Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EST Tue Dec 14 2021 Valid 12Z Sat Dec 18 2021 - 12Z Wed Dec 22 2021 ...Likely precipitation with significant amounts and strong southwesterly winds over the western mainland Saturday night through Monday... ...Overview... A significant pattern change is still expected over Alaska early next week. After a shortwave trough comes across the mainland Saturday, ridging from the surface to upper levels will set up across the Pacific, with its northern periphery spreading northeastward from the Bering Sea into Mainland Alaska and Northeast Pacific during Saturday-Monday. This pattern will lead to strong, moist, and unseasonably warm southwesterly flow over much of the state for the early part of next week, causing the potential for gusty winds and heavy precipitation along with much milder temperatures. ...Guidance Evaluation and Preferences... At the beginning of the period Saturday, there is reasonably good model agreement for a shortwave trough to track quickly across the mainland, which should spin up a weak surface low along the Gulf of Alaska coast tracking eastward, while another surface low weakens offshore of the southern Panhandle. Then, guidance continues to have a good signal for the aforementioned Pacific ridging coming into the state and directing southwesterly flow initially to the Aleutians/western mainland and spreading eastward. However, there are some model differences especially west of the state with the details of low pressure systems. The 12Z ECMWF and UKMET indicate two lows moving through the Bering Sea and eastern Siberia that eventually consolidate, along with timing differences. The 12/06Z GFS runs seemed to match well clustered ensemble means' low position best, so favored those along with lesser proportions of other deterministic guidance for the early part of the forecast period throughÂaroundÂMonday. By Tuesday and Wednesday, an upper low establishing itself to the north of Alaska may send energy southward and create a shortwave trough passing through the state cutting through the ridge axis (with additional ridging behind it moving into western Alaska early Wednesday). General trends since yesterday include a stronger shortwave, with the 12Z GFS and ECMWF agreeing reasonably well on its position, and the cold front moving faster across Alaska ahead of it. The 12Z CMC did not line up as well with the shortwave axis, but with the GFS/ECMWF agreement, felt it was best to trend stronger with the shortwave and a surface low strengthening offshore of the Alaska Panhandle by early Wednesday. The WPC model blend included the GEFS and EC ensemble means by day 7 and quickly increased their proportion to 50% ensemble means and 50% deterministic GFS/ECMWF on day 8 amid increasing model uncertainty. ...Weather/Hazard Highlights... Precipitation is likely for the Aleutians and the western mainland beginning Saturday given the increasing deep layer moist southwesterly flow into the state west of the Pacific ridge, which should spread into central parts of the mainland by Sunday. Over the western mainland from the Brooks Range southward, during Saturday night through Monday precipitation (of varying types, but mainly snow) may be heavy over terrain favored by strong southwesterly winds. Brisk low level winds are expected, which along the coast may even lead to areas of coastal flooding. Areas with a combination of snow and strong winds could see blizzard conditions. Precipitation and winds should trend lighter on Tuesday after the cold front passes. With the relatively warm inflow for Alaska, western areas of the mainland will see above normal temperatures on Saturday, increasing and spreading eastward through early next week. Temperatures could be 20 to as much as 40 degrees above average, with some temperatures rising above the freezing mark especially in the southwest part of the state. The Panhandle will be the main area with below normal temperatures through the first half of next week, while parts of the southeastern mainland could be around normal. Tate Hazards: - Heavy snow across portions of mainland Alaska, Fri-Sat, Dec 17-Dec 18. - Heavy snow across portions of mainland Alaska, Sun-Mon, Dec 19-Dec 20. - High winds across portions of mainland Alaska and the Aleutians, Fri-Sat, Dec 17-Dec 18. - High winds across portions of mainland Alaska, Fri-Mon, Dec 17-Dec 20. 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