Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 657 PM EST Mon Dec 13 2021 Valid 12Z Fri Dec 17 2021 - 12Z Tue Dec 21 2021 ...Likely precipitation with significant amounts and strong southwesterly winds over the western mainland Saturday night through Monday... ...Overview... Guidance continues to show a significant pattern change, from a west-to-east succession of shortwaves and associated surface systems to a strong Pacific upper ridge whose northern periphery will progress from the Bering Sea into Mainland Alaska and Northeast Pacific during Saturday-Monday. The gradient between the ridge and dynamics associated with a system tracking east of Kamchatka through eastern Siberia and into the Arctic should ultimately lead to strong westerly flow aloft over much of the state by next Tuesday. Flow ahead of the surface system will likely bring widespread precipitation and strong southwesterly low level winds into the western mainland during the latter half of the weekend into early next week. The pattern change will also promote a significant warmer trend for temperatures. ...Guidance Evaluation and Preferences... With today's UKMET trending deeper to recent runs of other models, clustering has further improved for the early Friday Bering Sea storm expected to weaken as it tracks into the far western mainland by early Saturday. The 12Z model consensus serving as today's forecast blend for at least the first half of the period continued the trend of recent days in being a little deeper--consistent with steadily improving track and timing clustering--though the models show signs of starting to stabilize for strength. This system should lead to a weak Gulf wave that tracks southeastward offshore the Panhandle during the weekend, after a leading weak wave that takes a similar track Friday into early Saturday. Individual model runs have been varying with details of the storm forecast to track from the western Pacific to east of Kamchatka and eventually into the Arctic. The guidance average has adjusted somewhat eastward versus yesterday when the system passes near Kamchatka, while some models now suggest there could be two lows that eventually consolidate, and there have been timing issues as well. For the latter the 00Z ECMWF appeared notably fast. Into day 7 Monday a primary emphasis on the 12Z operational models in the forecast blend had decent agreement from the means, and in spite of the low's track adjustment east of Kamchatka it still ended up near continuity northwest of the mainland by early Monday. The new 12Z ECMWF mean was close to this scenario as well. After early Monday the models have been showing increasing spread and run-to-run variability for the shape of the northern periphery of the strong Pacific upper ridge and details of shortwave energy. These differences aloft corresponded to variation in where a surface front sets up between the mainland and Aleutians/North Pacific. Decreasing confidence in specifics led to increasing total ensemble weight to 40 percent by day 8 Tuesday, with somewhat more 00Z ECMWF mean versus the 12Z GEFS mean as the former continues to be somewhat stronger and closer to the models for upper ridge strength. ...Weather/Hazard Highlights... Light to moderate precipitation and brisk to strong winds should accompany the system tracking from the central Bering Sea into the western mainland Friday-Saturday. Brisk winds could reach the southwestern coast of the mainland depending on exact low track. From late this week into the weekend a couple weak waves tracking southeastward from the Gulf of Alaska may produce light to moderate precipitation though consensus is trending noticeably lighter over the Panhandle. The flow ahead of a strong storm forecast to track from east of Kamchatka through eastern Siberia and into the Arctic should spread a broad area of moisture and strong winds across portions of the Aleutians, Bering Sea, and then the western mainland during the weekend into early next week. Over the western mainland from the Brooks Range southward, during Saturday night through Monday precipitation (of varying types) may be heavy over terrain favored by strong southwesterly winds. Areas with a combination of snow and strong winds could see blizzard conditions. Precipitation and winds should trend lighter on Tuesday. Aside from some below normal temperatures over the Panhandle, eastern mainland, and southern coast, the majority of the mainland should see temperatures trend to above normal levels with the highest anomalies likely to be over western areas around Sunday-Monday. Rausch Hazards: - Heavy precipitation across portions of mainland Alaska, Sat-Sun, Dec 18-Dec 19. - Heavy snow across portions of mainland Alaska, Sun, Dec 19. - High winds across portions of mainland Alaska and the Aleutians, Sat-Sun, Dec 18-Dec 19. - Much below normal temperatures across portions of mainland Alaska, Thu, Dec 16. Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html