EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 142 AM EST TUE DEC 29 2015 VALID 12Z FRI JAN 01 2016 - 12Z TUE JAN 05 2016 ...OVERVIEW... THROUGH AT LEAST THE WEEKEND THERE IS REASONABLE AGREEMENT FOR ERN NOAM MEAN TROUGHING DOWNSTREAM FROM A WRN CANADA RIDGE. THERE IS ALSO DECENT CONSENSUS WITH PROGRESSIVE SRN STREAM FLOW AND IN PARTICULAR A WELL DEFINED TROUGH THAT SHOULD HEAD TOWARD SRN CA/NWRN MEXICO BY THE START OF NEXT WEEK AND CONTINUE EWD THEREAFTER. ADDITIONAL TROUGH ENERGY MAY APPROACH THE WEST COAST TOWARD THE END OF THE PERIOD NEXT TUE. SURROUNDED BY THESE FEATURES AN INTERIOR WEST UPR LOW IS STILL FCST TO FORM LATE IN THE WEEK WITH VARIOUS PSBL PATHS THEREAFTER. SHRTWV DETAILS DOWNSTREAM FROM WRN CANADA BY EARLY NEXT WEEK BECOME PROBLEMATIC WITH THE COMBINATION OF THE UNCERTAIN PATH OF THE WRN UPR LOW AND EVOLUTION TOWARD SHARPER/MORE AMPLIFIED RIDGING OVER WRN CANADA THROUGH ALASKA. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... FAST FLOW WITHIN THE ERN NOAM MEAN TROUGH WILL MAKE IT DIFFICULT FOR GUIDANCE TO MAINTAIN MUCH SEMBLANCE OF CONTINUITY/CONSENSUS WITH INDIVIDUAL FRONTS/TROUGHS ASSOC WITH INDIVIDUAL SHRTWVS. THE BEST DEFINED FEATURE IS STILL AN UPR LOW FOR WHICH THERE IS GRADUALLY IMPROVING CONSENSUS TOWARD A TRACK E OF HUDSON BAY RATHER THAN FARTHER SWD CLOSE TO MAINE AS IN SOME PRIOR MODEL RUNS. THE 12Z GFS WAS THE LAST PIECE OF GUIDANCE TO SHOW THE MORE SWD TRACK. BY EARLY NEXT WEEK MODEL/ENSEMBLE SPREAD INCREASES AS NERN PAC ENERGY ROUNDS THE WRN CANADA RIDGE AND IN ITS WAKE RIDGING AMPLIFIES/SHARPENS INTO ALASKA. THIS EVOLUTION COMBINED WITH PSBL EMERGENCE OF THE INTERIOR WEST UPR LOW LEAD TO VARIOUS MODEL/ENSEMBLE MEMBER POSSIBILITIES FOR SHRTWV DETAILS. THE MOST COMMON THEME BY DAY 7 TUE... AS REPRESENTED IN LATEST GEFS/ECMWF ENSEMBLE MEANS... IS FOR SHRTWV ENERGY TO BEGIN DIGGING A LITTLE FARTHER WWD THUS RAISING HGTS ALOFT OVER THE EAST. THERE ARE STILL CONSIDERABLE TRACK DIFFS FOR THE WRN UPR LOW. AS A STARTING POINT FOR THE FIRST HALF OF THE PERIOD PREFERRED A COMPROMISE PRIMARILY INVOLVING THE 12Z ECMWF/12Z UKMET/18Z GFS. THIS SOLN REFLECTED IMPROVING AGREEMENT ON EXISTENCE AND STRENGTH OF THE UPR LOW WHILE RECENT ENSEMBLE MEANS APPEARED TO BE IN THE PROCESS OF TRENDING TOWARD THE OPERATIONAL GUIDANCE. 00Z GFS/UKMET TRENDS ARE IN THE DIRECTION OF THE MORE NRN 12Z ECMWF. ASIDE FROM DETAIL DIFFS WITH THE SFC REFLECTION AND A SPORADIC OPERATIONAL RUN THAT TAKES A MORE SRN PATH... THERE IS BETTER THAN AVG AGREEMENT AND CONTINUITY WITH THE SYSTEM EXPECTED TO REACH JUST OFFSHORE CA BY DAY 6 MON. A SLIGHTLY DEEPER VERSION OF THE 18Z GEFS/12Z ECMWF MEANS PROVIDED THE MOST CONSISTENT EVOLUTION. THE 12Z ECMWF AND NEW 00Z GFS ARE FAIRLY CLOSE TO THIS SOLN. BASED PRIMARILY ON THE DESIRE FOR AN INTERMEDIATE TRACK FOR THE WRN UPR LOW... THE FIRST HALF OF THE FCST CONSISTED MOSTLY OF THE 12Z ECMWF/UKMET AND 18Z GFS. THEN DEVELOPING UNCERTAINTIES WITH THE ULTIMATE PATH OF THIS LOW AS WELL AS FLOW ACROSS THE ERN 2/3 OF SRN CANADA AND NRN TIER U.S. LED TO AN EVEN BLEND OF THE 18Z GEFS/12Z ECMWF MEANS FOR DAYS 6-7 MON-TUE. ...SENSIBLE WEATHER HIGHLIGHTS... EXPECT BELOW AVG COVERAGE OF PCPN ACROSS THE LOWER 48 WITH ACTIVITY PRIMARILY CONFINED TO LAKE EFFECT SNOW... ONE OR MORE DAYS OF PCPN FROM THE RIO GRANDE VLY EWD... AND WITH MSTR COMING INTO CA/SWRN STATES AFTER SUN. IN THE CASE OF THE LATTER TWO THERE IS SIGNIFICANT UNCERTAINTY IN PRECISE TOTALS AT SOME LOCATIONS. POTENTIAL STILL EXISTS FOR SOME PCPN TO BE IN THE FORM OF SNOW NEAR THE BIG BEND REGION OF TX. BELOW NORMAL TEMPS WITH MINUS 10-20F OR SO ANOMALIES FROM THE INTERIOR WEST INTO THE SRN PLAINS AT THE START OF THE PERIOD FRI SHOULD MODERATE WITH TIME. FARTHER EWD EXPECT TEMPS TO TREND A LITTLE WARMER AS WELL... THOUGH FROM ONLY MODERATELY BELOW NORMAL READINGS. NRN TIER LOCATIONS SHOULD BE ABOVE NORMAL MOST OF THE PERIOD. RAUSCH