EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 200 AM EST SAT JAN 13 2018 VALID 12Z TUE JAN 16 2018 - 12Z SAT JAN 20 2018 ...OVERVIEW... THE BEGINNING OF THE MEDIUM RANGE PERIOD ON TUESDAY WILL FEATURE A PRONOUNCED EASTERN U.S. TROUGH ALOFT AND AN EXPANSIVE SURFACE HIGH SETTLING SOUTH FROM CANADA. A NARROW RIDGE AXIS OVER THE INTERMOUNTAIN REGION IS FORECAST TO BREAK DOWN GOING THROUGH THE MIDDLE OF THE WEEK AS A FEW WEAK SHORTWAVE DISTURBANCES TREK ACROSS THE CENTRAL U.S. THE PATTERN SHOULD BECOME SLIGHTLY LESS AMPLIFIED TO CLOSE OUT THE WEEK WITH THE EASTERN TROUGH LIFTING OUT ACROSS THE ATLANTIC AND WEAK RIDGING OVER THE PLAINS. A STRONGER TROUGH SETTLES IN OVER THE WEST COAST BY THURSDAY AND RESULTS IN HEIGHT FALLS ACROSS MUCH OF THE WESTERN U.S. BY THE END OF THE FORECAST PERIOD. ...GUIDANCE ASSESSMENT/PREFERENCES... MODELS AND ENSEMBLES ARE IN REASONABLE AGREEMENT THROUGH EARLY WEDNESDAY ON THE SYNOPTIC SCALE ACROSS THE CONTINENTAL U.S. BEFORE NOTEWORTHY DIFFERENCES EMERGE REGARDING THE BREAK DOWN OF THE RIDGE AND THE RATE OF DEPARTURE FOR THE UPPER TROUGH OVER THE EAST COAST. A GENERAL COMPROMISE OF THE GFS/ECMWF/CMC ALONG WITH SOME OF THE EC MEAN APPEARED TO WORK WELL THROUGH DAY 4. THE ECMWF HAS MORE PRECIPITATION OVER EAST COAST FOR THE MIDDLE OF THE WEEK WITH A SURFACE LOW FORMING CLOSER TO THE COAST THAN THE GFS, AND THIS TREND WITH THE ECMWF HAS BEEN CONSISTENT OVER THE PAST 24 HOURS. A BLEND OF THE GEFS/ECMWF ENSEMBLE MEANS SEEM TO BEST REPRESENT THE MORE PREDICTABLE WEATHER FEATURES FOR THE SECOND HALF OF THE FORECAST PERIOD, ALONG WITH LESS OF THE DETERMINISTIC GFS AND ECMWF. THE PROSPECTS FOR AN UNSETTLED WEATHER PATTERN ACROSS THE WESTERN U.S. FOR THE MIDDLE TO END OF THE WEEK REMAIN WELL NOTED IN THE GUIDANCE, WITH HEAVY RAIN, MOUNTAIN SNOW, AND WINDY CONDITIONS BECOMING INCREASINGLY LIKELY. THERE IS ALSO A GOOD SIGNAL FOR AT LEAST ONE COASTAL LOW TO PRIMARILY EFFECT THE MID ATLANTIC AND NORTHEAST U.S., WITH SOME UNCERTAINTY REMAINING WITH THE PROXIMITY OF THE COASTAL LOW TRACK AND INLAND EXTENT OF PRECIPITATION AS SHORTWAVE ENERGY ROTATES THROUGH THE AMPLIFIED MID-UPPER LEVEL TROUGH POSITION. ...WEATHER/THREATS HIGHLIGHTS... A WELL DEVELOPED CLIPPER SYSTEM IS FORECAST TO DELIVER ANOTHER ROUND OF SNOWFALL TO THE GREAT LAKES, FOLLOWED BY A FRESH SURGE OF ARCTIC AIR BEHIND A REINFORCING COLD FRONT. FRIGID CONDITIONS ARE EXPECTED ACROSS THE UPPER MIDWEST AND EXTENDING SOUTHEAST TO THE OHIO VALLEY ON TUESDAY, WITH HIGHS 20 TO 30 DEGREES BELOW NORMAL FOR MID JANUARY. OVERNIGHT LOWS COULD EASILY REACH THE MINUS 20S ACROSS PARTS OF NORTH DAKOTA AND MINNESOTA, AND SOME RECORD LOW TEMPERATURES WILL BE POSSIBLE WHERE RADIATIONAL COOLING IS MAXIMIZED UNDER THE SURFACE HIGH. ABOVE AVERAGE TEMPERATURES ARE EXPECTED NEXT WEEK FOR THE SOUTHWESTERN U.S. AND EXTENDING INTO THE GREAT BASIN. PRECIPITATION INCREASES AGAIN OVER THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA TUESDAY WITH MULTIPLE ROUNDS OF UNSETTLED CONDITIONS FOR THE COAST THROUGH FRIDAY. EAST OF THE CASCADES, THERE SHOULD BE AT LEAST SOME RESPITE BETWEEN THE FIRST AND SECOND SYSTEMS. THE SYSTEM TOWARDS THE END OF THE WEEK ADVECTS MORE MOISTURE INTO THE REGION WITH SOME LOCATIONS POSSIBLY RECEIVING A FEW INCHES OF RAIN AND OVER A FOOT OF MOUNTAIN SNOW. D. HAMRICK