Preliminary Forecasts
Issued by NWS

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052
FXUS02 KWNH 240643
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
142 AM EST FRI NOV 24 2017

VALID 12Z MON NOV 27 2017 - 12Z FRI DEC 01 2017

...PATTERN ASSESSMENT AND MODEL CHOICES...

CONFIDENCE VARIES WIDELY DEPENDING ON THE SCALE OF INTEREST AND
OTHER ASPECTS OF THE FORECAST.  THERE IS GOOD AGREEMENT REGARDING
THE IDEA OF PROGRESSIVE MEAN FLOW ACROSS THE LOWER 48 FOR MOST OF
NEXT WEEK, TO THE SOUTH OF A CANADIAN UPPER LOW.  THE MAJORITY OF
GUIDANCE IS SIGNALING THAT A LARGER SCALE AND SLOWER MOVING MEAN
TROUGH MAY BEGIN TO APPROACH THE WEST COAST BY DAY 7 FRI, AS
SUPPORTED BY RIDGING THAT DEVELOPS OVER OR JUST EAST OF 160W.  AS
FOR THE LOWER CONFIDENCE DETAILS, MODELS AND INDIVIDUAL ENSEMBLE
MEMBERS HAVE DIFFERED SIGNIFICANTLY BETWEEN EACH OTHER AND RUN TO
RUN REGARDING SHORTWAVES EMBEDDED WITHIN THE PROGRESSIVE MEAN FLOW
WITH SOME DIFFERENCES NOTED FOR THE EASTERN PACIFIC TROUGH NEAR
THE END OF THE PERIOD AS WELL.

REGARDING LEADING SHORTWAVE ENERGY REACHING WESTERN NORTH AMERICA
BY EARLY MON, GFS RUNS CONTINUE TO OUTPACE OTHER SOLUTIONS
INCLUDING THE GEFS MEAN ACROSS SOUTHERN CANADA.  FROM THE START OF
THE PERIOD THIS PRECLUDED USE OF THE OPERATIONAL GFS IN THE
FORECAST BLEND.  THE 00Z GFS HAS TRENDED FAVORABLY SLOWER BUT IS
STILL ON THE LEADING SIDE OF THE ENVELOPE AS THE 00Z UKMET/CMC
MAINTAIN CONTINUITY WITH THE PRIOR CONSENSUS.  MEANWHILE THE
12Z-18Z CYCLES OF OPERATIONAL MODEL RUNS HAVE TRENDED MORE
STRONGLY TOWARD A SEPARATE SHORTWAVE/CLOSED LOW CROSSING THE
SOUTHERN HALF OF THE WEST AND CONTINUING INTO THE PLAINS (12Z
GFS/UKMET FASTEST AND 12Z ECMWF SLOWEST).  NEW 00Z RUNS MAINTAIN A
REASONABLE DEGREE OF CONTINUITY ON EXISTENCE OF THIS FEATURE BUT
NOT SO MUCH FOR TIMING, WITH 12Z/23 ENSEMBLES A LOT MORE VARIED
THAN THE CURRENT OPERATIONAL RUNS.

THE NEXT SHORTWAVE IN THE SERIES SHOULD REACH THE WEST COAST
AROUND LATE TUE-WED.  ALREADY BY THAT TIME FRAME THE MODELS AND
ENSEMBLES DEVELOP DISAGREEMENTS IN AMPLITUDE, SHARPNESS, AND
WHERE/HOW MUCH STREAM SEPARATION COULD OCCUR, WITH DIFFERENCES
INCREASING MORE GREATLY OVER THE REST OF THE FORECAST.  THIS
SPREAD AND THE DESIRE TO MAINTAIN REASONABLE CONTINUITY ARGUE FOR
ADJUSTING THE FORECAST EMPHASIS LATER IN THE PERIOD TOWARD THE
ENSEMBLE MEANS UNTIL BETTER CLUSTERING ARISES.  FOR THE UPPER
TROUGH NEARING THE WEST COAST LATE NEXT WEEK, ECMWF/ECMWF MEAN
RUNS HAVE GENERALLY BEEN STEADIEST OVER THE PAST COUPLE DAYS.
18Z/00Z GEFS MEANS HAVE MADE A NOTABLE TREND TOWARD THE ECMWF MEAN
FOR THE DEPTH OF THE TROUGH, HELPING TO INCREASE CONFIDENCE
SOMEWHAT.  12Z AND NEW 00Z CMC RUNS ARE SIMILAR IN PRINCIPLE AS
WELL.  RECENT GFS RUNS HAVE TENDED TO HAVE A SHARPER/SLOWER
DEPICTION OF THE TROUGH.

BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FORECAST
INCORPORATED IDEAS FROM THE 12Z ECMWF/UKMET/CMC AND 18Z GEFS MEAN
FOR DAYS 3-4 MON-TUE.  A MORE EVEN MODEL/ENSEMBLE MEAN WEIGHT
(INTRODUCING THE 12Z ECMWF MEAN) SERVED AS THE DAY 5 WED STARTING
BLEND ON THE WAY TO AN EXCLUSIVE ENSEMBLE MEAN SOLUTION BY DAYS
6-7 THU-FRI.


...WEATHER HIGHLIGHTS...

ACROSS THE WEST, PROGRESSIVE EARLY-MID WEEK SYSTEMS MAY GENERATE A
COUPLE PERIODS OF RAIN/HIGH ELEVATION SNOW.  WITH UPPER LEVEL
ENERGY POTENTIALLY CLOSING OFF A LOW THAT CROSSES THE FOUR CORNERS
REGION, THE FIRST SYSTEM IS LIKELY TO BE THE MORE PRODUCTIVE ONE.
LATE PERIOD EASTERN PACIFIC TROUGHING ALOFT MAY BRING ANOTHER AREA
OF MOISTURE TO THE NORTHWEST TOWARD THE END OF THE WEEK.  THE
WASHINGTON OLYMPICS/CASCADES HAVE THE BEST PROBABILITY OF SEEING
THE HIGHEST 5-DAY PRECIP TOTALS BUT UNCERTAINTY IN SHORTWAVE
DETAILS ALOFT ALLOW POTENTIAL FOR OTHER AREAS TO SEE FOCUSED
ACTIVITY AS WELL.  LEADING SYSTEM EMERGING FROM THE WEST MAY
GENERATE AN AXIS OF MEANINGFUL RAINFALL ACROSS THE CENTRAL PLAINS
AROUND MIDWEEK.  AMOUNTS WILL DEPEND ON STRENGTH/TIMING OF THE
UPPER SYSTEM.  THE NORTHEAST QUADRANT OF THE LOWER 48 SHOULD SEE
LIGHT TO PERHAPS LOCALLY MODERATE PRECIP IN ASSOCIATION WITH
FRONTAL PASSAGES.  EXPECT SNOW CHANCES TO BE CONFINED TO NEAR THE
CANADIAN BORDER.

DOMINANCE OF PACIFIC FLOW ALOFT WILL PROMOTE NEAR TO ABOVE NORMAL
TEMPS OVER MUCH OF THE LOWER 48 DURING THE PERIOD.  THE MOST
ANOMALOUS WARMTH WILL EXTEND FROM THE INTERIOR WEST THROUGH THE
CENTRAL U.S. MON AND TO SOME EXTENT INTO TUE.  MAX AND/OR MIN
READINGS AS HIGH AS 20-30F ABOVE NORMAL SHOULD CHALLENGE DAILY
RECORD VALUES AT SOME LOCATIONS.  DOUBLE-DIGIT ANOMALIES WILL
LIKELY TAPER OFF IN COVERAGE FOR THE LATTER HALF OF THE WEEK.

RAUSCH

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