Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 752 PM EDT Thu May 20 2021 Valid 12Z Mon May 24 2021 - 12Z Fri May 28 2021 ...Overview... The best clustering of latest guidance continues to show that a mean low over the Aleutians/Bering Sea and downstream ridge over parts of the mainland (also building into the northeastern Pacific around midweek or so) will be the dominant upper features during the period, but some important specifics vary. A persistent uncertainty is what role individual waves emerging from the western Pacific will play--including whether they merge with the mean low over the Aleutians/Bering or remain separate. This pattern will tend to focus episodes of precipitation from the Aleutians into the Alaska Peninsula and eventually farther east along the southern coast. ...Guidance/Predictability Assessment... The most notable adjustment in most guidance compared to 24 hours ago is a farther westward trend with the mean low over the Aleutians/Bering Sea from about Tuesday onward. This trend corresponds to earlier incorporation of a leading wave that tracks out of the western Pacific, and among the means was first suggested in Wednesday's 12Z ECens. Today's 12Z GFS appears to have been a relative hiccup relative to surrounding runs. It pulls the initial upper low well eastward of other models/means while the 06Z and new 18Z versions compare better to consensus. Later in the period another Pacific wave provides some divergence in solutions, with the 12Z ECMWF particularly aggressive in bringing a strong low into the Alaska Peninsula and beyond on day 8 Friday. Other models offer different varieties of track/timing/strength but with the prevailing theme of being much less extreme. Short-range guidance has finally become more convergent for compact upper low/shortwave energy reaching the western mainland, ultimately feeding into the overall Gulf/Panhandle upper trough by early Monday. This evolution ended up being somewhat of a compromise among the wide range of prior solutions--closer to earlier GFS runs in terms of dropping it into the trough versus lingering as an upper low over the mainland per some ECMWF runs, but with a track adjusted somewhat northeast of GFS runs from a day or so ago. Some details of flow over the mainland are still uncertain in the extended period with models differing in specifics of Arctic flow that could reach into at least northern areas. A blended approach that holds onto general mean ridging over the mainland provides the best consistency for the time being, while awaiting better agreement for low-predictability shortwaves. Based on guidance comparisons through arrival of the 12Z ECMWF, today's forecast started with the 12Z ECMWF/CMC/UKMET and 06Z GFS for the early part of next week. Then the forecast trended to 40-50 percent total ensemble (12Z GEFS/00Z ECens) input for Thursday-Friday with lingering minority contributions from the 06Z GFS, 12Z CMC, and last two ECMWF runs (to temper the 12Z run's Alaska Peninsula low by Friday). ...Weather/Threats Highlights... Initial low pressure and associated frontal system over the Aleutians/Bering Sea as well as one or more trailing systems that may merge into the overall mean low should bring periods of unsettled weather to areas from the Aleutians through at least the Alaska Peninsula and eventually southern coast. Some moisture may extend into parts of the western and southern mainland as well. Fronts/embedded waves may provide localized enhancement of precipitation and winds at times. The best signal for highest totals during the five-day period exists from the Alaska Peninsula to Kenai Peninsula and over favored terrain just north of Bristol Bay. The Panhandle and portions of the southern mainland as well as parts of the North Slope will tend to see below normal high temperatures during the period. Other locations will likely experience above normal highs with the greatest anomalies over the west, though some moderation may occur after midweek. Above normal readings will be more prevalent for lows, with the Panhandle and parts of the eastern mainland tending to be the primary exception. Rausch Hazards: No significant hazards are expected over Alaska during this forecast period. 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