Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Tue Jul 30 2019 Valid 12Z Sat Aug 03 2019 - 12Z Wed Aug 07 2019 There is a loose consensus that ridging will prevail at the surface and aloft to the south of the mainland while some degree of upper troughing persists over parts of the Bering Sea and eastern Asia. This pattern would focus highest precipitation totals for the five-day period from the eastern Aleutians through the northern two-thirds or more of the mainland. Beyond this expected mean pattern there are multiple embedded uncertainties that limit confidence in specifics. The strong system expected to cross the northern mainland is one such forecast problem. There is good agreement for its existence and potential to bring an episode of strong winds/areas of heavy precipitation, but agreement and continuity for timing have been poor thus far. As of the 00-06Z cycles, the 00-06Z GFS/00Z ECMWF and their ensemble means (plus the CMC mean) provided the majority cluster--significantly faster than yesterday but not as fast as the 00Z UKMET or even faster CMC. New 12Z guidance including the ECMWF has come in with stronger clustering in favor of faster timing than the aforementioned model/ensemble grouping but slower than the 00Z CMC. Today's forecast started with the 00-06Z model/ensemble cluster given the already faster adjustment from continuity and the GFS being the only 12Z run available at the start of forecast preparation. Given the adjustments in the full array of 12Z guidance, an average of the new runs would provide a more likely--though still not truly confident--solution. Behind this system there are hints of waviness over the Bering in response to shortwave energy within the upstream mean trough aloft. The 06Z GFS/00Z ECMWF offer a reasonable starting point to reflect this waviness relative to the weaker ensemble means. The 00Z ECMWF mean has the best detail among the means. Specifics become even more uncertain with upstream energy possibly closing off an upper low that reaches the Bering, with considerable spread and run-to-run variability. In addition there is also the possibility for higher latitude energy to reach the mainland from Siberia and the Arctic. These issues favor a steady increase of ensemble weight with time. Based on the guidance available through arrival of the 12Z GFS, the forecast started with a 06Z GFS/00Z ECMWF blend for day 3 Sat. After that time incorporation of ensembles (06Z GEFS/00Z ECMWF means) led to an even model/mean blend by day 6 Mon and 100 percent ensemble means by day 8 Wed. 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 - Heavy rain across portions of mainland Alaska, Fri, Aug 2. - Heavy rain across portions of mainland Alaska, Sat, Aug 3. - High winds across portions of mainland Alaska, Sat, Aug 3. 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