Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 808 PM EDT Wed Jul 28 2021 Valid 12Z Mon Aug 02 2021 - 12Z Fri Aug 06 2021 ...Heavy rainfall possible for parts of western and northern Alaska... ...Overview and Guidance/Predictability Evaluation... The latest guidance continues to favor a strong amplification of an upper trough with embedded closed low located between Siberia and the Aleutians while upper ridging builds downstream over portions of the mainland and into northwestern Canada. With ample moisture advecting into the Mainland within this flow pattern, this will yield favorable conditions for enhanced rainfall - possibly with significant amounts over the West and North. This setup will also help maintain an area of low pressure over the eastern Gulf of Alaska near the Panhandle/southern Mainland coastline; which may also keep periods of cloud cover and light rain over the area. Unfortunately, the degree of spread on both large and small-scale details have not improved significantly with the last couple of model cycles. However, the ECWMF did have downward trend with the heights for midweek over central Alaska while the GFS has not been consistent. The ensemble means are closer to the ECWMF with the overall evolution. Confidence with this forecast continues to be lower for much of the extended period. The preferred blend for this forecast used roughly and 80/20 weighting of the 12Z ECWMF/GFS/CMC/UKMET to the ensemble means initially and into the middle period then shifting more toward the ECWMF late in the period. ...Weather/Threats Highlights... One or more periods of potentially heavy rainfall will be possible across the western and northern portions of the Mainland. Although the spread in the guidance lows the confidence a bit on when and where, the best time frame appears to be around Sunday into Monday- this is when areas of enhanced moisture will likely be in proximity of the favored higher terrain of Seward Peninsula and Brooks Range. Some of the highest totals may occur across these locations and additional rain may linger beyond Monday. A leading shortwave and surface system may produce one period of focused precipitation over the northwest early in the weekend while the combination of a stalled front and possibly some western Pacific tropical moisture included within southerly flow ahead of a Bering Sea storm could provide the added enhancement thereafter. Some of the rainfall totals may be close to climatological extremes. Mostly light to scattered rain is expected at times for the Panhandle ahead of the lingering northeastern Pacific upper low. For much of the eastern half of the Mainland and points into the northwestern Canada will have warming temperature as the upper ridge builds during this period. Daily temperatures across the northern Coast will likely be higher than seasonal average for late July into early August; while near or below normal daily maximums are observed elsewhere across the state. Southern portions of the Panhandle could see pockets of warmer readings. Overall, the daily minimums are expected to be above normal, with only a few localized areas below normal. Campbell Hazards: - Heavy rain across portions of mainland Alaska, Sat-Sun, Jul 31-Aug 1. 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