EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1059 AM EST WED JAN 20 2016 VALID 12Z SAT JAN 23 2016 - 12Z WED JAN 27 2016 THERE CONTINUES TO BE HIGH CONFIDENCE THAT A MAJOR EAST COAST STORM WILL OCCUR BETWEEN THE SHORT RANGE AND MEDIUM RANGE PERIODS...STARTING THURSDAY AND ENDING SUNDAY. THERE HAVE BEEN...HOWEVER...SOME TRENDS OVER THE PAST COUPLE DAYS THAT APPEAR TO AFFECT THE FORECAST FOR THE START OF THE MEDIUM RANGE FORECAST. LOOKING AT THE TRENDS OF THE SURFACE LOW TRACKS/QPF DISTRIBUTION...THERE HAS BEEN A SOUTHWARD/SLOWER TREND OF BOTH OVER THE PAST 48 HOURS. THIS HAS IMPLICATIONS FOR WHERE THE NORTHERN BOUNDARY OF THE SNOWFALL OCCURS...WITH A VERY SHARP GRADIENT BETWEEN VERY HEAVY AND VERY LITTLE SNOW POSSIBLE. RIGHT NOW...THIS AREA LIES OVER SOUTHERN AND CENTRAL NEW YORK AND NEW ENGLAND. FARTHER SOUTH...THE HEAVIER SNOWFALL TOTALS HAVE ALSO SHIFTED SOMEWHAT SOUTHWARD OVER THE PAST COUPLE DAYS AS WELL AS THE ISSUE OF MIXING/CHANGEOVER. THE STATE OF VIRGINIA LIES WITHIN THIS ZONE. FOR NOW. WHAT THE TRENDS WILL BE OVER THE NEXT COUPLE DAYS WILL SORT OUT THE DETAILS. WITH INDIVIDUAL OPERATIONAL MODEL RUNS...AS OPPOSED TO THE SUITE OF ENSEMBLES...THERE ARE TRENDS THAT HAVE OCCURRED WITH RESPECT TO WHAT WAS OCCURRING WITHIN THE LARGER ENSEMBLE SYSTEM. FOR EXAMPLE...THE OPERATIONAL GFS OVER THE PAST 48 HOURS HAS REMAINED LARGELY CONSISTENT...WHILE THE OVERALL TREND HAS BEEN SOUTH/SLOW. THE ECMWF HAS VACILLATED SOUTH/NORTH WHILE THE UKMET HAS BEEN FARTHER SOUTH AND THE CANADIAN HAS ALSO COME SOUTH BUT REMAINS ON THE NORTHERN SIDE OF THE SOLUTIONS. THESE ARE ALL SOLUTIONS THAT HAVE OCCURRED IN THE 3 TO 4 DAY FORECAST RANGE. THERE IS ALSO ANOTHER IMPORTANT CONSIDERATION IN HOW QUICKLY THE STORM EVOLVES AND EXITS ON DAYS 3 AND 4. BOTH THE 06Z GFS AND 00Z ECMWF HAVE THE MAIN SURFACE LOW FARTHER WEST THAN THE MEANS SHOW ON SUNDAY MORNING/DAY 4. THIS DIFFERENCE HAS AN IMPORTANT POSSIBLE EFFECT ON IMPACT IF THE CURRENT OPERATIONAL RUNS ARE ON THE SLOWER END OF THE SOLUTIONS. THE SPEED OF THE SYSTEM AFFECTS HOW LONG THE SNOWFALL CONTINUES VERSUS A FASTER ENDING TIME. IN GENERAL...THIS PARTICULAR SYSTEM IS A RELATIVE SLOW MOVER IN THAT SNOWFALL COULD LAST AS LONG AS 36 HOURS AS OPPOSED TO PERHAPS MORE TYPICAL JANUARY SYSTEMS THAT LAST FROM 12 TO 18 OR PERHAPS 24 HOURS. SO...DESPITE THE GENERAL AGREEMENT BETWEEN A COMPLEX SUITE OF MANY MODELS/ENSEMBLES...THERE ARE SOME SUBTLE BUT IMPORTANT DIFFERENCES THAT CCOULD HAVE LARGE IMPACTS ON THE MOST POPULOUS AREA OF THE COUNTRY. GENERAL OVERVIEW... THE EAST COAST SYSTEM WILL OCCUR THRU SATURDAY INTO SUNDAY...DAYS 3/SATURDAY AND 4/SUNDAY OF THE MEDIUM RANGE FORECAST. THIS SYSTEM WILL BE GONE FOLLOWING DAY 4/SUNDAY FOLLOWED BY A SIGNIFICANT RISE IN HEIGHTS IMPLYING A WARMUP ONCE THE SURFACE HIGH MOVES OFF THE SOUTHEAST COAST ON DAYS 5 AND 6/MONDAY AND TUESDAY. A SIGNIFICANT UPPER TROUGH WILL MOVE ACROSS THE CENTRAL WEST COAST ON DAY 3/SATURDAY AND WILL PROGRESS EASTWARD ACROSS THE GREAT BASIN/ROCKIES ON SUNDAY/DAY 4 TO THE MISSISSIPPI VALLEY ON MONDAY/DAY 5. THIS SYSTEM WILL BE ASSOCIATED WITH A COLD FRONT WITH MOSTLY MARITIME AIR IN ITS WAKE CROSS THE SOUTHWEST US AND REACH THE SOUTHERN PLAINS ON DAY 5/MONDAY. LOW PRESSURE IS EXPECTED TO DEVELOP IN THE MID MISSISSIPPI VALLEY AND CROSS THE GREAT LAKES BY DAY 6/TUESDAY. THE ASSOCIATED COLD FRONT WILL MOVE EAST TO THE EAST COAST ON DAY 6/TUESDAY AND WILL CONTINUE EASTWARD OFFSHORE TO THE GULF COAST BY WEDNESDAY/DAY 7. MODEL CHOICES AND CONFIDENCE... THERE IS GENERAL AGREEMENT BETWEEN THE ENSEMBLE MEANS THROUGH DAY 7. IN PARTICULAR... THE LATEST VERSIONS OF THE 06Z GFS AND THE 00Z ECMWF WITH THEIR SUBTLE BUT IMPACTFUL DIFFERENCES AS DISCUSSED EARLIER...AS WELL AS THE GEFS/ECMWF ENSEMBLE MEANS WERE REASONABLY SIMILAR ACROSS MUCH OF THE NATION THROUGH DAY 6. THE OPERATIONAL RUNS START TO DIVERGE ON DAY 7 BUT THEY ARE NOT UNREASONABLE WHEN COMPARED TO THE MEANS. THEREFORE...THE FORECAST MAPS WERE NOT DIFFICULT TO COMPOSE WITH A 60/40 BLEND OF OPERATIONAL/MEANS FOR DAYS 3 THROUGH 6 AND THE REVERSE FOR DAY 7. KOCIN