Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 627 PM EDT Wed Aug 25 2021 Valid 12Z Sun Aug 29 2021 - 12Z Thu Sep 02 2021 ...Pattern Overview and Model Guidance... A split flow pattern is expected to be in place with a strong polar vortex and associated mid-level westerly flow over the Arctic, and a slow moving (almost Omega block) progression of shortwaves over the North Pacific. The storm system initially over the eastern Gulf will exit over British Columbia and weaken, followed by a building ridge over the Gulf and extending into central Alaska. By the middle of next week, it appears likely a large scale upper trough and potential closed low could develop across the Bering and the Aleutians. In terms of the 12Z model guidance suite, there has been some improvement in the degree of model spread compared to the past couple of days. By Tuesday, the GFS becomes stronger with a shortwave trough south of the Aleutians and its ridge extends farther north into Alaska compared to the model consensus, and the CMC is more robust with height falls from the polar vortex over the North Slope towards the end of the forecast period. However, these differences are not considered egregious and some of their solutions were incorporated into the forecast process, along with a slightly higher component of the 12Z ECMWF and increasing use of the EC/GEFS means by the second half of the forecast period. ...Sensible Weather... Improving weather can be expected for much of the state, including the panhandle region, by Sunday as the Gulf low moves inland and weakens, with some lingering showers remaining south of Juneau. The next significant round of rainfall arrives for the Aleutians and eastward to the Kenai Peninsula for the beginning to middle of next week in conjunction with the slow moving storm system south of the region. There is still some significant model spread regarding the placement of the main QPF axis, so some adjustments in future forecasts can be expected going forward. Another area of significant rainfall and mountain snow is also likely for portions of interior western Alaska for Sunday and Monday. In terms of temperatures, below average readings are likely across much of the Interior through the beginning of the week, with a moderation trend likely through Wednesday as the upper level ridge tries to build back in. Temperatures should continue to remain on the cool side for coastal portions of southern Alaska where clouds and rainfall will be more prevalent. Hamrick Hazards: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat, Aug 28. - High winds across portions of mainland Alaska, Sat, Aug 28. - Much below normal temperatures across portions of mainland Alaska, Sat, Aug 28. - High significant wave heights for coastal portions of mainland Alaska, Sat, Aug 28. - Heavy rain across portions of mainland Alaska, Wed, Sep 1. 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