Preliminary Forecasts
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FXUS02 KWNH 050642
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 AM EST MON DEC 05 2016

VALID 12Z THU DEC 08 2016 - 12Z MON DEC 12 2016

...OVERVIEW...

ONCE AN AMPLIFIED TROUGH CROSSES THE EAST LATE THIS WEEK MOST
GUIDANCE SHOWS MEAN FLOW ACROSS THE LOWER 48 EVOLVING TOWARD A
FLAT/BROADLY CYCLONIC CONFIGURATION WHILE HGTS ALOFT TREND LOWER
OVER CANADA.  THIS PATTERN IS CONSISTENT WITH TELECONNECTIONS
ASSOC WITH A PERSISTENT RIDGE/CORE OF POSITIVE HGT ANOMALIES OVER
OR NEAR NWRN ALASKA.  FOR A MULTI-DAY TIME FRAME TELECONS SHOW
ABOVE AVG PRECIP POTENTIAL ALONG PORTIONS OF THE WEST COAST AND
WELL BELOW NORMAL TEMPS CNTRD OVER THE NRN PLAINS.  LOW AMPLITUDE
NATURE OF THE MEAN FLOW MID-LATE PERIOD AND COMPLEXITIES OF SRN
ALASKA/NRN-NERN PAC FLOW WILL LIKELY CONTRIBUTE TO A PERIOD OF LOW
PREDICTABILITY FOR FEATURES EMBEDDED WITHIN THE LONGWAVE PATTERN.


...GUIDANCE EVALUATION/PREFERENCES...

FROM DAY 3 THU INTO EARLY DAY 6 SUN THE UPDATED FCST STARTS WITH A
BLEND OF HALF 18Z GEFS MEAN AND THE REMAINDER 12Z ECMWF/ECMWF
MEAN.  THE 12Z ECMWF IS PHASED OUT AFTER LATE SUN IN FAVOR OF AN
EVEN BLEND OF GEFS/ECMWF MEANS BY DAY 7 MON.  THIS SOLN PROVIDES
FAIRLY GOOD CONTINUITY AS GUIDANCE IS STILL IN THE PROCESS OF
RESOLVING DETAILS OF THE INITIAL TROUGH CROSSING THE EAST AND
CONFIDENCE IN SPECIFICS RAPIDLY DECREASES UPSTREAM.

MULTI-DAY TRENDS FOR THE ERN TROUGH AND ASSOC SFC EVOLUTION
CONTINUE TO POINT TOWARD AN INTERMEDIATE SOLN.  ECMWF RUNS HAVE
BEEN ON THE SLOWER SIDE BUT GRADUALLY TRENDING FASTER WHILE LATEST
GFS RUNS HAVE TRENDED AWAY FROM AN AGGRESSIVE SPINUP OF MID LVL
ENERGY/SFC LOW PRES OVER NEW ENGLAND AND THE CANADIAN MARITIMES AS
SEEN IN SOME EARLIER RUNS.  NEW 00Z GUIDANCE THUS FAR STILL
RECOMMENDS ADJUSTING THE 12Z ECMWF/ECMWF SOMEWHAT E-NE WITH SFC
DEVELOPMENT BY DAY 4 FRI.

MEANWHILE GUIDANCE SPREAD FOR LOW PRES OFF THE PAC NW COAST AS OF
DAY 3 THU IS UNUSUALLY HIGH FOR A DAYS 3-4 FCST.  GFS RUNS
CONTINUE TO LIE IN THE ERN SIDE OF THE GUIDANCE SPREAD... ALBEIT
WITHIN THE FULL ENSEMBLE ENVELOPE.  THE PREFERRED BLEND REFLECTS
THE STRONG MAJORITY OF GUIDANCE THAT KEEPS THE SYSTEM FARTHER AWAY
FROM THE PAC NW LATE THIS WEEK.  CONSENSUS WAITS UNTIL AROUND DAY
5 SAT TO BRING THE SFC LOW TO NEAR VANCOUVER ISLAND.

