Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 642 PM EST Sun Dec 12 2021 Valid 12Z Thu Dec 16 2021 - 12Z Mon Dec 20 2021 ...Overview... Models and ensemble means maintain a mean upper low over or near the Sea of Okhotsk for most of the period while advertising a pattern change downstream. Initially there will be progressive flow across the North Pacific and Bering Sea/Alaska with a succession of shortwave troughs and ridges, with the troughs supporting Bering Sea systems and waves over the Gulf of Alaska. Then from the weekend into the start of next week a strong upper ridge will build over the central and northern/northeastern Pacific. The northern extension of this ridge will progress from the Bering Sea through the mainland. Expect a strong jet to set up between the ridge and upstream low/trough. As this new pattern takes shape a strong storm should track from near Kamchatka to northwest of the mainland. ...Guidance Evaluation and Preferences... Continuity is good in principle with the initial Bering Sea system and leading front nearing the mainland as of the start of the period early Thursday as well as the wave that reaches the Gulf of Alaska by early Friday. Most models have suggested a separate wave to the south will stay sufficiently far south not to have an influence on the southern Panhandle, with the UKMET being the exception. Meanwhile a slower trend in latest GFS runs toward the prior majority has led to better timing agreement for the system forecast to track from the western Aleutians through the Bering Sea and into the mainland Thursday-Saturday. Today's forecast continues recent deeper trends seen in most models, though the UKMET was noticeably weaker than the others. Also some latitude spread arises by early Saturday. Consensus still shows a wave near the southeastern coast and Panhandle during the weekend. As the large scale pattern changes during the latter half of the period, the past couple days of model runs have been consistent in showing a stronger Bering Sea into mainland upper ridge versus the ensemble means, with the ECMWF mean tending to be a little stronger than other means. There have been some differences in timing of strong low pressure tracking from near Kamchatka to northwest of the mainland, as well as for how much shortwave energy could try to dampen the northern side of the ridge. However the recent model continuity for the upper ridge through at least Sunday, plus the past couple ECMWF runs and 12Z GFS/CMC being sufficiently agreeable overall and fitting within the general ensemble mean template, favored keeping a majority weight of operational guidance even into day 8 Monday. The 12Z models, with the least weight on the UKMET, provided the starting point for the forecast during the first half of the period. With guidance comparisons offering continued support for operational runs thereafter, by day 8 Monday the blend maintained 75% operational model weight (12Z GFS/CMC and 12Z/00Z ECMWF) with the rest provided by the 00Z ECMWF mean given its stronger solution for portions of the upper ridge. ...Weather/Hazard Highlights... Each of two Bering Sea systems and subsequent Gulf of Alaska waves will bring generally light to moderate precipitation to the Aleutians, western/southern parts of the mainland, and the Panhandle during the latter half of the week. Briefly heavier activity may be possible over a few isolated locations along the southern coast and Panhandle. The system tracking from the western Aleutians to the mainland Thursday-Saturday may produce an episode of brisk to strong winds over the Bering Sea, and possibly into parts of the western mainland depending on the exact evolution/track of the low. Flow ahead of a storm forecast to track from near Kamchatka to northwest of the mainland should spread a broad area of moisture and strengthening winds across portions of the Aleutians, Bering Sea, and eventually the mainland during the weekend into the start of next week. Some locations over favored terrain may see enhanced precipitation in that time frame. Below normal temperatures over the eastern part of the state on Thursday will give way to warmer temperatures moving in from the southwest, with western areas likely to be above normal by the end of the week. Anomalies for min temperatures should be greater relative to those for highs. Departure of Saturday low pressure may lead to a brief dip in temperatures over the mainland but then the flow ahead of the storm well to the west should renew the overall warming trend early next week, with the northwestern half of the mainland possibly reaching well above normal values. The Panhandle will tend to remain below normal while a mix of above/below normal readings prevail in-between. 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