Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EDT Mon Sep 09 2019 Valid 12Z Fri Sep 13 2019 - 12Z Tue Sep 17 2019 The very start of the forecast period on Fri will feature a strong but weakening Bering Sea system nearing the western mainland and a leading frontal wave emerging over the Gulf of Alaska. Behind this overall system the focus turns to what has been the most significant forecast problem in recent days, a shortwave forecast to reach the western Pacific around Thu. Models and ensembles have depicted a variety of possible evolutions at the surface and aloft with effects extending downstream through the Bering Sea/Aleutians and the mainland/Gulf of Alaska. At the moment an overwhelming majority of guidance (00Z ECMWF/UKMET/CMC/NAVGEM and the ensemble means) agrees that the upper trough and likely embedded closed low will be sufficiently progressive to bring a fairly strong storm system northeastward through the Aleutians/Bering Sea and likely into the mainland. The CMC differs in stalling it near the western coast. In contrast recent GFS runs keep the western Pacific upper low more separated from the westerlies, ultimately leading to a completely different pattern downstream. The new 12Z GFS has trended somewhat faster but is still significantly slower than remaining solutions. The behavior of guidance leading up to this point suggests additional changes are possible, but for preparing a specific deterministic forecast it appears straightforward to start with an operational blend of the ECMWF cluster early in the period and then gradually incorporate the 06Z GEFS/00Z ECMWF means into the forecast. While they offer differing details a minority component of the ECMWF/CMC still provide a little enhancement to the means through the end of the period. Remaining 12Z guidance provides ongoing support for the favored majority cluster. Over the Arctic the primary forecast consideration is to tone down the combined depth/southward extent of the 00Z ECMWF upper low that approaches the northern mainland by next Tue. Previous runs did not depict this upper low. However the GEFS/ECMWF means show a steady lowering of heights aloft through the period so there seems to be potential for a feature to come into the picture over the Arctic by late in the period. The preferred blend yields a weak upper low to depict this possibility, with depth less extreme than the 00Z ECMWF/CMC. The leading system will produce late-week enhanced precipitation along the southern coast and Panhandle. The next storm, expected to track from the western Pacific through the Bering Sea and into the mainland, would bring a broad field of brisk winds along with potential for enhanced precipitation most likely to extend from the Aleutians into the southwestern mainland, southern coast, and eventually the 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 - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Sep 12-Sep 13 and Sun-Mon, Sep 15-Sep 16. 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