EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1012 AM EST MON JAN 29 2018 VALID 12Z THU FEB 01 2018 - 12Z MON FEB 05 2018 15Z UPDATE... FORECAST WAS NOT CHANGED SIGNIFICANTLY FROM OVERNIGHT ISSUANCE. MODELS CONTINUED TO SHOW GENERAL LARGE SCALE AGREEMENT TO JUSTIFY HEAVY USE OF DETERMINISTIC GUIDANCE THROUGH DAY 5. THUS, THE WPC FORECAST FOR DAYS 3-5 WAS BASED ON A BLEND OF THE 00Z ECMWF/CMC/06Z GFS. MODELS BEGIN TO SHOW SOME SMALL SCALE DIFFERENCES DURING DAYS 6-7. ONE OF THE MORE SIGNIFICANT DIFFERENCES NOTED DURING THIS TIME FRAME IS THE AMPLITUDE OF PACIFIC SHORTWAVE ENERGY CROSSING THE CREST OF THE RIDGE AND APPROACHING THE PAC NW. A TREND WAS NOTED AMONG RECENT DETERMINISTIC RUNS TOWARD SOMEWHAT GREATER AMPLITUDE WITH THIS FEATURE, BUT WOULD LIKE TO SEE MORE RUN-TO-RUN CONSISTENCY HERE. ADDITIONALLY, SHORTWAVES ROTATING AROUND THE HUDSON BAY UPPER LOW SHOW SOME TIMING DIFFERENCES BY DAYS 6-7 WHICH HAVE SOME DEGREE OF INFLUENCE ON THE CHARACTER OF THE BROAD UPPER TROUGH ACROSS THE GREAT LAKES/EASTERN U.S. GIVEN THESE CONSIDERATIONS, ENSEMBLE MEAN (00Z ECENS/NAEFS) WEIGHTING WAS INCREASED SIGNIFICANTLY DURING THE DAY 6-7 PERIOD. OVERALL, THIS RESULTED IN A FORECAST FAIRLY CONSISTENT WITH CONTINUITY. THE MOST SIGNIFICANT ADJUSTMENT WAS AN INCREASE IN THE FORWARD SPEED OF THE COLD FRONT CROSSING THE MS VALLEY/EASTERN U.S. EARLY NEXT WEEK, WHICH HAD SOME BROAD SUPPORT AMONG THE GUIDANCE. RYAN PREVIOUS DISCUSSION (ISSUED AT 0657 UTC)... ...OVERVIEW AND GUIDANCE PREFERENCES... THE MEDIUM RANGE PERIOD FLOW ALOFT WILL FEATURE RIDGING CENTERED NEAR THE WEST COAST AND TROUGHING SPREAD DOWNSTREAM OVER MOST OF THE REST OF THE LOWER 48. ENSEMBLES MEANS ARE IN GENERAL AGREEMENT OVERALL AND A COMPOSITE BLEND OFFERS A GOOD STARTING POINT FOR THE MEDIUM RANGE FORECAST IN LIEU OF CHASING TIMING/AMPLITUDE DETERMINISTIC MODEL DIFFERENCES. ...WEATHER HIGHLIGHTS/THREATS... IN THE WEST, A SERIES OF PROGRESSIVE IMPULSES WILL SUPPORT ADDITIONAL PERIODS OF MODEST PRECIPITATION INCLUDING HEAVIER TERRAIN ENHANCED SNOWS ACROSS THE NWRN US AND ESPECIALLY THE NRN ROCKIES WITH ADDED NRN STREAM IMPULSES OF UNCERTAIN TIMINGS. EXPECT ANOTHER ROUND OF PRECIPITATION (MOSTLY SNOW) ALONG THE DIVIDE VIA UPSLOPE FLOW NEXT WEEKEND IN THE WAKE OF A CLIPPER SYSTEM. UPSTREAM...THERE IS ALSO SOME MODEL SIGNAL TO BRING ADDITIONAL MODEST ERN PACIFIC SYSTEM ENERGY AND PCPN TO THE PACIFIC NW AGAIN BY DAYS 6/7. DOWNSTREAM...A LEAD FRONT REACHES THE EAST-CENTRAL US BY THU. THE MAIN SFC LOW CENTER WILL TRACK THROUGH SRN CANADA WITH MOST OF THE PRECIPITATION TO START...BUT INCREASED GULF MOISTURE WILL SUPPORT AN EXPANDED PRECIPITATION AREA ALONG/AHEAD OF THE COLD FRONT ACROSS THE E-CENTRAL THEN ERN US. ENOUGH COLD AIR SHOULD BE IN PLACE TO THE NORTH OF THE FRONT TO SUPPORT A SWATH OF SNOW PARALLEL TO ANY SFC LOW TRACK OR AT LEAST PARALLEL TO THE UPPER TROUGH AXIS PROPAGATION. HOWEVER...THE MODELS AND ENSEMBLES ARE NOW TRENDING AWAY FROM LOW DEVELOPMENT. EARLIER RELEASED WPC PROGS PLUGGED IN A FRI MID-ATLANTIC WAVE FOR CONTINUITY AS THERE WAS A SMALL CLUSTER OF SUPPORTING ENSEMBLES AND THE 12 UTC CANADIAN...BUT THE 00 UTC CANADIAN US NOW MUCH MORE PROGRESSIVE WITH THE FRONT. NEXT WEEKEND, A CLIPPER SYSTEM WILL SPREAD SOME GENERALLY LIGHT SNOW TO THE CENTRAL STATES INTO THE GREAT LAKES WITH RAIN INCREASING AND EXPANDING INLAND FROM THE GULF COAST AS RETURN FLOW RESUMES...WITH WIDESPREAD MODERATE PCPN EARLY NEXT WEEK INTO THE ERN STATES. SCHICHTEL