Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 756 PM EDT Sun Oct 23 2022 Valid 12Z Thu Oct 27 2022 - 12Z Mon Oct 31 2022 ...Heavy mountain snows and coastal rain threat across the Alaska Panhandle into southern coastal sections late this week... ...Overview... Guidance maintains a slowly evolving pattern over higher latitudes, with an Arctic ridge/closed high to the north of retrograding northern mainland energy that should ultimately settle over or near the Seward Peninsula as an upper low--yielding a Rex Block configuration. Progressive/energetic mean flow should 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 as of the start of the period early Thursday, lingering in this vicinity through Friday and possibly into Saturday, bringing a threat for significant precipitation along the Panhandle and parts of the southeastern coast. Confidence remains well below average for upstream systems and their effects. These features include one that will likely reach the Bering Sea by Friday, another that should track south of the Aleutians during the weekend, and a final one that may reach the western Bering Sea by Sunday-Monday. ...Guidance/Predictability Assessment and Preferences... The most agreeable aspects of the forecast are the initial Gulf of Alaska system late this week and the overall Arctic/northern-central mainland pattern. A consensus looks reasonable for resolving lingering minor strength/position scatter for the Gulf system. The 12Z GFS/UKMET/ECMWF and latest ensemble means cluster quite well for the idea that an upper low should settle over or near the Seward Peninsula by Friday or Saturday onward. The new 18Z GFS shifted its overall pattern a little south of consensus though. Most of the rest of the forecast continues to have well below average confidence. Models and individual ensembles still diverge widely for eventual evolution of the system forecast to reach the Bering Sea on Friday. This spread relates to significant differences for the handling of individual shortwaves and possible interaction within separate streams emerging from eastern Asia and western Pacific. The CMC has been consistently on the very deep and slow side for the surface low from Friday onward, with the UKMET joining it through the end of its run. The 12Z/22 ECMWF had a similar idea but the past two runs as of 12Z today are more progressive and not as deep. The past 12-18 hours of GFS runs have been trending northward from a very suppressed track, closer to the 12Z ECMWF. Enough ensemble members side with the progressive scenario that the respective GEFS/ECens/CMCens means have maintained that idea, with low pressure reaching or consolidating over the Gulf by late Saturday/Sunday. At that time the latest GFS runs lean to the extreme side with their southward track toward or even a bit south of the southern Panhandle. On the other hand the 12Z ECMWF trended northward from its prior run, closer to the means. Ultimately favor a solution that leans increasingly more toward the ensemble means after early Friday, with just enough ECMWF/GFS input to provide a little added detail where possible. This provides the best possible continuity while awaiting any shift in dominant clustering that could provide enough confidence for a significant change in the preferred forecast scenario. GFS/ECMWF runs suggest that upstream Pacific energy could support an elongated surface system that tracks to the south of the Aleutians to some degree during the weekend. Poor agreement among ensemble members leads to a rather weak depiction in the means. Latest ECMWF runs are the farthest north, with remaining guidance keeping the system far enough south to have minimal effect on the Aleutians. Finally, models/means offer a decent signal for another system to reach the western Bering Sea by day 8 Monday. The 12Z CMC is a fast extreme while remaining solutions generally agree better than average for a forecast that far out in time. Guidance preferences led to starting with a 12Z model composite for day 4 Thursday, followed by a quicker than usual transition to incorporate increasing weight of the GEFS/ECens/CMCens means thereafter. By the end of the period the means accounted for 75 percent of the blend with lingering input from the 12Z ECMWF/GFS. ...Weather/Hazards Highlights... Gulf of Alaska low pressure, persisting from Thursday through Friday and possibly into Saturday, will likely support an episode of heavy rain and higher elevation snow across the Panhandle and parts of the southeastern coast during the latter half of the week. Confidence remains much lower than average for the details regarding a trailing system that should reach the Bering Sea by Friday and generally track eastward thereafter. There is still a minority scenario of a strong and slower Bering Sea storm that would produce a significant wind/wave threat over the Bering, as well as meaningful precipitation to the southwest corner of the mainland. The current more likely option would have the system bring a brief period of rain across the Aleutians and then gravitate toward the long-term mean low over the Gulf of Alaska, leading to a rebound of enhanced precipitation over the Panhandle and possibly southeastern coast during the weekend. Locations seeing significant totals from the first event may be sensitive to any additional activity during the weekend. An upstream system has a low probability of brushing the Aleutians with some rain but is more likely to track far enough south to have minimal effect. Another system may reach the western Bering Sea by next Monday. The forecast pattern should help to establish below normal high temperatures over an increasing majority of the state, with the greatest anomalies over southern areas. Locations near the northern coast of the mainland may remain somewhat above normal though. Low temperature anomalies should be more moderate, but with below normal readings also becoming common over the southern half of the mainland. Expect the North Slope to maintain above normal lows while the Panhandle may see a mix of above/below average lows depending on location. 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