Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 644 PM EDT Sat May 16 2020 Valid 12Z Wed May 20 2020 - 12Z Sun May 24 2020 ...Guidance/Predictability Assessment... Latest models and ensemble means display better than average agreement and continuity with most aspects of the overall pattern from midweek through next weekend. The dominant focus will be on a tightly wound and deep storm system that is forecast to track into the Aleutians/southern Bering Sea Wed-Thu and slowly weaken as it drifts gradually eastward toward the southwestern Alaska Peninsula. The flow around this system will produce areas of strong winds. In addition the slow system motion and leading front(s) with possible embedded waves could support a period of strong precipitation enhancement most likely to focus along the Kenai Peninsula/eastern Alaska Peninsula and possibly parts of the southwest corner of the mainland. Between this system and a quickly departing Gulf of Alaska upper low as of early Wed, an upper ridge will likely build into the Gulf and the mainland by Thu. This ridge should push northeast with time to yield an axis from western Canada to the northeast corner of the mainland and into the Arctic next weekend. Guidance preferences sided with a blend of the 12Z GFS/ECMWF with modest input from the 12Z CMC from day 4 Wed into early day 6 Fri. After early Fri the blend phased out the CMC as it strayed farther northwest than most solutions for the southern Bering system. Meanwhile detail uncertainties favored a gradually increasing contribution from the 12Z GEFS/00Z ECMWF means, reaching 30-40 percent total by days 7-8 Sat-Sun. Late in the period the 00Z/12Z ECMWF runs became mixed over which run compared better to the model/mean consensus depending on location, so the blend included both runs next weekend. Maintaining a fair degree of operational GFS/ECMWF input helped to strengthen the downstream ridge aloft versus the means, reflecting recent stronger trends in most guidance. The primary adjustment from continuity for the Aleutians/Bering Sea storm was a slight northward trend. The GFS was in the southern part of the spread (close to continuity) while the UKMET tracked north of the ECMWF (which along with its mean started the northward trend overnight). Within the fairly high-confidence large scale pattern, confidence decreases later in the period for one or more possible frontal waves reaching the Gulf. Such details may take a number of days to resolve satisfactorily. ...Weather/Hazard Highlights... The initially deep storm system reaching the Aleutians/Bering Sea will first bring precipitation and areas of strong winds across the Aleutians. Then for a period of time after Wed heavier activity should focus more narrowly over southeastward-facing terrain along the Kenai Peninsula/Alaska Peninsula, where models and ensembles show the most common signal for highest five-day totals. Kodiak Island and parts of the southwestern mainland could see some enhancement as well. By next weekend the location/duration of any lingering moderate-heavy precip--including the degree of eastward extent--will depend upon lower confidence small scale wave details over the Gulf of Alaska. While the general surface pattern over the mainland looks stable through the period, a stationary front draped across the central or south-central mainland through midweek should quickly dissipate as upper ridging builds overhead and to the east, leaving a mere trough that remains through the weekend. Highs will tend to be above normal over the central mainland and somewhat cooler than normal over the North Slope region and southern mainland/Panhandle. Expect greater coverage of above normal anomalies for morning lows. Rausch 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