EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1206 AM EST WED JAN 10 2018 VALID 12Z SAT JAN 13 2018 - 12Z WED JAN 17 2018 ...ICE STORM TO EXIT THE NORTHEAST SATURDAY... ...OVERVIEW... A DYNAMIC SYSTEM IN THE EAST ON SATURDAY WILL EXIT INTO THE CANADIAN MARITIMES BY SUNDAY AS THE WEATHER TURNS QUIET FOR A COUPLE DAYS OVER THE LOWER 48. A CLIPPER SYSTEM WILL WHISK DOWN THROUGH THE GREAT LAKES NEXT WEEK AND A PACIFIC SYSTEM WILL PUSH INTO WA/OR/NORCAL TUESDAY AS THE UPPER RIDGE YIELDS TO STRONG ONSHORE FLOW. ...GUIDANCE ASSESSMENT/PREFERENCES... FOR THE SYSTEM IN THE EAST SAT/SUN, THE MESSY CONSENSUS HAS (FOR SEVERAL DAYS) POINTED TO A LOW NEAR THE WV/MD PANHANDLES WHICH LIES NEAR THE 12Z ECMWF/CANADIAN (AND 18Z GEFS MEAN) AND REMAINS THE PREFERENCE. THE 18Z/12Z GFS RUNS WERE STILL A BIT QUICKER WHILE THE UKMET WAS SLOWER. THOUGH THE 12Z ECMWF ENSEMBLE MEAN JUMPED A BIT QUICKER FROM AN OTHERWISE STABLE POSITION, IT MAY BE A RESULT OF NORTHERN LOWS (PERHAPS TRIPLE POINT LOWS) TUGGING THE MEAN ISOBAR TO THE NORTHEAST. PARENT LOW CLUSTERING STILL RESIDED NEAR THE ECMWF HRES. SYSTEM WILL PULL THROUGH MAINE EARLY SUNDAY AND THEN AWAY FROM NEW ENGLAND. IN ITS WAKE WILL BE HIGH PRESSURE AND THEN A CLIPPER SYSTEM. ENSEMBLES HAVE TRENDED MUCH STRONGER (BUT STILL ONLY ABOUT 1024MB) WITH THIS SYSTEM BUT GIVEN THE HIGHER PRESSURE ON EITHER SIDE THIS ONLY MEANS THAT THE SPREAD HAS DECREASED ENOUGH SO THAT THEY CAN RESOLVE THE FEATURE AMONG THE 20/50 MEMBERS. USED A CONSENSUS APPROACH FOR THE FORECAST NEXT WEEK WHICH INCLUDED A MINORITY GFS WEIGHTING, BUT OTHERWISE LEANED ON THE ECMWF AND ECMWF ENSEMBLE MEAN (AND GEFS MEAN). IN THE WEST, QUESTION HAS BEEN HOW LONG THE RIDGE MAY HOLD BEFORE THE PACIFIC FLOW PUSHES IT INLAND. THE ECMWF ENSEMBLE MEAN WAS QUICKEST TO INDICATE SUCH AN EVOLUTION BUT HAS CONTINUED TO TREND A BIT SLOWER. THE GEFS MEAN HAS BEEN MORE CONSISTENT WITH A SLOWER EROSION OF THE RIDGE BUT THE 18Z/12Z GFS WERE MUCH MORE AGGRESSIVE (AND FLATTER) TO BREAK DOWN THE RIDGE. THOUGH THIS REMAINS A POSSIBILITY, PUT MORE WEIGHT IN THE ECMWF WHICH WAS A BIT SLOWER THAN ITS ENSEMBLE MEAN AND WAS ALSO CLOSE TO THE CANADIAN. BY NEXT WEDNESDAY, ALL ENSEMBLES SHOW THE BREAKDOWN OF THE RIDGE WITH THE LEAD SYSTEM LIKELY PUSHING THROUGH THE INTERIOR WEST AS ANOTHER LOOMS OFFSHORE. ECMWF-LED CONSENSUS SERVED WELL AS A STARTING POINT. ...WEATHER HIGHLIGHTS... WELL ABOVE AVERAGE TEMPERATURES WILL BE SHUNTED OUT TO SEA AS THE SYSTEM MOVES THROUGH NEW ENGLAND ON SATURDAY, ENDING THE BRIEF WARM SPELL. HIGH PRESSURE WILL RETURN TEMPERATURES TO BELOW AVERAGE BY ABOUT 10-20 DEGREES. CLIPPER SYSTEM WILL BRING LIGHT SNOW TO THE GREAT LAKES (WITH LOCAL ENHANCEMENT) AND ANOTHER PUSH OF COLDER AIR (SUB-ZERO HIGHS FOR MUCH OF THE DAKOTAS AND MINNESOTA ON MONDAY). TO THE WEST, ABOVE AVERAGE TEMPERATURES WILL RULE INTO NEXT WEEK THANKS TO THE UPPER RIDGE. PRECIPITATION CHANCES WILL INCREASE ALONG THE COAST STARTING MONDAY AND THEN WORK THEIR WAY INLAND NEXT TUE-WED THROUGH THE CENTRAL/NORTHERN ROCKIES. SNOW LEVELS WILL BE ON THE HIGHER SIDE TO START BUT THEN DROP BEHIND THE FRONT. FRACASSO