EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1110 AM EDT THU OCT 27 2016 VALID 12Z SUN OCT 30 2016 - 12Z THU NOV 03 2016 ...OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT... A RELATIVELY FAST-MOVING AND ACTIVE FLOW REGIME WILL PERSIST FROM THE WESTERN U.S. INTO THE NORTHERN PLAINS/GREAT LAKES/NORTHEAST DURING THE MEDIUM RANGE, WHILE RIDGING GRADUALLY EXPANDS FROM THE SOUTHEASTERN STATES TO THE SOUTHERN PLAINS. AN ELONGATED RIDGE AXIS EXTENDING FROM THE NORTH PACIFIC INTO ALASKA EARLY IN THE PERIOD WILL GRADUALLY PROPAGATE EASTWARD THROUGH THE MEDIUM RANGE, INTO WESTERN CANADA BY TUESDAY. THE RIDGE WILL SERVE TO DEFLECT STRONGER FLOW AND ASSOCIATED UPPER-LEVEL ENERGY FROM THE PACIFIC EITHER NORTH INTO ALASKA OR SOUTH INTO THE U.S. WEST COAST. THE RESULT, AS DISCUSSED YESTERDAY, IS THE POTENTIAL FOR GREATER THAN AVERAGE POTENTIAL FOR RUN-TO-RUN DISCONTINUITIES IN MODEL SOLUTIONS. THIS CONTINUES TO BE OBSERVED, WITH RATHER LARGE CHANGES NOTED BY DAY 6-7 AMONG CONSECUTIVE RUNS OF THE GFS. GIVEN THE RUN-TO-RUN VARIATION IN MODELS, PARTICULARLY THE GFS, TRIED TO LEAN TOWARD CONSISTENCY IN THIS FORECAST PACKAGE. THE 00Z GFS/GEFS WERE MUCH MORE SIMILAR TO THE 00Z ECMWF/ECMWF ENSEMBLE MEAN THROUGH MUCH OF THE MEDIUM RANGE THAN WERE THE 06Z GFS/GEFS. THE 06Z GFS/GEFS WERE MUCH QUICKER WITH THE ARRIVAL OF THE WEST COAST TROUGH ON TUESDAY, AND ALSO CLOSED THE TROUGH OFF INTO A DEEP UPPER LOW ACROSS THE SOUTHWEST BY WED (WHICH IS NOT SHOWN IN THE 00Z GFS OR ECMWF). ADDITIONALLY, DIFFERENCES IN THE STRENGTH OF THE WESTERN CANADA RIDGE IN THE 06Z GFS RESULT IN A MUCH DEEPER OCEANIC STORM WEST OF BRITISH COLUMBIA BY DAY 7 RELATIVE TO THE 00Z GFS/ECMWF. THUS, THE WPC FORECAST PACKAGE THIS CYCLE LEANED HEAVILY ON THE 00Z GUIDANCE MENTIONED ABOVE. MORE WEIGHT WAS PLACED ON THE DETERMINISTIC 00Z GFS/ECMWF IN THE DAY 3-4 TIME FRAME, WITH MORE WEIGHT GRADUALLY SHIFTING TO THE ENSEMBLE MEANS DURING THE DAY 5-7 PERIOD. ONE ISSUE NOTED WITH THE 00Z GFS WAS ITS DEPICTION OF A STRONG POLAR HIGH NOSING MUCH FARTHER SOUTH INTO THE NORTHERN ROCKIES/NORTHERN PLAINS BY NEXT WED/THURS. TO REDUCE THE IMPACT OF THIS FEATURE, THE INFLUENCE OF THE 00Z DETERMINISTIC GFS IN THE FORECAST WAS REDUCED SIGNIFICANTLY BY DAY 6-7. ...WEATHER HIGHLIGHTS/THREATS... WELL ABOVE AVERAGE TEMPERATURES ARE FORECAST ACROSS THE SOUTHERN ROCKIES AND CENTRAL TO SOUTHERN PLAINS INTO THE MID/LOWER MS VALLEY AND SOUTHEAST WITH SCATTERED HIGH TEMPERATURE RECORDS EXPECTED. TEMPERATURES ARE FORECAST TO BE 10-20 DEGREES ABOVE AVERAGE DAYS 3-5 FROM PORTIONS OF THE SOUTHERN PLAINS TO THE LOWER MS AND TN VALLEYS. THE PEAK WARM ANOMALIES DRIFT EAST INTO THE SOUTHEAST BY DAYS 6-7 /WED AND THURS NEXT WEEK AS UPPER LEVEL RIDGING EXPANDS OVERHEAD. BELOW AVERAGE TEMPERATURES ARE EXPECTED IN AREAS ALONG AND NEAR THE WEST COAST, ESPECIALLY DAYTIME MAXIMA, THANKS TO UPPER-LEVEL TROUGHING AND CLOUDINESS/PRECIPITATION. PRECIPITATION WILL BE FOCUSED ALONG THE WEST COAST, CONTINUING INLAND IN TANDEM WITH EACH OF THE UPPER-LEVEL TROUGHS CROSSING THE GREAT BASIN AND THE ROCKIES. PRECIPITATION COULD BE HEAVY ACROSS PORTIONS OF COASTAL CALIFORNIA AND THE SIERRAS, ESPECIALLY SUNDAY THROUGH MONDAY. THE SURFACE CYCLONE FORECAST TO CROSS THE NORTHERN PLAINS TO THE UPPER MS VALLEY/UPPER LAKES DURING THE DAY 4-5 TIME PERIOD WILL RESULT IN RAIN FROM PORTIONS OF THE NORTHERN HIGH PLAINS TO THE UPPER MIDWEST. RAIN AND THUNDERSTORMS MAY INCREASE ACROSS THE WESTERN GULF COAST DURING THE MEDIUM RANGE AS WELL DUE TO LOW-LEVEL FLOW OFF THE GULF OF MEXICO AND A PERSISTENT ZONE OF CONVERGENCE NEAR THE COAST. RYAN