EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 128 AM EST MON JAN 18 2016 VALID 12Z THU JAN 21 2016 - 12Z MON JAN 25 2016 ...GRADUALLY INCREASING CONFIDENCE OF WINTER STORM TO IMPACT THE EAST COAST DURING THE FRIDAY-SUNDAY TIMEFRAME... ...PATTERN OVERVIEW... SPLIT FLOW WILL BE IN PLACE WITH A POTENT SOUTHERN STREAM SYSTEM INITIALLY CROSSING THE SOUTHERN PLAINS ON THURSDAY MORNING. THIS PARTICULAR FEATURE WILL BE ONE TO FOCUS ON GIVEN POSSIBLE WINTRY IMPACTS ACROSS THE EASTERN U.S. DURING THE FRIDAY TO SUNDAY TIME SPAN. THE GUIDANCE TRANSITIONS THIS UPPER TROUGH MORE INTO A CLOSED LOW AS IT BARRELS THROUGH THE SOUTHEASTERN U.S. AND CAROLINAS. A RAPIDLY DEEPENING SURFACE CYCLONE EMERGES OFF THE CAROLINA COAST BY FRIDAY AFTERNOON/EVENING WITH THE LOW GENERALLY LIFTING NORTHEASTWARD TOWARD THE OPEN ATLANTIC. IN ITS WAKE...THE PATTERN WILL BE RATHER PROGRESSIVE WITH A LONGWAVE TROUGH MIGRATING FROM THE WEST COAST ON 23/0000Z INTO THE MIDDLE MS VALLEY AS A LOW-AMPLITUDE SHORTWAVE BY SUNDAY. HEIGHTS BEGIN TO BUILD OVER THE WEST BY LATE IN THE PERIOD WITH THE PACIFIC JET GENERALLY AIMED AT BRITISH COLUMBIA. ...GUIDANCE EVALUATION/UNCERTAINTY ASSESSMENT/WPC PREFERENCES... AS EARLY AS DAY 3/THURSDAY MORNING...MODELS DIVERGE WITH THE AMPLITUDE OF THE INITIAL SHORTWAVE ANCHORING THE SOUTHERN PLAINS. THE 18Z/12Z GFS SOLUTIONS CONTINUED TO BE MORE AGGRESSIVE WITH MUCH LOWER HEIGHTS BEING DEPICTED. THERE IS A STRONG CONSENSUS IN SLIDING THIS SHORTWAVE EASTWARD WITH DEFINITIVE INTENSIFICATION NOTED IN ALL MODELS. AS NOTED PREVIOUSLY...THE 18Z/12Z GFS WERE MORE AMPLIFIED BUT ALSO BECOME QUICKER. A MAJORITY OF THE ENSEMBLE SPAGHETTI CONTOURS WERE MORE IN FAVOR OF THE SLOWER 12Z ECMWF/ECMWF ENSEMBLE MEAN SOLUTION. THIS INCLUDES THE 12Z CMC BUT ALSO THE 18Z/12Z GEFS MEANS WHICH FURTHER SUGGESTS THE GFS FORECAST IS A FASTER OUTLIER. REGARDING SURFACE LOW DEVELOPMENT...MODELS AGREE ON SHEARING AN INITIAL WAVE LIFTING TOWARD THE LOWER OH VALLEY WITH MORE FOCUS ON THE COASTAL CYCLOGENESIS UNFOLDING ALONG THE COASTAL CAROLINAS MIDDAY FRIDAY. EVALUATION OF THE ENSEMBLE LOW PLOT STILL INDICATES PLENTY OF UNCERTAINTY IS AT HAND. BASED ON THE 12Z ECMWF ENSEMBLE MEMBERS...THE SCATTER AT 23/1200Z STRETCHES FROM THE DELMARVA PENINSULA DOWN TO THE NC/SC COASTAL BORDER. THE 12Z ECMWF SITS ON THE NORTHERN SIDE OF THE SPREAD AMONG THAT BROAD CLUSTER. AS A WHOLE...THE FORECAST WAS DRIVEN BY A COMBINATION OF THE 12Z ECMWF/ECMWF ENSEMBLE MEAN AND 18Z GEFS MEAN THROUGH DAY 5. BEGAN TO SIGNIFICANTLY REDUCE CONTRIBUTIONS OF THE 12Z ECMWF BY DAY 5 ONCE IT STARTED SITTING ON THE NORTHERN EDGE OF ITS ENSEMBLE SPREAD. TIMING DIFFERENCES EMERGE WITH HOW QUICKLY THE SYSTEM ACCELERATES OFFSHORE WITH THE GEFS SOLUTIONS RACING OUT TO SEA. FOCUSED MUCH OF THE BLEND ON THE 12Z ECMWF ENSEMBLE MEAN THROUGH THE REMAINDER OF THE PERIOD. SUCH A BLEND WILL ALSO WORK FOR UPSTREAM LOCATIONS WITH THE DEAMPLIFYING FLOW ACROSS THE WESTERN/CENTRAL U.S. FOR THE WEEKEND. ENSEMBLE MEANS ALSO ALIGN REASONABLY WELL WITH THE RIDGE SETTING UP OVER THE WEST COAST IN TIME FOR NEXT MONDAY. ...SENSIBLE WEATHER HIGHLIGHTS... THE TEMPERATURE FORECAST ACROSS THE CONUS WILL FEATURE SOME BELOW NORMAL READINGS IN THE VICINITY OF THE UPPER LOW TRACKING FROM THE SOUTHERN PLAINS TO THE EAST COAST. THIS WILL PARTICULARLY BE THE CASE WITHIN THE AXIS OF MODERATE TO HEAVY PRECIPITATION. CURRENT TEMPERATURE DEPARTURES MAY BE IN THE 5 TO 10 DEGREE RANGE. MORE NOTABLY...BUILDING HEIGHTS ACROSS THE CENTER OF THE COUNTRY WILL FEATURE ABUNDANT WARMTH WITH FORECAST ANOMALIES POSSIBLY 15 TO 20 DEGREES ABOVE CLIMATOLOGY FROM THE ROCKIES TO THE GREAT PLAINS ON FRIDAY/SATURDAY. TWO REGIONS OF ACTIVE PRECIPITATION ARE LIKELY FOR THE FIRST FEW DAYS OF THE MEDIUM RANGE PERIOD. THE AFOREMENTIONED SYSTEM MOVING FROM THE GREAT PLAINS TO THE EASTERN SEABOARD WILL PRESENT A THREAT FOR HEAVY RAINFALL ACROSS THE SOUTHERN U.S. WITH WINTRY PRECIPITATION LOOKING MORE LIKELY FROM THE OH VALLEY THROUGH THE MID-ATLANTIC AND INTO NEW ENGLAND. THE CURRENT WPC WINTER WEATHER PROBABILITIES DO SHOW A FAIRLY STRONG SIGNAL FOR THE DAY 4-6 TIME PERIOD OF SNOW AFFECTING THIS SECTOR OF THE COUNTRY. ACROSS THE WEST COAST...SUSTAINED ONSHORE FLOW WILL SUPPORT MODERATE/HEAVY RAINFALL OVER THE LOWER ELEVATIONS WITH SNOW POSSIBLE ACROSS LOCAL MOUNTAIN RANGES...PARTICULARLY OVER THE WA CASCADES WHERE PROBABILITIES ARE HIGHEST THROUGH FRIDAY. THIS ACTION WILL SLIDE SOUTHWARD ALONG THE WEST COAST REACHING SOUTHERN/CENTRAL CA BY LATE FRIDAY. RUBIN-OSTER