EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 153 AM EST SUN JAN 14 2018 VALID 12Z WED JAN 17 2018 - 12Z SUN JAN 21 2018 ...PATTERN OVERVIEW... A LARGE EASTERN U.S. TROUGH IN PLACE AT THE BEGINNING OF THE FORECAST PERIOD ON WEDNESDAY IS FORECAST TO LIFT OUT BY THE END OF THE WEEK, FOLLOWED BY A BUILDING LOW AMPLITUDE RIDGE EAST OF THE ROCKIES. THE LARGE ARCTIC 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 AIR MASSES CLASH. A SECOND PACIFIC SYSTEM MAY BEGIN AFFECTING THE WEST COAST BY SUNDAY. ...GUIDANCE ASSESSMENT/PREFERENCES... THE GUIDANCE IS IN RELATIVELY GOOD AGREEMENT ON THE SYNOPTIC SCALE TROUGHS NEAR BOTH COASTS FOR WEDNESDAY AND THURSDAY, BUT THE MODELS ARE STILL STRUGGLING ON A COUPLE OF LEAD SHORTWAVE IMPULSES AHEAD OF THE MAIN WESTERN U.S. TROUGH AS THEY TRAVERSE A LOW AMPLITUDE RIDGE OVER THE PLAINS, WITH THE SOUTHERN STREAM IMPULSE LIKELY BEING THE STRONGER OF THESE. THE UKMET APPEARED TOO STRONG WITH THE NORTHERN STREAM SHORTWAVE AND DID NOT HAVE MUCH ENSEMBLE SUPPORT. THE LARGER TROUGH REACHING THE WEST COAST BY FRIDAY, ALONG WITH THE SURFACE LOW, IS HANDLED WELL AMONG THE GUIDANCE THROUGH EARLY SATURDAY, AFTER WHICH THE GFS IS FASTER AND MUCH STRONGER WITH THE 500MB LOW OVER THE PLAINS. 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. 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 CMC INDICATING A STRONGER STORM SYSTEM. ...SENSIBLE WEATHER... THE EASTERN HALF OF THE NATION WILL START OFF QUITE COLD ON WEDNESDAY COURTESY OF THE ARCTIC HIGH SETTLING IN OVER THE REGION AND THE PRONOUNCED UPPER LEVEL TROUGH NEAR THE EAST COAST. HIGH TEMPERATURE ANOMALIES ON THE ORDER OF 15 TO 25 DEGREES BELOW AVERAGE ARE EXPECTED FROM TEXAS TO THE OHIO VALLEY, WITH READINGS STRUGGLING TO GET OUT OF THE TEENS FROM IOWA TO THE CENTRAL APPALACHIANS. THE MAJOR CITIES OF THE NORTHEAST U.S. SHOULD BE SPARED THE BRUNT OF THIS ARCTIC SURGE IN CONTRAST TO THE EXTREMELY COLD WEATHER ABOUT A WEEK AGO. A STEADY MODERATION TREND IN TEMPERATURES COMMENCES BY THE END OF THE 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 ON WEDNESDAY 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. PARTS OF NEW ENGLAND CAN ALSO EXPECT SIGNIFICANT SNOW FROM A COASTAL LOW THAT DEVELOPS OFFSHORE, WITH SOME SNOW LIKELY EXTENDING SOUTHWARD INTO PARTS OF THE MID-ATLANTIC REGION. D. HAMRICK