Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EST Sun Jan 16 2022 Valid 12Z Thu Jan 20 2022 - 12Z Mon Jan 24 2022 ...Heavy precipitation threat along the southern coast and Panhandle late week into the weekend... ...Overview... The overall forecast is consistent in showing a much wetter and warmer regime for a majority of Alaska from late this week onward, as eastern Pacific upper ridging extends its influence farther northward into the mainland and western Canada while troughing (with at least one embedded upper low) prevails to the west. Heaviest precipitation should be along the southern coast and Panhandle Friday into Saturday but some locations may see significant totals thereafter. Specifics over this area, and magnitude/coverage of precipitation farther north, will depend on yet to be resolved details for two main North Pacific systems that are most likely to track into the southeastern Bering Sea. ...Guidance/Uncertainty Assessment... To start the period, latest model runs have trended somewhat weaker and faster with low pressure expected to reach the Gulf of Alaska or Panhandle by the start of the period early Thursday. Most ensemble means are actually a tad stronger than the 12Z model average so a slight account for their solutions could be feasible given continuity. Specifics have been dependent on low-predictability smaller scale details. There is still some debate over the ultimate evolution of elongated mainland trough energy as of early Thursday. The southeastern side of the spread for the trough/upper low (now represented by the 12Z UKMET) continues to see a gradual loss of support from consensus given persistent western solutions in the ensemble means (but with a trend toward more troughing as models have been showing in recent days) and a general westward trend in the ECMWF over the past couple days. The exact depth and position of the upper low retrograding into the Bering Sea will be crucial in determining the evolution of the first North Pacific system as it is drawn into the southeastern Bering Sea and beyond. For now prefer to lean away from the deep and westward solution seen in the 12Z ECMWF around Friday-Saturday, in favor of surface low pressure/troughing holding closer to the western coast of the mainland. Some GFS runs have been showing a less extreme variation of the idea though, so it is not completely out of the question. Guidance also diverges for specifics of the next Pacific system that is most likely to reach the eastern Aleutians/southeastern Bering Sea by Sunday-Monday. The 00Z ECMWF mean was on the east/southeast side of the spread by day 8 Monday while the new 12Z run adjusted somewhat northwest in the direction of earlier runs. There is also increasing timing spread at that time with the 00Z GFS and 12Z CMC straying quite fast while more recent GFS runs as well as ECMWF plus 12Z CMC/GEFS means are on the slower side--forming the greater majority cluster. Guidance comparisons led to starting with 80 percent weight of the 12Z operational models early, followed by increasing total 12Z GEFS/00Z ECMWF mean weight to half by days 7-8 Sunday-Monday. Preference to lean away from 12Z ECMWF specifics over the Bering Sea around Friday-Saturday led to reduced weight of that run in that time frame, while the ensemble component tilted a little more toward the GEFS late when the 00Z ECMWF mean became somewhat questionable. ...Weather/Hazard Highlights... A wave forecast to reach the Gulf of Alaska or Panhandle by the start of the period early Thursday may produce an initial episode of precipitation over the Panhandle and perhaps a little farther northwestward. Then guidance continues to show a strong signal for an atmospheric river event aimed at the southern coast and Panhandle from Friday into the weekend. GFS/ECMWF runs still depict a strong southerly low level flow of moisture ahead of a North Pacific into southeastern Bering system bringing precipitable water values as high as 3-4 standard deviations above normal for a time while the corresponding means are now plus 2-3 standard deviations. Also consistent is the idea that peak values should be around Friday followed by some decrease but still remaining well above normal. Heavy rainfall along the southern coast and Panhandle on Friday may focus more locally over the Panhandle during the weekend. Less extreme but still meaningful precipitation may extend farther west into the Alaska Peninsula. There is a less concentrated depiction of moisture ahead of the next Pacific/southeastern Bering system from the weekend into early next week, with best focus over the Alaska Peninsula and parts of the southern coast while amounts should decrease somewhat over the Panhandle as the upper ridge strengthens. The extent of this trend is uncertain at this time though. The flow of moisture ahead of both systems may also spread precipitation well north across the mainland. Significant totals are possible over some terrain-favored areas over the southern and western mainland but with a lack of confidence in specifics at this time. These systems will also bring episodes of precipitation and brisk to strong winds to portions of the Aleutians. Expect the North Slope and parts of the western mainland to see below normal temperatures extend through Friday or Saturday while above normal readings prevail over most other parts of the state. Regardless of above/below normal anomalies, there should be a gradual warming trend. More pronounced warming during the weekend and early next week will likely expand the coverage of much above normal readings across a majority of the state. 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