Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 649 PM EST Sun Dec 18 2022 Valid 12Z Thu Dec 22 2022 - 12Z Mon Dec 26 2022 ...Anomalous cold should ease across the Panhandle late week but continue across the eastern Interior through the weekend... ...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 a possible focus of an upper low in the Bering Sea later in the period. At the surface, strong high pressure in the northern mainland and east of the state as the period begins Thursday should shift away with the pressure gradient weakening somewhat, as 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 Thursday-Friday and shifting into the Panhandle late this week and early next week, with heavy snow a possible 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 day 4/Thursday even deterministic models agree in principle on an upper low atop the western Aleutians while vorticity is strung out west-east across the mainland. Then for Friday-Saturday additional energy dives west-southwest from north of the Yukon/Northwest Territories. The GFS/ECMWF are similar for this while the UKMET appeared to be an outlier with its energy farther east of other guidance, while the CMC is slower tracking the energy west, as both keep the ridge extending from the Chukchi Sea high stronger. Ensemble guidance shows considerable spread with that upper high/upper low interface so this remains uncertain. By next Sunday-Monday, ensemble means show a developed upper low centered over the southeastern Bering near the southwestern mainland and Alaska Peninsula. The upper low placements in each deterministic model differ notably and indicate there may be multiple upper low centers. As one can imagine this also causes considerable variation in the placement of surface lows from model to model and run to run. 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 70 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 by Thursday) surface high pressure over the northern mainland and in far northwestern Canada and deep northerly flow from the Arctic will cause continued colder than average temperatures especially for the southern two-thirds of the eastern mainland into the Panhandle through Thursday. Low temperatures are expected to fall into the -40 to -50 F range in the colder spots of the eastern/southeastern Interior and below zero on Thursday for the interior side of the Panhandle. By late next week the Panhandle should trend closer to normal in terms of temperatures but the eastern mainland will likely stay quite cold through the weekend, while over the southwestern mainland/Alaska Peninsula the potentially forming upper low should lead to a colder trend for the weekend. Moderate to perhaps heavy precipitation is forecast for the Alaska Peninsula/Kodiak Island areas around Thursday as a shortwave helps to focus moisture there, and gusty winds could be a threat as well. Then the Panhandle is also likely to see moist return flow for more widespread precipitation beginning late week and lasting through early next week (when Southcentral Alaska can also expect increasing precipitation). With the cold air in place to start the period and sufficient cold possibly lasting, much of this could fall as snow over land areas, for potentially heavy snow. But confidence is low given the uncertainties in the timing and placement details given the surface lows diverging in model guidance. Tate 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