EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1154 AM EDT MON OCT 30 2017 VALID 12Z THU NOV 02 2017 - 12Z MON NOV 06 2017 ...COLDER AND STORMY PATTERN FOR THE WEST LATER THIS WEEK INTO NEXT WEEK... ...OVERVIEW, GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES... AMPLIFIED FLOW OVER THE PACIFIC OCEAN AND ALASKA INTO WESTERN NORTH AMERICA WILL SUPPORT A TROUGH DIGGING INTO THE WEST LATER THIS WEEK THROUGH THE WEEKEND WITH ZONAL TO SOUTHWESTERLY FLOW ACROSS THE CENTRAL/EASTERN STATES. WITHIN THIS MEAN PATTERN THERE WILL BE TWO PRIMARY SYSTEMS OF INTEREST. FIRST IS A GREAT LAKES-EASTERN CANADA LOW THAT WILL BRING A COLD FRONT THROUGH THE EAST LATE THIS WEEK, WITH THE FRONT STALLING OVER CENTRAL/SOUTH-CENTRAL AREAS. MEANWHILE THE DIGGING WEST COAST TROUGH WILL EVENTUALLY EJECT ENERGY EASTWARD, PUSHING A STRONG COLD FRONT THROUGH THE WEST BY THE WEEKEND AND INTO THE PLAINS/GREAT LAKES SUN-MON AS PARENT LOW PRESSURE TRACKS FROM THE NORTHERN PLAINS INTO CANADA. THIS SYSTEM WILL PULL THE STALLED CENTRAL-EASTERN U.S. FRONT NORTH AS A WARM FRONT. BY DAY 7 MON UPSTREAM SHORTWAVE ENERGY WILL LIKELY DROP INTO THE GULF OF ALASKA/NORTHEAST PACIFIC AND COULD BEGIN TO INFLUENCE EJECTING WEST COAST TROUGH ENERGY. OVERALL THE GUIDANCE SHOWS BETTER THAN AVERAGE AGREEMENT/CONTINUITY, ALLOWING AN UPDATED CONSENSUS BLEND TO YIELD A FORECAST VERY SIMILAR TO PREVIOUS ISSUANCE. PRIMARY DETAIL DIFFERENCES OF NOTE INCLUDE 00Z/06Z GFS RUNS LEAVING THE STALLED EASTERN FRONT SOMEWHAT FARTHER NORTH THAN MOST OTHER SOLUTIONS AROUND DAY 5 SAT, THE 00Z GFS BECOMING A LITTLE SHARPER/MORE AMPLIFIED THAN THE MAJORITY WITH THE SOUTHERN HALF OF THE WEST COAST UPPER TROUGH AROUND LATE SAT-SUN (A LITTLE EXTREME BUT WITHIN THE FULL ENSEMBLE ENVELOPE), AND 00Z-06Z GEFS MEANS BECOMING SOMEWHAT WEAKER/SOUTHWARD THAN CONSENSUS WITH LOW PRESSURE FORECAST TO BE NEAR NORTHERN ONTARIO BY EARLY DAY 7 MON. THE GEFS MEANS ARE ALSO ON THE WEAK SIDE WITH THE LATE PERIOD ENERGY DROPPING INTO THE GULF OF ALASKA. ECMWF/ECMWF MEAN RUNS HAVE BEEN BETTER DEFINED WITH THIS ENERGY FOR MULTIPLE RUNS AND THE GFS STARTED LATCHING ONTO THE CURRENT CONSENSUS IDEA WITH THE 12Z/29 RUN. OFF THE EAST COAST SUN-MON THE GFS AND TO SOME EXTENT GEFS MEAN ARE STRONGER THAN MOST OTHER GUIDANCE WITH ENERGY FEEDING INTO A MODEST WEAKNESS ALOFT AND AS A RESULT SHOW MORE INVERTED SURFACE TROUGHING/LOW PRESSURE. BASED ON THESE CONSIDERATIONS THE UPDATED