Prognostic Meteorological Discussion
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FXUS02 KWBC 040646
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
145 AM EST SUN DEC 04 2016

VALID 12Z WED DEC 07 2016 - 12Z SUN DEC 11 2016

...OVERVIEW...

AN AMPLIFIED/COLD UPR TROUGH WILL PROGRESS FROM THE W-CNTRL U.S.
THROUGH THE EAST MID-LATE WEEK.  UPSTREAM FLOW WILL BE FLATTER IN
THE MEAN WITH SOME DEGREE OF CYCLONIC SHAPE PERSISTING OVER THE
LOWER 48 AS SUGGESTED BY TELECONNECTIONS RELATIVE TO FAIRLY STRONG
RIDGING FCST TO BE JUST NW OF ALASKA.  MODELS/ENSEMBLES ARE STILL
IN THE PROCESS OF RESOLVING IMPORTANT DETAILS OF THE CNTRL-ERN UPR
TROUGH WHILE THE SMALLER SCALE OF UPSTREAM FEATURES FURTHER LOWERS
FCST CONFIDENCE LATER IN THE PERIOD.


...MODEL EVALUATION/PREFERENCES...

FROM DAY 3 WED INTO EARLY DAY 4 THU THE FCST STARTS WITH A BLEND
OF HALF 12Z ECMWF AND THE REMAINDER 12Z-18Z GFS.  THERE IS
CONSIDERABLE MODEL/ENSEMBLE SPREAD WITH WEAK LOW PRES FCST OFF THE
EAST COAST EARLY IN THE PERIOD.  A STARTING POINT BTWN THE
SLOWER/SWWD ECMWF AND SOMEWHAT FARTHER NEWD GFS PROVIDES THE BEST
ACCOUNT FOR THE DISPARITY IN THE GUIDANCE.  SOME INDIVIDUAL RUNS
SUCH AS THE 12Z UKMET AND NEW 00Z CMC ARE QUITE PROGRESSIVE AND
THE NEW 00Z GFS/GEFS MEAN ARE FASTER THAN THE 12Z/00Z RUNS SO
FASTER TIMING MAY BE THE MORE PROBABLE ADJUSTMENT.  THE INITIAL
BLEND AVERAGES OUT TIMING DIFFS FOR THE DEEP CNTRL CONUS TROUGH...
WITH RECENT FASTER TRENDS IN THE ECMWF BRINGING ITS SOLN CLOSER TO
THE MIDDLE OF THE SPREAD.  LEADING FRONTAL EVOLUTION OVER THE EAST
IS SOMEWHAT COMPLEX WITH AN INITIAL BNDRY LIKELY DISSIPATING AFTER
EARLY WED FOLLOWED BY A REFORMING FRONT REACHING A SIMILAR
LOCATION EARLY THU.  MEANWHILE RECENT GFS RUNS ARE ON THE ERN SIDE
OF THE SPREAD WITH STRONG LOW PRES OFF THE PAC NW COAST AT LEAST
INTO THU.  CONSENSUS LIES MORE IN LINE WITH THE 12Z ECMWF WED-THU.

THE REMAINDER OF THE FCST FAVORS A 75/25 WEIGHTING OF THE 18Z GEFS
MEAN VS 12Z ECMWF MEAN.  OVER THE EAST THERE ARE STILL SIGNIFICANT
DIFFS IN HOW ENERGY EVOLVES ON THE ERN SIDE OF THE UPR TROUGH.
GFS RUNS ARE AMONG THE DEEPEST WITH MID LVL EVOLUTION/SFC
DEVELOPMENT FROM NEW ENGLAND INTO THE CANADIAN MARITIMES.  THE
NEXT CLOSEST SOLN IS THE 00Z UKMET THAT IS ESSENTIALLY A WEAKER
VERSION OF THE GFS.  THE 12Z ECMWF SHOWS STRONGEST DEVELOPMENT
FARTHER NEWD.  THE ECMWF MEAN IS ON THE SRN/SWRN SIDE OF THE
CURRENT SPREAD FOR NEW ENGLAND LOW PRES AROUND FRI AND MULTI-DAY
TRENDS HAVE BEEN TOWARD FARTHER EWD SFC DEVELOPMENT.  THUS PREFER
LEANING MORE TO THE 18Z GEFS MEAN AS THE BEST ALTERNATIVE TO LOWER
CONFIDENCE OPERATIONAL MODEL RUNS.

