EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1055 AM EST SAT DEC 05 2015 VALID 12Z TUE DEC 08 2015 - 12Z SAT DEC 12 2015 ...HIGH-END WET SEASON PRECIPITATION PATTERN FOR THE PACIFIC NORTHWEST COAST... ...OVERVIEW... MIGRATORY PACIFIC SYSTEM FREQUENCY WILL BE ON THE INCREASE --- AND GRADUALLY CARVING OUT A DEEP-LAYERED TROUGH ACROSS THE GREAT BASIN AND FOUR CORNERS REGION. WORKING NORTHEASTWARD FROM THE BASE OF THIS EMERGING TROUGH AXIS OUT WEST--- A SERIES OF PROGRESSIVE MIGRATORY SURFACE CYCLONES BEGIN TRACKING ACROSS THE CENTRAL PLAINS...MIDWEST AND WORK THEIR WAY SLOWLY ERODING A STABLE SURFACE RIDGE DRAPED ACROSS THE OHIO VALLEY...APPALACHIA AND THE MID-ATLANTIC STATES. ...MODEL PREFERENCES... ALONG WITH THE 5/00Z ECENS/NAEFS MEANS---PREFER TO MAINTAIN A DETERMINISTIC APPROACH (THE 5/00Z ECMWF) UNTIL THE 10/00Z TO 10/12Z---TO ENSURE RELATIVELY GOOD CONSISTENCY. OBVIOUSLY...THE COMPLEXITY OF THE PATTERN HAS ITS ROOTS IN THE NORTHEAST PACIFIC AND THE SUB-960MB CYCLONE IN THE GULF OF ALASKA. TO GIVE THIS SOME PERSPECTIVE---THE SUB-960MB VALUE IS A COMPILATION OF THE ENSEMBLES AND NOT INDIVIDUAL MODEL RUNS. SOME OF WHICH HAVE COME IN MUCH DEEPER/STRONGER (TO 938MB) AND THE TRIPLE-POINT LOWS ENTERING THE WEST CENTRAL COAST OF NORTH AMERICA-- IN THE 'PALTRY' SUB-980MB RANGE. WITH THESE PROJECTED AND VERY SIGNIFICANT VALUES --- WANT THE BEST CONTINUITY I CAN GET FOR TIMING SOME OF THE SENSIBLE WEATHER IMPLICATIONS MOVING WEST TO EAST WITH THE 'TRACK-ABLE' SHORTWAVES. ...UNCERTAINTY ASSESSMENT... NOT A BIG SURPRISE IN THIS PATTERN OFF THE PACIFIC NORTHWEST COAST THAT THE DETERMINISTIC SOLUTIONS ARE IDENTIFYING THE 'TRACK-ABLE' PACIFIC SHORTWAVES MIGRATING ACROSS THE NORTHERN ROCKIES. BUT THE RUN-TO-RUN VARIANCE---IS NOT STELLAR BEYOND THE 10/12Z TIME FRAME. THIS WILL FORCE THE NATIONAL FORECAST TO BECOME A BLEND OF THE ECENS/NAEFS MEANS ---PRIMARILY FOR DAY 6-7 TO HANDLE THE INCREASED FREQUENCY OF ORGANIZED PACIFIC SHORTWAVE ENERGY. FROM A SYNOPTIC SCALE PERSPECTIVE---AND IN A GENERAL DESCRIPTIVE SENSE (DAY-WISE)---THE ECMWF/UKMET HAS BEEN MORE RELIABLE WITH THE 3-SYSTEM APPROACH MIGRATING ACROSS THE LOWER 48 DURING THE FIRST HALF OF THE FORECAST PERIOD. THE SHORTWAVE EXITING THE NORTHEAST COAST (EARLY TUESDAY) ...FOLLOWED BY A 'LEAD' PACIFIC SHORTWAVE AND ITS COLD FRONT SPREADING DOWNWIND OF THE NORTHERN DIVIDE (TUESDAY)...AND A THIRD SYSTEM...AGGRESSIVELY CARVING OUT THE