Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Tue Aug 15 2023 Valid 12Z Sat Aug 19 2023 - 12Z Wed Aug 23 2023 ...Overview... Expect a wavy front to meander over the northern mainland from the weekend into next week to the south of a deep Arctic upper low. Meanwhile, a system forecast to track across the western/northern Bering Sea (originally associated with Tropical Storm Lan currently near Japan) should bring increased moisture that may produce enhanced rainfall, especially over the northwestern mainland, from interaction with the front and favored terrain. Behind this system most guidance shows a mean trough aloft persisting west of the mainland, potentially supporting unsettled weather through the middle of next week. Farther south there is a lot of uncertainty regarding individual waves that may track over the North Pacific/eastern Aleutians into the Gulf of Alaska. ...Guidance Evaluation/Preferences... Regarding the western/northern Bering Sea system, general trends of the operational models and means have continued to be toward the faster side. The GFS/UKMET are the fastest among the 12Z guidance, due in part to those runs pulling off some energy from the southern part of the supporting upper shortwave just east of Kamchatka early in the period. While the 06Z GFS was also rather fast but did not depict this flow separation, remaining guidance is not as fast as the 12Z GFS/UKMET. The 12Z GEFS mean becomes weak and slow though. This leaves the past two ECMWF runs (more 12Z than 00Z), 12Z CMC, and 12Z CMC mean/00Z ECMWF mean as the best cluster to represent this system. This provides a moderately faster trend without going to the extreme side of the envelope. The new 18Z GFS has trended somewhat slower and the 12Z ECMWF mean is a tad faster/northward than the prior run. The upper low north of the mainland has a fairly wide dispersion in the guidance. Most solutions show varying degrees of northward/northeastward progression with time. The 06Z/12Z GFS differ from this scenario by depicting a counterclockwise wobble that ultimately brings the feature close to the northwestern mainland again by the middle of next week. The 12Z CMC mean is the one other piece of guidance that has some element of those GFS runs. The 18Z GFS has switched to a northeastward track. The North Pacific/Aleutians into Gulf of Alaska area (including the Bering Sea by midweek) is quite uncertain. The GFS and CMC have strayed to the fast side with a leading wave which yesterday's consensus and the 00Z ECMWF/ECMWF mean had depicted over the eastern Aleutians as of early Saturday. The 12Z UKMET is still rather slow like that scenario but the ECMWF has adjusted to an intermediate timing that looks best given the wide spread. There are also various ideas for a separate wave that could reach the eastern Aleutians/southern Coast (12Z ECMWF/CMC) or stay a little south (12Z/18Z GFS). Within the broad mean trough expected by day 8 next Wednesday, there is also a lot of spread for specifics of the upper pattern and associated surface reflection. Prefer a blended/mean approach to yield at most a wave near the Aleutians at that time. With primary emphasis on the Bering Sea system, the starting blend employed the past two ECMWF runs (more 12Z than 00Z), 12Z CMC, and 30 percent total from the 12Z CMCens/00Z ECens means from day 4 Saturday into Day 7 Tuesday. Decreasing confidence in details thereafter led to a transition toward at least half means (some GEFS mean included after the main Bering Sea system passes by) with lingering ECMWF input split more evenly between its two runs. ...Weather/Hazards Highlights... Potential exists for some locally moderate to heavy rainfall over the northwestern mainland during the weekend into early next week, with some focus along a wavy surface front and moisture being contributed by flow ahead of the system forecast to track over the western and northern Bering Sea. Favored terrain over the Seward Peninsula and western Brooks Range may also see some enhancement. The direction and strength of winds over the western mainland are still uncertain. Most solutions keep the low track far enough west to support southerly low level winds, but there is a minority scenario that would have a faster and farther east low that could tilt winds more southwesterly and with greater strength. Continue to monitor forecasts for latest trends. Behind this system, the general pattern may support continued areas of lighter precipitation over the mainland into the middle of next week. Some areas from the eastern Aleutians to southern coast may see one or more episodes of rainfall, but with low confidence for timing and amounts. The northern mainland should see the greatest negative anomalies for high temperatures during the period. The rest of the mainland should see a mix of above/below normal anomalies for highs but with somewhat greater coverage of below normal readings. Low temperatures should lean more above normal, with just scattered pockets of modestly below normal anomalies. Rausch Hazards: - Heavy rain across portions of mainland Alaska, Fri-Sun, Aug 18-Aug 20. 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