Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 652 PM EST Wed Nov 24 2021 Valid 12Z Sun Nov 28 2021 - 12Z Thu Dec 02 2021 ...Arctic cold over much of the Interior through the weekend followed by moderation over the east... ...Increasing precipitation along the southern coast and Panhandle early-mid week... ...Overview and Guidance Evaluation/Preferences... In principle the past day of guidance has remained fairly agreeable and consistent with the large scale forecast that involves a developing eastern Bering Sea/western mainland upper trough helping to direct a consolidating North Pacific storm northeastward into the Gulf of Alaska, followed by the system weakening in place as upper troughing (with one or more embedded closed lows possible) persists. This storm may produce strong winds over some areas and will likely bring an episode of moderate to heavy precipitation to the southern coast and Panhandle. This pattern that evolves after an initial deep upper low is centered over the south-central mainland early Sunday should keep western areas cold while allowing for some moderation to the east next week. Farther west, a weak system should quickly drop southeastward over the western Bering Sea/Aleutians around Tuesday followed by a building upper ridge that reaches the same region by early next Thursday. Over the past day the models have generally trended closer together for the North Pacific into Gulf evolution, especially by way of the GFS trending south of the Alaska Peninsula and the 12Z ECMWF nudging back to the west some by Tuesday. Detail differences remain though. These issues include initial low pressure just south of the Aleutians on Sunday (no clear clustering per se at the moment) and storm track/possible frontal waviness into the Gulf (12Z CMC becoming an eastern extreme with the primary low, 12Z GFS stronger with an extra wave that the 18Z run dampened somewhat). From Sunday into Tuesday a 12Z model composite represented the most common ideas of guidance well. The main refinement from continuity was a somewhat north/northwest surface low position by early Tuesday. This blend also worked well elsewhere early in the period and then incorporating some of the 12Z GEFS/00Z ECMWF means with the GFS, 00Z/12Z ECMWF, and decreasing input of the CMC accounted for the typically increasing detail spread by Wednesday-Thursday. There are various possibilities for exactly how flow within the western mainland upper trough may evolve later in the period while throughout the forecast there is some continued ambiguity over how much connection there will be between upper troughing west of the Canadian Archipelago and the mainland mean trough. Also spread increases for the weak system reaching the western Bering Sea by Tuesday, with the 12Z CMC at least looking rather extreme with its eastward progression given the persistence of the pattern to the east. Timing differences for the upper ridge reaching the Bering Sea/Aleutians by day 8 Thursday are fairly typical for that far in advance. ...Weather/Hazard Highlights... Through the weekend expect many areas to see very cold temperatures, especially over the southwestern mainland where readings may be 20F or more below normal. Similar anomalies may extend eastward across the south-central Interior. The Panhandle into far southeast corner of the mainland as well as parts of the north could see slightly above normal lows though. The transition of the upper pattern from a mainland upper low toward a north-south trough aligned over the western mainland should support a moderating trend over eastern areas (with pockets of moderately above or below normal readings) while keeping the west well below normal. Weakening low pressure over the Gulf of Alaska this weekend may maintain light to moderate precipitation over the southeastern coast and Panhandle. A wave tracking well to the south may bring some moisture into the southern Panhandle as well. Intensifying North Pacific low pressure should bring precipitation and brisk to strong winds across the Aleutians (winds initially from the east and then turning northerly/northwesterly) and possibly into the Alaska Peninsula and Kodiak Island early in the week. A storm track into the Gulf should increase precipitation from near the Kenai Peninsula through the Panhandle Tuesday onward. While not perfect yet, today's guidance has improved its agreement versus yesterday with respect to coverage and magnitude of locally moderate to heavy totals. The weak system dropping through the western Bering Sea/Aleutians may produce an area of organized but generally light precipitation around Tuesday or so. By next Thursday southerly winds may strengthen over the western Aleutians ahead of a storm emerging from Asia. Rausch Hazards: - Heavy precipitation across portions of mainland Alaska, Mon, Nov 29. - High winds across portions of mainland Alaska, Mon, Nov 29. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Nov 30-Dec 1. - Much below normal temperatures across portions of mainland Alaska, Fri-Mon, Nov 26-Nov 29. 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