Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 549 PM EST Tue Nov 24 2020 Valid 12Z Sat Nov 28 2020 - 12Z Wed Dec 02 2020 ...Stormy pattern from the weekend into next week favoring heaviest precipitation over the Panhandle/southern coast... ...Overview and Guidance/Predictability Assessment... Latest models and ensembles maintain well above average agreement and continuity regarding the large scale pattern evolution. Moderately progressive flow during the weekend will bring one deep storm from near/south of the Alaska Peninsula into the Gulf of Alaska/southern coast during the weekend, followed by another strong storm reaching the Aleutians by Sun. As this second system tracks along/south of the Aleutians the upper flow should evolve into a more amplified and slower regime featuring an upper trough axis near or just west of the western coast of the mainland and an upper ridge over western Canada. This transition should direct the second storm toward the southwestern mainland during the first half of next week and potentially lead to a longer-duration Panhandle/southeastern coast heavy precipitation event compared to that expected ahead of the first system. A 12Z operational model blend represented consensus/continuity well for the Sat-Sun part of the forecast whose primary emphasis is the storm near the Alaska Peninsula/southern coast. Then spread and run-to-run variability for specifics required some adjustments to guidance input Mon-Wed along with modest ensemble mean inclusion (more 00Z ECMWF mean than 12Z GEFS mean due to the former being deeper with the second storm). The favored forecast removed the 12Z UKMET by early Mon and the CMC by early Tue due to straying enough to the slow side of the envelope for the second storm that their inclusion could adversely dilute the blend. Thus far ECMWF runs have provided the best intermediate anchoring, favoring greater weight versus other solutions (GFS a bit on the faster side but within reason). ECMWF input incorporated both 12Z and 00Z runs Mon-Wed to account for the developing detail differences. By day 8 Wed there is a decent model/ensemble mean signal for an area of low pressure to reach somewhere south of the Alaska Peninsula but with a lot of latitude spread. The centroid of this spread currently appears to be closest to the 00Z ECMWF--south of the 12Z GFS and north of the 12Z ECMWF. ...Weather/Threats Highlights... The forecast is agreeable and consistent for the first deep storm of interest, expected to track along/just south of the Alaska Peninsula and into the Gulf of Alaska during the weekend. This system should bring a period of enhanced precipitation to areas from the Peninsula across the southern coast and into the Panhandle, along with strong winds. The second storm is still on track to bring its wind/precip field into the Aleutians on Sun and continue eastward/northeastward during the first half of next week. There is some timing uncertainty but decent agreement on a surface low path crossing the Alaska Peninsula or Bristol Bay. Gradual amplification of the upper pattern should promote greater northward extent of moisture with this second system but heaviest precipitation emphasis will likely remain along the southern coast and Panhandle. Although the low pressure will weaken as it tracks inland by Tue-Wed, the upper pattern evolution continues to signal some persistence of a deep-layer flow of moisture into the southern coast and Panhandle, leading to heavy precipitation over portions of this region. Specifics remain uncertain given spread and variability for the magnitude and duration of best moisture feed. However the potential exists for a significant event since some individual solutions depict precipitable water anomalies reaching up to 2-4 standard deviations above normal for a time. Continue to expect above to well above normal temperatures over the state from the weekend through midweek, with a few isolated pockets of below normal readings over the Panhandle and/or far western mainland one or more days serving as the possible exceptions. Min temps should generally be farther above climatology than max temps. Pattern transition to a slower and more amplified regime will likely lead to increasing warm anomalies during next week and more coverage of readings 20F or more above normal. 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 the Alaska Panhandle and mainland Alaska, Sat-Mon, Nov 28-Nov 30. - Heavy precipitation across portions of the Alaska Panhandle, Tue, Dec 1. - High winds across portions of mainland Alaska, Fri-Mon, Nov 27-Nov 30. - High winds across portions of mainland Alaska and the Aleutians, Fri-Tue, Nov 27-Dec 1. 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