Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 325 PM EDT Mon Jul 23 2018 Valid 12Z Fri Jul 27 2018 - 12Z Tue Jul 31 2018 Latest guidance continues to show decent agreement in the overall pattern evolution that highlights amplified/large scale features. An upper low/trough near and south-southwest of the Alaska Peninsula as of early day 4 Fri will quickly open up while dynamics emerging from eastern Asia evolve into a Bering Sea closed low aloft with corresponding surface development. At the same time a mean ridge aloft will extend from western Canada through the eastern/northern mainland at the start of the period. The majority of guidance suggests the northern half of the ridge should be fairly persistent through the period while the southern part may ultimately retrograde into the northeastern Pacific. The best solution cluster provides reasonable continuity for the Bering system but some details remain in question, thus tempering confidence in the ultimate evolution. There are at least a couple bundles of energy aloft that will support surface development and there is a minority scenario (as offered by recent UKMET runs and 06Z GFS) that would have the trailing energy amplify enough to pull surface low pressure as far south as the Aleutians for a time. This solution is in the southern part of the full ensemble spread. On the other side of the spectrum there are some models/members that track into the extreme northern Bering Sea. So far the ensemble means and ECMWF have been steadiest with this system in principle, though the latest cycle deepens the system versus continuity--probably a reasonable trend. Another aspect of this system being monitored is what role Tropical Storm 14W (Wukong) may play. There is still a minority signal (such as from the 06Z parallel FV3 GFS) that the system could travel fast enough for it to be the primary area of low pressure as it interacts with northern stream energy. At the very least associated moisture may stream across the northern Pacific and eventually reach the Alaska Peninsula and vicinity. The overall pattern evolution will favor a period of moisture focus over/near the Peninsula with duration and extent depending on specifics that have lower predictability at extended time ranges. To the east of this evolution, some operational runs are now showing a stronger hint that some of the initial upper low/trough energy near or south/southwest of the Peninsula may try to push into the downstream ridge. Latest ensemble means have trended a bit weaker with the ridge, reflecting some members that have similar ideas. At this point prefer a blended/ensemble mean approach that maintains ridge strength to a reasonable degree. The one item of note at higher latitudes is that by days 7-8 Mon-Tue the 00Z ECMWF becomes an extreme to outlier solution relative to the ensemble spread as it develops cyclonic flow over the Arctic/northern mainland versus the consensus ridge. Above considerations led to starting the early part of the period with a blend of half 00Z ECMWF and the rest 00Z CMC and 00Z/12Z GFS. Gradual divergence of model details favored a steady increase of 06Z GEFS/00Z ECMWF ensemble weight with time, reaching 80 percent total by late in the forecast. The 00Z ECMWF provided a reasonable blending companion over the Bering at that point but its questionable solution over the Arctic precluded more than just a modest inclusion. Rausch WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html