EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 901 AM EST TUE DEC 20 2016 VALID 12Z FRI DEC 23 2016 - 12Z TUE DEC 27 2016 ...SIGNIFICANT WINTER STORM POSSIBLE FOR THE INTERIOR WEST AND ROCKIES EASTWARD INTO THE PLAINS AND UPPER MIDWEST FRIDAY INTO MONDAY... OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A POTENTIALLY ROBUST WINTER STORM MAY PUSH THROUGH THE WESTERN STATES LATER THIS WEEK AND LIFT THROUGH THE CENTRAL/NORTHERN PLAINS AND UPPER MIDWEST SUNDAY INTO MONDAY. THE ENSEMBLES HAVE CONTINUED TO SHOW INCREASED SUPPORT FOR A DIGGING WESTERN TROUGH TO SPUR CYCLOGENESIS OVER THE GREAT BASIN BY FRIDAY AND DEEPEN AS IT TURNS NORTHEASTWARD TOWARD THE UPPER GREAT LAKES EARLY NEXT WEEK. WITH THE CYCLONE, THE 00Z CANADIAN/00Z ECMWF APPEAR SLOW, WITH THE 06Z GFS THE BEST MATCH TO THE CURRENT ENSEMBLE MEAN SOLUTIONS. BY THE END OF THE PERIOD, THE ENSEMBLE SPREAD NEAR AND OFF THE WEST COAST INCREASES YET AGAIN TO ABOVE AVERAGE VALUES, WITH THE POSSIBILITY OF ANOTHER CLOSED LOW. DESPITE THE UNCERTAINTY, BELIEVE A COMPROMISE OF THE 06Z GFS AND ENSEMBLE MEANS SHOULD SERVE US WELL AS A STARTING POINT. OVERALL, A COMPROMISE OF THE 00Z CANADIAN/00Z UKMET/06Z GFS/00Z ECMWF WAS ACCEPTABLE FRIDAY INTO SATURDAY BEFORE SWITCHING TO A COMPROMISE OF THE 06Z GFS/00Z NAEFS MEAN/00Z ECMWF ENSEMBLE MEAN SOLUTION THEREAFTER FOR THE WIND GRIDS, PRESSURES, AND 500 HPA HEIGHT PREFERENCE. THE TEMPERATURES, RAIN CHANCES, CLOUDS, DEW POINTS, AND WEATHER WERE MORE HEAVILY BASED ON THE ENSEMBLE MEANS. WEATHER HIGHLIGHTS ~~~~~~~~~~~~~~~~~~ WESTERN STORM SHOULD SPREAD RAIN AND MOUNTAIN SNOW OVER A LARGE AREA FRIDAY/SATURDAY EVEN DOWN TO SOUTHERN CALIFORNIA WHILE THE WEAKENING UPPER LOW TO THE SOUTH SHOULD CONTRIBUTE TO SOME MODEST RAIN/SNOW OVER THE AZ/NM AS WELL. HOW FAR NORTH THE PRECIPITATION GETS INTO WA/ID IS STILL IN QUESTION. SHOULD THE SYSTEM SMARTLY DEEPEN ON THE LEE SIDE OF THE ROCKIES, BLIZZARD CONDITIONS WOULD BE POSSIBLE AND PERHAPS WIDESPREAD ON CHRISTMAS DAY FROM WYOMING NORTHEASTWARD INTO THE DAKOTAS. A WEAKER SYSTEM WOULD STILL SPREAD SNOW FROM THE ROCKIES NORTHEASTWARD, BUT WITH LESS WIND AND LESSER AMOUNTS/COVERAGE. WARM SECTOR PRECIPITATION SHOULD FOCUS ALONG/AHEAD OF THE FRONT FROM THE ARKLATEX TO THE MIDWEST. COLDER TEMPERATURES SHOULD AT LEAST BRIEFLY FOLLOW BEHIND THE SYSTEM WITH EXPECTED WARMING AHEAD OF THE FRONT (PERHAPS 10-20F) -- THOUGH HIGH PRESSURE SLIDING THROUGH NEW ENGLAND ON SUNDAY MAY KEEP THE AREA CLOSER TO AVERAGE. SOUTHEAST RIDGE SHOULD KEEP SOUTHERN AL/GA AND FLORIDA ABOVE AVERAGE BY SEVERAL DEGREES, WHICH IN FL APPROACHES SUMMER-LIKE HEAT. THE EKDMOS 90TH PERCENTILE MAX TEMP VALUES WERE NEAR OR ABOVE DAILY RECORD MAX TEMPS FOR MOST LONG TERM STATIONS IN THE SUNSHINE STATE, SO WOULD NOT BE SURPRISED TO SEE MORE DAILY RECORDS AT LEAST TIED IF NOT BROKEN. ROTH/FRACASSO