Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EST Sat Dec 31 2022 Valid 12Z Wed Jan 04 2023 - 12Z Sun Jan 08 2023 ...Overview and Guidance/Predictability Assessment... Most aspects of the large scale evolution in the Wednesday-Sunday period continue to be fairly agreeable and consistent, while individual models/ensemble members exhibit various embedded uncertainties. Consensus maintains the idea of a western Canada mean ridge extending northwestward into the mainland to some degree while a mean upper low wobbles across the northwestern Pacific/Aleutians, most likely progressing east-southeast of the Aleutians by next Sunday. The flow between these two primary features will likely contain multiple shortwaves that could reach into/cross the mainland, with one or more surface systems tracking north-northwest over the northeastern Pacific, while the Aleutians mean low aloft should produce multiple surface systems over/near that region. To a greater degree than yesterday, guidance is suggesting an upper weakness could develop near the northern coast of the mainland, between the Canada/Alaska ridge and a separate ridge closer to the North Pole. There is still a decent amount of spread and run-to-run variability for individual waves tracking over the northeastern Pacific. The first feature of note has had some degree of loose persistence in the guidance, reaching near the Gulf of Alaska by the start of the forecast early Wednesday, but with a fair amount of spread/inconsistency for specifics. The 12Z ECMWF/CMC/UKMET have all gravitated toward this system reaching near Kodiak Island as of 12Z Wednesday while the 12Z GFS is on the slow side. The old 00Z ECMWF was deeper/eastward and the 06Z GFS was fairly close to the 00Z ECMWF. Preference was for a blend of the past two ECMWF runs, 06Z GFS, and 12Z UKMET/CMC as a starting point. This brought the low a little west of continuity toward Kodiak Island, which the new 12Z ECMWF mean happens to support. Immediately behind this system, the 12Z ECMWF/UKMET/CMC have extraneous waves near/offshore of the Panhandle with minimal ensemble mean support. A blended solution helps to downplay those questionable waves. Into late week there has been a continued signal for a well-defined storm to track northward toward or just west of Haida Gwaii and then weaken as it continues into or west of the Panhandle. The means start to become diffuse with this system by early Friday, corresponding to when the operational models show increasing divergence. Preferred an intermediate and still slightly offshore depiction of this system, with the new 12Z ECMWF mean essentially favoring a compromise between the past two ECMWF runs into early Friday before the mean completely loses definition. Over and north of the mainland, a general blend provides a good way to introduce the potential upper weakness near the northern coast. The 12Z ECMWF/CMC show the lowest heights within this weakness while the ensemble means hint at the feature but in much more subtle form. The ensemble means maintain some degree of mean ridging aloft over most of the mainland through day 8 Sunday while a lot of smaller-scale shortwave uncertainties arise in the operational runs. The old 00Z ECMWF had one of the lower confidence solutions in closing off an upper low. At least the new run has trended more open and progressive with that shortwave energy. From the start of the period onward, operational models still show a seemingly random assortment of individual surface lows/frontal systems across the Aleutians and surrounding waters in association with the large scale mean low. At least the ensemble means have been stable and agreeable with the overall area of low pressure. The old 00Z ECMWF/CMC suggested there would be some question marks over whether the mean low could persist over the Aleutians into day 8 Sunday versus shifting more east-southeast over the Pacific as the GFS and ensemble means have been showing. However the 12Z ECMWF/CMC runs have adjusted toward the majority scenario, for the time being improving confidence for that aspect of the forecast. Based on considerations at the start of the period, day 4 Wednesday started with a blend of the past two ECMWF runs, 06Z GFS, and 12Z UKMET/CMC. With the rapid dissipation of this system, the blend then transitioned toward the 12Z GFS versus the 06Z run over the next couple days. Then over the latter half of the period the forecast increased 12Z GEFS/00Z ECMWF mean input to reach 40-50 percent while eventually phasing out the 00Z ECMWF late due to its undesirable mainland forecast (leaving the 12Z ECMWF/GFS/CMC). ...Weather/Hazards Highlights... Expect the overall pattern to promote periods of precipitation along the southern coast and Panhandle through the period. Depending on specifics of individual systems, such as a leading one near Kodiak Island early Wednesday and a trailing one that may have some influence on the southern Panhandle late in the week, there could be some pockets of locally enhanced activity at times. On the other hand there may also be some drier areas between systems. Most of the remainder of the mainland will likely experience dry conditions during except for perhaps very light/scattered snow with any passing upper shortwaves. The Aleutians will see an unsettled pattern Wednesday into the weekend as multiple waves/frontal systems bringing varied wind and precipitation (mostly rain) focus. Confidence in exact specifics over and near the Aleutians remains quite low though. Best consensus suggests that low pressure emphasis may shift farther to the east-southeast by next Sunday. This could increase easterly low level flow over the eastern Aleutians/Alaska Peninsula at that time. Much of the state should see above normal temperatures through the end of the week, especially for lows. With readings trending gradually cooler with time late week into the weekend, some areas of slightly below normal highs may develop over parts of the southern half of the mainland. The shape/depth of the possible upper weakness near the northern coast of the mainland will play some role for temperature trends over the North Slope. Meanwhile the Aleutians should start out below normal and then trend a little warmer. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook 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