Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 641 PM EDT Wed Oct 28 2020 Valid 12Z Sun Nov 01 2020 - 12Z Thu Nov 05 2020 ...Guidance/Predictability Assessment... Over the past day guidance has noticeably trended toward a slower/deeper upper low and surrounding trough over the southern mainland and Gulf of Alaska from Sun into the first half of next week. This trend leads to more persistent surface low pressure near the southeastern coast. Upstream Arctic/northeastern Siberian energy will likely drop into the mean trough by around the middle of next week, quickly followed by an amplified upper ridge approaching the mainland from the west. This late-period evolution should help to push low pressure to the southeast after midweek. Meanwhile a system should track south of the Aleutians early in the period, followed by some uncertainty over whether some of its energy could reach the Gulf/Panhandle or remain farther south. From day 4 Sun into early day 6 Tue the 12Z ECMWF/CMC/UKMET and 06Z GFS offered the best clustering for the overall forecast. The 12Z GFS differed from consensus in being faster already at the start of the period with the system south of the Aleutians, ultimately leading to a storm track into the Gulf instead of south of the Panhandle (with varying ideas for specifics). The 12Z GEFS mean had a more subdued depiction of a frontal wave that merges into the overall circulation. Interestingly the 12Z ECMWF mean (arriving after forecast preparation) trended toward the GEFS scenario. Forecast preference favored the dominant operational model cluster led by the 12Z ECMWF given the tendency for operational runs to reflect important details somewhat better than the means early in the extended period. The ECMWF mean trend does temper confidence a bit though. Meanwhile there was good agreement for the deep upper trough/low settling over the southern coast and vicinity from Sun into the first part of next week, while a blend also provided a good depiction of the upstream energy expected to drop into the mean trough. Late day 6 onward added in some of the 12Z GEFS/00Z ECMWF means but still no more than 30 percent total as blending the ECMWF/CMC/GFS already toned down uncertain details within an agreeable pattern. Day 8 Thu split the ECMWF influence among the 12Z and prior 00Z runs as the new 12Z run strayed to the aggressive side of the spread with the strength of the upper ridge building into the mainland. An intermediate position for the upper ridge axis by day 8 looked reasonable, given mixed trends including the 12Z ECMWF and recent GEFS means trending somewhat faster versus the consistent and slower ECMWF mean as well as the 06Z/12Z GFS holding back on the extremely fast 00Z GFS timing. Guidance agreed well with a strong gradient reaching the western Aleutians and spreading across the Bering Sea next Wed-Thu ahead of a front forecast to push into the western Bering by next Thu. ...Weather/Threats Highlights.. The dominant focus for heavy precipitation will be over the Panhandle through Sun. Some moisture/precip should linger into next week near persistent northeastern Gulf low pressure but with less extreme totals. Parts of the southeastern third of the mainland may see periods of mostly light precipitation. Shortwave energy originating from northern Russia and the Arctic may bring some light activity to the western mainland as it drops into the mean trough aloft by Tue-Wed. The western/central Aleutians may be in the northern periphery of the moisture shield associated with North Pacific low pressure early in the period. By next Wed-Thu expect a broad area of precipitation and enhanced winds to spread across the western half of the Aleutians and an increasing portion of the Bering Sea, ahead of a deep low forecast to track near Kamchatka with associated frontal system extending into the western Bering. The state will see a wide spectrum of temperature anomalies during the period. Anticipate much below normal readings over the southern/southeastern parts of the mainland, with this cold air eventually reaching the Panhandle. On the other hand the North Slope will likely see above to much above normal temperatures with more modest positive anomalies extending over portions of the far western mainland. By next Thu the upper ridge approaching from the west should promote a warming trend over western/central areas. Rausch Hazards: - Heavy precipitation across portions of the Alaska Panhandle, Sat-Sun, Oct 31-Nov 1. - Much below normal temperatures across portions of the Alaska Panhandle and mainland Alaska, Sat-Tue, Oct 31-Nov 3. 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