Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 639 PM EDT Mon May 04 2020 Valid 12Z Fri May 08 2020 - 12Z Tue May 12 2020 ...Guidance/Predictability Assessment... Models/ensembles show relatively good consensus at the large scales through the extended forecast period, with a persistent trough/upper-level low south of Alaska across the North Pacific, and an upper-level ridge axis building from western Canada into mainland Alaska next week. This setup will result in a Rex Block pattern, that will persist through the end of the extended forecast period. Even at smaller scales, deterministic model consensus is above average during days 4-5 (Fri-Sat) as one low pressure system slowly weakens south of the Gulf of Alaska and another stronger system crosses the western Aleutians and moves into the western Bering Sea. The above average consensus during this time frame lent to the use of a 12Z ECMWF/GFS blend during days 4-5. By days 6-8 (Sun-Tue), the preferred storm track does not change substantially, but guidance spread gradually increases through time. Spread is highest days 7-8 with respect to another surface low forecast to move into the western Bering Sea. Also by that time, the GFS was a bit quicker than other model/ensemble guidance to weaken the western side of the upper ridge, allowing more shortwave energy to affect western portions of mainland Alaska. This solution does not have overwhelming support among ensemble guidance at this time, and thus emphasis on the GFS was reduced substantially through time. Given the decent large scale agreement among the ensembles, opted to gradually increase weighting of ECENS/GEFS ensemble means through time, with majority weight on the means by days 7-8. ...Weather/Hazard Highlights... Persistent low pressure south of the Gulf of Alaska will keep onshore flow and precipitation, along with gusty winds, in the picture for the Alaska Peninsula and portions of southern mainland Alaska through the forecast period, especially Fri as a weakening frontal boundary approaches. The western/central Aleutians will be subject to rain and gusty winds Fri-Sun in association with a low pressure system. Rising heights across mainland Alaska as the upper ridge builds will promote warming temperatures. High temperatures are forecast to be 5 to 15 deg F (even 20 deg in some cases) above average across much of mainland Alaska through much of the forecast period, with temperatures forecast to rise into the 60s (possibly even surpassing 70) for portions of the Interior. Ryan 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 mainland Alaska, Thu-Fri, May 7-May 8. - Heavy precipitation across portions of mainland Alaska and the Aleutians, Fri-Sat, May 8-May 9. - Much above normal temperatures across portions of the Alaska Panhandle and mainland Alaska, Fri-Mon, May 8-May 11. - Much above normal temperatures across portions of mainland Alaska, Thu-Mon, May 7-May 11. 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