ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 210 PM EST SAT DEC 09 2017 VALID 12Z WED DEC 13 2017 - 12Z SUN DEC 17 2017 MODELS/ENSEMBLES SHOW A GENERAL CONSENSUS THAT PERSISTENT UPPER RIDGE ACROSS WESTERN CANADA AND INTO ALASKA SHOULD BEGIN TO WEAKEN SOME BY LATE NEXT WEEK, ALTHOUGH DIFFERENCES REMAIN AS TO HOW QUICKLY AND TO WHAT DEGREE THIS OCCURS. AT LEAST DURING THE EARLY PORTION OF THE MEDIUM RANGE, MODELS SHOW AVERAGE AGREEMENT WITH RESPECT TO A SURFACE LOW INITIALLY CROSSING THE GULF OF AK ON DAY 4 (WED) THAT THEN MOVES QUICKLY NORTH INTO SOUTHERN MAINLAND AK ON DAY 5 (THU) BEFORE WEAKENING. WHILE SOME SPREAD REMAINS, THE ECMWF AND GFS SEEMED REASONABLY WELL CLUSTERED WITHIN CONSENSUS, AND A HEAVILY DETERMINISTIC BLEND INCLUDING THESE TWO SOLUTIONS SERVED AS A FORECAST STARTING POINT DURING DAYS 4-5. FROM DAY 6 (FRI) ONWARD, SPREAD BEGINS TO INCREASE WITH RESPECT TO THE NEXT SYSTEM CROSSING THE ALEUTIANS. MODELS SHOW QUITE A BIT OF RUN-TO-RUN VARIABILITY ESPECIALLY BY DAYS 7-8 (SAT-SUN) SURROUNDING WHETHER THE TRIPLE POINT LOW TAKES OVER AS THE PRIMARY LOW, TAKING A MORE EASTERLY TRACK AS SHOWN BY THE ECMWF, OR WRAPS NORTH INTO THE BERING SEA AS THE ECENS/GEFS MEANS SUGGEST. GIVEN THE UNCERTAINTY, THE FORECAST WAS TRENDED TOWARD SIGNIFICANTLY HEAVIER ECENS/GEFS WEIGHTING BY LATE NEXT WEEK. ADDITIONALLY, ENSEMBLE MEANS ARE ALL A BIT SLOWER TO WEAKEN THE UPPER RIDGE RELATIVE TO THE DETERMINISTIC GUIDANCE, SO MORE ENSEMBLE WEIGHTING IN THE FORECAST WOULD SUPPORT A SLIGHTLY STRONGER/MORE PERSISTENT RIDGE THAN INDICATED BY MOST DETERMINISTIC SOLUTIONS. IN TERMS OF SENSIBLE WEATHER, THE PERSISTENT ONSHORE FLOW AND RAPID SUCCESSION OF SHORTWAVES AND SURFACE CYCLONES WILL RESULT IN A CONTINUED STORMY PERIOD FOR SOUTHERN AND SOUTHEASTERN AK, WITH HEAVY PRECIPITATION POSSIBLE ESPECIALLY IN FAVORED WINDWARD TERRAIN AREAS. FARTHER NORTH, SOMEWHAT DRIER CONDITIONS WILL PREVAIL NORTH OF THE ALASKA RANGE, BUT STILL EXPECT SCATTERED SNOW SHOWERS THROUGH MUCH OF THE PERIOD AS ROUNDS OF SHORTWAVE ENERGY PASS OVERHEAD. GRADUAL WEAKENING OF UPPER RIDGING BY LATE NEXT WEEK SHOULD RESULT IN DECREASING TEMPERATURES FROM ABOVE NORMAL ACROSS MUCH OF THE STATE SOUTH OF THE BROOKS RANGE, TO CLOSER TO CLIMATOLOGICAL NORMS AS ARCTIC AIR IS ABLE TO SPREAD FARTHER SOUTH ACROSS MUCH OF THE STATE. RYAN