EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 154 AM EST MON DEC 12 2016 VALID 12Z THU DEC 15 2016 - 12Z MON DEC 19 2016 ...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... GUIDANCE OFFERS SIMILAR IDEAS FOR THE EXPECTED MEAN PATTERN... EVOLVING TOWARD AN AMPLIFIED POSITIVELY TILTED TROUGH BY THE WEEKEND BTWN A NERN PAC/NWRN NOAM RIDGE AND STRENGTHENING SRN GULF/CARIBBEAN RIDGE STARTING TO BUILD INTO THE ATLC. CONUS TROUGHING SHOULD BECOME MORE SHALLOW BY EARLY NEXT WEEK AS A STRONG JET CROSSING THE NRN PACIFIC COMES INTO THE PICTURE. WITHIN THIS MEAN FLOW THERE IS STILL MORE THAN THE USUAL DEGREE OF UNCERTAINTY FOR FCST DETAILS ORIGINATING FROM A TROUGH/PSBL EMBEDDED CLOSED LOW EXPECTED TO BE OFF THE WEST COAST AS OF EARLY THU. THUS FAR SOLNS CONTINUE TO DISPLAY GREATER THAN DESIRED SPREAD AND NOTABLE TRENDING EVERY 12-24 HRS. EARLY IN THE PERIOD LATEST RUNS SHOW A GREATER NWD PUSH OF MSTR/WARMER AIR OVER THE N-CNTRL WEST IN RESPONSE TO A MORE AMPLIFIED OFFSHORE TROUGH AND DOWNSTREAM RIDGE ALONG WITH AN ASSOC FARTHER NWD TRACK OF SFC LOW PRES. THIS SYSTEM THEN AFFECTS THE FCST E OF THE ROCKIES FRI ONWARD. OVER THE PAST COUPLE DAYS THERE HAS BEEN A GOOD SIGNAL FOR LOW PRES TO TRACK FROM ABOUT THE CNTRL PLAINS NEWD THROUGH THE GRTLKS AND/OR NORTHEAST BUT WITH SIGNIFICANT DIFFS IN SYSTEM STRENGTH AND TIMING. BEYOND UNCERTAINTIES WITH HANDLING OF THE INITIAL ERN PAC DYNAMICS... ONCE THE SYSTEM REACHES THE CNTRL U.S. THERE IS ALSO THE QUESTION OF HOW MUCH NRN STREAM INTERACTION THERE WILL BE. IN A POSITIVE TREND THE 00Z GUIDANCE THUS FAR AND OLD 12Z ECMWF HAVE BECOME NOTABLY WEAKER THAN THE VERY DEEP SYSTEM SEEN IN YDAYS 00Z GFS/ECMWF RUNS. BY SUN-MON SPECIFICS OF FAST NRN PAC FLOW WILL BEGIN TO INFLUENCE THE FCST WHILE INDIVIDUAL SOLNS SHOW VARYING DEGREES OF FLOW SEPARATION THAT MAY OCCUR WITHIN THE OVERALL MEAN TROUGH. THESE DIFFS LEAD TO THE SPREAD IN FRONTAL TIMING/WAVINESS OVER THE ERN STATES LATE IN THE PERIOD. A 60/40 BLEND OF THE 12Z ECMWF MEAN AND 18Z GEFS MEAN OFFERED THE BEST APPROACH TO ACCOUNT FOR SOME ADJUSTMENTS IN LATEST GUIDANCE OVER THE WEST WHILE MAINTAINING A FAIRLY CONSISTENT FCST FOR THE SYSTEM EMERGING FROM THE PLAINS. LATE IN THE PERIOD THIS SOLN PROVIDED AN INTERMEDIATE ERN U.S. FRONTAL TIMING BTWN THE SLOW 18Z GFS AND FASTER 12Z ECMWF. ...WEATHER HIGHLIGHTS/THREATS... EARLY IN THE PERIOD EXPECT SOME LAKE EFFECT SNOW IN THE WAKE OF A DEEP UPR LOW TRACKING ACROSS SERN CANADA. OTHERWISE THE DOMINANT FOCUS WILL BE ON THE SYSTEM THAT WILL INITIALLY AFFECT THE WEST AND THEN