Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 654 PM EDT Sun Jul 26 2020 Valid 12Z Thu Jul 30 2020 - 12Z Mon Aug 03 2020 ...Overview and Guidance Evaluation... Today's guidance has carried over some aspects of the forecast pattern evolution from continuity but with some meaningful adjustments for details within an area of expected troughing/closed low(s) between the Bering Sea and Northeast Pacific, while providing some improved clarity for what could be a fairly strong system tracking across the Bering Sea next weekend. The ideas of an east-west upper ridge building over the mainland and northwestern Canada along with eastward departure of an initially deep Arctic upper low have maintained reasonable consistency. Since yesterday the operational GFS/ECMWF/UKMET runs have all trended toward a more pronounced southeastward pull of Bering Sea dynamics into the Northeast Pacific, leading to an upper low and surface system where prior consensus had suggested a somewhat farther north and east-west elongated pattern at the surface and aloft. The ensemble means hint at lower heights reaching into the northeastern Pacific but appear to be a cycle or two behind in depicting the full detail of this trend. Corresponding to this adjustment the latest majority cluster has trended a little stronger with the east-west upper ridge over the mainland. The 12Z CMC differs somewhat from the other models so there is still some lingering uncertainty. Another question mark is how energy within a leading upper low off to the southeast ejects from its initial position. Solutions range between a northward track that would affect portions of the southern coast/Panhandle to a more sheared/eastward evolution that would produce lesser effects. Meanwhile the guidance has improved its clustering for how upper dynamics (likely in the form of a closed low) should track from Siberia/Kamchatka into the Bering Sea, supporting a fairly strong surface system crossing the Bering next weekend. Among the ensemble means available up to the time of forecast preparation only the 12Z GEFS mean reflected the operational model evolution while the 12Z CMC/00Z ECMWF means provided at best a vague hint of this system--though ultimately leading to an eastern Bering upper low like the other solutions late in the period. The new 12Z ECMWF mean has adjusted to the operational model cluster to provide further confidence in the majority scenario. Typical guidance error currently suggests an intermediate timing, between the slow 12Z ECMWF and fast 12Z CMC. Ahead of this system there is still low confidence in specifics of a western Pacific wave that may get picked up and brush the Aleutians around Fri or so. Latest trends are toward a faster wave that may become embedded along a warm front versus a slower pickup that could allow the wave to merge with the parent low. Preference to reflect the majority operational model cluster for specific details led to using the 12Z GFS/ECMWF/UKMET for the first part of the forecast period. The blend transitioned to nearly even model/ensemble weight by days 7-8 Sun-Mon, with a higher proportion going to the 12Z GEFS mean versus the 00Z ECMWF mean due to comparisons for the system tracking over the Bering Sea next weekend. By the latter half of the period the operational ECMWF component was split between the 12Z run and prior 00Z version due to some aspects of the new run becoming less appealing at that time. ...Weather Highlights/Threats... Initial southeastern Bering Sea low pressure and wavy frontal system extending to the south and southeast should bring some focused precipitation to parts of the Alaska Peninsula/southwest corner of the mainland early in the period. Some moisture may reach the rest of the southern mainland and Panhandle late week into the weekend but precip intensity and duration are quite uncertain due to low predictability of medium-smaller scale aspects of flow aloft. Some precipitation may also accompany a front that should drop down across the northern half of the mainland and then stall. Low pressure expected to track across the Bering Sea next weekend may bring a period of precipitation and stronger winds with some effects possibly reaching the western mainland by late in the period. Expect below normal high temperatures over approximately northern and southern thirds of the mainland as well as the Panhandle. Above normal highs should prevail over central latitudes but likely with a gradual cooler trend over time. A larger portion of the state should see above normal lows with the North Slope most likely to be the exception with some below normal lows. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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