DEEP-LAYERED TROUGH ALONG THE WEST COAST (THURSDAY). BUT EVEN THE DETERMINISTIC ECMWF WAS NOT PICTURE PERFECT WITH ITS DEPICTION OF THE DAY 5 'LEAD' PACIFIC WAVE MIGRATING ACROSS THE GREAT LAKES. THOUGH IT IS DEFINITELY TRENDED STRONGER...IT TAKES ON MORE OF A CONSOLIDATED OPEN-WAVE NATURE. THE 5/06Z GFS---A CLOSE SECOND---AND THE 5/00Z UKMET/CANADIAN SLIGHTLY DEEPER AND MORE AGGRESSIVE ACROSS THE SOUTHEAST STATES BY 10/12Z. PREFER TO STAY WITH SOMETHING CLOSER TO THE DETERMINISTIC ECMWF TODAY...OWING TO ITS PACIFIC ORIGINS...THE SYSTEM WITH BE MORE OF A DRY FROPA BUT WINDY (MODIFIED-PACIFIC) AIRMASS INTRUSION FOR THE MISSOURI VALLEY AND OZARKS. THAT SAID A SHARPER...DEEPER 500-MB SOLUTION EAST OF THE MS VALLEY SEEMED TO BE A NON-FACTOR SENSIBLE WEATHER WISE. BEYOND DAY 5---THERE ARE AT LEAST TWO MORE INTENSE SHORTWAVES IN THE SEQUENCE--- AND WILL MIGRATE THROUGH THE BASE OF THE PACIFIC TROUGH (THURSDAY). THE LEAD WAVE EXITING THE DIVIDE AND THE FOLLOW-UP SHORTWAVE TRACKING ACROSS CALIFORNIA. FOR THESE TWO 'TRACK-ABLE' WAVES...THE DETERMINISTIC 5/00Z GFS/ECMWF MIGHT BE DECENT STARTING POINT TO IDENTIFY WITHIN THE ENSEMBLE MEANS. IN OTHER WORDS...USE THESE SOLUTIONS AS A ROUGH BASELINE ---THEN ADJUST AS NEEDED FOR SPACING ETC --- AS THE ENTIRE TROUGH BECOMES MORE FIRMLY ENTRENCHED OVER THE FOUR CORNERS AND GREAT BASIN. ...SENSIBLE WEATHER HIGHLIGHTS... ACROSS THE WEST---A TUESDAY-WEDNESDAY FRONTAL PROGRESSION THROUGH NORTHERN CALIFORNIA SHOULD CUT OFF THE HIGH-END PRECIPITATION SEQUENCE ---FOR A BRIEF TIME --- WITH MORE OF A COLD-SECTOR SHOWER AND BLUSTERY NORTHWEST FLOW PATTERN FOR THE PACIFIC NORTHWEST. SUBSEQUENT PACIFIC ENERGY GRADUALLY FOCUSES SOUTHWARD --- ALLOWING THE SOUTHWEST QUARTER OF THE NATION --- TO RECEIVE A PORTION OF THE DEEP NORTHEAST PACIFIC TROUGH AND PRECIPITATION PRODUCTION. FOR THE DIVIDE AND PLAINS---LEAD DOWNSTREAM PACIFIC ENERGY SPREADING ACROSS THE GREAT BASIN AND NORTHERN DIVIDE---WINDY AND VERY MILD. EVEN A SECOND SHORTWAVE (AROUND THURSDAY) PASSES WITH TEMPERATURES (BEHIND IT) THAT ARE CLOSE TO 10F ABOVE NORMAL. GREAT LAKES AND NORTHEAST--- EXITING CYCLONE OFF THE NORTHEAST COAST OFFERS SOME ONSHORE EAST TO NORTHEAST FLOW AND MODIFIED-CANADIAN HIGH PRESSURE. BUT OVERALL---MILDER MID-DECEMBER WEATHER THROUGH MID-WEEK THAN IS THE NORM. BY WEDNESDAY AND THURSDAY...THE PACIFIC INFLUENCES (MID-LEVEL MOISTURE AND WINDS) SHOULD BE APPROACHING FROM THE WEST. VOJTESAK