EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1057 AM EST MON JAN 01 2018 VALID 12Z THU JAN 04 2018 - 12Z MON JAN 08 2018 ...OVERVIEW/GUIDANCE ASSESSMENT AND PREFERENCES... THE MEDIUM RANGE PERIOD STARTS OUT WITH DEEP TROUGHING ACROSS THE EASTERN STATES AND A RAPIDLY DEEPENING CYCLONE IN THE WESTERN ATLANTIC WITH SHORTWAVE ENERGY MOVING INTO THE WEST. EVENTUALLY THIS PATTERN TRANSITIONS INTO MORE ZONAL/MEAN FLOW ACROSS THE LOWER 48 WITH BUILDING TROUGHING OFFSHORE IN THE PACIFIC BY THE END OF THE PERIOD. REGARDING THE MODELS, THERE IS FAIRLY GOOD AGREEMENT ON THE OVERALL PATTERN THROUGH DAY 5...WITH DIFFERENCES, PARTICULARLY IN THE DETAILS, ARISING THEREAFTER. THE BIGGEST DIFFERENCES EXIST WITH EVENTUAL PROGRESSION OF SHORTWAVE ENERGY INTO THE WEST EARLY IN THE PERIOD AND BUILDING TROUGHING ACROSS THE EASTERN PACIFIC BY DAY 7. THE OPERATIONAL GFS IS THE QUICKEST TO PROGRESS SHORTWAVE ENERGY INTO THE WEST AND ALSO THE QUICKEST/FARTHEST EAST OF THE OPERATIONAL RUNS WITH TROUGHING/POSSIBLE CLOSED LOW SOMEWHERE OFF THE SOUTHWEST COAST BY DAY 7. RUN TO RUN VARIABILITY IN THE MODELS HOWEVER IS PARTICULARLY HIGH SO CONFIDENCE IN ANY PARTICULAR SOLUTION AT THIS POINT IS VERY LOW. FOR THIS REASON, THIS CYCLE OF THE WPC PROGS STUCK WITH A BLEND OF PREVIOUS SHIFT CONTINUITY AND THE OPERATIONAL ECMWF/GFS DAYS 3 AND 4, EVENTUALLY TRANSITIONING TO A MOSTLY EC MEAN/GEFS MEAN BLEND WITH CONTINUITY BY DAY 7. ...WEATHER HIGHLIGHTS/THREATS... CONFIDENCE CONTINUES TO INCREASE THAT PORTIONS OF NEW ENGLAND SHOULD SEE FAIRLY SIGNIFICANT SNOWFALL FROM A RAPIDLY DEEPENING CYCLONE EXPECTED TO TRACK FROM THE WESTERN ATLANTIC INTO THE CANADIAN MARITIMES THURSDAY INTO FRIDAY, WITH A PERIOD OF STRONG WINDS ACROSS THE NORTHEAST CAUSING BLOWING SNOW AND DANGEROUSLY COLD WIND CHILLS. THERE IS ALSO SOME POTENTIAL FOR SNOWFALL ALONG THE MID-ATLANTIC COAST WITH AMOUNTS VERY SENSITIVE TO THE EXACT TRACK OF THE LOW. BEHIND THE SYSTEM, MUCH OF THE EASTERN HALF OF THE COUNTRY WILL SEE VERY COLD TEMPERATURES WITH AFTERNOON HIGH VALUES INTO THE WEEKEND AS MUCH AS 20 TO 30 DEGREES BELOW NORMAL, WITH RECORD LOWS POSSIBLE. FROM SUNDAY ONWARD, TEMPERATURES ACROSS THE EAST SHOULD MODERATE FROM WEST TO EAST AS PACIFIC FLOW EVENTUALLY PUSHES INTO THE REGION. ELSEWHERE, THE WESTERN U.S. SHOULD SEE A MORE UNSETTLED PERIOD, AS WEAKENING SHORTWAVE ENERGY PROMOTES RAIN/MOUNTAIN SNOWS ACROSS THE PACIFIC NORTHWEST...WITH SOME OF THIS MOISTURE ALSO EXTENDING EASTWARD INTO THE ROCKIES. PROGRESSION OF THIS FLOW ALOFT SHOULD BRING A FRONTAL SYSTEM INTO THE EAST-CENTRAL U.S. BY THE START OF NEXT WEEK, WITH AN INCREASE IN PRECIPITATION POTENTIAL FROM THE PLAINS/MISSISSIPPI VALLEY AND EASTWARD. EXPECT SNOWFALL IN THE NORTHERN LATITUDES OF THE UPPER MIDWEST AND GREAT LAKES, WITH RAIN ACROSS THE SOUTHERN REGIONS, AND A TRANSITION ZONE SOMEWHERE IN BETWEEN. ACROSS CALIFORNIA AND THE SOUTHWEST, THERE IS SOME POTENTIAL FOR AN INCREASE IN PRECIPITATION BY DAY 7 BUT HOW MUCH OF THIS MOISTURE MAKES IT ONSHORE BY DAY 7 IS STILL IN QUESTION DUE TO MODEL DIFFERENCES IN THE UPPER TROUGHING OFFSHORE. SANTORELLI