EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1104 AM EDT SAT OCT 28 2017 VALID 12Z TUE OCT 31 2017 - 12Z SAT NOV 04 2017 ...COLDER AND SNOWIER FOR THE NORTHWEST NEXT WEEK... 15 UPDATE... NO SIGNIFICANT CHANGES WERE MADE TO THE FORECAST FROM OVERNIGHT. CONFIDENCE IS A BIT ABOVE AVERAGE WITH RESPECT TO THE LARGE SCALE FLOW PATTERN ACROSS NORTH AMERICA THROUGH THE MEDIUM RANGE. REX BLOCK OVER THE CENTRAL PACIFIC IS FORECAST TO BECOME BETTER ESTABLISHED BY LATE NEXT WEEK. TELECONNECTIONS ASSOCIATED WITH THE POSITIVE HEIGHT ANOMALIES OVER THE ALEUTIANS AND THE NEGATIVE HEIGHT ANOMALIES OVER THE CENTRAL PACIFIC BOTH FAVOR TROUGHING DIGGING INTO THE PACIFIC NORTHWEST WITH ZONAL TO SLIGHTLY ANTICYCLONIC FLOW ALONG THE EASTERN SEABOARD. THESE ANOMALY CENTERS HAVE A FAIRLY STRONG CORRELATION TO ARCTIC AIR SPILLING SOUTH ALONG THE EASTERN SLOPES OF THE NORTHERN ROCKIES AS WELL AS THE NORTHERN PLAINS, AND PERHAPS CROSSING THE CONTINENTAL DIVIDE AND REACHING THE NORTHWESTERN STATES. A CLIPPER-TYPE SYSTEM MOVING FROM BRITISH COLUMBIA/ALBERTA INTO THE MIDWEST AND GREAT LAKES DAYS 3-5 WILL BE THE LEADING EDGE OF COLDER AIR. A BLEND LARGELY BASED ON THE 00Z ECMWF/GFS SERVED AS A STARTING POINT FOR THE FORECAST FOR DAYS 3-4 (TUE-WED). AFTER THAT TIME, 00Z ECENS/NAEFS ENSEMBLE WEIGHTING WAS BOOSTED, FAIRLY SUBSTANTIALLY BY DAYS 6-7 (FRI-SAT). GREATEST AREA OF UNCERTAINTY SURROUNDS INTENSE SHORTWAVE ENERGY DIGGING INTO THE PAC NW BY DAYS 6-7, AND TO WHAT DEGREE THIS ENERGY UNDERCUTS THE STRONG NORTH PAC RIDGE. 00Z ECMWF WAS THE MOST AGGRESSIVE WITH UNDERCUTTING THE RIDGE AND LOWERING HEIGHTS WEST INTO THE PAC, WHILE THE 00Z/06Z GFS ALONG WITH THE ENSEMBLE MEANS WERE LESS AGGRESSIVE. TELECONNECTIONS SUGGEST THAT MOST SIGNIFICANT HEIGHT FALLS SHOULD NOT PROGRESS TOO FAR WEST BENEATH THE RIDGE, SO THE 00Z ECMWF SOLUTION WAS DEEMPHASIZED QUICKER RELATIVE TO THE GFS BY DAYS 6-7. RYAN PREVIOUS DISCUSSION... ...OVERVIEW... MAIN DRIVER TO THE CONUS PATTERN WILL BE IMPRESSIVELY STRONG UPPER RIDGING OVER WESTERN ALASKA WHICH FAVORS TROUGHING DIGGING SOUTHWARD INTO THE PACIFIC NORTHWEST AND RATHER ZONAL FLOW ACROSS THE CENTRAL/EASTERN STATES. A STORM SYSTEM WILL TREK ACROSS THE US/CANADIAN BORDER WED/THU AND DRAG ITS COLD FRONT OFF THE NORTHEAST AND MID-ATLANTIC COASTS FRIDAY. TAIL END OF THIS FRONT WILL TRY TO OOZE OVER THE DIVIDE AS ARCTIC HIGH PRESSURE SLIDES SOUTHEASTWARD OUT OF FAR NW CANADA THROUGH ALBERTA/SASKATCHEWAN. ...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES... 12/18Z GFS AND 12Z ECMWF OFFERED GOOD CLUSTERING NEAR THEIR ENSEMBLE MEANS THROUGH THE ENTIRE PERIOD OVER MOST OF THE CONUS. A BLEND OF THE DETERMINISTIC MODELS EARLY YIELDED TO ABOUT A 60/40 SPLIT BY NEXT FRI/SAT BETWEEN THE ENSEMBLES AND GFS/ECMWF, RESPECTIVELY. ONE AREA OF LESS CONFIDENCE INCLUDED OFF THE WEST COAST WHERE AN UPPER LOW WILL PAUSE AROUND 32N/124W BEFORE SLIDING EASTWARD AND WEAKENING. THE ECMWF WAS SLOWER WITH THIS PROGRESSION THAN THE 12Z GFS/UKMET/CANADIAN WITH THE ECMWF ENSEMBLE MEAN MOSTLY IN BETWEEN. OPTED TO RELY ON THE ECMWF EPS AS A MIDDLE GROUND SOLUTION. THOUGH THE ENSEMBLES WERE RATHER ALIGNED OFF THE WEST COAST WITH THE TROUGH AXIS, THE STRENGTH OF THIS LIKELY UPPER LOW IS UNCLEAR. STRENGTH OF THE UPSTREAM UPPER RIDGE OVER ALASKA (+3 TO +3.5 STND DEV 500MB HEIGHTS EVEN IN THE ENSEMBLE MEANS) SUGGESTS THE POTENTIAL FOR A DEEP SYSTEM NOT UNLIKE THE GFS/CANADIAN/ECMWF. ...WEATHER THREATS/HIGHLIGHTS... UPPER PATTERN WILL BRING MUCH COLDER AIR TO THE NORTHERN ROCKIES AND EVENTUALLY WA/OR AS THE COLD FRONT SLOWLY SPILLS OVER THE DIVIDE. RAIN AND SNOW WILL BE ON THE INCREASE ESPECIALLY BY NEXT WEEKEND. TEMPERATURES WILL TREND COLDER, LIKELY 10-20F BELOW AVERAGE FOR NORTHERN MONTANA. BACK TO THE EAST, WEAKENING COLD FRONTAL BOUNDARY ON TUESDAY COMBINED WITH AN UPPER LEVEL SHORTWAVE WILL ACT TO EXPAND SHOWERS INTO/THROUGH THE LOWER/MID-MS VALLEY INTO THE OHIO VALLEY AHEAD OF THE NEXT FRONT. THIS WILL PUSH EASTWARD FRI/SAT INTO THE I-95 CORRIDOR. FRACASSO