FORECAST INCORPORATED ASPECTS OF THE 00Z-06Z GFS AND 00Z ECMWF/CMC FOR THE FIRST HALF OF THE PERIOD, FOLLOWED BY 60-70 PERCENT WEIGHTING OF THE 00Z ECMWF/ECMWF MEAN VERSUS THE 00Z-06Z GFS/06Z GEFS MEAN. ...WEATHER THREATS/HIGHLIGHTS... MUCH COLDER AIR WILL SETTLE INTO MUCH OF MONTANA 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 WHEN SNOW LEVELS DROP TO ABOUT 2000-3000 FT IN THE PAC NW. TEMPERATURES ABOUT 10-20F BELOW AVERAGE ARE LIKELY FOR MUCH OF MONTANA AND SOME OF THE NORTHERN PLAINS AS OF THU-FRI WITH THE CHILLY AIR BLEEDING WESTWARD AND SOUTHWARD THROUGH OR/WA/ID AND CALIFORNIA THIS WEEKEND. SNOW IS LIKELY TO BE HEAVY OVER NW MONTANA AT TIMES AS UPSLOPE FLOW STRENGTHENS BY FRIDAY WITH A 1036-1040MB HIGH OVER ALBERTA. PRECIPITATION WILL EXPAND THROUGH THE WEST/INTERMOUNTAIN WEST AS THE UPPER TROUGH AND PACIFIC FRONT WORK INLAND. BY NEXT WEEKEND UP TO SEVERAL INCHES OF PRECIPITATION ARE LIKELY FOR MUCH OF NORTHERN CALIFORNIA INTO SW OREGON, ESPECIALLY OVER THE COASTAL RANGES, KLAMATH MOUNTAINS, NORTHERN SIERRAS, ETC. WITH SEVERAL FEET OF SNOW POSSIBLE FOR THE FAVORED HIGHER TERRAIN. EVEN SOUTHERN CALIFORNIA COULD SEE SOME PRECIPITATION NEXT WEEKEND DEPENDING ON THE DEPTH OF THE UPPER TROUGH AND INFLUX OF MOISTURE. FARTHER INLAND ACTIVITY SHOULD REACH AT LEAST AS FAR SOUTH AS THE CENTRAL ROCKIES. REINFORCING HIGH PRESSURE TRACKING SOUTHEAST FROM ALASKA MAY BRING HIGHS DOWN TO 20-30F BELOW NORMAL OVER PARTS OF MONTANA BY SUN-MON. BACK TO THE EAST, A WARM FRONT WILL MOVE THROUGH THE NORTHEAST ON THU WITH AN AXIS OF LIGHT TO MODERATE RAIN ALONG/AHEAD OF THE COLD FRONT FROM EASTERN TEXAS NORTHEASTWARD THROUGH THE GREAT LAKES AND PARTS OF THE NORTHEAST. THE SOUTHERN PART OF THE FRONT WILL LINGER THROUGH THE MID-MISSISSIPPI VALLEY/TENNESSEE VALLEY/CAROLINAS WITH SCATTERED SHOWERS ALONG THE BOUNDARY FRI-SAT BEFORE LIFTING BACK TO THE NORTH AS THE WESTERN SYSTEM CROSSES THE NORTHERN PLAINS ON SUN. THE COLD FRONT TRAILING FROM THIS SYSTEM MAY GENERATE RAINFALL AS FAR SOUTH AS THE MID MS VALLEY DAYTIME MON WITH A FARTHER SOUTHWARD EXTENT SHORTLY THEREAFTER. SOUTHERN TIER STATES WILL SEE ABOVE NORMAL TEMPS THROUGH THE PERIOD WITH A FEW RECORD HIGHS POSSIBLE OVER TEXAS AND SCATTERED RECORD WARM LOWS ELSEWHERE. FRONTAL PROGRESSION WILL LEAD TO VARIABLE TEMPS OVER THE NORTHERN HALF OF THE EAST, WITH GREATEST ANOMALIES OF THE ABOVE NORMAL VARIETY EARLY AND LATE IN THE PERIOD. RAUSCH/FRACASSO