDITING TO ENHANCE DETAIL.


...WEATHER HIGHLIGHTS...

WITH SOME IMPORTANT DETAILS STILL IN DOUBT, GENERALLY EXPECT THE
SYSTEM ASSOC WITH THE AMPLIFYING CNTRL-ERN U.S. UPR TROUGH TO
GENERATE SOME LOCALLY MDT-HVY RNFL OVER PARTS OF THE CNTRL-SRN
PLAINS/W-CNTRL GULF COAST ON WED WITH ACTIVITY THEN SPREADING
EWD-NEWD TO THE EAST COAST BY LATE IN THE WEEK.  AMPLIFIED/BLOCKY
NATURE OF THE LARGE SCALE PATTERN COULD PROMOTE FAIRLY SLOW SYSTEM
MOTION AND LINGERING PRECIP OVER PARTS OF THE EAST INTO THE
WEEKEND.  COMBINING THE VARIOUS POSSIBILITIES FOR SYSTEM EVOLUTION
THE LWR MS VLY/CNTRL GULF COAST REGION APPEARS TO HAVE THE BEST
POTENTIAL FOR SEEING HIGHEST RNFL TOTALS WITH THIS SYSTEM.  SOME
LOCATIONS EXTENDING INTO THE MID ATLC/NORTHEAST, AND PERHAPS
WITHIN ANOTHER BAND FARTHER NW OF THIS AXIS, MAY SEE SIGNIFICANT
AMOUNTS AS WELL.  WARM SECTOR CONVECTION MAY BE STRONG TO SEVERE
WITH GUIDANCE DETAILS STILL TOO UNCERTAIN TO SPECIFY MOST LIKELY
AREAS TO BE AFFECTED.  THE COLD SECTOR OF THIS SYSTEM WILL BRING
BELOW NORMAL TEMPS FROM THE HIGH PLAINS INTO THE EAST WITH SOME
AREAS SEEING ONE OR MORE DAYS OF AT LEAST MINUS 10F ANOMALIES FOR
HIGHS.

AHEAD OF THE UPR TROUGH AMPLIFYING TOWARD THE WEST COAST, ABOVE TO
MUCH ABOVE NORMAL TEMPS WITH MANY ANOMALIES OF PLUS 10F OR GREATER
WILL PROGRESS FROM THE WEST COAST STATES THROUGH THE INTERIOR WEST
AND INTO THE ROCKIES/HIGH PLAINS.  BASED ON LATEST FCST FOR TEMPS
THERE STILL APPEARS TO BE A BETTER CHANCE FOR SCATTERED DAILY
RECORD WARM LOWS THAN RECORD HIGHS.  ARRIVAL OF THE UPR TROUGH
WILL BRING NEAR TO BELOW NORMAL READINGS TO THE WEST COAST FOR
NEXT WEEKEND.  MOST PRECIP SHOULD BE IN THE LGT-MDT RANGE OF
INTENSITY AND BE CONFINED MOSTLY TO THE NRN-CNTRL WEST COAST INTO
THE NRN ROCKIES, THOUGH SOME LGT/SCT ACTIVITY FARTHER S LATE IN
THE PERIOD CANNOT BE RULED OUT.

RAUSCH

$$





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