Prognostic Meteorological Discussion
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000
FXUS02 KWBC 281600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST SUN DEC 28 2014

VALID 12Z WED DEC 31 2014 - 12Z SUN JAN 04 2015

...OVERVIEW...

THE LARGE SCALE FCST REMAINS ON TRACK... WITH AN EXPECTED
MIGRATION OF GREATEST POSITIVE HGT ANOMALIES FROM THE EXTREME NERN
PAC TO OR BEYOND NWRN MAINLAND ALASKA SUPPORTING A FLATTER TREND
WITH THE DOWNSTREAM MEAN TROUGH EXTENDING SWWD FROM CANADA INTO
THE LOWER 48.  ON A MULTI-DAY AVG BASIS THIS EVOLUTION SHOULD
BRING TEMPS CLOSER TO NORMAL OVER THE SRN HALF OF THE WEST BUT
MAINTAIN BELOW NORMAL READINGS FROM THE NORTHWEST INTO THE GRTLKS
AND SWD INTO THE PLAINS.  THERE IS STILL A FAIR AMOUNT OF
UNCERTAINTY IN THE EMBEDDED DETAILS THOUGH.  SOME TIMING DIFFS
PERSIST FOR THE UPR LOW EJECTING FROM THE SOUTHWEST AS WELL AS
WITH A FRONT DROPPING INTO THE NRN TIER LATE THIS WEEK... AND
DEVELOPING COMPLEXITIES WITH NERN PAC/ALASKA FLOW MAY HAVE AN
INFLUENCE OVER THE CONUS BY LATE WEEK/WEEKEND WHEN THE DETAILS OF
STREAM INTERACTION WILL LIKELY PLAY A CRUCIAL ROLE IN HOW THE SFC
LOW PRESSURE EVOLVES OVER THE ERN HALF OF THE CONUS.


...GUIDANCE EVALUATION/PREFERENCES...

TRACK DIFFS FOR THE SWRN UPR LOW HAVE IMPROVED BUT TIMING ISSUES
REMAIN.  IN RECENT DAYS THE MOST EXTREME SOLNS WHETHER ON THE FAST
OR SLOW SIDE OF THE ENVELOPE HAVE TENDED TO GRAVITATE TOWARD THE
INNER HALF OR 2/3 OF THE SPREAD.  THUS WOULD TEND TO LEAN AWAY
FROM THE 00Z CMC THAT IS ON THE FAST SIDE AS WELL AS THE 00Z/06Z
PARALLEL GFS WHICH TREND SLOWER THAN MOST OTHER GUIDANCE AFTER 12Z
FRI.  IT APPEARS TO BE AROUND FRI-FRI NIGHT WHEN THE EXACT DETAILS
OF NRN STREAM FLOW WILL BECOME CRITICAL WITH RESPECT TO HOW
QUICKLY THE EJECTING SYSTEM CONTINUES NEWD AND HOW THE SFC PATTERN
EVOLVES OVER THE ERN HALF OF THE CONUS.  UNFORTUNATELY IT IS
AROUND DAYS 4-5 THU-FRI WHEN SIGNIFICANT COMPLEXITIES DEVELOP WITH
FLOW OVER AND JUST S OF ALASKA... AND ISSUES OVER THIS AREA REACH
DOWN TO THE NWRN QUADRANT OF THE CONUS AROUND THE TIME THAT WILL
BE CRITICAL FOR DETERMINING WHAT HAPPENS WITH THE EJECTING SWRN
SYSTEM.  PARALLEL GFS RUNS THAT ARE SLOW WITH THE EJECTING SWRN
UPR LOW ARE STRONGEST WITH A COMPACT UPR LOW THAT COULD PASS TO
THE S OF ALASKA... WITH SOME OTHER SOLNS NOT EVEN SHOWING SUCH A
LOW.  THEREFORE THE GFS-P SCENARIO THAT ALSO RESULTS IN A FAST NRN
TIER SFC WAVE AFTER FRI IS CONSIDERED LESS PROBABLE.  FARTHER EWD
THE 00Z ECMWF BECOMES LESS FAVORED BY DAYS 6-7 SAT-SUN AS IT SHOWS
GREATER SEWD AMPLITUDE ALOFT THAN THE MAJORITY OF ENSEMBLE
MEMBERS... OR WHAT THE STRENGTHENING RIDGE OFF THE SOUTHEAST...
WOULD RECOMMEND.

