EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 148 AM EST SAT MAR 10 2018 VALID 12Z TUE MAR 13 2018 - 12Z SAT MAR 17 2018 ...INCREASINGLY WET PATTERN FOR THE WESTERN U.S. NEXT WEEK... ...LINGERING UNCERTAINTY FOR WESTERN ATLANTIC LOW PRESSURE... ...PATTERN OVERVIEW/MODEL ASSESSMENT/PREFERENCES... FROM TUE INTO THU EXPECT GRADUAL PROGRESSION OF AMPLIFIED MEAN FLOW AS A DEEP EASTERN PACIFIC TROUGH AND EMBEDDED LOW APPROACH THE WEST COAST, WESTERN U.S. RIDGE REACHES THE PLAINS, AND THE DOWNSTREAM TROUGH REACHES THE EAST COAST. THEN BY FRI-SAT MOST GUIDANCE EXPECTS THE WEST COAST UPPER LOW TO OPEN UP WITH OVERALL WESTERN TROUGHING TO BECOME MORE POSITIVELY TILTED WHILE FLOW TO THE EAST EVENTUALLY BECOMES SOMEWHAT FLATTER--ALBEIT WITH STILL SIGNIFICANT HEIGHT RISES REACHING THE EASTERN STATES. ON THE LEADING EDGE OF THE EASTERN MEAN TROUGH ALOFT, THERE ARE STILL SOME PERSISTENT UNCERTAINTIES WITH THE COMPLEX DETAILS ALOFT THAT WILL AFFECT THE EVOLUTION OF THE WESTERN ATLANTIC SYSTEM EXPECTED TO TRACK INTO THE CANADIAN MARITIMES BY WED. ENSEMBLE SPREAD REMAINS SIGNIFICANT BUT OPERATIONAL RUNS AND ENSEMBLE MEANS GENERALLY SHOW BETTER CLUSTERING THAN WHAT WAS SEEN 24 HOURS AGO. AN AVERAGE AMONG THE 18Z GFS AND 12Z ECMWF/CMC ALONG WITH THE 18Z GEFS/12Z ECMWF MEANS REPRESENTED CONSENSUS THROUGH THE 12Z/18Z CYCLES. THE 12Z AND NEW 00Z UKMET RUNS ARE A BIT ON THE WESTERN AND/OR STRONGER SIDE OF THE SPREAD WITH SOME 12Z CMC ENSEMBLES BEING THE OTHER SOURCE LEANING A BIT ON THE WESTERN SIDE OF THE ENVELOPE. THE NEW 00Z CMC STRAYS TO THE SOUTH AND THEN EAST OF MOST GUIDANCE. THROUGH MID-PERIOD THERE IS RELATIVELY GOOD AGREEMENT FOR THE OVERALL UPPER TROUGH NEARING THE EAST COAST BUT SOME SPREAD DEVELOPS FOR THE EMBEDDED UPPER LOW THAT CROSSES THE NORTHEAST. AT THE SAME TIME THERE IS REASONABLY GOOD AGREEMENT/CONTINUITY IN PRINCIPLE FOR THE DEEP UPPER LOW OVER THE EASTERN PACIFIC TO DRIFT TOWARD THE CENTRAL WEST COAST WHILE LEADING HEIGHT FALLS STEADILY PUSH INTO THE WEST AND DISPLACE THE INITIAL WESTERN RIDGE INTO THE PLAINS. CONFIDENCE REMAINS LOW FOR INDIVIDUAL PIECES OF ENERGY ALOFT THAT WILL ROTATE AROUND THE CLOSED LOW CENTER AND SUPPORT MOVEMENT/DEVELOPMENT OF ONE OR MORE SURFACE LOWS ALONG WITH THEIR ASSOCIATED FRONTS. DURING THE FIRST HALF OF THE PERIOD THE MOST COMMON THEME IS FOR THE OVERALL AREA OF LOW PRESSURE TO DRIFT SOUTHWARD WITH TIME. A MODEL/ENSEMBLE COMPROMISE PROVIDES THE BEST WAY TO ACCOUNT FOR THIS GENERAL EVOLUTION WHILE ACCOUNTING FOR THE LOW CONFIDENCE IN ANY SPECIFIC SOLUTION. INDIVIDUAL MODELS AND ENSEMBLES DISPLAY A FAIRLY RAPID INCREASE IN SPREAD FOR LARGER SCALE ASPECTS OF THE FORECAST DURING DAYS 6-7 FRI-SAT. THIS SPREAD IS DRIVEN BY SHORTWAVE DIFFERENCES IN NORTHERN PACIFIC FLOW THAT APPROACHES THE WEST COAST MEAN TROUGH, HOW LEADING WESTERN ENERGY EJECTS INTO THE CENTRAL U.S./SOUTHERN CANADA, AND SHORTWAVE DIFFERENCES ACROSS CENTRAL-SOUTHERN CANADA. AMONG RECENT GUIDANCE THE 12Z GFS BECOMES ONE OF THE FASTEST AMONG THE ENTIRE ENSEMBLE SPREAD TO BRING THE WEST COAST