Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 751 PM EDT Thu Aug 15 2024 Valid 12Z Mon Aug 19 2024 - 12Z Fri Aug 23 2024 ...Multiple rounds of heavy rain and strong winds for portions of western Alaska through next Thursday... ...Overview... Guidance still shows the amplified and energetic pattern that becomes established in the short range, with a series of systems tracking southwest to northeast across the Bering Sea, continuing into the extended period next week but then finally slackening by next Friday. One system in this series will track away from the northern coast on Monday after producing areas of heavy rain/high winds on Sunday, leaving the extratropical reflection of Typhoon Ampil as the dominant storm of interest next week. This latter system will likely produce another episode of high winds and enhanced precipitation over western areas. Some of the shortwave energy crossing the mainland early in the week may drop southeastward over the Panhandle Tuesday-Wednesday, followed by some upper ridging ahead of extratropical Ampil. Consensus agrees upon a transition to flat or broadly cyclonic mean flow aloft from Siberia through Alaska by next Friday. ...Guidance Evaluation/Preferences... Guidance continues to differ somewhat regarding exactly how Typhoon Ampil will interact with approaching northern stream dynamics passing over Kamchatka on Monday. Latest ECMWF-initialized machine learning (ML) models provide much greater support for the faster/deeper evolution seen in the GFS/ECMWF, yielding a track over the western Bering Sea by early Tuesday in contrast to a weaker and farther southeast track in the 12Z CMC/UKMET. The 12Z CMCens mean is a compromise at least halfway toward the non-CMC idea as well. The GFS is a bit on the deeper side of the dynamical spread (in the 960s mb at its strongest), though the new 12Z ML models offer a little more potential for that possibility (versus most being in the 970s in the 00Z cycle). Even with these differences early in the week, the UKMET/CMC join the consensus track over the western Bering Sea by early Wednesday. Thereafter, continuity and relative agreement look good as the system continues quickly northeastward over the Bering Strait or far eastern Siberia--gradually weakening as the upper level support likely opens up. Behind this system, models/ensembles generally agree on the pattern evolving toward broadly flat to cyclonic flow from Siberia across the Bering Sea and most of Alaska by next Friday. Overall there is enough similarity to support medium or greater confidence in the large scale pattern at that time, but embedded details have much lower predictability/confidence. Shortwave specifics on the southern edge of the westerlies will influence how much frontal waviness could develop over the North Pacific with some potential effects on the Aleutians. Sporadic operational runs have been showing such waviness (but with other runs suppressed) while the signal in the ML models has been more diffuse, and the ensemble means keep the front fairly suppressed without any defined waves. High uncertainty favors a model/mean mix by next Friday. Above forecast considerations led to starting the days 4-5/Monday-Tuesday part of the forecast with a blend of the 12Z GFS/ECMWF. The forecast incorporated a small weight of the 12Z CMC by day 6/Wednesday as it joined the favored cluster for extratropical Ampil. Rapidly increasing detail uncertainties led to splitting ECMWF input among the 00Z/12Z runs starting day 7/Thursday and increasing total ensemble weight (12Z GEFS/00z ECens) to 40 percent by Friday. This solution provided reasonable continuity for the track of extratropical Ampil while yielding a deeper trend based on gradually improved clustering and ML model support. ...Weather/Hazards Highlights... Before the start of the extended period, the system tracking away from the northern coast on Monday should bring a period of high winds and heavy rainfall primarily to northern and western areas on Sunday. Heaviest rain should be along the western Brooks Range, while strongest winds will likely be across favored terrain and near the western coast. The Days 3-7 Hazards Outlook reflects these threats. Confidence has improved somewhat today on the idea of extratropical Ampil tracking over the western Bering Sea by Tuesday and continuing northeastward across the Bering Strait/eastern Siberia around Wednesday. This storm may produce a broad area of strong winds across the Bering Sea and extending into the western mainland, with the Hazards Outlook depicting best potential for high winds over the western mainland on Wednesday. Extent of high winds should be less expansive than with the Sunday storm. Also, guidance shows less extreme moisture anomalies ahead of extratropical Ampil versus the Sunday system, so rainfall should be somewhat less extreme but still significant at some locations. The late-week pattern looks to maintain unsettled weather but with less pronounced wind/precipitation focus. Precipitation may be in the form of snow at highest elevations of the Brooks Range and Alaska Range. Temperatures will vary with system progression but highs will likely be below normal over a majority of the state for most of next week. Coolest readings should be around Monday as a leading trough passes over the state and then again late in the week as flat to cyclonic flow aloft becomes established. Exceptions with some above normal highs include parts of the east and Panhandle around Wednesday-Thursday as an upper ridge crosses the region, and areas along the northern coast. Anomalies for morning lows will tend to alternate between above and below normal in a west-to-east progression. 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