EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1110 AM EST THU DEC 07 2017 VALID 12Z SUN DEC 10 2017 - 12Z THU DEC 14 2017 ...A MIDWEST/GREAT LAKES THROUGH NERN US HEAVY SNOW THREAT... ...PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT.. THE 06 UTC GFS/GEFS AND 00 UTC ECMWF/ECMWF ENSEMBLES OFFER A WELL CLUSTERED FORECAST SOLUTION THAT IMPLIES BELOW NORMAL UNCERTAINTY OVERALL DESPITE MODEST RUN TO RUN VARIABLILITY. A FAVORED COMPOSITE SOLUTION LOCKS IN A FULL LATITUDE WRN NORTH AMERICAN RIDGE/ERN NORTH AMERICAN TROUGH PATTERN ALOFT. THIS WILL CONTINUE A SYNOPTIC PATTERN OVER THE U.S. WITH MILD/DRY CONDITIONS IN THE WEST...A TRANSITION ZONE OF TEMPS IN THE PLAINS TO MS VALLEY...AND COLD FLOW INTO THE ERN THIRD OF THE NATION WHERE HEAVY SNOW POTENTIAL WILL FOCUS FROM THE MIDWEST/GREAT LAKES TO THE NRN MID-ATLANTIC/NORTHEAST. THIS PATTERN WILL BE HIGHLIGHTED BY A SERIES OF POTENT SHORTWAVES AND ATTENDANT CLIPPER SYSTEMS/SNOW SWATHS THAT WILL DIG TO THE LEE OF THE WRN NOAM RIDGE TO THE UPPER MIDWEST AND MIDWEST/GREAT LAKES THEN OUT ACROSS THE ERN US WITHIN THE ERN US TROUGH. REINFORCED COLD AIR ADVECTION WILL ALSO ENHANCE LAKE EFFECT SNOWS IN THE WAKE OF SYSTEM PASSAGES THAT DO OFFER SOME TIMING VARIANCE. IN THE WAKE OF A NOW SHORT RANGE DEEPENED LOW TO EFFECT NEW ENGLAND AND CANADIAN MARITIMES THIS WEEKEND...ANOTHER UPSTREAM LOW ALSO HAS AMPLE POTENTIAL TO FURTHER DEVELOP AS A COASTAL STORM OFF THE NERN US TUE-WED AS ENERGY TRANSFERS OFFSHORE. MODELS AND ENSEMBLES ARE TRENDING TO SUPPORT A HEAVY SNOW THREAT IN THE COLD AIR ALONG/NORTH OF THE LOW TRACK FROM THE NRN MID-ATLANTIC TO THE NORTHEAST. MOISTURE IS RELATIVELY MODEST...BUT THE LOW AND SUPPORT ALOFT SEEM QUITE FAVORABLE. SCHICHTEL