Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Mon Oct 25 2021 Valid 12Z Fri Oct 29 2021 - 12Z Tue Nov 02 2021 ...Heavy precipitation is forecast for the Alaska Peninsula and Southcentral Alaska with gusty winds from the Aleutians to Southcentral late week into the weekend... ...Overview... The medium range period will begin with an upper low initially centered northwest of the state that should drift west with time, causing ridging to build into portions of Interior Alaska from the east. Additionally, persistent troughing is likely in the Northeast Pacific with an upper low centered just south of the Aleutians, helping maintain surface lows with strong moist onshore flow that will cause precipitation and gusty winds for southern parts of the mainland this weekend. ...Guidance/Predictability Assessment... Model guidance begins in fairly good agreement with the deep surface and upper-level low pressure system in the Northeast Pacific, with minor differences typical of the forecast time frame. Despite the differences being relatively small, small variations in the surface low track closer to or farther from the Southcentral Alaskan coast could impact the start time and amounts of heavy precipitation and winds there. A multi-model blend heavy on the 12Z GFS/ECMWF and initially including the 12Z UKMET and CMC was able to be used for this feature early in the period. Farther upstream, model agreement starts to go somewhat awry as energy from a possibly closed upper low moves east to the south of the Kamchatka Peninsula and may combine with energy from what is currently Tropical Storm Malou. Malou has its own forecast differences, with the 12Z UKMET/CMC slower than recent GFS and ECMWF runs, and it appears the Joint Typhoon Warning Center takes a track in between these clusters. The evolution of the energy possibly combining is uncertain and thus confidence is low. The UKMET seems to be an outlier as it tracks a shortwave quickly east, while the CMC becomes out of phase by Sun over the Aleutians as it appears to combine the western energy with the initial upper low more quickly than other guidance, so those models were quickly removed from the forecast blend. Another apparent issue is with the 12Z GFS, which somewhat like the CMC produces one upper low, which is centered over the Aleutians around Monday. These scenarios remain possible but not favored for now. Also, around Sunday/Monday the EC mean was a bit out of phase with the CMC/GEFS mean, but came back around to cluster well with other guidance by the end of the period. Thus the most prudent compromise at this point for the middle to latter part of the forecast period was a blend of the 12Z ECMWF, 06Z GFS, and 12Z GEFS mean, and increasing weighting of the 00Z EC mean by the end. ...Weather/Hazard Highlights... After some leading precipitation possible across the southern part of Alaska on Friday, precipitation is likely to increase substantially over the weekend for the Alaska Peninsula into Southcentral Alaska with the deep low pressure system spinning over the Northeast Pacific providing moist onshore flow to the region. Some strong southerly winds are also possible in those areas, while gusty northerly winds are expected on the backside of the low for the Aleutians Friday into Saturday. Precipitation should continue into early next week. Farther north, some snow is likely to spread across much of the western and central parts of the state with the moist inflow from the low pressure system. Temperatures across much of interior Alaska will be near to slightly above normal through the end of the workweek, while colder than average temperatures especially in terms of highs are forecast over southwestern Alaska into the Panhandle. By the weekend into early next week, temperatures should gradually warm as heights aloft rise as the mean upper low shifts away from the state and upper ridging builds in. Low temperatures are currently expected to be 10-25F above normal for much of the interior, with highs generally 5-15F above average. Tate 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 Hazards: - Heavy precipitation across portions of mainland Alaska, Sat-Mon, Oct 30-Nov 1. - High winds across portions of mainland Alaska and the Aleutians, Fri-Sat, Oct 29-Oct 30. 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