Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 406 PM EDT Fri Oct 11 2019 Valid 12Z Tue Oct 15 2019 - 12Z Sat Oct 19 2019 The majority of guidance shows similar ideas for the large scale flow evolution over the course of the Tue-Sat period but with a number of meaningful question marks for embedded features. Consensus shows an axis of low heights (containing one or more closed lows) from the Bering Sea to the northeast Pacific/Gulf of Alaska into late week while an upper ridge initially near the northern coast of the mainland lifts into the Arctic. Flow around the southern side of the ridge should bring one or more bundles of energy into/across the mainland. There is reasonable agreement on amplification of western-central Pacific troughing aloft Thu-Sat with the combination of the trough and persistent Bering upper low energy providing support for low pressure development Right from the start models/ensembles have been diverse and inconsistent for the path of extratropical Hagibis. At the time of forecast preparation (arrival of the 12Z GFS) there was pronounced spread in forecasts valid early Tue, highlighted by 00Z ECMWF ensemble members ranging from the northern Bering Sea to the North Pacific. Ensemble means had adjusted somewhat toward the slow Bering Sea scenario but the 00Z ECMWF/CMC and to some extent 00Z GFS represented the faster southern cluster that looked more probable before guidance became more mixed over the past couple cycles. Thus the initial forecast was based on the 00Z ECMWF/CMC/GFS early in the period. However the new 12Z ECMWF added to the northern Bering cluster, leaving the CMC as the only southern solution. So now there is increased confidence in the 12Z GFS/ECMWF/UKMET and ensemble means. After Tue gradually increasing weight of the 06Z GEFS/00Z ECMWF means looked reasonable to balance the agreeable aspects of the large scale pattern with less confident details. Some combination of extratropical Hagibis and fast-moving upstream energy should reflect as low pressure over the Bering on day 5 Wed with energy feeding into low pressure that lingers over or near the Gulf of Alaska. Then by Thu-Sat individual models and ensemble members differ considerably for low pressure evolution in association with the amplifying Pacific upper trough. The means do show better than average clustering for the surface low, reaching around Bristol Bay next Sat--which favors tilting the forecast more toward the means at that time. The upper ridge forecast to be near the northern coast of the mainland as of early Tue has trended a little slower on its way into the Arctic. Guidance seems to diverge for the ridge's path more than was the case 24 hours ago. To the south of the ridge there is also considerable uncertainty for the details of retrograding shortwave energy across the mainland. These issues also recommend an initial model blend that transitions more toward the ensemble means with time. Extratropical Hagibis and the generally active nature of North Pacific flow should produce one or more episodes of strong winds/high waves, while precipitation of varying intensity will likely extend across the Aleutians/Alaska Peninsula and southern coast/Panhandle. 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 Alaska Hazards... - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Oct 15-Oct 16. - High winds across portions of the Aleutians, Mon-Thu, Oct 14-Oct 17. - High significant wave heights for coastal portions of the Alaska Panhandle, Wed-Thu, Oct 16-Oct 17. - High significant wave heights for coastal portions of the Aleutians, Mon-Thu, Oct 14-Oct 17. 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