Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 548 PM EST Mon Nov 23 2020 Valid 12Z Fri Nov 27 2020 - 12Z Tue Dec 01 2020 ...Stormy pattern from late this week into next week... ...Overview and Guidance/Predictability Assessment... Today's operational models and ensemble means display remarkably good agreement in principle through the entire extended period. Behind a rapidly weakening Gulf of Alaska wave on Fri, two very strong storms tracking from near the Aleutians into the Gulf and/or southern mainland will be the dominant features of the forecast. The first should reach the Gulf or southern coast by around day 6 Sun. The second may take a slightly farther northwest track toward the Alaska Peninsula/southwestern mainland by early day 8 Tue, in response to a western Canada upper ridge that may be somewhat stronger and more persistent than that ahead of the first system. Both systems will promote an episode of focused precipitation and strong winds. The primary evolution of note at higher latitudes will be an initial weakness aloft over the North Slope, followed by retrogression of embedded energy/incorporation of upstream flow to yield a north-south upper trough near 180 longitude by the end of the period. Model/mean solutions were close enough to allow for using exclusively operational models through the entire period to provide the best detail possible while still accounting for uncertainty to some degree. The main fine-tuning early in the period involved focusing on the 12Z ECMWF and CMC/UKMET Fri into Sat as the GFS was a bit south of consensus with the weakening Gulf wave and on the northern side of the spread for the first deep Aleutians system. By Sun the CMC was slightly on the northern side of the spread for the first storm and then by Tue the GFS strayed modestly northward with the second. However in both cases typical guidance spread/error ranges left both solutions within the realm of possibility. Also the GFS was somewhat on the faster side of the spread for the second storm but the 12Z ECMWF/ECMWF mean adjusted modestly faster, narrowing the gap. Thus the latter half of the forecast used half 12Z ECMWF with the other half divided evenly between the GFS and CMC. ...Weather/Threats Highlights... Rapidly weakening Gulf of Alaska wave on Fri may support moderate precipitation along the southern coast and possibly into the Panhandle. Expect the first deep storm of interest to track from the southern Bering Sea/Aleutians into the Gulf of Alaska Fri-Sun, accompanied by strong winds along with focused precipitation extending from the Aleutians through the Alaska Peninsula and southern coast/Panhandle. Specific effects along/near the Peninsula in particular remain uncertain due to sensitivity to the exact track of the surface low. The second storm will likely bring its wind/precip field into the Aleutians on Sun and continuing eastward/northeastward Mon-Tue. Current forecast of a track slightly northwest of the first storm would spread more wind/moisture into the southern mainland by late period versus the first system. Guidance is suggesting some slowing of the upper pattern by next Tue, which would result in a more persistent deep-layer flow of moisture into the Panhandle and vicinity at that time. Exact amounts differ among the guidance but there is good consensus on highest five-day totals from near the Kenai Peninsula through the Panhandle, with lesser but still meaningful totals extending back through the Alaska Peninsula and Aleutians. Much of the state will likely see above normal temperatures through the period. In most cases the anomalies for min temps should be greater than those for max temps. Min temps could exceed 20F above normal at some locations on one or more days. A few pockets of slightly below normal highs could exist near the western coast of the mainland mid-late period, close to the axis of the upper trough forecast to evolve near 180 longitude. A few below normal highs are also possible over parts of the Panhandle during the period and localized areas over the south/southeast on Fri. 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 Hazards: - Heavy precipitation across portions of mainland Alaska and the Aleutians, Thu-Fri, Nov 26-Nov 27 and Sun, Nov 29. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Nov 28-Nov 30. - High winds across portions of the Alaska Panhandle and mainland Alaska, Fri-Mon, Nov 27-Nov 30. - High winds across portions of mainland Alaska and the Aleutians, Thu-Mon, Nov 26-Nov 30. 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