Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EST Tue Nov 08 2022 Valid 12Z Sat Nov 12 2022 - 12Z Wed Nov 16 2022 ...Heavy precipitation threat along and near the southern coast through the weekend into early next week... ...Overview... Most guidance shows a long-term mean ridge prevailing over Mainland Alaska and northwestern Canada, with shortwaves periodically flowing around or through the ridge. The combination of this ridge and an upper trough/low from the Bering Sea southward at least through the weekend will favor a persistent flow of moisture into portions of the southern coast and favored terrain over the far southern mainland. In varying ways, models/ensembles show the trough splitting after the start of the week--likely favoring a lighter trend for the southern coast precipitation. Meanwhile by next Monday-Wednesday the guidance shows a broad area between the western half of the Pacific and Bering Sea where one or more potentially strong storms could develop. ...Guidance/Predictability Assessment and Preferences... At the start of the period early Saturday most guidance shows elongated low pressure over the Bering Sea, reflecting the system expected to be over the Aleutians late in the short range. The 12Z CMC differs from consensus in depicting its surface low over the Alaska Peninsula. A notable change in the forecast appears by Sunday-Monday when models since the 00Z cycle have been signaling a compact but potentially strong wave tracking northward from the Pacific across the Alaska Peninsula and possibly into the mainland. There is still significant spread for timing and track (though a bit less now that the 12Z/18Z GFS runs are somewhat east of the prior runs that tracked the storm into the Bering Sea) with latest CMC runs generally on the eastern edge of the spread due to being farthest east with the upper trough axis. The 12Z ECMWF is on the fast side with the surface low. Prefer to introduce this system in some form given the guidance signals, with a compromise among the 12Z GFS/UKMET and 00Z ECMWF as the best intermediate cluster. Over the Arctic, latest guidance is showing more potential for some shortwave energy to brush parts of the North Slope during the weekend. Details of this energy and reflection at the surface are still fairly uncertain though. By the early to middle part of next week there is reasonable agreement that the initial trough from the Bering Sea southward will split, with the northern part progressing into/around the downstream ridge and the rest dropping into a mid-latitude upper low. There is typical detail spread for exactly how this occurs and for the day-to-day shape of the mean ridge. This favors greater input of the ensemble means later in the period. Another mean trough should head into the western Bering Sea and western-central Pacific, but so far guidance has exhibited a lot of spread and run-to-run changes for one or more surface lows that could be on the strong side. An average of the 12Z CMCens/00Z ECens means (western Bering) and 12Z GEFS mean (western Aleutians) provided a decent starting point for this system valid early next Wednesday given the spread, with the 12Z GFS being the one solution that fit within this template. The fact that the new 12Z ECMWF mean (arriving after forecast preparation) jumped northward to be closer to the operational run keeps confidence low. Preferences for the first half of the period led to starting the forecast with a blend of the 12Z GFS/UKMET and 12Z/00Z ECMWF runs, with subsequent manual adjustment to bring the Sunday-Monday surface low and associated winds somewhat closer to the individual model forecasts. The rapidly increasing detail spread thereafter led to transitioning the forecast to 75 percent ensemble means by day 8 Wednesday with the GFS providing the remaining input. ...Sensible Weather and Potential Hazards... The broad southerly mean flow during the weekend into the start of next week will favor the potential for significant precipitation along the southern coast of the mainland and some of the favored terrain just inland. Several inches of liquid in the form of higher elevation heavy snow and coastal heavy rain will be possible. Flow ahead of an initial Bering Sea system, as well as a rapidly developing northward-moving Pacific wave that could ultimately track into or near the western mainland by Sunday-Monday, will help to focus this activity. In addition this latter system may produce an area of gale to storm force winds offshore. Some precipitation may extend northward through the mainland and as far east as the Panhandle, with lesser amounts than expected along the southern coast in both cases. Expect the rain/snow to trend lighter by Tuesday-Wednesday. With low confidence in the details, a system reaching the Aleutians/Bering Sea by Tuesday-Wednesday may produce a broad area of precipitation along with some strong winds. The forecast pattern will support a broad area of above to much above normal temperatures across the state through the period. The only exceptions are likely to be some below normal readings over the southern two-thirds of the Panhandle and perhaps in a few locations over the eastern mainland. Rausch Hazards: - Heavy precipitation across portions of mainland Alaska, Fri-Mon, Nov 11-Nov 14. 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