Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 PM EST Mon Dec 10 2018 Valid 12Z Fri Dec 14 2018 - 12Z Tue Dec 18 2018 Guidance agrees well in principle that there should be mean troughing aloft over the Bering Sea and western Mainland Alaska during a majority of the period. Amplified upstream flow will likely bring a ridge into the Bering Sea by the first half of next week, ultimately narrowing the mean trough. Within the mean trough an initial upper low should drift to the north/northwest. Then additional energy should cross the Bering Sea and feed into the mean trough axis persisting over the southwestern mainland. Questions over how this energy interacts with two deep Pacific storms (both accompanied by strong winds) lead to uncertainty in the ultimate track of the systems and effects for locations over the northeastern Pacific/Gulf of Alaska. For the deep storm forecast to track south of the Aleutians around Thu-Fri, guidance continues to show some latitude spread/variability for the low track in this time frame thus tempering confidence in precise effects on the Aleutians. Current majority clustering--including the 06Z GFS/00Z ECMWF and their ensemble means--provides a good intermediate solution relative to recent spread, between farther north earlier ECMWF runs and farther south 12Z/09 GFS, early in the period. As the system heads into the northeastern Pacific by the weekend the guidance tracks have been diverging due in part to differences in how strongly Canadian ridging may extend into the mainland. Latest GFS/ECMWF runs have backed off yesterday's idea of a stronger ridge extending into the mainland (yielding a farther south low track) while the ensemble means have provided a more consistent depiction of flow aloft over recent days. In fact by early day 7 Mon the 06Z GFS/00Z ECMWF actually bring the surface low into the vicinity of the Kenai Peninsula faster than the ensemble means. To tone down continuity adjustments the preference is to lean somewhat more toward the ensemble means at that time frame in contrast to the operational runs early in the period. Influence from upstream Bering Sea/Aleutians flow provides additional uncertainty. Locations from the Kenai Peninsula to the Panhandle have the best potential to see the highest precipitation amounts from this system. Over the past day the majority guidance cluster has made a stronger and northward adjustment with the second Pacific system. The northward trend is not yet enough for the system to have a meaningful influence on the Aleutians but toward the end of the period allows the system to reach the northeastern Pacific south of the Alaska Panhandle by early day 8 Tue with a general north or northwest track after that time. Elsewhere, clustering has improved for the path of the upper low initially over the western mainland as ensemble means have trended toward a little more north-northwest motion while latest ECMWF runs are slower than the 00Z/09 run. This leaves the 00Z CMC as the only solution to keep the feature wobbling over the extreme northern Bering Sea for multiple days. Lately there have been signals that weak energy aloft over the western Bering Sea early in the period may generate a compact late week/weekend surface low but with insufficient agreement among models to depict a feature. Agreement is still not ideal but enough solutions have such a wave to merit at least a weak reflection at this time with the 06Z GFS/00Z ECMWF serving as the most reasonable operational model template for position. Based on guidance comparisons the forecast blend started with a nearly even blend of the 06Z GFS/00Z ECMWF/00Z ECMWF mean for day 4 Thu into day 5 Sat. While not as pronounced as yesterday, the GEFS mean still appears too weak with the initial northern Pacific storm. The GEFS/ECMWF means become more comparable late Sat onward to allow for GEFS mean inclusion in the rest of the forecast. The blend still tilts toward the operational runs day 6 Sat but then rapidly trends toward 70 percent ensemble mean influence for days 7-8 Mon-Tue. The GEFS mean has lower weight than the ECMWF mean due to being slower than other solutions with the western Pacific pattern late in the period. Rausch WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html