Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EST Thu Mar 11 2021 Valid 12Z Mon Mar 15 2021 - 12Z Fri Mar 19 2021 ...Overview and Guidance/Uncertainty Assessment... Today's models and ensembles continue to advertise an increasingly blocky pattern from the Bering Sea through a large portion of the eastern Pacific. However after about early Tuesday solutions diverge for the shape and position of significant features, and the differences appear to be at least as great if not more so than over the past couple days. Early in the week there is decent continuity with the axis of a sharpening and digging upper trough reaching about 160W longitude by early Tuesday and possibly including an embedded low not far from Bristol Bay. After that time the forecast complexity increases with the inter-related issues of how upper ridging that builds into the Bering Sea and vicinity evolves, what proportion of Tuesday trough energy drops into an upper low well south of the mainland, as well as the shape and position of that upper low. Issues with the ridge also affect the character of higher latitude flow and its influence on the mainland. Early in the period the 12Z GFS has trended favorably toward (but not yet quite to) other models that have been more progressive than the GFS with the short range wave that develops near the Alaska Peninsula and then continues into the Gulf. Already by day 5 Tuesday some greater surface differences arise from low-predictability shortwave details on the east side of the upper trough, with the 12Z CMC more aggressive than other operational runs with the idea of another wave tracking north into the Gulf. The 12Z parallel GFS and 06Z operational GFS had variations of such a wave though. Then the ECMWF/ECMWF mean/UKMET are quickest to drop the greatest proportion of trough energy into the Pacific upper low and to position this low farther south. The GFS/GEFS mean and to varying degrees CMC/CMC mean have more energy lingering near the southwestern mainland/Peninsula for a time, leading into an upper low that is generally farther northward and in some cases more east-west elongated. These differences correspond to the ECMWF/ECMWF mean building the Bering ridge partially into the mainland with a high center getting close to the western coast of the mainland, in contrast to GFS/GEFS/CMC mean runs that are quicker to close of the Bering ridge and have the upper high drifting into or near Siberia. In terms of sensible weather, which direction the pattern consolidates toward (or between the two extremes) will determine if the southern coast trends drier (ECMWF) or there is a continued potential for one or more waves that will bring episodes of wind/precipitation to areas over and near the Gulf of Alaska. Given the uncertainty of forecast specifics and if anything more divergence in solutions over the past day or so, the deterministic forecast blend trended toward an even mix of the 12Z GFS/GEFS mean and 12Z ECMWF/00Z ECMWF mean by the latter half of the period to reflect an intermediate solution for the Bering Sea ridge/Pacific upper low as well as flow to their north along with corresponding surface features. Early in the week the forecast started with a 12Z operational model blend that removed the CMC by day 5 Tuesday. ...Weather/Hazards Highlights... The wave likely to be over the Gulf of Alaska at the start of the period Monday should focus a brief period of enhanced precipitation over the southeastern coast and Panhandle early in the week. For the rest of the week confidence decreases for specifics of precipitation coverage and amounts from the Panhandle westward as well as for the surface/wind pattern. The preferred intermediate forecast would depict a trend toward lighter and more scattered precipitation within a full envelope ranging between dry conditions mid-late week and one or more episodes of possibly meaningful/focused precipitation Tuesday onward. The surface pattern would feature strongest low pressure staying well southward over the Pacific with only weak features rotating around the northern side of the overall circulation. The remainder of the mainland should be fairly dry though a weakening front during the latter half of the week could bring some precipitation. The Aleutians will see a decrease in precipitation amounts/coverage through the first half of the week followed by an increase in moisture/winds over at least the western Aleutians as a front approaches later in the week. Expect above normal min/max temperatures to be the most common over the North Slope through most of next week. Elsewhere there should be greater coverage of below normal readings for highs while anomalies for morning lows will likely be more balanced between above/below normal if not tilted a bit more toward the warmer side of climatology. Rausch No significant hazards are expected over Alaska during this forecast period. 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