Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 710 PM EDT Sun Oct 04 2020 Valid 12Z Thu Oct 08 2020 - 12Z Mon Oct 12 2020 ...Overview and Guidance/Predictability Assessment... A strong and massive Bering Sea storm as of early Thu should slowly trend weaker as it drifts closer to the southwestern coast of the mainland. Ahead of this system the consensus still shows late week upper trough elongation into the northeastern Pacific, supporting development of an associated wave. The combination of the elongated trough/possible embedded low lifting northward and eventual approach of the Bering Sea upper low should bring the surface low northward into the Gulf of Alaska. Meanwhile most guidance has held onto yesterday's trends of taking an initial eastern mainland upper ridge and closing off the northern part into an upper high that quickly tracks into the Arctic. Around this upper high, shortwave/upper low energy may progress southwestward into the mainland by next weekend. There is a fair amount of agreement that some combination of this energy as well as what remains of the Bering upper low and possibly surrounding/incoming flow should comprise a mean trough aloft whose axis reaches about 160W longitude by day 8 Mon. Today the operational guidance is more mixed regarding progression of the initial Bering system at the surface and aloft. There are also considerable differences regarding the details of the trough/upper low that may reach parts of the mainland from the northeast. On the positive side latest guidance appears to have simplified the northeast Pacific surface low forecast somewhat. Meaningful differences remain with specifics but at least models have really dampened the prior signal for involvement from any northward-tracking waves ahead of the low that breaks off from the front reaching the Alaska Peninsula by early Thu. Finally, the 12Z GFS/CMC offer potential for a strengthening mid-latitude Pacific system to affect the Panhandle by day 8 Mon. Thus far the signal is too low in other guidance to incorporate that idea into the forecast. A 12Z operational model blend represented consensus well as of early day 4 Thu. Solutions diverged after that time, as has been the case recently. The forecast quickly adjusted to primary weight on the 12Z/00Z ECMWF runs and 06Z GFS with a small ensemble component (a bit more 00Z ECens than 12Z GEFS) through mid-period followed by a more even model/mean weight late. This approach helped to maintain continuity for moderately slow progression of the Bering system (versus the faster 12Z GFS/CMC; GEFS mean hedged between the ideas, showing some early progression but also holding an implied upper low back) and to a reasonable extent over the northeastern Pacific/Gulf. Meanwhile the 12Z ECMWF/06Z GFS inclusion meshed with WFO AFG coordination supporting some degree of upper trough/low arriving from the northeast--also suggested by the past couple ECMWF means. This feature supported introduction of a leading cold front that pushes southward over parts of the mainland. ...Weather/Threats Highlights... Guidance is still in the process of trying to resolve important details for enhanced precipitation potential from the eastern Alaska Peninsula across the southern coast and into the Panhandle. The most common theme is for some locally higher totals to focus along the eastern Peninsula to the Kenai Peninsula in association with the overall pattern evolution at the surface and aloft late this week, with signals regarding coverage/amounts being more mixed across locations to the east. Confidence further decreases for specifics during the weekend and early next week. The upper trough/low forecast to reach into the mainland from the northeast could eventually promote some scattered precipitation during the latter half of the period while the Aleutians will likely see continued brisk and unsettled conditions. The forecast continues to show above normal temperatures over most of the state. The Panhandle will be the main exception with below normal readings. Pockets of the southern mainland may see modestly below normal highs. The upper trough/closed low reaching into the mainland from the northeast by the weekend may begin a cooling trend, with highs at some locations over the north possibly declining toward normal depending on the depth of the upper feature. 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