Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 659 PM EST Mon Dec 19 2022 Valid 12Z Fri Dec 23 2022 - 12Z Tue Dec 27 2022 ...Cold air in place and moist return flow could lead to heavy snow across the Panhandle late this week... ...Overview... An upper-level high well to the northwest of the mainland late this week is forecast to weaken and shift a bit south into Siberia by early next week, as energies undercut this high and track across the mainland with an eventual likely focus of an upper low in eastern parts of the Bering Sea later in the period. At the surface, strong high pressure in the northern mainland as the period begins Friday should shift away with the pressure gradient weakening somewhat, while the general low track sets up over/near the Aleutians, northern Pacific, and Gulf of Alaska. While the details of these surface lows remain nebulous, the overall idea from the pattern and model guidance is that precipitation is likely across the Alaska Peninsula/Kodiak Island into Friday and shifting into the Panhandle late this week and early next week, with heavy snow a threat. ...Guidance/Predictability Assessment and Preferences... The overall pattern of a Chukchi Sea closed upper high with the main flow to the south is generally locked in in the model guidance, as evidenced by the fairly agreeable ensemble means on the broad scale. For days 4-5 (Friday/Saturday), considerable vorticity is forecast to dive west-southwest from the Arctic north of the Yukon/Northwest Territories to potentially form a closed upper low somewhere in the vicinity of the southeastern Bering Sea. By around day 5 there are already some timing and placement differences the energy/upper low likely in part due to interaction with energy already in the vicinity from the short range period as well as an additional round or two of energy coming in from upstream, and ensemble guidance shows considerable spread with the upper high/upper low interface. The GFS/ECMWF/UKMET were reasonably agreeable on a farther west position north of the Aleutians, but the CMC was farther east around the Bristol Bay/Alaska Peninsula/southwestern mainland on day 5 (12Z Saturday). However, the CMC actually agreed better with the ensemble means (particularly the EC and CMC means, as the GEFS was weaker) with the position, so this remains uncertain. These deterministic differences continue through the rest of the period with the positions of individual shortwaves and upper lows, and as one can imagine this also causes considerable variation in the placement of surface lows from model to model and run to run. But at least there is general agreement for the upper trough axis southwest-northeast around the Bering to the southern and eastern mainland and surface low tracks from the Aleutians and staying south of the mainland coast. The WPC forecast utilized a mainly deterministic model blend for day 4 but with a gradual ramp-up of the ensemble mean guidance as the period progresses to over half percent by day 8, given the increasing spread and thus decreasing confidence in the details. ...Weather/Hazards Highlights... Very cold air funneling into the state in response to strong (but weakening through Friday) surface high pressure over the northern mainland and deep northerly flow from the Arctic will cause continued colder than average temperatures especially for much of the eastern mainland through late week. Low temperatures are expected to fall into the -30s and -40s F in the colder spots of the eastern/southeastern Interior. Meanwhile, ridging sneaking back into the Panhandle will lead to moderating temperatures in that area compared to the cold short range. Then as troughing/possible upper lows affect the southwestern mainland/Alaska Peninsula, this should lead to a colder trend there for the weekend into early next week. For Friday-Saturday, cold air to start the period but increasing moisture directed at the Panhandle could lead to widespread heavy snow there. Widespread precipitation is likely to continue through early next week for the Panhandle and also make it into Southcentral Alaska, but moderating temperatures by that time may lead to some rain rather than just snow by Christmas Day and beyond. Areas around the Alaska Peninsula/Kodiak Island may also see light to moderate precipitation especially early in the period. However, timing and placement details of enhanced precipitation still have somewhat low confidence given the surface low placements diverging in the model guidance. Tate Hazards: - Heavy precipitation across portions of the Mainland Alaska, Fri-Mon, Dec 23-Dec 26. - Heavy precipitation across portions of Mainland Alaska, Thu, Dec 22. - Heavy precipitation across portions of Mainland Alaska, Sun-Mon, Dec 25-Dec 26. - Heavy snow across portions of the Alaska Panhandle, Fri-Mon, Dec 23-Dec 26. - High winds across portions of Mainland Alaska, Thu, Dec 22. - Much below normal temperatures across portions of Mainland Alaska, Thu-Sat, Dec 22-Dec 24. 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