Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1226 PM EST Fri Dec 21 2018 Valid 12Z Tue Dec 25 2018 - 12Z Sat Dec 29 2018 ...A Widespread Heavy Precipitation Threat for the Alaskan Southern Tier... ...Guidance/Predictability Assessment and Preference... While the non-06z GFS guidance showed reasonable agreement at 500 hPa, surface issues continue to plague the pressures forecast with the system moving through the northeast Pacific Tuesday and Wednesday and with the longwave trough moving from the 180th meridian late Tuesday before splitting as it passes by 160W early Friday. To deal with the uncertainty, the pressures and 500 hPa heights used a compromise of the 00z Canadian, 00z ECMWF, 06z GFS, and 00z UKMET early on. From Wednesday onward, once the UKMET run reached its end, used increasing amounts of the 00z NAEFS mean and 00z ECMWF ensemble mean. The remainder of the grids were a more even blend of the above deterministic and ensemble mean guidance. This appeared to lead to reasonable day-to-day continuity. ...Weather/Threats Highlights... The main storm track into next week includes a series of organized low pressure systems and frontal systems that track along/south of the Aleutians and the Alaska Peninsula into the Gulf of Alaska. These lows present both a maritime hazard in the form of gale to storm force winds and a periodic heavy precipitation threat throughout the next week for the Aleutians/AK peninsula/Southwest Alaska and southern/southeastern Alaska. Individual system timing and strength remains quite variable in guidance, but the overall storm track seems reasonable. A lead system exiting the eastern Gulf of Alaska Monday is replaced upstream with a set of lows from the Aleutians that are supported by an amplifying mid-upper level mean trough. The lows are expected to increasingly effect the full southern tier of Alaska and offshore into the Bering Sea as well as the Gulf of Alaska next week. Activity may increase over time next week with stormy conditions including widespread heavy to excessive precipitation channel into the state between the aforementioned supporting mean trough aloft and a building downstream upper ridge. The inland threat would be enhanced into coastal communities and over favored southern tier terrain. Quite cold and dry conditions are expected early into mid next week over the North Slope/Interior and Bering Strait. From Wednesday onward, shortwave trough/closed low energy aloft tends to favor a weakness aloft that may support some limited light snows. Roth/Schichtel 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