Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Sat Sep 02 2023 Valid 12Z Wed Sep 06 2023 - 12Z Sun Sep 10 2023 ...Overview... Features within a moderately progressive pattern carrying over from the short range time frame should generally begin to trend slower as they amplify into what appears to be a longer term mean trough aloft setting up over and south of the mainland. An embedded low within a leading trough reaching the western mainland by midweek should track over the southern coast/Gulf of Alaska late in the week, with accompanying Gulf low pressure helping to focus precipitation along the southern coast and Panhandle. Then a strong system tracking across the Bering Sea Friday into Saturday may track near the Alaska Peninsula/Kodiak Island later in the weekend. This system will likely produce a broad area of precipitation and brisk to strong winds, with primary precipitation emphasis along the southern coast and extending into the Panhandle by next weekend. Continue to expect below normal high temperatures over most areas during the forecast period. ...Guidance Evaluation/Preferences... The latest array of guidance generally supports keeping the forecast close to yesterday's continuity with some minor refinements in response to recent trends. A composite of 12Z model guidance with greater emphasis on the ECMWF/GFS relative to the CMC/UKMET represented the forecast evolution well early-mid period, while increasing 12Z GEFS/CMCens and 00Z ECens input to 30-60 percent total by days 7-8 next weekend (and limiting operational models to the GFS/ECMWF by day 8) accounted for increasing detail uncertainty in operational model runs and the relative stability of recent means for the Bering Sea storm track. Continuity is very good for the upper trough reaching the western mainland by early Wednesday. By Thursday-Friday there is a better signal today (first suggested by earlier ECMWF/ECMWF mean/CMC runs) that the embedded upper low will drop to the southern coast/Gulf of Alaska. Beyond this refinement, the primary change since yesterday is that the upper trough to the south has become somewhat slower and sharper, allowing for increased potential of some moisture/surface wave focus into the Panhandle for a time around the end of the week. 00Z GFS/ECMWF runs were most pronounced with this idea while the 12Z runs have backed off some, due in part to slightly faster shortwave progression. For now prefer a fairly weak depiction of such a wave, with a track near the southern Panhandle. This aspect of the forecast has low predictability for the time being, given the sensitivity of specifics to precise shortwave amplitude and timing. For the Bering Sea system expected to track near the Alaska Peninsula and Kodiak Island by next Sunday, guidance shows typical spread and variability but with the full envelope of solutions tending to become more narrow with time. Latest consensus has trended a bit farther north with the low track over the Bering Sea around Friday, while as of early Saturday the models are now clustered closer together near the consistent ensemble means over the southeastern Bering Sea. The 12Z GFS was a notable deep extreme over the Bering Sea (favorably tempered in the 18Z run). Especially after early Saturday the 12Z CMC strayed to the slow side, holding in place a concentrated moisture focus over the eastern Alaska Peninsula and Kenai Peninsula--an evolution not supported by other solutions that advertise a more steady eastward progression of moisture. This favored excluding the CMC from the forecast after Saturday. Otherwise there is better than average agreement among the models and means for a surface low position just south of the Alaska Peninsula/Kodiak Island by early Sunday. An upper low initially north-northwest of the mainland continues to be troublesome in terms of its ultimate path and possible influence in the forecast. Current possibilities range from a counterclockwise curl that could bring it close to the northwestern mainland (12Z CMC and 12Z ECens mean), or retrograding (18Z GFS), or dropping well south across the central or eastern Bering Sea and possibly having some effect on the system to the southeast by next weekend (last two ECMWF runs). The current blend preference yields a weakening low that does not reach much south of the Bering Strait. Needless to say, confidence in any specific solution is minimal. ...Weather/Hazards Highlights... The leading system reaching the mainland by early Wednesday and forming a deep-layer low over the southern coast/Gulf of Alaska later in the week will produce the best rain and high elevation snow focus Wednesday-Thursday along the southeastern coast and Panhandle, with some areas of enhanced totals also possible over parts of the Alaska Range and perhaps extending into eastern parts of the Interior. At least the southern Panhandle could see meaningful precipitation extend into Friday depending on the uncertain strength and path of a northeastern Pacific surface wave. Flow behind the separate areas of mainland and Gulf of Alaska low pressure could produce some brisk to strong winds for a brief time over the eastern Bering Sea and parts of the western mainland. Confidence continues to increase in concept for a fairly vigorous system forecast to track across the Bering Sea during the latter half of next week and most likely end up just south of the Alaska Peninsula/Kodiak Island by next Sunday. Expect this system to spread an area of precipitation and brisk to strong winds across the Aleutians/Bering Sea during Thursday-Friday into Saturday. Meanwhile from Friday through the weekend, moisture should spread across the far southwestern mainland followed by the southern coast and eventually the Panhandle. Specifics over these areas are still somewhat uncertain, with southwestern mainland precipitation coverage as well as magnitude of precipitation totals along the southern coast/Panhandle being sensitive to the exact low track (and associated low level flow direction/strength). New guidance runs over the coming days will help to resolve these details. Expect below normal high temperatures over much of the state next week. Coolest readings will probably be mid-late week as western mainland upper troughing transitions to a southern coast/Gulf of Alaska upper low, with a modest rebound during the weekend. Low temperatures will tend to average slightly warmer then normal, though with some areas seeing modestly below normal readings on one or more days--most likely over the southwest and then progressing into the Interior Thursday into the weekend. 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