DIFFS THAT ARISE WITH ENERGY TRACKING S OF THE ALEUTIANS AROUND
DAY 4 THU ARE THE STARTING POINT FOR INCREASING SPREAD THAT IS
OBSERVED FROM THE PAC INTO THE LOWER 48 AFTER DAY 5 FRI.  THE FACT
THAT SHRTWV ENERGY COMING INTO THE NORTHWEST ON FRI IS HEADING
TOWARD THE MEAN TROUGH POSN DURING THE WEEKEND WOULD SEEM TO FAVOR
MAINTAINING A STRONGER REFLECTION THAN SEEN IN THE 12Z ECMWF/CMC
MEANS.  EVOLUTION OF PLAINS TO E-CNTRL U.S. SFC LOW PRES SAT-SUN
WILL BE SENSITIVE TO EXACTLY HOW MID LVL ENERGY IS DISTRIBUTED...
A LOW PREDICTABILITY ENDEAVOR TO RESOLVE AT THAT DISTANT OF A TIME
FRAME.  ENSEMBLE GUIDANCE THROUGH THE 12Z/18Z CYCLE SHOWS BEST
POTENTIAL FOR A DEFINED SFC LOW TO TRACK INTO THE GRTLKS RATHER
THAN INTO THE SOUTHEAST AS IN THE 12Z ECMWF.  BEHIND THIS SYSTEM
GUIDANCE IS SIGNALING THE ARRIVAL OF ANOTHER FEATURE TO THE WEST
COAST BY SUN.  AGAIN CONFIDENCE IS LOW IN THE SPECIFICS.


...WEATHER HIGHLIGHTS/THREATS...

VERY COLD TEMPS OVER THE WRN-CNTRL STATES AS OF WED WILL PUSH EWD
WITH TIME.  DURING WED-THU SOME LOCATIONS FROM THE ERN SLOPES OF
THE ROCKIES INTO THE NRN-CNTRL PLAINS SHOULD SEE HIGHS 20-30F
BELOW NORMAL.  COLD AIR REACHING THE EAST LATE IN THE WEEK SHOULD
BRING A COUPLE DAYS OF MINUS 10-20F ANOMALIES.  A WARMING TREND
WILL COMMENCE BEHIND THE CORE OF COLDEST AIR BUT CNTRL-ERN U.S.
TEMPS SHOULD TEND TO REMAIN SOMEWHAT BELOW NORMAL THROUGH NEXT
WEEKEND WITH THE NRN PLAINS LIKELY TO SEE THE MOST PERSISTENT COLD
ANOMALIES.  THERE MAY BE A BAND OF PCPN ACCOMPANYING THE LEADING
COLD FRONT... FOLLOWED BY MEANINGFUL LAKE EFFECT SNOW IN STRONG
CYCLONIC FLOW DURING THE LATTER HALF OF THE WEEK.  POTENTIAL
SYSTEM NEXT WEEKEND MAY SPREAD A SHIELD OF PCPN FROM THE PLAINS
INTO THE EAST WITH SNOW N/RAIN S.

AFTER WED EXPECT A MULTI-DAY PERIOD OF PCPN FOCUSED OVER WINDWARD
TERRAIN FROM THE NRN-CNTRL WEST COAST INTO THE ROCKIES.  HIGHEST
TOTALS WILL LIKELY BE FROM THE PAC NW COAST AND CASCADES INTO THE
NRN SIERRA NEVADA RANGE.  TEMPS OVER THE WEST WILL MODERATE FROM A
CHILLY START TO THE PERIOD WED BUT SHOULD REMAIN COOL ENOUGH TO
KEEP SNOW LEVELS FAIRLY LOW.  DEPENDING ON OFFSHORE SFC LOW
TRACK/STRENGTH THE PAC NW MAY SEE A PERIOD OF ENHANCED WINDS
MID-LATE WEEK.

RAUSCH

$$





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