Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Mon Aug 15 2022 Valid 12Z Fri Aug 19 2022 - 12Z Tue Aug 23 2022 ...Bering Sea/Aleutians/Alaska Peninsula storm to bring a multi-day threat for heavy precipitation along and near the southern coast... ...Overview... The dominant focus of the forecast continues to be on a strong storm that is expected to be over the Aleutians/Bering Sea at the start of the period early Friday and may linger over or near the Alaska Peninsula through the weekend and early next week as it slowly weakens. This system should support a persistent pattern favorable for heavy precipitation over some areas along the southern coast of the mainland and possibly into favored terrain a little farther inland. There will likely be a period of brisk to strong winds over the Bering Sea and North Pacific as well. Meanwhile guidance continues to show significant differences and day-to-day variability for details of upper flow from the Arctic into parts of the mainland. ...Guidance/Predictability Assessment... Consistent with previous days, latest operational models suggest some type of interaction among a couple upper features over the Bering Sea late week into the weekend with the overall upper low persisting into next week. Specifics of this interaction provide some of the ongoing uncertainty in the forecast. As has also been the case thus far, guidance shows some trends from previous forecast. The most notable adjustments today are a somewhat farther north surface low track as of Friday and then a slower trend during the weekend and early next week. This would have the system potentially stalling over or near the central Alaska Peninsula for multiple days instead of reaching at least to Kodiak Island as in previous forecasts. The 12Z ECMWF/UKMET/CMC aligned well in principle and fit the ensemble mean template. The 12Z GFS looked weak and disjointed with the overall evolution, and eventually strayed well west of consensus, while the 06Z version compared better to other models/means. The new 18Z GFS has come back closer to the majority. Meanwhile significant spread and day-to-day variability persist for Arctic into northern-central mainland flow. Latest ECMWF/CMC runs have backed away from yesterday's scenario of an Arctic trough/upper low dropping into the mainland, though with the 12Z GEFS/CMCens and 00Z ECens means maintaining a modest weakness over the far northwest. In contrast, by around days 7-8 Monday-Tuesday the 12Z GFS/ECMWF/CMC have all trended toward a fairly strong ridge building over parts of the Arctic/northern mainland. The new 12Z ECMWF mean hints at more ridging around this time frame but not nearly to the degree of the operational models. Guidance comparisons led to starting the first half of the forecast with a blend of the 06Z GFS and 12Z ECMWF/CMC/UKMET. Trending the operational model composite toward a nearly even weight of models and 12Z GEFS/00Z ECMWF means by day 8 Tuesday continued to provide a good balance between detail and larger scale pattern agreement for the Alaska Peninsula storm and surrounding flow, while staying conservative over northern areas where confidence in the mean pattern is fairly low. ...Weather/Hazard Highlights... Slow movement of the strong storm system tracking from the Aleutians/Bering Sea to around the Alaska Peninsula should lead to a very wet pattern over multiple days across the southern coast and perhaps into favored terrain over the far southern mainland, from late this week through the first half of next week. Currently the most agreeable focus for highest precipitation totals is near the Kenai Peninsula, with significant totals potentially extending as far west as parts of the Alaska Peninsula. Recent slower trends with the system have delayed arrival of moisture to the southeastern coast and Panhandle, leading to lower amounts over those areas. Some areas along the southern coast may see several inches or more of total precipitation during the period. This storm will also produce a broad area of brisk to strong winds across the Bering Sea/Aleutians/North Pacific, especially late this week into the weekend. Gradual weakening of the storm will likewise yield gradually lower wind speeds. Some areas of precipitation are possible over the rest of the mainland but with low confidence in specifics due to uncertainty of important upper flow details. Surface high pressure over the Arctic should keep North Slope highs below normal for most of the period while clouds and precipitation keep southern areas rather cool during the day as well, with below normal readings extending into at least the northern Panhandle. However highs may trend to moderately above normal levels over middle latitudes of the mainland. Expect lows to be somewhat below normal in the west and above normal in the east late this week, with moderately above normal lows becoming more widespread thereafter. Rausch Hazards: - Heavy rain across portions of mainland Alaska, Thu-Mon, Aug 18-Aug 22. 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