Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 623 PM EDT Wed Oct 27 2021 Valid 12Z Sun Oct 31 2021 - 12Z Thu Nov 4 2021 ...Overview and Model Guidance... The medium range period will begin on Sunday with a rather pronounced upper ridge over western Canada and an elongated trough axis extending from the Bering to the northern Gulf. The ridge axis is expected to hold firm through mid-week over eastern Alaska and into Canada, whilst a large upper low with multiple shortwave perturbations takes up residence over the Bering and the Aleutians. The potential exists for another organized storm system to develop over the Gulf by the end of the forecast period. Model agreement is greatest with respect to the anomalous upper ridge near the west coast of Canada and extending northward to the Arctic through mid-week. However, differences in the guidance are apparent as early as this weekend across the western Bering regarding the broad upper low evolution. The 12Z ECMWF/CMC are out of phase with a broad ridge over this region in contrast to the UKMET/GFS/GEFS mean with more of a trough. These differences continue into early next week with more in the way of shortwave energy lifting north across western mainland Alaska. Elsewhere across the forecast domain, model differences appear less significant. The WPC forecast was primarily derived from a GFS/ECMWF/UKMET blend along with some previous WPC continuity, and this included some use of the ensemble means early in the forecast period owing to the differences noted above. ...Weather/Hazard Highlights... Heavy precipitation is expected for southern portions of mainland Alaska for Sunday and into Monday as a steady surge of moisture advects northward from the northeast Pacific, although the intensity of the atmospheric river event is expected to wane some by late on Sunday with the heaviest rainfall likely on Saturday. The Kenai Peninsula and the coastal mountain ranges as far east as the Yukon border appear to have the greatest prospects for excessive rainfall that could result in flooding and run-off concerns, and very heavy snowfall for the higher elevations. Some strong southerly winds are also expected in those areas where gusts could exceed 40 mph. Precipitation should continue through the middle of next week, albeit abating some 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. In terms of temperatures, most locations across central and eastern portions of the mainland should enjoy above normal readings next week, with some areas up to 20 degrees above normal. The upper trough over the Bering will tend to keep readings closer to normal or perhaps slightly below for the western portion of the mainland, and this also holds true for the Aleutians. Hamrick Hazards: - Heavy precipitation across portions of southern mainland Alaska, Sat-Mon, Oct 30-Nov 1. - High winds across portions of southern mainland Alaska and the Aleutians, Sat, Oct 30. - High winds across portions of southern mainland Alaska, Sat-Sun, Oct 30-Oct 31. 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