Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 650 PM EST Wed Dec 18 2019 Valid 12Z Sun Dec 22 2019 - 12Z Thu Dec 26 2019 ...Below average temperatures expected across parts of mainland Alaska into next week... ...Two potentially strong storms may track near the Aleutians, bringing precipitation and strong winds... ...Wet pattern along the southeastern coast/Panhandle... Today's models and ensemble means display a number of similar themes but there are ongoing uncertainties for important specifics. One aspect of relative convergence compared to yesterday is with the development and persistence of mean troughing aloft especially over the northern and western mainland--initiated by an upper ridge that migrates from the northern Bering across eastern Siberia from late weekend into early next week. 24 hours ago operational guidance had a better signal for this evolution than the ensemble means but since then the means have trended toward more mainland troughing. Farther south the guidance continues to show a very active North Pacific/Aleutians stream that should include two vigorous wind/precipitation-producing systems. Also this pattern will likely support mean low pressure over the Gulf of Alaska and a persistent focus for precipitation along the southeastern coast/Panhandle. For the system likely to be over the southern Bering at the start of the forecast early Sun, guidance remains divided over the ultimate track of the parent low. Current guidance majority including the 12Z GFS/CMC/UKMET/NAVGEM and latest GEFS/ECMWF means would have the initial surface low linger over the Bering at least into day 5 Mon versus the past couple ECMWF runs that whisk it into the northeastern Pacific. Some GFS runs keep the parent low over the Bering longer than consensus though. Based on history this area is notorious for such sensitivity, favoring a consensus/continuity approach until guidance trends emphatically suggest otherwise. Strength has been a less contentious issue, with the majority of guidance still leaning a little weaker than GFS runs. The trailing system that may affect the Aleutians around Tue-Wed/Christmas Eve-Christmas Day still has a decent signal in the guidance but exhibits considerable spread for strength/track/timing. The most basic question mark comes down to whether and for what duration the upper dynamics will be a mere open wave or close off an embedded upper low. Considering this energy originates from fast/flat flow emerging over the western Pacific just a day or so before development, this seems to be a detail too small in scale to have much predictability several days out in time. Therefore the ultimate preference for the deterministic forecast is a model/ensemble compromise--more GEFS mean due to a deeper low than the ECMWF mean into day 7 and inclusion of enough operational input (but not the very slow 12Z CMC) to enhance the means though not to the extent of relying too much on any one model scenario. By day 8 Thu the GEFS mean may be too slow to transfer energy into the Gulf of Alaska though. Behind this system the models/means depict another general area of low pressure, perhaps consisting of multiple centers, that will more likely track to the south of the Aleutians after midweek. A general guidance blend provides a reasonable starting point given low confidence in specifics that many days out in time. Across the northeastern Pacific/Gulf of Alaska, solutions agree on one system just off the southeastern coast/Panhandle early Sun. Then a wave along the first Bering system's front should reach the Gulf early next week. Model/ensemble consensus recommends a low track somewhat north of the past couple ECMWF runs, though at least the 12Z ECMWF did trend favorably from the 00Z version. Whether from persistence or re-development (including some support from energy within the mainland upper trough) the guidance suggests northern Gulf low pressure extending into Wed/Christmas Day. Then energy associated with the second Aleutians storm should arrive by day 8 Thu with another well-defined surface low. Perhaps reflecting slightly greater predictability around this time, the past two ECMWF runs trend closer together over the Gulf after showing very different low tracks/depth upstream. As noted earlier the guidance shows much better overall agreement today for the upper trough over the mainland. It remains to be seen whether some of the embedded energy will extend as far westward into the Bering as some GFS runs and to some extent the 12Z GEFS mean suggest--or whether energy in the southern mainland will be as deep as in ECMWF runs. Given modest spread in location, a consensus approach still looks good for positioning the upper low over/near the Canadian Archipelago. Flow around this circulation should support a couple cold fronts that will brush the extreme northern mainland. Expect the pattern evolution to yield a westward shift in the axis of below normal temperatures compared to the start of the period, while anomalies may become a little less extreme with time. Based on the composite of forecast preferences, today's blend started with components of the 12Z GFS/ECMWF/CMC early in the period. This yielded an evolution close to the ensemble means/continuity in principle while maintaining better detail/strength. The forecast then adjusted to a 12Z GFS/GEFS mean and 12Z-00Z ECMWF compromise for days 6-7 followed by introduction of the 00Z ECMWF mean from late day 7 onward. Total ensemble mean weight reached 50 percent by day 8. 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: - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon, Dec 23. - High winds across portions of the Aleutians, Sun-Mon, Dec 22-Dec 23. - Much below normal temperatures across portions of mainland Alaska, Sat-Mon, Dec 21-Dec 23. - High significant wave heights for coastal portions of mainland Alaska and the Aleutians, Mon-Tue, Dec 23-Dec 24. 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