SPREAD PRECIP OF VARIOUS TYPES ACROSS THE ERN 2/3 OF THE COUNTRY. EARLY IN THE PERIOD MUCH OF THE WEST SHOULD SEE AT LEAST SOME PCPN WITH HIGHEST TOTALS LIKELY TO BE OVER WINDWARD TERRAIN IN THE SIERRA NEVADA RANGE WITH LESSER RELATIVE MAXIMA IN THE NRN-CNTRL ROCKIES. DEPENDING ON EXACTLY HOW THE SYSTEM EVOLVES THERE MAY ALSO BE A BRIEF PERIOD OF ENHANCED PCPN ALONG THE COASTAL RANGES OF SRN CA. THERE REMAINS A CONTINUED POTENTIAL FOR INPUT FROM TWO SEPARATE AREAS OF LOW LATITUDE MSTR NEAR AND WELL E OF HAWAI'I. MOST PCPN SHOULD BE SNOW EXTREME NORTH WHILE FARTHER SWD SNOW LVLS SHOULD START OUT FAIRLY HIGH BUT DECREASE WITH TIME AS TROUGHING ALOFT COMES INLAND. MOST OF THE WEST SHOULD TREND DRIER DURING THE WEEKEND/EARLY NEXT WEEK BUT FAST NRN PAC FLOW MAY BEGIN TO BRING MSTR TO PARTS OF THE PAC NW. WRN MSTR WILL FIRST STREAM ACROSS THE NRN TIER LATE THIS WEEK IN THE FORM OF SNOW WITH LOW LVL UPSLOPE FLOW PSBLY INCREASING AMTS OVER THE NRN HIGH PLAINS. FROM FRI NIGHT INTO THE WEEKEND LOCATIONS FROM THE OH VLY/MID ATLC INTO THE NORTHEAST MAY SEE A PROGRESSION FROM SNOW/WINTRY MIX TO RAIN AS WARM FLOW PUSHES NEWD AHEAD OF THE PLAINS-GRTLKS SYSTEM BUT INITIALLY INTERACTS WITH LINGERING COLD AIR. BEST POTENTIAL FOR MOSTLY SNOW CONTINUES TO EXTEND FROM THE NRN PLAINS INTO THE UPR GRTLKS. IN THE WARM SECTOR THERE MAY BE ONE OR MORE BANDS OF HVY RNFL OVER THE TN/OH VLY REGION AND PSBLY EXTENDING INTO THE LWR MS VLY. EVEN IF THE SFC LOW IS NOT AS STRONG AS SOME EARLIER MODEL RUNS SUGGESTED THERE MAY BE A SUFFICIENTLY TIGHT SFC GRADIENT TO PROMOTE A PERIOD OF STRONG WINDS. FOR TEMPS... CNTRL-SRN PARTS OF THE WEST SHOULD BE QUITE WARM THU INTO EARLY FRI WITH SOME PSBL RECORD WARM MINS AND MAYBE AN ISOLD RECORD HIGH... IN CONTRAST TO 10-20F BELOW NORMAL READINGS ACROSS FAR NRN AREAS. SWD PUSH OF COLD AIR WILL BRING BELOW NORMAL TEMPS ACROSS MUCH OF THE WEST FOR THE WEEKEND. ERN STATES WILL BE VERY COLD THU-FRI WITH SOME MINUS 20-30F ANOMALIES AND POTENTIALLY SCT RECORD COLD MIN/MAX READINGS FROM THE MIDWEST INTO NORTHEAST... FOLLOWED BY A RAPID WARMUP AHEAD OF THE PLAINS/GRTLKS SYSTEM AND THEN ANOTHER PUSH OF COLD AIR AFTER FRONTAL PASSAGE. THE SOUTHEAST/FLORIDA WILL LIKELY SEE 2-3 DAYS OF ABOVE NORMAL TEMPS. SOME LOCATIONS IN THE EXTREME NRN PLAINS MAY BE AT LEAST 25-30F BELOW NORMAL FRI-SAT WITH MODESTLY LESS EXTREME ANOMALIES PUSHING SWD THROUGH THE PLAINS SAT-SUN. TREND TOWARD FLATTER MEAN TROUGHING ALOFT SHOULD PROMOTE A WARMING TREND... THOUGH STILL WITH BELOW NORMAL TEMPS... OVER THE NRN TIER BY NEXT MON. RAUSCH