Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Sat Sep 17 2022 Valid 12Z Wed Sep 21 2022 - 12Z Sun Sep 25 2022 ...Overview... Latest guidance agrees fairly well with the general pattern but some important differences develop over the Pacific by next weekend. An initial upper trough over and extending to the south-southwest of the mainland, along with a leading wavy surface front, will quickly depart after Wednesday. Then upstream troughing should settle into position along an axis from near the Bering Strait southward into the northern Pacific. The upper low anchoring the mean trough will likely bring a vigorous surface system (though not nearly as strong as Merbok's remnants) across Siberia toward the Bering Strait while the associated frontal system sweeps through the Bering Sea/Aleutians and eventually weakens as it moves into the mainland. An embedded wave may track near the southern coast late in the week. By next weekend there are widening differences in the shape and longitude of the Pacific part of the upper trough. These differences decrease confidence in location and magnitude of moisture reaching the southern coast and Panhandle within what should be an overall wet pattern for that part of the state. ...Guidance/Predictability Assessment... Latest guidance has done well in gravitating toward a common solution during the first half of the period (Wednesday to early Friday), favoring a composite of the 12Z operational models for that part of the forecast. The primary detail consideration involves the ultimate incorporation of Typhoon Nanmadol into the larger scale flow, with the GFS differing from other guidance in such a way as to result in a more pronounced frontal wave that reaches near the southwest coast of the mainland by early Thursday and Kenai Peninsula by early Friday. The most notable large scale trend over the past couple days is a northward adjustment for the surface low that emerges from Siberia. From late Friday through next weekend the models and ensemble members increasingly diverge for the shape and position of the central/southern part of the upper trough extending south from the Bering Strait. Differences with the strength/position of eastern Pacific ridging as well as for upper dynamics tracking out of eastern Asia both contribute to this spread. Latest ensemble means favor an intermediate solution between the western upper trough axis of the past two ECMWF runs and farther east 12Z GFS. The 12Z CMC is close to the 12Z GFS trough axis but pulls surface low pressure a little west of the 12Z GFS by day 8 Sunday. There is also uncertainty over whether Arctic energy could curl into the core of the upper trough next weekend. This aspect of the forecast has low predictability given the general evolution and typical guidance errors at distant time frames. The overall preference is to transition the operational model blend from the first part of the period toward a model/mean mix that ultimately contains 60 percent total weight of the models (evenly split among the 12Z GFS/ECMWF/CMC) and the rest 12Z GEFS/00Z ECens by day 8 Sunday. The new 12Z ECMWF mean offers good support for this approach. ...Weather/Hazards Highlights... A leading wavy front will focus enhanced precipitation over the far southeast corner of the mainland through the Panhandle on Wednesday. Farther west, strong low pressure tracking from Siberia toward the Bering Strait, along with its associated frontal system, will spread a broad area of moisture and brisk to strong winds across the Bering Sea/Aleutians and into parts of the western-southern mainland from Wednesday into Thursday. Then a potential frontal wave tracking near the southern coast around late week could provide additional enhancement within a mean pattern that will favor areas of heavy precipitation along the southern coast and Panhandle. Uncertainty with the specifics of the forecast upper trough lead to decreasing confidence in the coverage and intensity of precipitation over these areas by next weekend. Meanwhile the upper trough should help to maintain scattered areas of light to locally moderate precipitation elsewhere over the mainland through next weekend. The expected pattern will support below average high temperatures across the southern mainland and Panhandle, while central/northern parts of the mainland should see at least a mix of above/below normal highs if not more above normal readings. Morning lows will be mostly above normal with the highest anomalies tending to be over the North Slope. Any pockets of slightly below normal lows will tend to be over the far south. Rausch 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