THE FCST BECOMES INCREASINGLY DIFFICULT MID-LATE PERIOD WITH
INDIVIDUAL MODELS AND ENSEMBLES SHOWING A VARIETY OF POSSIBILITIES
FOR INDIVIDUAL SHRTWV FEATURES.  THE GEFS/ECMWF MEANS REFLECT THE
TELECONNECTION-FAVORED LARGE SCALE PATTERN FAIRLY WELL AND THUS
OFFER THE MOST PLAUSIBLE SOLNS OVERALL.  THE MOST PROMINENT
UNCERTAINTY IN THE SAT-MON PERIOD IS THE EVOLUTION OF MID LVL
ENERGY AND CORRESPONDING SFC REFLECTION FROM THE HIGH PLAINS INTO
THE E-CNTRL U.S..  THUS FAR OPERATIONAL MODEL RUNS HAVE BEEN QUITE
VARIABLE ESPECIALLY AT THE SFC.  SIMILAR TO YDAY THE MOST COMMON
SCENARIO APPEARS TO BE FOR SFC LOW PRES TO TRACK INTO THE UPR
GRTLKS BY DAY 6 SUN.  FROM THAT POINT THE LOW MAY CONTINUE ONWARD
OR DECELERATE FOR A TIME WHILE WAITING FOR UPSTREAM ENERGY.


...WEATHER HIGHLIGHTS/THREATS...

COLD AIR OVER THE CNTRL U.S. ON DAY 3 THU... WITH TEMPS 15-30F
BELOW NORMAL... WILL PUSH INTO THE EAST FOR LATE THU THROUGH SAT
WITH CONSIDERABLE COVERAGE OF MINUS 10-20F ANOMALIES OVER
CNTRL-SRN AREAS.  EXPECT LESS EXTREME READINGS OVER THE
GRTLKS/NORTHEAST.  AT THE SAME TIME THE PLAINS WILL TREND WARMER
EXCEPT FOR FAR NRN AREAS WHICH MAY REMAIN WELL BELOW NORMAL INTO
AT LEAST EARLY NEXT WEEK.  BRISK AND COLD CYCLONIC FLOW ACROSS THE
GRTLKS SHOULD PROMOTE AREAS OF LAKE EFFECT SNOW DURING THU-FRI.
FROM THE WEEKEND INTO EARLY NEXT WEEK... POTENTIAL SYSTEM EMERGING
FROM THE HIGH PLAINS DURING THE WEEKEND AND RETURN FLOW AROUND
HIGH PRES TRACKING FROM THE ERN STATES INTO THE ATLC MAY BRING
SOME SNOW ACROSS THE NRN PLAINS/GRTLKS AND PSBLY INTO THE
NORTHEAST WHILE RAIN WILL BE PSBL OVER SRN AREAS OF THE E-CNTRL
U.S..  DETAIL UNCERTAINTIES SFC/ALOFT LEAD TO LOW CONFIDENCE IN
EXACT LATITUDE OF THE RAIN-SNOW LINE AT THIS TIME.

OVER THE WEST... THE MEAN PATTERN WILL FAVOR A WINDWARD FOCUS FOR
ENHANCED PCPN FROM THE PAC NW AND NRN HALF OF CA EWD INTO THE
ROCKIES.  FOR THE 5-DAY PERIOD AT LEAST SEVERAL INCHES OF LIQUID
EQUIV ARE PSBL OVER PORTIONS OF THE PAC NW COAST/CASCADES AND NRN
CA/SIERRA NEVADA RANGE WITH LESSER AMTS REACHING THE ROCKIES.
POTENTIAL REMAINS FOR A PERIOD OF STRONG WINDS OVER THE PAC NW
LATE THIS WEEK.  MODESTLY BELOW NORMAL TEMPS ON THU SHOULD RETURN
TO WITHIN A FEW DEGS ABOVE/BELOW NORMAL FOR FRI AND THEN TEND TO
LEAN MORE BELOW NORMAL THAN ABOVE THEREAFTER.

RAUSCH

$$





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