Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 735 PM EDT Thu Mar 25 2021 Valid 12Z Mon Mar 29 2021 - 12Z Fri Apr 02 2021 ...Powerful Bering Sea storm to reach Mainland Alaska early next week... ...Guidance/Uncertainty Assessment... The majority of model/ensemble guidance shows good agreement and continuity regarding the strong Bering Sea system forecast to track into the western mainland around Monday-Tuesday followed by a transition of low pressure emphasis to the Gulf of Alaska as the supporting upper trough (extending south into the Pacific) ultimately settles over the mainland and northeastern Pacific. Farther west there is a general theme in the guidance for a trough to approach/reach the western Pacific while some degree of mean ridging builds over the North Pacific into the Aleutians/Bering Sea. However solutions differ in the strength and position of this ridge depending on how energy within the trough evolves/ejects onward. For about the first half of the period the primary consideration for the operational model blend used as the basis of the manual forecast was to phase out 12Z CMC input after Tuesday, as that model developed its Gulf low well southwestward of the dominant clustering of other models and ensemble means. There was minimal disagreement for the strong Bering Sea storm. After midweek the forecast blend tilted about 70 percent total toward the past two ECMWF runs and 00Z ECMWF mean relative to the 12Z GFS/GEFS mean. This scenario reflected recent trends over the past couple days toward stronger North Pacific into Bering Sea mean ridging during the latter half of the week, with ECMWF/ECMWF mean runs showing greater lead time in reflecting this ridge. The 12Z ECMWF (and recently arrived 12Z ECMWF mean) have backed off a bit from the prior cycle for strength of this ridge, supporting some degree of compromise from the strongest side of the envelope. This latest adjustment yielded closer agreement among the ECMWF/GEFS means--which still suggest the operational GFS may be too flat with the ridge. ...Weather/Hazards Highlights... Early in the week the strong Bering Sea storm forecast to track into the western mainland will support an episode of high winds/waves along with enhanced snow/precipitation over favored mainland terrain. As supporting upper dynamics continue eastward and then settle over the mainland/northeastern Pacific, expect a wave along the trailing front to develop and reach the Gulf of Alaska by early Wednesday. Low pressure should linger over or near the Gulf into late week. The Gulf system will likely bring a period of heavy precipitation to areas from the southeastern coast through the Panhandle. There are still timing and magnitude details to be worked out though. Farther west one or more waves along a mean frontal boundary around the northern periphery of a deep-layer ridge centered over the central Pacific may produce precipitation over parts of the Bering Sea and vicinity from about midweek onward, but confidence in any specifics is currently in the lower half of the spectrum. On Monday expect below normal temperatures over the eastern mainland/Panhandle while above normal readings (especially for morning lows) prevail over western areas with the approach of the Bering Sea low and trailing front. The above normal min readings will continue overspreading the state over the next day or so but clouds and precipitation will tend to keep daytime highs near to below normal. The upper trough settling over the area by mid-late week will yield below normal temperatures over many locations in the mainland and Panhandle by the latter half of the week, with anomalies tending to be more below normal for highs. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Mar 30-Mar 31. - Heavy snow across portions of mainland Alaska, Sun-Mon, Mar 28-Mar 29. - High winds across portions of mainland Alaska and the Aleutians, Sun-Mon, Mar 28-Mar 29. 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 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