Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Tue Oct 26 2021 Valid 12Z Sat Oct 30 2021 - 12Z Wed Nov 03 2021 ***Heavy precipitation is expected for south-central Alaska this weekend with atmospheric river event*** ...Overview and Model Guidance... The medium range period will begin on Saturday with an upper low initially centered south of the Alaska Peninsula, and an upper trough over eastern Siberia. An upper level ridge should be in place across northwest Canada, and deep southerly flow between the ridge and the upper low is expected to produce heavy precipitation for portions of southern Alaska. Model guidance begins in fairly good agreement with the deep surface and upper-level low over the Gulf of Alaska, with some minor differences regarding potential mesoscale features. There are greater model differences upstream across the Bering with the ridge axis, and the 12Z CMC shows significant phase differences as early as Saturday and even more apparent by early next week, with the CMC the only operational model suggesting a trough and closed low over western Alaska, whereas the ensemble means strongly support an extension of the ridge from western Canada. The WPC forecast was primarily derived from a GFS/ECMWF/UKMET blend along with some previous WPC continuity, and gradually increasing contributions from the ensemble means by the second half of the forecast period. ...Weather/Hazard Highlights... Precipitation is forecast to increase substantially over the weekend from the Alaska Peninsula to western portions of the southeast Panhandle, with the deep low pressure system over the Gulf of Alaska providing a prolonged moist onshore flow to the region. The rain will be quite heavy for windward terrain, with the potential for 3 to 6 inches over the weekend, and very heavy snow for the higher terrain. 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 on Saturday. Precipitation should continue into early next week, albeit abating in intensity. Farther north, light 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. By the weekend into early next week, temperatures should gradually warm as upper level ridging sets in as the upper low shifts away from the state. Low temperatures are currently expected to be 10-15F above normal for much of the interior, with highs generally 5-15F above average. Hamrick 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