Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Fri Mar 26 2021 Valid 12Z Tue Mar 30 2021 - 12Z Sat Apr 03 2021 ...Guidance/Uncertainty Assessment... Latest models and ensemble means continue to show a mean trough aloft settling over the mainland and northeastern Pacific mid-late week. The northern part of the shortwave will initially support weakening mainland low pressure from the deep storm forecast to track over the Bering Sea in the short term, while energy farther south should bring deepening low pressure into the Gulf of Alaska by early Wednesday. Some northern stream energy digging into the overall trough may reinforce the mean trough and associated surface low over the Gulf later in the week. Farther west, most guidance suggests that a couple shortwaves and associated surface systems will track around the northern periphery of a deep layer ridge that builds over the northern Pacific/Aleutians/Bering Sea mid-late week. However agreement and continuity are poor with respect to the details of these shortwaves and surface systems. The broad consensus of guidance does show the overall pattern flattening some by early next weekend, which would at least promote a trend toward more shallow/broad mean trough over and southeast of the mainland. Similar to yesterday, the favored forecast blend emphasized the 12Z GFS/ECMWF/UKMET for about the first half of the period and then quickly transitioned toward about half models (12Z/00Z ECMWF and 12Z GFS) and half means (12Z GEFS/00Z ECens) for days 7-8 Friday-Saturday. The CMC continues to be slower than other solutions to bring developing low pressure into the Gulf. Otherwise there is fairly good continuity/clustering with the systems over the mainland and Gulf, though the latter has trended somewhat deeper over the past day. Lingering detail differences are fairly small in scale and thus have low predictability. The progressive shortwaves around the upstream ridge yield low confidence for specifics of individual waves expected to track across the Bering Sea. Best potential for one of these waves to reach the western mainland will be toward early next weekend. Unfortunately the current envelope at that time is vary wide-ranging, from another deep and impactful northern Bering storm (00Z ECMWF and 12Z CMC) to a weak and suppressed wave (12Z GFS). The 06Z GFS and 12Z ECMWF are between the two extremes. Ensemble means signal a most likely track reaching the western mainland around 60-65N latitude. ...Weather/Hazards Highlights... Expect the coverage and intensity of precipitation over most of the mainland to decrease with time from Tuesday onward as mainland low pressure weakens. Meanwhile a developing frontal wave should track into the Gulf of Alaska by midweek and linger for at least a couple days, bringing a period of heavy precipitation to areas from the southeastern coast through the Panhandle. Highest totals are likely around midweek followed by a steady decrease as low pressure weakens and upper troughing slowly broadens. Waves along a mean frontal boundary around the northern periphery of a deep-layer ridge that extends from the Pacific into the Bering Sea may produce precipitation areas that extend from the Bering Sea to locations anywhere between the eastern Aleutians and western mainland. Any moisture reaching the mainland should take until Friday-Saturday to arrive. At that time the wide spread of guidance for possible track/strength of low pressure keeps confidence low for resolving the magnitude and coverage of precipitation and other sensible weather effects. On Tuesday, morning lows will still be above to well above normal over at least the western half of the mainland but highs over the state will be more moderate with near to somewhat below normal readings. From midweek onward the arrival of upper troughing will spread below normal highs over much of the state with morning lows tending to be less extreme including some pockets of modestly above normal. Possible flattening of the mean flow by next Saturday would start a gradual warming trend. Rausch Hazards: - Heavy precipitation across the Alaska Panhandle into the southern coastal ranges of mainland Alaska, Tue-Wed, Mar 30-Mar 31. 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 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