Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Sun Aug 23 2020 Valid 12Z Thu Aug 27 2020 - 12Z Mon Aug 31 2020 ...Overview and Guidance Evaluation/Preferences... Behind a late-week system tracking from the Alaska Peninsula/Kodiak Island across the Gulf of Alaska and Panhandle, guidance continues to advertise a significant large-scale pattern change toward a mean upper low over the Bering Sea and downstream ridge aloft building over the Northeast Pacific and eastern mainland/western Canada. There is good agreement that a strong North Pacific into Bering Sea storm will usher in this change, with a broad area of wet/windy conditions. However there are still question marks with the specifics of system's evolution and greater uncertainty over the role of a possible upstream wave that could influence the forecast by next Sun-Mon. For the dominant North Pacific into Bering Sea system, multi-day trends continue to be toward slower northward progression. Other details differ in some guidance, such as the 12Z GFS reflecting separate waves to a greater extent than earlier runs or the 12Z ECMWF/UKMET/CMC early-mid period. The 06Z GFS compared better to the majority 12Z cluster. By day 8 Mon the CMC strays a bit to the fast side with the main surface low. Upstream, over recent days the guidance has been showing very inconsistent signals regarding a potential wave that could either influence the leading system or merge into its circulation. Very strong solutions such as the 00Z ECMWF (which was due to questionably fast ejection of Typhoon Bavi remnants and phasing with northern stream energy) have tended to adjust back to a weaker/suppressed wave in later runs. The 12Z ECMWF and other 12Z models do indeed favor a less influential wave that tracks around the southern side of the main system--possibly near the Aleutians/Alaska Peninsula by next Sun-Mon. Overall clustering remains good for the system tracking across the Gulf of Alaska Thu-Fri. Especially at the start of the forecast there is still evidence of a modest slower trend. By day 5 Fri there are noticeable differences in exact strength/position of the surface low but the very small scale leads to low predictability at that time frame. Farther north, the consensus of guidance (discounting the 12Z CMC's tiny upper low north of the mainland) has trended toward more upper ridging that extends into the northern mainland from north of Russia late this week. Then the evolution trends toward the previously advertised col region that develops near or just north of the northern coast. Above considerations led to using the 06Z GFS and 12Z ECMWF/UKMET/CMC as the starting point for the forecast late this week before trending toward a model/mean blend, reaching 40-65 percent means (12Z GEFS/00Z ECens) by days 7-8 Sun-Mon. A little manual editing improved definition of the possible late-period wave south of the main Bering system. Today's blend produced some eastward adjustment for the Bering system around next weekend but with minimal change in progression of the leading fronts. ...Weather Highlights/Threats... Continue to expect a broad area of precipitation and stronger winds to accompany the storm forecast to track from the North Pacific into the Bering Sea during the period. Recent trends have held back the northward progression of impacts a bit but there is still reasonable consensus that enhanced precip/winds should reach the Alaska Peninsula and then extend into the southwestern third to half of the mainland over the course of the weekend. Some moisture could reach as far east as the Panhandle by Sun or Mon. Significant precip totals will be possible along the southern coast and over some areas of favored terrain in the southwestern mainland. The system tracking from near Kodiak Island to the Panhandle Thu-Fri will bring some focused precipitation to areas along the southern coast and Panhandle. There will also be a broader area of scattered precip over portions of the mainland late this week. A drier trend will progress west to east behind this system and before the approach of moisture ahead of the Aleutians/Bering Sea storm. Below normal highs will prevail most of the period from the North Slope through the eastern and southern-southeastern mainland and Panhandle. Western areas will see the best potential for above normal highs. There will be greater coverage of above normal readings for low temperatures. Expect highest anomalies for lows late this week to be over the northeastern mainland. Flow ahead of the Aleutians/Bering Sea storm should spread warmer min temperatures across much of the state during the weekend and early next week. Rausch 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 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