Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 754 PM EDT Fri Sep 01 2023 Valid 12Z Tue Sep 05 2023 - 12Z Sat Sep 09 2023 ...Overview... Latest forecasts continue to show a moderately progressive pattern containing a couple fairly strong upper troughs with possible embedded lows, and associated surface systems accompanied by areas of enhanced precipitation and winds. The first system should feature primary low pressure reaching the northwestern mainland by late Tuesday/Wednesday and then a triple point wave evolving over the Gulf of Alaska mid-late week. The next system will likely track across the Bering Sea during the latter half of the week. Compared to yesterday, guidance has come closer together for the depiction of this system but timing and latitude uncertainties still exist. Ahead of these features, a modest upper trough crossing the Panhandle around the start of the period on Tuesday will support a weak surface wave south of the Panhandle. The forecast pattern will favor below average high temperatures over most the state during the period. ...Guidance Evaluation/Preferences... Today's forecast represents mainly just refinements to continuity for significant features, as the best clustering of guidance has held onto most ideas of the preferred solution from 24 hours ago. Today's favored evolution was derived by blending the 12Z ECMWF/GFS/CMC/UKMET in order from more to less weight during days 4-5 Tuesday-Wednesday, followed by transitioning ECMWF input from the 12Z to 00Z run over the following couple days, removing the GFS after day 7 Friday, and incorporating 40-55 percent total 12Z GEFS/00Z ECens weight by days 7-8. The leading weak system south of the Panhandle as of early day 4 Tuesday shows very good clustering and continuity. For the system arriving into the mainland from the west by Wednesday, latest consensus is a tad weaker than yesterday but the overall evolution looks quite similar at the surface and aloft. After early Wednesday there are detail uncertainties within the overall upper trough that takes on a negative tilt, with various ideas for where an embedded upper low may be. Given the low predictability of such details several days out in time, will maintain the core of the trough over the mainland as suggested by a blended approach. The 12Z ECMWF and ECens mean (the latter not available yet at time of forecast preparation) and eventually the CMC track an upper low over the Gulf, so that possibility will be monitored in upcoming runs. There is better agreement in principle on the Gulf of Alaska surface wave though. To the north of this system, there is still lingering uncertainty with an Arctic upper low which the 12Z UKMET keeps near the northern coast of the mainland and incorporates into the overall upper trough but most other solutions keep farther away to the northwest. Lack of support for the UKMET scenario with this upper low led to its minimal weight in the forecast blend. Behind this system, ECMWF/ECens mean trends over the past 24-36 hours toward other guidance have improved clustering with respect to a fairly vigorous system expected to track across the Bering Sea during the latter half of the week. The latest GEFS/ECens/CMCens means are very close with their tracks latitude-wise, and the only timing issue among them is with the CMCens straying fast after day 7 Friday. Operational models are generally exhibiting typical spread and variability for time frame involved. The 00Z ECMWF was closer to the means while the 12Z run shifted farther north until wobbling closer to the means by the very end of day 8 Saturday. Meanwhile GFS runs since the 06Z/01 cycle have strayed farther south after early day 7 Friday and the 12Z CMC is somewhat slow for a time. The 06Z and 18Z GFS runs are on the deeper side of the envelope. Overall prefer a track closest to the GEFS/ECens means and operational runs that are closest to those means, with a blend of their ideas yielding a system whose track is close to continuity with slightly greater depth (between the latest models and means). ...Weather/Hazards Highlights... The system anchored by a surface low arriving into the northwestern mainland by early Wednesday should spread a broad area of precipitation across at least the western half of the mainland Tuesday through early Wednesday with moisture continuing eastward thereafter. Some terrain enhancement will be possible over western and southern areas, and the combination of flow aloft ahead of the upper trough axis and a surface wave developing over the Gulf of Alaska after early Wednesday should promote higher totals of rain/high elevation snow along the southeastern coast and Panhandle for a day or so mid-late week. This event still looks to provide meaningful precipitation but with amounts remaining below hazardous criteria. Flow behind this system could produce some brisk to strong winds for a brief time over the eastern Bering Sea and parts of the western mainland. Confidence is somewhat greater today with respect to the system forecast to track across the Bering Sea during the latter half of next week, though with continued uncertainty in specifics. This system should spread an area of precipitation and brisk to strong winds across the Aleutians/Bering Sea during Thursday-Friday, with best rainfall focus likely reaching the Alaska Peninsula and vicinity by late Friday or Saturday. Expect below normal high temperatures over much of the state next week. Coolest readings will probably be mid-late week when the deepest upper troughing is likely to be over the mainland. 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 into the interior Thursday into Saturday. Rausch No significant hazards are expected over Alaska during this forecast period. 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