Preliminary Forecasts
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FXUS02 KWNH 050646
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 AM EDT SUN JUL 05 2015

VALID 12Z WED JUL 08 2015 - 12Z SUN JUL 12 2015

...PATTERN OVERVIEW...

AN UNSEASONABLY DEEP UPR LOW INITIALLY NEAR THE CA COAST IS STILL
FCST TO EJECT INLAND... WHICH ALONG WITH NRN PAC FLOW SETTLING
INTO A TROUGH NEAR THE PAC NW COAST COAST... WILL ULTIMATELY LEAD
TO A NWD SHIFT IN THE CORE OF LOWEST HGTS WITHIN THE OVERALL WEST
COAST MEAN TROUGH.  THE GENERAL THEME OF THE DOWNSTREAM PATTERN
REMAINS SIMILAR AS WELL WITH LOW AMPLITUDE FLOW BTWN AN ELONGATED
SRN TIER RIDGE AND CYCLONIC FLOW OVER CANADA.  MOST GUIDANCE
CONTINUES TO SIGNAL THAT THE STRONGEST RIDGING ALOFT OVER THE SRN
TIER SHOULD BUILD OVER THE SRN PLAINS/SRN ROCKIES LATE IN THE
PERIOD.  AT THE SFC ONE FRONT SHOULD SET UP ACROSS CNTRL LATITUDES
OF THE CONUS E OF THE ROCKIES BEFORE DISSIPATING WHILE ANOTHER
MEAN BNDRY SHOULD EXIST NEAR THE CANADIAN BORDER.


...GUIDANCE EVALUATION/PREFERENCES...

ENSMEAN TRENDS OVER THE PAST 12-24 HRS TOWARD SOME OF THE
OPERATIONAL GUIDANCE EARLY IN THE PERIOD OVER ERN NOAM ALONG WITH
MODERATE SPREAD FOR THE CA COAST UPR LOW LEAD TO A MODEL/ENSEMBLE
MEAN CONSENSUS PROVIDING A REASONABLE FCST FOR THE DAYS 3-5
WED-FRI TIME FRAME.  IN-BETWEEN THOSE TWO AREAS THE BLEND
MITIGATES UNCERTAIN SHRTWV DETAIL DIFFS FROM INDIVIDUAL MODEL
RUNS.  INCREASING DIVERGENCE OF OPERATIONAL RUNS AFTER FRI FAVORS
A BLEND OF ENSEMBLE MEANS FROM THE 12Z ECMWF/NAEFS AND 18Z GEFS
CYCLE.

AT LEAST THROUGH DAY 5 FRI THE 12Z ECMWF/ECMWF MEAN AND 12Z
GFS/18Z GEFS MEAN SEEM TO OFFER THE BEST COMPROMISE FOR THE UPR
LOW NEAR THE CA COAST.  THE 18Z GFS/12Z CMC ARE SLIGHTLY FASTER...
AS IS THE NEW 00Z GFS.  THE 00Z CMC ADJUSTED A LITTLE SLOWER.  THE
APPROACHING NRN PAC FLOW APPEARS FAR ENOUGH REMOVED FROM THE UPR
LOW FOR A WHILE TO SUPPORT LEANING AWAY FROM THE FASTEST SOLNS
THOUGH OCCASIONALLY SLOW BIASES IN ECMWF-BASED GUIDANCE COULD
ARGUE FOR TIMING SLIGHTLY FASTER THAN THE 12Z ECMWF/ECMWF MEAN BY
FRI.  THE ULTIMATE TRACK/EVOLUTION OF ENERGY FROM THIS UPR LOW
BECOME INCREASINGLY UNCERTAIN AFTER FRI.  THERE HAVE ALSO BEEN
SOME FLUCTUATIONS OVER THE PAST COUPLE DAYS IN THE AMPLITUDE OF
TROUGHING THAT DEVELOPS NEAR THE PAC NW/BC COAST FRI ONWARD WITH
LATEST SOLNS ON AVG A LITTLE WEAKER.

