ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 207 PM EST SUN DEC 10 2017 VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017 MODELS/ENSEMBLES CONTINUE TO SHOW A GENERAL CONSENSUS THAT PERSISTENT UPPER RIDGE ACROSS WESTERN CANADA AND INTO ALASKA SHOULD BEGIN TO WEAKEN SOME BY LATE IN THE WEEK, ALTHOUGH SOME DIFFERENCES REMAIN AS TO HOW QUICKLY AND TO WHAT DEGREE THIS OCCURS. DURING THE EARLY PORTION OF THE MEDIUM RANGE, PREFERRED THE ECMWF SOLUTION FOR LOW PRESSURE MOVING FROM THE GULF INTO SOUTHERN AK THU. MODELS STILL SHOW SOME SPREAD HERE BUT THE ECMWF HAS BEEN GENERALLY CONSISTENT FOR SEVERAL RUNS NOW AND IS ALSO WELL CENTERED WITHIN THE ENSEMBLE CLUSTERING. FOR THE NEXT SYSTEM CROSSING THE ALEUTIANS THROUGH FRI, TOOK A BLENDED APPROACH TO THE FORECAST INCLUDING THE ECMWF/GFS AS WELL AS ECENS/NAEFS MEANS. AFTER FRI SPREAD BEGINS TO INCREASE MORE SUBSTANTIALLY, AND IT IS DIFFICULT TO JUSTIFY BUYING INTO ANY ONE DETERMINISTIC SOLUTION AT THIS TIME. DETERMINISTIC GUIDANCE SEEMS TO HAVE SHOWN A TREND TOWARD TRANSFERRING MOST OF THE ENERGY ASSOCIATED WITH THIS SYSTEM TO A POTENTIAL TRIPLE POINT LOW IN THE GULF THAT EITHER MOVES EAST TOWARD SOUTHEAST AK (AS SHOWN BY THE ECMWF) OR TURNS MORE TO THE NORTH (PER THE GFS). AT THIS TIME, PREFERRED AN ENSEMBLE APPROACH (INCLUDING THE ECENS/GEFS/NAEFS) FROM DAY 6 ONWARD, WHICH SHOWED A LOW STAYING CLOSER TO THE SOUTHERN MAINLAND AK COAST. THIS KEEPS THE FORECAST SOMEWHAT CONSISTENT WITH CONTINUITY WHILE ALSO ADJUSTING A BIT TOWARD THE LATEST TRENDS. BY EARLY NEXT WEEK, MODELS ALL SHOW ANOTHER SYSTEM MOVING INTO THE PICTURE, CROSSING THE WESTERN ALEUTIANS OR THE SOUTHERN BERING SEA. DETERMINISTIC MODELS ARE ALL OVER THE PLACE WITH THIS FEATURE, BUT ENSEMBLE MEANS ARE MUCH CLOSER TO ONE ANOTHER, SHOWING A CYCLONE NEARING THE WESTERN ALEUTIAN CHAIN BY 12Z NEXT MON. ADDITIONALLY, ENSEMBLES CONTINUE TO PERSIST WITH A SLIGHTLY STRONGER RIDGE AXIS ACROSS WESTERN NORTH AMERICA RELATIVE TO THE DETERMINISTICS, SO THE PREFERRED ENSEMBLE APPROACH LATER IN THE PERIOD WOULD FAVOR THIS SCENARIO OF A SLIGHTLY MORE SLOWLY WEAKENING RIDGE. IN TERMS OF SENSIBLE WEATHER, THE RAPID SUCCESSION OF SHORTWAVES AND SURFACE CYCLONES ACROSS THE ALEUTIANS, THE GULF, AND INTO SOUTHERN AK 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 IN THE WEEK SHOULD RESULT IN DECREASING TEMPERATURES, CLOSER TO OR EVEN A FEW DEGREES BELOW CLIMATOLOGICAL NORMS AS ARCTIC AIR IS ABLE TO SPREAD FARTHER SOUTH ACROSS MUCH OF THE STATE. RYAN