EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 141 AM EST SAT DEC 03 2016 VALID 12Z TUE DEC 06 2016 - 12Z SAT DEC 10 2016 ...OVERVIEW... DEEP/COLD UPR TROUGHING WILL PROGRESS FROM WEST TO EAST ACROSS THE LOWER 48 DURING THE PERIOD. GUIDANCE AGREES ON THE LARGE SCALE EVOLUTION BUT WITH SIGNIFICANT DISPARITY IN EMBEDDED DETAILS/TIMING OF THE OVERALL TROUGH AFTER WED. WHILE THIS TROUGH REACHES THE EAST LATE IN THE WEEK... UPSTREAM FLOW OVER THE MID-LATITUDE PACIFIC AND EXTENDING INTO THE U.S. WILL TREND FLATTER. THIS WILL LIKELY LEAD TO LOWER PREDICTABILITY FOR SHRTWV DETAILS BY LATE IN THE PERIOD. WELL AHEAD OF THE UPR TROUGH THE GUIDANCE IS CONTINUING TO MAKE SOME ADJUSTMENTS FOR THE SHRTWV ENERGY ORIGINALLY OVER NRN MEXICO AND CROSSING THE EAST ON TUE. ...MODEL EVALUATION/PREFERENCES... DAYS 3-4 TUE-WED CONSIST PRIMARILY OF THE 00Z ECMWF.. 12Z-18Z GFS.. AND 18Z GEFS MEAN. COMPARED TO OPERATIONAL GUIDANCE THE ECMWF MEAN CONTINUES TO APPEAR BEHIND THE CURVE IN PROPERLY DEPICTING THE DETAIL AND TIMING OF EJECTING NRN MEXICO ENERGY. THERE HAS BEEN SOMEWHAT OF A SLOWER TREND OVER THE PAST DAY IN THE OPERATIONAL MODELS AND THUS FAR THIS TREND SEEMS TO BE EXTENDING INTO THE NEW 00Z RUNS. ACROSS THE NRN TIER THE GENERAL TREND HAS BEEN TO EXTEND HGT FALLS SOMEWHAT FARTHER EWD. COMBINATION OF THIS TREND ALONG WITH SLOWER EJECTION OF THE ENERGY TO THE SE RESULTS IN A LOWER PROPORTION OF EARLY TUE TN VLY SFC LOW PRES MAKING IT INTO THE GRTLKS. INSTEAD THERE IS MORE EMPHASIS ON A SLOWER WRN ATLC WAVE LATE TUE ONWARD. TIMING SPREAD BEGINS TO DEVELOP WITH ENERGY CROSSING THE WEST WITH THE RESULTING BLEND STAYING WITHIN THE MIDDLE PORTION OF THE ENVELOPE. FOR DAYS 5-7 THU-SAT THE FCST USES A BLEND OF THE 18Z GEFS/12Z ECMWF MEANS AS RECENT ECMWF/UKMET RUNS LEAN TO THE SLOWER SIDE OF THE GUIDANCE SPREAD FOR THE CNTRL-ERN TROUGH AND GFS RUNS STRAY TOWARD THE FASTER SIDE. SO FAR THERE DOES NOT APPEAR TO BE A PRONOUNCED TREND FAVORING ONE SIDE OF THE SPREAD OR THE OTHER... THUS FAVORING THE ENSEMBLE MEANS WHICH KEEP THE OVERALL TIMING FAIRLY CLOSE TO CONTINUITY. BY THIS PART OF THE FCST THE ADJUSTMENTS MADE FOR COMPONENTS OF THE UPR TROUGH LEAD TO A GENERAL TREND TO DELAY DEVELOPMENT OF DEEPEST SFC LOW PRES UNTIL ERN CANADA/NRN NEW ENGLAND IN CONTRAST TO PRIOR DAYS WHEN SUCH POTENTIAL EXTENDED AS FAR WWD AS THE GRTLKS. WITH FLOW COMING INTO THE NORTHWEST ON THE SRN SIDE OF A STRONG NERN PAC SYSTEM... THE ECMWF/UKMET RUNS ARE ALSO ON THE SLOWER SIDE OF THE SPREAD WHILE THE ENSEMBLE MEANS PROVIDE A GOOD MIDDLE GROUND AND MINIMAL CHANGE IN CONTINUITY. AS ERN PAC INTO WRN-CNTRL U.S. FLOW FLATTENS DAY 7 SAT ENSEMBLE MEMBERS BECOME MORE DIVERSE IN REGARD TO LOCATING INDIVIDUAL SHRTWVS. TELECONNECTIONS ON RIDGING/POSITIVE HGT ANOMALIES NEAR/JUST NW OF ALASKA SUGGEST FLOW SHOULD RETURN TO SOME TYPE OF POSITIVELY TILTED CONUS TROUGH... PERHAPS SHORTLY BEYOND DAY 7... BUT THERE ARE VARIOUS WAYS THIS CAN BE ACHIEVED FROM A MULTI-DAY MEAN PERSPECTIVE. ...WEATHER HIGHLIGHTS/THREATS... THE UPR TROUGH PROGRESSING FROM WEST TO EAST ACROSS THE LOWER 48 WILL BE ACCOMPANIED BY MUCH BELOW NORMAL TEMPS... INITIALLY OVER THE WEST/PLAINS AND THEN REACHING THE EAST. EXPECT THE MOST EXTREME ANOMALIES TO BE OVER THE ERN SLOPES OF THE NRN-CNTRL ROCKIES INTO THE PLAINS TUE-THU WITH HIGHS 20-30F OR SO BELOW NORMAL. ANOMALIES FOR MINS SHOULD BE SOMEWHAT LESS EXTREME EXCEPT OVER MONTANA. AS THE COLD AIR PUSHES EWD AND SFC HIGH PRESSURE EVENTUALLY BUILDS INTO THE SOUTHEAST THE SRN HALF OF THE EAST WILL LIKELY BE COLDEST RELATIVE TO NORMAL WITH MINUS 10-20F ANOMALIES. WHILE THERE IS STILL SOME UNCERTAINTY IN SFC DETAILS... OVERALL PATTERN WILL FAVOR A PERIOD OF COLD AND BRISK CYCLONIC FLOW ACROSS THE GRTLKS/NORTHEAST AND ENHANCED LAKE EFFECT SNOW LATE IN THE WEEK. AHEAD OF THE COLD AIR THERE WILL BE AN AREA OF MOSTLY RAIN ACROSS PORTIONS OF THE EAST ON TUE. THEN UNCERTAINTY IN FRONTAL TIMING OVER THE EAST LEADS TO LOWER THAN DESIRED CONFIDENCE IN DETERMINING COVERAGE/INTENSITY OF RNFL BEFORE THE COLD AIR ARRIVES. ACROSS THE WEST... EARLY PERIOD TROUGH CROSSING THE REGION WILL BRING SOME AREAS OF SNOW TO FAVORED TERRAIN. THEN THE PRIMARY EMPHASIS WILL BE ON MSTR PUSHING INTO THE NORTHWEST BY THE LATTER HALF OF THE WEEK AND THEN EXTENDING INTO THE NRN ROCKIES. FAVORED LOCATIONS ALONG THE PAC NW/EXTREME NRN CA COAST AND THE CASCADES WILL LIKELY SEE THE HIGHEST PCPN TOTALS. WRN U.S. TEMPS WILL MODERATE COMPARED TO TUE-WED BUT THERE SHOULD BE ENOUGH LINGERING COLD AIR TO PRODUCE SNOW AT FAIRLY LOW ELEVS. RAUSCH