Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 629 PM EST Tue Dec 17 2019 Valid 12Z Sat Dec 21 2019 - 12Z Wed Dec 25 2019 ...Below average temperatures expected across the Alaska Interior through at least the weekend... ...Two potentially strong storms may track near the Aleutians, bringing precipitation and strong winds... Guidance depicts an active North Pacific and Aleutians/southern Bering pattern with two systems that operational models and to a lesser extent ensembles suggest could be quite strong. The first should reach the Aleutians and vicinity during the weekend followed by another around the middle of next week/Christmas Day. While the signal for each system is fairly good, spread and run-to-run variability with track/timing persist so confidence is somewhat lower for precise impacts in terms of precipitation and winds at any specific location. At higher latitudes the latest operational runs in particular show a tendency for a modest upper ridge to build into eastern Siberia by the start of the next week, resulting in troughing and possible persistence of below normal temperatures over parts of the mainland. The 12Z GEFS mean hints at this idea but overall the ensembles are more ill-defined with the pattern at these latitudes. Even farther north/northeast there is initially good consensus regarding an upper low over/near the Canadian Archipelago with some moderate divergence in location/orientation by mid-late period. During the early-middle part of the forecast preferences leaned to a blend of the 12Z ECMWF/GFS with somewhat more weight on the ECMWF. This solution represented consensus for the first Aleutians/southern Bering storm with many operational solutions depicting a mid-upper 960's mb low as of 12Z Sun. Consistent with some other storms over recent months the GFS was deepest. It could have partial merit but may be overdone given overwhelming clustering among the other models. Track has also been an issue with the 12Z GFS/ECMWF southern Bering path providing a good compromise between some earlier southward ECMWF runs (as well as WPC continuity to some degree) and more northward 12Z GEFS mean/CMC. One piece of the uncertainty puzzle is some fast-moving upstream energy that comes in underneath the system. Guidance spread for the second Aleutians storm required a transition in operational runs, as both the 12Z GFS and ECMWF strayed faster than their previous runs or the 12Z GEFS/00Z ECMWF means and 12Z CMC. This favored adjusting the operational part of the blend toward the 06Z GFS and 00Z ECMWF later in the period. The new 12Z ECMWF mean has trended faster though. The desire to maintain a reasonable amount of detail given operational model signals led to ensemble mean guidance reaching only 30-40 percent total weight by mid-late period. Elsewhere, multiple low pressure centers may exist over the Gulf of Alaska during the period--first supported by leading shortwave energy and then dynamics ahead of the first Aleutians/southern Bering storm generating a frontal wave. Depending on how upper troughing may evolve over the mainland (with a model/mean blend helping to navigate the detail uncertainty), some of the embedded energy could influence Gulf low pressure as well. Evolution over the Gulf should favor a precipitation focus over the southeastern coast and Panhandle. Meanwhile the cyclonic flow aloft over the Arctic may bring a cold front to the northern coast of the mainland during the first half of next week. The 12Z ECMWF ultimately wobbled toward the western side of the spread with the Arctic upper low next week, providing another reason to lean away from that solution late in the period. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - High winds across portions of the Aleutians, Sun-Mon, Dec 22-Dec 23. - Much below normal temperatures across portions of mainland Alaska, Fri-Mon, Dec 20-Dec 23. 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