UPPER LOW/TROUGH ENERGY INTO THE PLAINS WHILE THE 18Z GFS BECOMES QUITE FAST AND PHASED WITH NORTHERN TIER U.S./SOUTHERN CANADA SHORTWAVE ENERGY. THE ENSEMBLE MEANS PROVIDE A MORE STABLE FRAMEWORK FOR THE FORECAST BY FRI-SAT. ECMWF RUNS ARE NOT FAR FROM THE MEANS IN PRINCIPLE THOUGH OF COURSE DIFFER IN THE DETAILS. COMBINED MODEL-ENSEMBLE MEAN PREFERENCES EARLY-MID PERIOD LED TO INCLUDING ASPECTS OF THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF MEAN/CMC FOR THE STARTING BLEND FROM DAY 3 TUE INTO DAY 5 THU. THEN THE INCREASING GUIDANCE SPREAD SUPPORTED A RAPID TRANSITION TOWARD THE ENSEMBLE MEANS, ABOUT 60 PCT WEIGHTING BY EARLY DAY 6 FRI AND 90 PCT ON DAY 7 SAT WITH THE 12Z ECMWF SERVING AS THE MINIMAL OPERATIONAL INPUT AROUND THE END OF THE FORECAST PERIOD. ...WEATHER HIGHLIGHTS/THREATS... POTENTIAL REMAINS FOR MEANINGFUL SNOWFALL/WIND OVER PRIMARILY COASTAL NEW ENGLAND IN ASSOCIATION WITH THE WESTERN ATLANTIC SYSTEM LIKELY TRACKING INTO THE CANADIAN MARITIMES BY WED. IMPACTS WILL BE VERY SENSITIVE TO EXACT TRACK OF THE SYSTEM WHICH SO FAR HAS BEEN A GREATER THAN AVERAGE FORECAST CHALLENGE. THUS CONFIDENCE IS STILL NOT GREAT ON EXACT EFFECTS AT THIS TIME. EVEN IF NOT AS INTENSE AS ANY SNOW WITH THE ATLANTIC STORM, LONGER DURATION SNOW MAY PRODUCE DECENT ACCUMULATIONS FROM UPSTATE NEW YORK INTO NORTHERN NEW ENGLAND AS AN UPPER LOW CROSSES THE REGION. SOME LAKE EFFECT SNOW IS POSSIBLE FARTHER TO THE WEST AND SNOW MAY EXTEND DOWN ALONG THE WESTWARD FACING SLOPES OF THE APPALACHIANS. MOISTURE ARRIVING INTO THE WEST WILL INITIALLY BRING RAIN AND HIGHER ELEVATION SNOW INTO THE CENTRAL/NORTHERN WEST COAST AND EXTENDING INTO THE NORTHERN ROCKIES. WITH TIME THE AXIS OF GREATEST MOISTURE SHOULD TREND SOUTHWARD AS UPPER TROUGHING/EMBEDDED CLOSED LOW APPROACH THE COAST AND EVENTUALLY OPEN UP. EXPECT HIGHEST PRECIP TOTALS DURING THE PERIOD TO BE CENTERED OVER FAVORED TERRAIN IN NORTHERN-CENTRAL CA WHILE ONE OR MORE PERIODS OF ENHANCED ACTIVITY ARE ALSO POSSIBLE OVER SOUTHERN CA AND THROUGH THE GREAT BASIN/SOUTHWEST/NORTHERN-CENTRAL ROCKIES. ARRIVAL OF UPPER TROUGHING WILL HELP TO BRING DOWN SNOW LEVELS COMPARED TO THE START OF THE PERIOD TUE. BY FRI-SAT LEADING HEIGHT FALLS SHOULD SUPPORT PLAINS LOW PRESSURE IN SOME FORM. EXPECT FLOW BETWEEN THIS LOW AND DOWNSTREAM HIGH PRESSURE PROGRESSING INTO THE ATLANTIC TO BRING INCREASING MOISTURE AND PRECIP (MOSTLY RAIN) TO THE EAST-CENTRAL U.S. BY THE START OF THE WEEKEND. INITIALLY MODERATE WARMTH OVER THE WEST WILL PROGRESS EASTWARD IN LINE WITH THE UPPER PATTERN AND BECOME MORE PRONOUNCED ONCE REACHING THE CENTRAL U.S. WITH SOME LOCATIONS SEEING MIN AND/OR MAX READINGS AT LEAST 10-20F ABOVE NORMAL BY THU-SAT. AT THE SAME TIME COOLER AIR WILL BE MOVING INTO THE WEST WITH MAX TEMPS IN PARTICULAR TRENDING WELL BELOW NORMAL OVER AN INCREASING AREA, ESPECIALLY THE SOUTHERN HALF OF THE WEST. OVER THE EAST EXPECT AREAS OF MINUS 10-20F ANOMALIES FROM TUE INTO THU FOLLOWED BY A SIGNIFICANT WARMING TREND AS UPPER TROUGHING DEPARTS INTO THE ATLANTIC. RAUSCH