Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 603 PM EST Mon Dec 28 2020 Valid 12Z Fri Jan 01 2021 - 12Z Tue Jan 05 2021 ...Deep system in the western Bering late this week with strong winds and significant waves... ...Overview and Guidance/Uncertainty Assessment... An extremely deep area of low pressure in the far western Bering Sea Friday will split off its triple point low into the Gulf as the parent low slowly decays. The upper pattern will favor troughing in the Bering to remain into early next week with an extension into the Gulf. Upper ridging over the Interior Friday will slip northwestward into northeastern Russia to be replaced by troughing from NW Canada, ultimately merging with troughing in the eastern Bering. This keeps the storm track south of the Aleutians after the lead deep Bering system, with an aim generally on the Panhandle. Strong 200kt jet across the North Pacific later this week will translate eastward and weaken, perhaps to be reinvigorated next week off Japan. The 12Z guidance was in relatively good agreement, though the GFS/ECMWF clustered better with the 12Z GEFS and 00Z ECMWF ensemble means for the Fri-Sun period. Utilized a deterministic blend to start in order to maintain an appropriate depiction of the Bering low (930s mb, which was in between the deterministic models in the 920s to low 930s and the ensemble means in the 940s to around 950mb). Progressive flow beneath the upper low will keep that system and the next couple systems moving from west to east, with less agreement through the period (though the GFS/ECMWF were, perhaps by happenstance, closer than expected out to next Monday). Incorporated more ensemble weighting by next week to tame down any feature as cycle-to-cycle consistency has been lacking in the progressive pattern. To the north, guidance differs on how the bring in lower heights across the Interior (or North Slope), but ensemble consensus seems to take the bulk of height falls WSW toward the Seward Peninsula. This should get caught up with the leftover Bering troughing to move across Bristol Bay/AKPen but extending back along western Alaska next week. ...Weather/Hazards Highlights... Deep Bering system will bring at least a period of modest rain/snowfall and strong winds (40-50kts+, especially over the water) around the center. Significant waves are also possible (over 40ft). Please consult the Ocean Prediction Center for more information. Parent low will take several days to wind down, maintaining scattered precipitation for the Aleutians/Bering Sat-Tue. Triple point low detaching from the occlusion could provide more focus for rain/snow as it moves into the Panhandle late Fri into Sat. Trailing system may again bring more rain/snow to the Panhandle late Sun into Mon. Locally heavier amounts are possible. Temperatures near to above normal over western areas of the mainland Friday will turn colder this weekend into next week as colder air filters in from the east. Readings may be 10-15 degrees below normal north of the Alaska Range. For colder valleys this could mean low near 30 below and highs only around 10-20 below. For much of Southcentral into the Panhandle, near to above normal temperatures are expected. Fracasso Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Fri-Sat, Jan 1-Jan 2. - High winds across western Aleutians, Thu-Fri, Dec 31-Jan 1. - High significant wave heights for coastal portions of the western Aleutians, Thu-Fri, Dec 31-Jan 1. 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