Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Thu Dec 06 2018 Valid 12Z Mon Dec 10 2018 - 12Z Fri Dec 14 2018 The models and ensemble means agree remarkably well on the overall pattern during the period. Current consensus shows a persistent mean low aloft over or just south of the Bering Strait while progressive Pacific flow containing energetic shortwaves will promote a series of systems that track near the Aleutians and into the Gulf of Alaska. Guidance agrees that this pattern should focus heaviest precipitation for the overall period from the eastern Kenai Peninsula through the Alaska Panhandle. The Aleutians will likely be a secondary maximum for precipitation. Periods of strong winds will be another noteworthy hazard accompanying this series of storms. Looking at the forecast details, as of early day 4 Mon an operational model consensus provides somewhat better refined details than the ensemble means for evolution of a leading northern Pacific wave tracking south of the Alaska Peninsula and then reaching the Gulf. Specifics become somewhat more murky for a time upstream around Tue-Wed with varied ideas for how initial Bering sea low pressure may interact with the next shortwave aloft/surface wave traversing the northern Pacific. Some of the detail spread for track/strength persists as the feature settles into the Gulf Wed-Thu. Thus far there is better than average clustering and continuity for a very strong storm advertised to track along or just south of the Aleutians around days 7-8 Thu-Fri. It is impressive that latest ensemble means have trended deep enough to be in the 960's mb that far out in time. Most operational runs over recent cycles have been below 960mb with some even below 950mb. In light of typical uncertainty/verification for forecasts 7-8 days out in time, a model/mean blend that produces a central pressure in the upper half of the 950's mb seems to be a reasonable starting point. Farther north of the main storm track, expect a persistent surface reflection (trough and/or low) near the mean low aloft by the latter half of the period while northern parts of Mainland Alaska will remain under the gradient to the south of weak east-west surface ridging over the Arctic along 75-80N latitude. Based on guidance preferences, the forecast started with an operational model blend days 4-5 Mon-Tue with input from the 00Z ECMWF/06Z-12Z GFS/00Z CMC in order from highest to lowest weight. Then the forecast phased out the CMC while beginning to incorporate 06Z GEFS/00Z ECMWF mean data--30 percent total for day 6 and reaching 50 percent total by days 7-8 Thu-Fri. The 06Z GEFS mean was a little stronger with primary surface lows during the latter half of the period so it had more weight for the ensemble mean part of the forecast. Rausch WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html