EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 400 AM EST MON JAN 15 2018 VALID 12Z THU JAN 18 2018 - 12Z MON JAN 22 2018 ***PATTERN OVERVIEW*** THE LARGE UPPER LEVEL TROUGH THAT WILL BE OVER THE EASTERN U.S. THROUGH THE MIDDLE OF THIS WEEK WILL BEGIN LIFTING OUT AT THE BEGINNING OF THE MEDIUM RANGE PERIOD ON THURSDAY. THE ARCTIC SURFACE HIGH WILL MOVE OFFSHORE BY THE END OF THE WEEK AND ALLOW WARMER RETURN FLOW FROM THE SOUTHWEST BY THE WEEKEND. THIS PATTERN CHANGE WILL BE INITIATED BY LOWERING HEIGHTS OVER THE WESTERN U.S. AS A STRONG PACIFIC STORM SYSTEM MOVES INLAND, BRINGING WIDESPREAD SHOWERS AND MOUNTAIN SNOW FOR MUCH OF THE WEST COAST AND INTERMOUNTAIN WEST REGION. THIS STORM REACHES THE PLAINS BY SATURDAY WITH INCREASING PROSPECTS FOR SHOWERS AND THUNDERSTORMS TO DEVELOP AS A STRONG SURFACE LOW TRACKS NORTHEASTWARD FROM THE CENTRAL PLAINS TO THE GREAT LAKES REGION. A SECOND PACIFIC SYSTEM WILL LIKELY AFFECT THE WEST COAST BY SUNDAY AND INTO MONDAY WITH A BRIEF PERIOD OF UPPER LEVEL RIDGING BETWEEN SYSTEMS FOR THE WESTERN U.S. ***GUIDANCE ASSESSMENT/PREFERENCES*** THERE ARE SOME MODEST MODEL DIFFERENCES AT THE BEGINNING OF THE PERIOD REGARDING THE SHORTWAVE IMPULSES CROSSING THE SOUTHERN PLAINS AND ALSO THE UPPER MIDWEST, WITH THE UKMET BEING THE MOST PROGRESSIVE. THE GFS AND ITS ENSEMBLE MEAN ARE FARTHER OFFSHORE WITH THE COASTAL LOW OFF THE EAST COAST, AND THE ECMWF HAS TRENDED SOME IN THAT DIRECTION. THE LARGER TROUGH REACHING THE WEST COAST BY FRIDAY, ALONG WITH THE SURFACE LOW OVER THE PLAINS, IS HANDLED WELL AMONG THE GUIDANCE GOING INTO THE WEEKEND, AND CONFIDENCE HAS IMPROVED TO ABOVE AVERAGE FOR A FORECAST AT THIS TIME RANGE. THE ENSEMBLE MEANS PORTRAY A GOOD IDEA OF WHERE THE SURFACE FRONT WILL BE, WITH SLIGHTLY MORE WEIGHTING TOWARDS THE EC MEAN AND OPERATIONAL ECMWF GIVEN A MORE CONSISTENT TREND WITH THE ECMWF. AFTER A BRIEF RIDGE SETS IN OVER THE WEST BEHIND THE DEPARTING STORM SYSTEM, BOTH THE GFS AND ECMWF ARE INDICATING HEIGHT FALLS OVER THE EASTERN PACIFIC BY SUNDAY, WITH THE ECMWF AMONG THE SOUTHERN SOLUTIONS AND THE GFS WELL TO THE NORTH. THE EC MEAN INDICATED A REASONABLE POSITION FOR THIS SURFACE LOW. ***SENSIBLE WEATHER*** THE EASTERN HALF OF THE NATION WILL BEGIN TO HAVE A STEADY MODERATION TREND IN TEMPERATURES TOWARDS THE END OF THIS WEEK AND ESPECIALLY INTO NEXT WEEKEND ACROSS THE CENTRAL AND EASTERN U.S. AHEAD OF THE NEXT STORM SYSTEM, WITH READINGS EXPECTED TO BE ABOVE AVERAGE BY POTENTIALLY 10 TO 20 DEGREES. IN THE PRECIPITATION DEPARTMENT, THE WEST COAST WILL LIKELY GET REPEATED ROUNDS OF MODERATE TO LOCALLY HEAVY RAINFALL, WITH SEVERAL INCHES OF RAIN POSSIBLE WEST OF THE CASCADES. THE STORM SYSTEM REACHING THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA LATE SUNDAY AND INTO MONDAY WILL LIKELY QUALIFY AS AN ATMOSPHERIC RIVER EVENT WITH A PLUME OF SUBTROPICAL MOISTURE ORIENTED TOWARDS THE COAST. HEAVY SNOW IS LIKELY FOR THE HIGHEST ELEVATIONS OF THE CASCADES, SIERRA NEVADA, AND THE CENTRAL AND NORTHERN ROCKIES FOR THE END OF THE WEEK, AND AGAIN BY LATE SUNDAY. AN ACTIVE WEATHER PATTERN IS ALSO EXPECTED ACROSS THE CENTRAL U.S. BY SUNDAY AND INTO EARLY MONDAY OF NEXT WEEK. THE INTENSIFYING SURFACE LOW OVER THE CENTRAL AND NORTHERN PLAINS IS EXPECTED TO RESULT IN A SURGE OF GULF MOISTURE EXTENDING NORTHWARD TO THE GREAT LAKES REGION, WITH SCATTERED TO NUMEROUS SHOWERS AND SOME THUNDERSTORMS EXPECTED. RAINFALL AMOUNTS COULD EXCEED AN INCH BEFORE THE COLD FRONT PASSES THROUGH. IN ADDITION, THE POTENTIAL FOR A MAJOR WINTER STORM IS INCREASING ACROSS THE DAKOTAS AND ESPECIALLY OVER MINNESOTA AND WISCONSIN AS A DEFORMATION ZONE WILL LIKELY DEVELOP ON THE NORTHWEST SIDE OF THE SURFACE LOW. D. HAMRICK