FOR DAYS 3-5 WED-FRI THERE IS SUFFICIENT CLUSTERING AND ENSEMBLE
MEAN SUPPORT TO START WITH AN OPERATIONAL BLEND CONSISTING OF THE
00Z/06Z GFS AND 00Z ECMWF.  FROM LATE FRI ONWARD THE 06Z GEFS/00Z
ECMWF ENSEMBLE MEANS ARE GIVEN MORE WEIGHTING DUE TO THEIR MORE
GRADUAL RUN TO RUN ADJUSTMENTS FOR TIMING/EVOLUTION OF SFC
DETAILS.  BASED ON THE MEAN PATTERN IT SEEMS THAT ONCE THE SYSTEM
PASSES THE HIGH PLAINS IT SHOULD BECOME INCREASINGLY
PROGRESSIVE... WHICH THE MEANS AND 00Z-06Z GFS/00Z ECMWF AT LEAST
AGREE UPON IN PRINCIPLE.  THE MAJORITY SOLN THUS FAR HAS BEEN FOR
LOW PRESSURE TO TRACK THROUGH THE GRTLKS INTO SERN CANADA BUT
ISOLD PIECES OF GUIDANCE HAVE ALSO SUGGESTED POTENTIAL FOR A
PRIMARY SFC LOW W OF THE APLCHNS AND SECONDARY DEVELOPMENT SUCH AS
MOST RECENTLY DEPICTED IN THE 00Z CMC.  PREFER TO MAINTAIN THE
MAJORITY IDEA UNTIL THERE ARE STRONGER TRENDS AWAY FROM IT.


...SENSIBLE WEATHER HIGHLIGHTS...

THE UPR LOW INITIALLY OVER THE SWRN STATES SHOULD PRODUCE AREAS OF
PCPN FROM SRN CA THROUGH THE SRN ROCKIES MID-LATE WEEK... WITH
ENHANCED PCPN MOST LIKELY OVER FAVORED TERRAIN IN AZ/SRN UT.  ALSO
NOTE THAT SNOW LVLS SHOULD BE UNUSUALLY LOW WITH THIS SYSTEM.
MSTR WILL STREAM NEWD FROM THE SRN PLAINS WITH TIME AND BEST
POTENTIAL FOR HEAVIEST PCPN SHOULD EXTEND NEWD FROM THE SERN
PLAINS/W-CNTRL GULF COAST.  CONTINUED UNCERTAINTY WITH PRECISE
EVOLUTION SFC/ALOFT LEADS TO LESS THAN DESIRED CONFIDENCE
REGARDING DISTRIBUTION/INTENSITY OF VARIOUS WINTRY PCPN TYPES IN
THE NRN PART OF THE MSTR SHIELD.  MEANWHILE EXPECT SOME SNOW IN
THE VICINITY OF A COLD FRONT DROPPING INTO THE NRN TIER... WITH
NWLY FLOW ALOFT AND/OR LOW LVL UPSLOPE FLOW FORM THE E SUPPORTING
AREAS OF SNOW FROM THE NORTHWEST INTO NRN PLAINS.  RAIN SHOULD BE
CONFINED TO COASTAL/VERY LOW ELEVS OF THE PAC NW.

AS THE UPR LOW CROSSES THE SOUTHWEST EXPECT SOME HIGHS OVER THE
REGION TO BE 10-25F BELOW NORMAL MID-LATE WEEK.  SIMILARLY COLD
TEMPS OVER THE PLAINS TO START THE PERIOD SHOULD MODERATE
BRIEFLY... BUT AREAS ACROSS THE NRN TIER AND THEN SWD THROUGH THE
PLAINS SHOULD TREND COLDER AGAIN WITH THE COMBINATION OF THE FRONT
PUSHING SWD FROM CANADA THU ONWARD AND PASSAGE OF THE CNTRL-ERN
CONUS SYSTEM LATER IN THE PERIOD.  BY NEXT WEEKEND SERN TEMPS
SHOULD TREND ABOVE NORMAL IN THE WARM SECTOR OF THIS SYSTEM.

RAUSCH

$$




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