Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Mon May 02 2022 Valid 12Z Fri May 06 2022 - 12Z Tue May 10 2022 ...Overview... Expect somewhat ill-defined flow aloft over the mainland late this week into the weekend with a tendency toward weak ridging over the Interior. From late weekend into early next week a mean trough may develop near the west coast of the mainland and perhaps drift a little inland. Surface high pressure should track from the Arctic into the Canadian Archipelago while a wavy front will likely meander over the central mainland. Farther south, forecast details remain very uncertain as guidance shows a wide array of possibilities for how northern stream flow aloft could interact with an initial mid-latitude Pacific system. Greater interaction would lead to low pressure and associated moisture having more of an effect on some areas from the eastern Aleutians/Alaska Peninsula through the southern coast and Panhandle, while the opposite side of the guidance envelope with minimal interaction could lead to fairly dry conditions. Currently the most probable outcome is for precipitation to reach some of these locations with pockets of at least moderate totals. ...Guidance/Predictability Assessment... The 12Z GFS appeared to provide a resolution to lingering question marks with the upper low near the eastern Aleutians as of early Friday, trending toward the greater progression which the ECMWF/UKMET/CMC have been advertising over the past couple days. The 12Z GEFS mean still held it back some like prior GFS runs, and now the 18Z GFS has reverted to having it reach no farther east than the Alaska Peninsula. Preference at the time of forecast preparation was a compromise timing among 12Z models, with the ECMWF input split between the last two runs since the 12Z run was fastest. The combination of issues with this upper low and subsequent differences in how northern stream flow may interact with mid-latitude Pacific low pressure continues to result in widely divergent solutions for what ultimately happens to the system and what the effects between the eastern Aleutians and Panhandle would be. The 12Z ECMWF jumped to a more separated scenario that keeps the Pacific system and its moisture well southwest, in a manner similar to some prior CMC runs. This contrasts with the prior 00Z ECMWF that brought the system into optimal position to direct a pronounced flow of focused moisture into the southern coast. Regardless of details of the leading upper low, recent GFS runs and GEFS means plus ECMWF means through the 00Z cycle have generally favored bringing the system along a path south of the Alaska Peninsula and into the Gulf of Alaska, spreading moderate to locally enhanced areas of precipitation into the southern coast and Panhandle. The 12Z CMC/CMC mean and now the new 12Z ECMWF mean offer an alternative idea, keeping the Pacific system off to the southwest but still having enough dynamics aloft to develop a weak leading wave that could track over or near the Gulf and produce some precipitation. Based on guidance available through arrival of the 12Z ECMWF, above average uncertainty favored holding onto as much continuity as possible by way of trending the initial operational model blend toward 70 percent total means (12Z GEFS/00Z ECens) and the rest 12Z GFS by day 8 Tuesday. Guidance has been inconsistent for specifics of flow farther north over and near the mainland. The 12Z ECMWF strayed toward the faster side of the spread for the mid-late period upper trough but the new 12Z ECMWF mean trended somewhat east as well. The forecast from the aforementioned blend provided a conservative approach which at least reflects somewhat more definition than yesterday. ...Weather and Hazard Highlights... Mostly light precipitation from the eastern Aleutians across the southern coast and Panhandle late this week should become more scattered heading into the weekend. Significant uncertainty with flow details over and near the North Pacific maintains low confidence for the ultimate path of Pacific low pressure (and/or possible leading wave) and thus for how much moisture may reach areas from the eastern Aleutians through the Panhandle from later in the weekend through early next week. Possibilities range from fairly dry conditions to a period of significant moisture aimed at the southern coast. The most probable outcome at this time is for some precipitation to spread across areas from the Alaska Peninsula through the southern coast and Panhandle, with some locations reaching at least moderate intensity for a time. Scattered precipitation will be possible over the rest of the mainland during the period, tending to be more over the south late week into the weekend but then possibly expanding as an upper trough develops/approaches from the west. Temperatures should be fairly stable through the period. Expect below normal highs across a majority of the southern mainland and Panhandle as well as the North Slope, with moderately above normal highs over Interior areas. Morning lows will tend to have warmer anomalies, leading to greater coverage of above normal readings. The Panhandle, parts of the far eastern mainland, and areas near the northern coast have the best potential for below normal lows. Sufficiently warm temperatures combined with the remaining snow pack could present flooding concerns across portions of the Yukon River Basin in eastern Alaska. Rausch Hazards: - Flooding possible across portions of eastern interior Alaska, Thu-Mon, May 5-May 9. 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