Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 523 PM EST Sun Dec 11 2022 Valid 12Z Thu Dec 15 2022 - 12Z Mon Dec 19 2022 ...Synoptic Overview... An upper level trough with a strong surface low south of the eastern Aleutians and an upper high over the eastern Gulf are expected to be in place on Thursday. The mid-upper level ridge axis is forecast to build northward across much of the mainland going into the weekend, potentially setting up a blocky flow pattern. Farther west across the western Bering and far western Aleutians, a building trough will likely support a few surface lows that may affect portions of the Aleutians late in the forecast period. ...Guidance/Predictability Assessment and Preferences... The 12Z model guidance suite indicates a bit more than the average level of differences for a day 4 forecast on Thursday, especially with the position and timing of the low south of the Alaska Peninsula. The past two runs of the CMC have been much farther north with the upper low and the accompanying surface low, and has limited ensemble support. Later in the forecast period for Saturday through Monday, the GFS/GEFS mean has less of an upper ridge across the mainland and a trough from western Canada tries to build in. Both the ECMWF and ECENS mean, along with previous forecast continuity, support the idea of a strong ridge axis building north across western Alaska. The fronts/pressures forecast was primarily derived from a GFS/ECMWF/ECENS blend through Friday, and then mainly using ECMWF/ECENS through next Monday. There is the potential for significant changes in the extended forecast going forward given the level of uncertainty with storm systems across the North Pacific. ...Weather/Hazards Highlights... Moist onshore flow to the east of a quasi-stationary front is expected to produce copious coastal rain and very heavy mountain snow from the Alaska Peninsula to the St. Elias Mountains through Thursday night, followed by an abatement in intensity and coverage as a surface high likely builds in across the northern Gulf. Some light snow will likely make it well inland across portions of the Interior for the end of the week. Off and on showers will likely persist going into the weekend across much of the Aleutians with generally south or southeast flow continuing. Temperatures are expected to be generally above average across much of southern Alaska, and slightly below average for portions of the Interior valleys and North Slope. Hamrick 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