FOR THE FRONT THAT REACHES THE NORTHEAST BY MIDWEEK THE PAST 12-24
HRS OF ENSMEAN RUNS HAVE BROUGHT THEIR SOLNS IN LINE WITH HALF OR
MORE RECENT OPERATIONAL GUIDANCE THAT HAD BEEN SHOWING FASTER
TIMING.  CONFIDENCE IS LOW IN THE SPECIFICS BUT THERE IS SOME
AGREEMENT WITH THE GENERAL IDEA OF MIDWEST TO NRN MID ATLC
WAVINESS PASSING INTO THE WRN ATLC IN THE WED-FRI TIME FRAME.
SUPPORTING SHRTWV DETAILS WILL LIKELY TAKE INTO THE SHORT RANGE
TIME FRAME TO BE RESOLVED SATISFACTORILY.  GUIDANCE CONSENSUS
SUGGESTS THIS FRONT SHOULD DISSIPATE BY NEXT WEEKEND.

A DEVELOPING DISCREPANCY IN THE GUIDANCE ARISES BY MID-PERIOD AS
THE PAST COUPLE UKMET RUNS AND TO A SOMEWHAT LESSER EXTENT
GFS/GEFS RUNS SHOW INCREASINGLY STRONGER SERN RIDGING RELATIVE TO
THE 12Z ECMWF/ECMWF MEAN.  THE 00Z CMC HAS TRENDED STRONGER THAN
ITS 12Z RUN AS WELL.  THIS IDEA IS TOO NEW TO FOLLOW IN
CONTINUITY... AS OPPOSED TO THE MORE CONSISTENT SIGNAL OF THE
STRONGER SRN PLAINS/ROCKIES RIDGE NEXT WEEKEND... SO THE
INTERMEDIATE SOLN OFFERED BY THE PREFERRED BLEND SEEMS REASONABLE
AT THIS TIME.

FINALLY CONFIDENCE IS MODERATE AT BEST ALONG THE CANADIAN BORDER
GIVEN CURRENT SPREAD/VARIABILITY IN SHRTWV ENERGY THAT WILL
DETERMINE SWD/EWD EXTENT OF THE LEADING SFC FRONT... AGAIN
FAVORING THE CONSENSUS SOLN THROUGH DAY 5 FRI AND THE ENSEMBLE
MEANS THEREAFTER.


...SENSIBLE WEATHER HIGHLIGHTS...

ONE FAVORED AREA FOR ENHANCED RNFL SHOULD BE FROM THE
CNTRL/S-CNTRL PLAINS INTO THE NRN MID ATLC OR ERN GRTLKS...
INITIALLY WITH FOCUS PROVIDED BY A WAVY SFC FRONT AND ONE OR MORE
SHRTWVS ALOFT.  DEVELOPING UNCERTAINTY REGARDING THE AMPLITUDE OF
SERN RIDGING BY LATE IN THE WEEK LOWERS CONFIDENCE IN FAVORED
LATITUDE FOR CONVECTION AT THAT TIME.  WITHIN THE AFOREMENTIONED
AREA... LOCATIONS SEEING CLOUDS/RNFL DURING THE DAYTIME HOURS MAY
SEE SIGNIFICANTLY BELOW NORMAL MAX TEMPS ESPECIALLY EARLY IN THE
PERIOD.  CURRENT EXPECTATION IS FOR TEMPS TO TREND TOWARD
SEASONABLE HIGHS AND NEAR/ABOVE NORMAL LOWS OVER THE CNTRL-ERN
STATES NEXT WEEKEND BUT WITH WARMER POTENTIAL FOR SOME LOCATIONS
UNDER A STRONGER SERN RIDGE SCENARIO.  MEANWHILE THE UPR LOW
EJECTING INTO THE WEST FROM JUST OFF THE CA COAST IS STILL
EXPECTED TO ENHANCE RNFL OVER FAVORED TERRAIN INLAND FROM THE
CNTRL WEST COAST AND THEN INTO THE NRN ROCKIES AND PSBLY PLAINS.
ALSO ANTICIPATE ONE OR MORE EPISODES OF CONVECTION WITH VARYING
INTENSITY OVER THE CNTRL-SRN ROCKIES AND DIURNAL ACTIVITY OVER
PARTS OF THE SOUTHEAST.  EVOLUTION OF FLOW ALOFT OVER THE WEST
SHOULD SUPPORT A SIGNIFICANT COOLING TREND IN THE NORTHWEST AFTER
SOME LOCATIONS SEE HIGHS 10-20F ABOVE NORMAL WED-THU.  THE SRN
HALF OF THE WEST SHOULD SEE BELOW NORMAL HIGHS MOST DAYS.

RAUSCH

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