Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 744 PM EDT Sat Jul 29 2023 Valid 12Z Wed Aug 02 2023 - 12Z Sun Aug 06 2023 ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... Guidance over the past day has shown some trends toward intermediate ideas compared to very contrasting solutions from 24 hours ago across Mainland Alaska and the Bering Sea. During the first half of the period, latest GFS runs have adjusted toward a somewhat farther westward extent of southern mainland upper ridging, while most other guidance has adjusted toward bringing somewhat more shortwave energy into the Bering Sea early in the period (though not as strong as GFS runs have been showing) thus eroding eastern Bering Sea upper ridging that the majority cluster had been showing. GFS runs through the 12Z cycle have been persistently on the fast side with the upper low/surface system tracking across Kamchatka and into Siberia late next week into the weekend (and with leading moisture/height falls reaching the western mainland) but the ECMWF/ECMWF mean have nudged somewhat faster. By late in the forecast period the GEFS mean and CMCens/ECens are closer to each other for the overall pattern than they were yesterday. Consensus suggests that by next Sunday the upper ridge should take on an orientation from western Canada northwestward as height falls gradually advance across the Bering Sea but likely not with the strength/northward amplitude seen in the new 18Z GFS. On the other hand, operational models and ensembles still vary in seemingly random fashion for the upper low initially forecast to be south of the Panhandle and most likely to wobble north/west to some degree underneath the mainland ridge. The resulting means of the 12Z GEFS/CMCens and 00Z ECens means all ended up clustering fairly well for the upper low and the 12Z GFS was the closest operational run to the means. The 12Z ECMWF and especially CMC track the upper low farther north, with somewhat more influence on the Panhandle and/or the southern coast. The new 12Z ECens mean takes a slightly wider arc for a while in deference to the operational run but ends up in a similar position as the old run toward the end of the period next weekend. Today's forecast employed a more general blend to capture guidance's apparent attempt toward convergence for some aspects of the Bering Sea into mainland pattern while tempering the more extreme attributes that have lower confidence. The first half or so of the period incorporated the 12Z ECMWF, GFS, UKMET, and CMC from more to less weight (UKMET/CMC being equal) along with a small component of the 12Z CMCens/00z ECens due to their similarities for the northeastern Pacific upper low and greater preference versus the GEFS mean over/west of the mainland. Increasing detail differences in the models and improved comparisons of the GEFS mean to other guidance later in the period led to a days 7-8/Saturday-Sunday blend employing 70-75 percent total weight of the three ensemble means with lingering input from the 12Z ECMWF. ...Weather/Hazards Highlights... The North Slope/Brooks Range region and Southcentral may see some rainfall during the mid-late week time frame, with the majority of activity tending to be fairly light. Depending on the character of Bering Sea shortwave energy rounding the mainland upper ridge, far northwestern/northern areas could see a separate area of rainfall. Meanwhile a wavy front should bring some light to moderate rainfall to the Aleutians and perhaps as far eastward as the southwestern Alaska Peninsula. Then expect a Kamchatka into Siberia storm system to push a front into the Bering Sea and Aleutians by next weekend, with some focused rainfall accompanying the front. Moisture ahead of the front may reach into the far western mainland at that time but with low confidence for timing and precise rainfall amounts. To what degree the upper low initially south of the Panhandle ultimately influences the weather along the Panhandle/southern coast is still quite uncertain, with the range of potential tracks leading to no influence at all or some increase of moisture--but even in the latter case, rainfall would most likely be light. Evolution/movement of the upper ridge over the mainland during the period should favor above normal highs especially over the Interior, with greatest anomalies tending to drift west to east with time. Some pockets of below normal highs will be possible over parts of the North Slope/Brooks Range mid-late week and the southeastern coast/northern Panhandle most days. Much of the state should see above normal lows through the period. Rausch 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