Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 339 PM EDT Thu Oct 24 2019 Valid 12Z Mon Oct 28 2019 - 12Z Fri Nov 01 2019 ...Heavy coastal rain and mountain snow for southeastern coastal areas and much above normal temperatures for the central and northern mainland early next week... A highly amplified upper pattern will be in place at the start of the medium range period, with a strong closed low in the eastern Bering Sea and a well-defined ridge downstream from the panhandle northward through the eastern part of the mainland. Aside from relatively minor differences handling individual short waves and associated surface lows rotating through the trough, the models are in good agreement showing broad southerly flow encompassing much of the mainland through Day 4. The amplified nature of the pattern will allow moisture from the subtropics to be transported northward, and where it impinges on the southeastern coast, there will be very heavy rain and high elevations snows. In addition to the precipitation, the southerly flow will allow a much warmer than normal air mass to overspread the state (except for the panhandle). Readings across the Brooks Range and North Slope may exceed 20 degrees above normal from Monday through Wednesday of next week. After Day 4, the models struggle to handle the interaction of the strong Bering Sea closed low and a system in the eastern Siberian Sea. The 12Z ECMWF shows the two systems phasing Day 5, while the GFS and CMC keep them well separated. The latest ensemble guidance doesn't really support this ECMWF solution, so we generally favored a blend of the GFS/CMC and the 12Z GEFS (and 00Z ECMWF ensembles, which also don't support the 12Z deterministic ECMWF run). The atmospheric river impacting the southeastern coast will move slowly eastward into the northern part of the panhandle Day 5 and gradually weaken as it runs into the downstream upper ridge. During the middle to latter part of next week, a short wave is expected to rotate around the leftover Bering Sea upper trough toward the Aleutians or Gulf of Alaska. There are timing and track differences among the guidance, with the 12Z GFS most progressive and 12Z ECMWF slowest. The Canadian is initially a decent compromise on Wednesday, but then quickly lifts northeastward as the system gets swept up by an amplifying trough in the arctic stream. The GFS tends to have a fast bias with mid-latitude features, so we are favoring a slower solution; at the moment this seems to be best encapsulated by the 00Z ECMWF ensemble mean. This system may bring a renewed outbreak of widespread precipitation for the southwestern and south-central coastal sections Thursday into Friday, but considering the model spread, expect the details surrounding the associated sensible weather to change with future model runs. Klein 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 Alaska Hazards... - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sun-Wed, Oct 27-Oct 30. - Heavy rain across portions of mainland Alaska, Sun-Mon, Oct 27-Oct 28. - Much above normal temperatures across portions of mainland Alaska, Sun-Thu, Oct 27-Oct 31. 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