Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon Oct 24 2022 Valid 12Z Fri Oct 28 2022 - 12Z Tue Nov 01 2022 ...Heavy mountain snows and coastal rain threat across the Alaska Panhandle into southern coastal sections late this week... ...Overview... Guidance continues to show a general Rex Block configuration over the Arctic and Mainland Alaska from late this week through at least the weekend. There is somewhat more uncertainty today regarding details of the Arctic ridge and mainland upper low (most likely near the Seward Peninsula) and persistence of this pattern beyond the weekend. Progressive/energetic mean flow will prevail across the North Pacific and Bering Sea, with a tendency for mean troughing over the Gulf of Alaska. This pattern will initially support Gulf low pressure Friday-Saturday with the threat for significant precipitation along the Panhandle and parts of the southeastern coast tapering off as the low weakens. Confidence remains well below average for upstream system(s) tracking over and to the east of the Bering Sea/Aleutians Friday onward. Models and means show a somewhat slower and larger scale mean trough aloft taking shape from the western Bering Sea southward early next week, possibly bringing a system into the central Aleutians by next Tuesday. ...Guidance/Predictability Assessment and Preferences... A consensus approach looks good for the Friday-Saturday Gulf low, with a 12Z model blend resolving lingering detail differences (such as the GFS extending lowest pressures farther east at some valid times). Farther north through the period, after about early Saturday the operational models seem to display more spread and run-to-run variability than yesterday regarding the location of the Arctic upper ridge and some of the upper low details over or near the mainland. Latest GFS runs have been particularly erratic, while the ECMWF has been better with the overall upper low in spite of some variability for the ridge. Thus far the ensemble means have been more agreeable and consistent through the period, favoring a tilt to the means once differences develop. Models and individual ensemble members continue to be quite varied for shortwaves (and their interaction) within separate streams of flow emerging from eastern Asia and the western Pacific as of the start of the forecast period. This low predictability pattern leads to low confidence in determining a most-likely specific scenario among the various depictions offered for one or more surface lows tracking from the Bering Sea/Aleutians into the northeastern Pacific. A slower/northern Bering low per the 12Z ECMWF/CMC is still somewhat in the minority but within the full ensemble spread, and the 12Z CMC mean (plus 12Z ECMWF mean arriving after forecast preparation) have nudged a bit slower than prior ensemble mean clustering. Still, the ensemble means are consistent in having low pressure settle into the Gulf from Sunday onward with a gradual weakening trend into Tuesday. In principle the 12Z GFS (and somewhat north/northwest 18Z run) are the closest to the general ensemble mean evolution. The GFS strength as well as the 12Z run's eastward track near the Panhandle by Sunday remain question marks though. The high uncertainty in the forecast favored holding onto the ensemble means and partial GFS input as the best way to maintain continuity until there is better confidence in a significant change. Farther upstream, a more suppressed trend in the 12Z ECMWF increases confidence that trailing frontal waviness should remain well south of the Aleutians during the weekend. The 12Z CMC is on its own with having such a wave eventually track into the Gulf. Then there is decent continuity and agreement with the general idea of western Bering/Pacific upper trough amplification by Monday-Tuesday. However model solutions vary with important details such as whether there is an open trough versus an embedded low and the potential interaction with a mid-latitude Pacific surface wave. At the moment the ensemble means are remarkably close in having a surface low near the central Aleutians as of early next Tuesday. Agreement and continuity overall favored using the 12Z GEFS/CMCens and 00Z ECMWF means as the primary basis of most of the forecast, with 60-75 percent total weight of these means used for days 5-8 Saturday-Tuesday. Day 4 Friday was able to employ 70 percent operational guidance to provide optimal detail for the initial Gulf low. Operational input was more GFS for days 5-6 due to its closer comparison to continuity and the means, and then more ECMWF days 7-8 as its Arctic/mainland pattern compared better. ...Weather/Hazards Highlights... Gulf of Alaska low pressure persisting into Friday-Saturday should gradually weaken, supporting a lighter trend for initial heavy rain and higher elevation snow across the Panhandle and perhaps into the far southeastern coast. Confidence is still below average for the details regarding a trailing system (or two separate waves) that should reach the Bering Sea/Aleutians by Friday and generally track eastward thereafter. This system should initially bring some precipitation across the Bering Sea/Aleutians and may ultimately enhance precipitation over the Panhandle/southeastern coast during the weekend. A lower probability, but still possible, scenario would have a northern wave bring moderate amounts to parts of the southwestern mainland and then less moisture to the Gulf region. Locations seeing significant totals from the first event may be sensitive to any additional activity during the weekend. Expect western/central parts of the Aleutians and Bering Sea to see increasing precipitation with the system forecast to reach near the central Aleutians by early next Tuesday. Below normal high temperatures should persist over a majority of the state including the Panhandle through the period, with the greatest anomalies over southern areas. Locations near the northern coast of the mainland may remain somewhat above normal. Low temperature anomalies will likely be more moderate, but still below average over most areas aside from the North Slope. The Panhandle may see a mix of above/below average lows depending on location. Rausch Hazards: - High winds across portions of mainland Alaska and the Aleutians, Thu, Oct 27. 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