Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon Jun 26 2023 Valid 12Z Fri Jun 30 2023 - 12Z Tue Jul 04 2023 ...Overview... Continue to expect fairly cloudy and at times wet conditions over a decent portion of the state, but with persistent guidance spread and variability for specifics of individual systems or flow patterns that will determine the distribution, intensity, and timing of rainfall. At least there is somewhat of a majority scenario at the moment for eastward progression of a Gulf of Alaska upper low late this week into the weekend while a consolidating system tracks just south of the eastern Aleutians/Alaska Peninsula. An upper ridge northeast of the mainland by Friday may briefly build its influence southwest over the mainland before it connects with ridging ahead of the North Pacific system. Meanwhile there are lingering uncertainties with digging Arctic flow likely to yield an upper trough over Siberia and the Bering Sea, with some combination of flow on the west side of this trough and energy emerging from over/near the western Pacific possibly supporting a well-defined system reaching the Aleutians/Bering Sea by early next week. ...Guidance Evaluation/Preferences... For the various features/patterns over and closely surrounding the mainland, there is at least a relative majority of solutions that offer a coherent evolution. Most guidance has been nudging faster with the late week Gulf of Alaska upper low for the second day in a row, while the 12Z/18Z GFS runs are a southern extreme with the upper low track. The 12Z GEFS mean and 06Z GFS are closer to most other models. Associated surface low pressure should rapidly weaken early in the period. Behind this feature, GFS/ECMWF/UKMET runs have been clustering toward consolidation of Aleutians/southern Bering shortwave energy into a fairly strong upper low/surface system just south of the eastern Aleutians/Alaska Peninsula from Friday through the weekend. However the 12Z/18Z GFS runs are somewhat on the deep/slow side with the overall system. The 12Z CMC ends up being a southern extreme. The ensemble means still appear to be in the process of trending toward the definition seen in the models, while the 12Z GEFS and latest ECMWF means closest to the best model cluster (06Z GFS/00Z-12Z ECMWF/12Z UKMET). This system should weaken as it nears Kodiak Island by around next Monday. Meanwhile the flow aloft over the mainland has been an ongoing challenge. Essentially corresponding to majority ideas for the above systems, the most likely scenario has easterly flow over much of the mainland Friday followed on Saturday by a ridge extending farther southwest over the mainland from an upper high just west of the Canadian Archipelago. Then this ridge may connect with separate ridging that builds ahead of the North Pacific system. There is still some debate over the relative influence of this overall ridge and the trough that digs to the west. At the very least, the 12Z CMC is the most extreme model in terms of how far eastward the trough extends into the mainland. Farther west, model runs are still showing a variety of possibilities for how flow on the western side of the Siberia/Bering Sea trough may evolve as well as how dynamics may emerge into the western Pacific from within or underneath a blocky pattern. The very uncertain relative interaction of these streams may keep confidence in specifics quite low for a while. Operational models are actually showing more spread than yesterday for associated low pressure that could reach the Aleutians/Bering Sea by next Monday-Tuesday (or not, per the very suppressed 12Z GFS) which is actually more reflective of uncertainty for upper flow details. However ECMWF runs continue to show such a system, even with different run-to-run details for its dynamic support, and the ensemble means are slowly getting deeper with the system (ECMWF mean being the strongest for the second day in a row) to suggest incremental reduction in member spread and/or greater strength in some members. These considerations maintain at least enough confidence to keep the system in the forecast with an intermediate strength. Based on the best consensus of guidance available at the time of forecast preparation, the early part of the forecast started with a blend of the 06Z GFS and 12Z ECMWF/UKMET. Day 6 Sunday started a transition toward incorporating some 12Z GEFS/00Z ECens means while also splitting ECMWF input between the 12Z and 00Z runs as its details began to diverge. Then days 7-8 Monday-Tuesday increased total ensemble mean weight to 50-65%. ...Weather/Hazards Highlights... The southern half or so of the Panhandle may see a brief period of somewhat enhanced rainfall around the end of the week as the upper low tracks eastward across the Gulf of Alaska. Then the system currently expected to consolidate just south of the eastern Aleutians and track east-northeastward through the weekend/beginning of next week may bring a period of moderate to heavy rainfall from the Alaska Peninsula/Kodiak Island eastward across the southern coast. Meanwhile the pattern evolution across the mainland will support some pockets of enhanced moisture during the period, which combined with possible shortwave impulses could lead to episodes of locally moderate to heavy rainfall. Specifics are not easily resolved several days out in time though, so monitor forecasts into shorter time frames when there may be better clarity on location and magnitude of any heavier activity between late this week and early next week. The late-period storm forecast to affect the Aleutians/Bering Sea would bring an increase of winds and rainfall across those areas early next week. It should continue to be on the cool side across much of the state for high temperatures, with most areas below normal and greatest negative anomalies likely to be over southern areas including the Panhandle. Some pockets of above normal highs will be possible late week into the weekend over portions of the northern coast and the Seward Peninsula. Low temperatures should be moderate, with areas of below/above normal readings. Warmest lows should be over the northeastern mainland late this week. Rausch - Heavy rain across Kenai Peninsula Borough of Alaska, Sun-Mon, Jul 2-Jul 3. 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