ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 315 PM EST TUE DEC 19 2017 VALID 12Z SAT DEC 23 2017 - 12Z WED DEC 27 2017 ...WET CONDITIONS LIKELY ACROSS THE SOUTH-CENTRAL AK COAST THIS WEEKEND... A HIGHLY AMPLIFIED PATTERN IS IN PLACE WITH A STRONG POSITIVE HEIGHT ANOMALY EXTENDING UP ACROSS WESTERN CANADA INTO FAR EASTERN AK. THE 12Z GFS ADVERTISES 500-MB HEIGHT ANOMALIES AROUND 2 TO 2.5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY FOR MID/LATE DECEMBER. FARTHER UPSTREAM...A ROBUST UPPER LOW SHOULD INITIALLY BE SPINNING OVER THE CENTRAL ALEUTIANS ON SATURDAY MORNING WITH THE CIRCULATION CENTER LIFTING UP INTO THE BERING SEA. GENERAL WEAKENING IS LIKELY AS THE SYSTEM BEGINS TO SHEAR BASED ON SOME OF THE AVAILABLE GUIDANCE. AN ADDITIONAL SHORTWAVE IS FORECAST TO PUSH TOWARD THE GULF OF AK BY SUNDAY WITH THIS FEATURE RAPIDLY DISSIPATING AS IT ENCOUNTERS THE STOUT DOWNSTREAM RIDGE. DETAILS BEYOND THESE BECOME FUZZY AS AN ADDITIONAL LOWERING OF HEIGHTS IS LIKELY BACK TOWARD THE KAMCHATKA PENINSULA. DURING THE DAY 6-8...DECEMBER 25-27 TIMEFRAME...OPERATIONAL/ENSEMBLE GUIDANCE AGREE ON ESTABLISHING ANOTHER POWERFUL TROUGH WHICH TAKES AIM ON THE WESTERN ALEUTIANS JUST BEYOND THE END OF THIS FORECAST PERIOD. THE OVERALL PATTERN WILL BE BEST SHAPED WITH AN ENSEMBLE MEAN BASED APPROACH BEYOND DAY 5/DECEMBER 24 GIVEN THE POOR RUN-TO-RUN MODEL CONTINUITY. ADDITIONALLY...THERE ARE SOME SITUATIONS WHERE THE 06Z/00Z GFS DEPICTED AN AREA OF HIGH PRESSURE WHILE THE 00Z ECMWF SUGGESTED A DEEP LOW IN ITS PLACE. THROUGH THE END OF THE WEEKEND...TOOK A MULTI-MODEL APPROACH WITH DECREASING AMOUNTS OF THE 06Z GFS/00Z ECMWF EARLY BEFORE REVERTING TO SOLELY ENSEMBLE MEANS BY NEXT WEEK. THIS WAS LED BY 50 PERCENT OF THE 00Z ECMWF ENSEMBLE MEAN WITH THE REMAINING CONTRIBUTIONS COMING FROM THE 06Z GEFS/00Z NAEFS ENSEMBLE MEANS. GIVEN THE EXPANSIVE ARCTIC HIGH SURGING SOUTHWARD ACROSS FAR EASTERN AK INTO THE YUKON TERRITORY...A MARKED COOLING TREND IS EXPECTED. HIGHS SHOULD STRUGGLE TO REACH THE ZERO DEGREE MARK ACROSS EAST-CENTRAL AK AS THE COLDER AIR SPILLS IN FROM THE POLAR REGIONS. DIURNAL TEMPERATURES SHOULD BE ON THE LOWER SIDE ACROSS MUCH OF AK GIVEN THE VAST CLOUD COVER EXPECTED FOR MUCH OF THE PERIOD. REGARDING PRECIPITATION...HEAVY RAIN/SNOW WILL BE LIKELY ALONG THE SOUTH-CENTRAL AK COAST AS A STRONG SURGE OF PACIFIC MOISTURE SHOOTS NORTHWARD AHEAD OF THE DEEP SURFACE CYCLONE. 12Z GFS 850-700 MB MOISTURE FLUX ANOMALIES SHOULD BE ON THE ORDER OF 4 TO 5 STANDARD DEVIATIONS ABOVE AVERAGE. 12-HOUR PRECIPITATION TOTALS MAY EXCEED 2 INCHES IN MANY LOCATIONS AS ADVERTISED BY THE 12Z GFS/ECMWF. IN ADDITION TO THE PRECIPITATION...WIND GUSTS REACHING THE 50 TO 60 KNOT RANGE WILL BE POSSIBLE BEFORE SLACKENING AS THE PARENT CYCLONE DECREASES IN STRENGTH. RUBIN-OSTER