Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Sun Aug 11 2024 Valid 12Z Thu Aug 15 2024 - 12Z Mon Aug 19 2024 ...Overview... The guidance consensus continues to show mean troughing aloft from the Arctic through the Bering Sea and into the North Pacific as of late this week, with the portion from the Bering Sea southward likely progressing gradually eastward with time while another trough may reach the northwestern Pacific by next Monday. Dynamics within the leading trough should support well-defined surface system tracking from the Bering Sea northeastward late this week, spreading a broad area of precipitation and brisk/strong winds from the Bering Sea into portions of the mainland, with some potential for trailing energy/waviness to produce additional precipitation. However, confidence remains lower than desired for some details of the primary system and especially for any trailing features. Meanwhile an upper ridge extending into Southcentral as of Thursday will drop slowly southeastward through the Panhandle Friday into the weekend. Expected western Pacific tropical development may ultimately interact with the emerging northwestern Pacific upper trough to yield an extratropical storm system near the western Aleutians by early next week. ...Guidance Evaluation/Preferences... Dynamical and machine learning (ML) guidance continues to show difficulties in resolving the evolution of and interaction among a leading medium/smaller-scale shortwave reaching the eastern Aleutians/southeastern Bering Sea as of early Thursday and incoming upper trough energy just to the west. Differences aloft have led to a variety of solutions for associated surface low pressure. Recent ECMWF runs had been on the weaker and faster/eastward side of the spread for the system due to less interaction among upper features and a more open overall upper trough, but the 12Z version adjusted closer to recent GFS/CMC runs and their means in principle. The 00Z ECens mean showed some tendencies of the earlier operational run but the 12Z ECens mean has trended back some. At least for the moment this offers a reasonable cluster for depicting this system's definition. 00Z MLs support a 980s mb depth while the system is over the Bering Sea (a tad weaker than the 12Z dynamical runs at their strongest point) while generally leaning somewhat faster with northeastward progression. The 12Z UKMET was weaker with the leading shortwave and stronger with trailing energy, leading to a farther south and then east surface system versus the favored majority cluster. Guidance still varies widely for what happens immediately behind this system. Some recent GFS runs, including from 00Z/06Z/18Z, have been sufficiently strong/concentrated with trailing dynamics to produce a second defined system that tracks from the Aleutians northeastward into the mainland. 00Z MLs are split, with about half suggesting some variation of this, while other guidance limits the surface reflection to a weaker and more suppressed frontal wave. High uncertainty regarding this aspect of the forecast recommends the more conservative frontal wave scenario, with such a wave settling into its typical southern coast/Gulf location by late in the period as upper troughing approaches. Also of note, most guidance suggests tempering the depth/sharpness of upper troughing that the 12Z ECMWF/CMC and 18Z GFS bring into the Gulf of Alaska/northeastern Pacific by next Monday. There is a decent signal in the guidance for a developing tropical system to track near Japan late this week and recurve thereafter, likely becoming extratropical as it interacts with northern stream upper troughing late in the forecast. There is a lot of latitude spread and run-to-run variability for the system's track by next Monday, favoring a model/ensemble mean compromise for track and depth. Guidance comparisons led to starting today's forecast with a blend of the 12Z GFS/ECMWF and lesser weight of the 12Z CMC for the first half of the period, followed by incorporating 30-45 percent total weight of the ensemble means (12Z GEFS/CMCens and 00Z ECens). Also the ECMWF part was split between the 12Z/00Z runs by day 8 Monday. This solution helped to maintain decent larger scale continuity compared to the past couple days while awaiting any trends pronounced enough to merit significant changes. ...Weather/Hazards Highlights... The storm expected to track from the Bering Sea northeastward late this week should spread a broad area of windy conditions and rain of varying intensity from Bering Sea into parts of the mainland. Western and southern portions of the mainland should see relatively greater totals of rain, with favored southward-facing terrain likely to see the heaviest rainfall. Strongest winds should be over the Bering Sea into the far western mainland. While a relative majority cluster for depicting this storm has emerged in the guidance at the moment, detail predictability still looks below average--tempering confidence in some specifics of impacts at particular locations. In addition, predictability/confidence are even lower for any trailing frontal wave(s) that could produce additional rainfall focus from the Aleutians into the mainland. Regardless of specifics, as the overall upper trough progresses eastward, moisture focus should push more into Southcentral and the Panhandle by the weekend/early next week. Some mainland precipitation could be in the form of snow at highest elevations, with somewhat better potential in the latter half of the period with the approach/arrival of upper troughing. The forecast pattern will favor a trend toward below normal highs over most of the state, with coolest anomalies likely to be during next weekend. Exceptions include the northern coast of the mainland, the eastern interior Thursday into Saturday, and parts of the southern two-thirds of the Panhandle. Morning lows will vary, with above normal readings late this week (aside from a cool start over the east/Panhandle Thursday) followed by a cooler trend from west to east during the weekend and early next week. 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