Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Tue Jul 27 2021 Valid 12Z Sat Jul 31 2021 - 12Z Wed Aug 04 2021 ...Heavy rainfall possible for parts of western and northern Alaska... ...Overview and Guidance/Predictability Evaluation... The majority of guidance continues to show strong amplification/deepening of an upper trough with embedded closed low between Siberia and the Aleutians while upper ridging builds downstream over portions of the mainland and into northwestern Canada. This upper ridge could keep an upper low meandering off the Panhandle/Haida Gwaii for most of the period. The developing pattern over and west of the mainland should be favorable for a significant amount of moisture (possibly a combination of western Pacific tropical moisture and another source from west-southwest of Kamchatka) to reach parts of the western and northern mainland. However individual model runs show a disconcerting degree of spread for important large scale aspects of this evolution--which historically tend to have better predictability than what the models currently suggest in this case. Therefore confidence is currently lower than desired for the location, duration, and magnitude of what could be a significant rainfall event over some areas. General preference for the overall forecast evolution is closest to the ensemble means in principle as they have been the most agreeable and stable for the potentially strong Bering Sea storm system along with its supporting dynamics, plus the downstream ridge. Note that the means have generally been showing a trend toward a stronger ridge over the past couple days. Recent ECMWF runs have been closest to the means in a broad sense but have also been on the extreme side of guidance for the duration and magnitude of high precipitable water values over the western mainland--up to a few standard deviations above climatology. GFS runs have tended to be southward and become more negatively tilted with the upper low/trough versus the means and other guidance while pulling downstream ridging farther northwestward than other guidance. The 06Z/12Z GFS evolution tends to favor southwestern areas. Early in the period the 06Z run did compare better to consensus than the 12Z run. The 18Z GFS has adjusted closer to the majority cluster. Meanwhile the latest CMC runs have tended to shift the overall pattern east of consensus, leading that model to spread a broader and eastward area of heavy rainfall over the mainland. Aside from the 12Z ECMWF's faster trend, the more common trend has been slower for the upper low off the southern Panhandle and Haida Gwaii. This corresponds to the recent stronger trend for the ridge that builds to the north, with the average of ensemble means suggesting the low could meander over the northeastern Pacific through the end of the period. Based on guidance comparisons, today's forecast started with a 70 percent model/30 percent mean blend for days 4-5 Saturday-Sunday. Models consisted of 00Z/12Z ECMWF runs along with the 06Z GFS and 12Z UKMET while the 12Z GEFS/00Z ECens provided the mean input. The blend quickly transitioned to 70 percent means and 30 percent total 00Z/12Z ECMWF by days 7-8 Tuesday-Wednesday. ...Weather/Threats Highlights... Expect one or more episodes of potentially heavy rainfall over parts of the western and northern mainland. The best overlap of guidance in terms of enhanced moisture is around Sunday-Monday while favored terrain over the Seward Peninsula and Brooks Range currently has the best probability for highest totals during the period. Potential exists for other areas over the west/north to see significant rain as well as before and/or after the most favored days. 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. Deceleration of the front extending from the Bering storm may support persistence of rain late in the period. It is still within the realm of possibility that this event could produce some rainfall amounts approaching climatological extremes. The Panhandle may see mostly light/scattered rain at times ahead of the lingering northeastern Pacific upper low. The upper ridge building over the eastern mainland/northwestern Canada should promote a warmer trend for highs over eastern/interior areas. The northern coast of the mainland may also see above normal highs. Below normal highs should prevail elsewhere, though the southern Panhandle could see pockets of warmer readings. Min temperatures will tend to be above normal over most of the state with only a few localized areas below normal. Rausch Hazards: - Heavy rain across portions of mainland Alaska, Sat-Mon, Jul 31-Aug 2. 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