Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Mon Jul 22 2019 Valid 12Z Fri Jul 26 2019 - 12Z Tue Jul 30 2019 Over and near the mainland the primary forecast uncertainty regards the evolution/path of the upper low (and surrounding flow) forecast to be just west of the mainland at the start of the period Fri. Low confidence details of the southern periphery of an Arctic ridge may have a significant influence on the path of the upper low itself while progressive North Pacific flow will play a role in energy that may elongate to the southeast of the low and cross the Gulf of Alaska. For days 4-5 Fri-Sat the 06-12Z GFS and 00Z ECMWF/CMC cluster fairly well with each other and 06Z GEFS/00Z ECMWF-CMC ensemble means so those operational models provide the foundation for the early portion of the forecast. After Sat guidance steadily diverges for upper low due in part to differences in the degree to which the Arctic ridge extends into the mainland and northwestern Canada. What trends exist seem to be pointing toward slower upper low progression. The 00Z ECMWF mean reflects this trend and has support from the 06Z GEFS/00Z CMC mean, while the 12Z GFS is slower than the 06Z run and the 00Z CMC has the upper low meandering over the far northern Bering for the entire rest of the period. Interestingly operational ECMWF runs have been fairly consistent but with three of the past four runs becoming faster than the means. This leaves the 06Z GEFS/00Z ECMWF means as the best starting point for the late-period forecast given the increasing spread with time, with a model/mean blend providing the transition between the operational model-focused day 5 Sat forecast and the all-means forecast by day 8 Tue. Farther upstream the guidance diverges over the North Pacific/Aleutians/southern Bering late in the period, especially after 12Z Mon. The most likely scenario as reflected by the latest ensemble means and GFS runs would have a warm front approaching if not reaching the western Aleutians. Based on the ensemble mean position of the upper trough by this time there is the possibility that anchoring low pressure could end up being a little farther east than depicted by the means, but not nearly to the extent of the 00Z ECMWF. Thus preferences lean to the means exclusively by day 8 Tue as with the evolution over the mainland/Gulf of Alaska. The overall pattern should support fairly broad coverage of precipitation potential for at least portions of the period. Best signal for highest 5-day accumulation exists over the southern mainland and Panhandle but some northern areas may see meaningful totals as well. Especially after Sat, guidance spread for specifics is sufficiently great to keep confidence low for pinpointing the position and timing of any locally significant rainfall on a particular day. 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 No significant hazards are expected over Alaska during this forecast period. 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