Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 PM EDT Sun Jun 17 2018 Valid 12Z Thu Jun 21 2018 - 12Z Mon Jun 25 2018 The most consistent and confident part of the forecast is for western Canada mean ridging that may extend into Mainland Alaska to some degree. Other aspects of the forecast either display some continuity adjustments from yesterday or at least maintain enough spread/detail uncertainty to temper confidence. Within mean troughing aloft initially over the western coast of the mainland and portions of the Bering Sea, today's guidance has gravitated toward emphasizing upper low energy over/near the western mainland more than a separate farther westward feature dropping southward into the Bering. Thus the forecast represents an adjustment to this scenario relative to the intermediate longitude of the overall trough axis (and a little more of a Bering surface reflection) when greater spread existed yesterday. Meanwhile there is still a decent model/ensemble signal for low pressure tracking from the mid-latitude Pacific into the northeastern Pacific/Gulf of Alaska during the period. Individual model runs continue to suggest that low pressure track and intensity will be sensitive to the relative strength/possible interaction among a leading shortwave aloft and trailing energy. GFS runs are strongest with the leading feature aloft, leading to a farther northward track. On the other hand the 00Z CMC ultimately places more emphasis on trailing energy which results in a more southern track. The 12Z GFS has backed away from the northern edge of the envelope a bit but ensemble means do favor a solution in the middle to northern half of the full solution envelope. Overall an average among the GFS/ECMWF and their ensemble means offered a reasonable depiction of the most likely scenario. That said, new 12Z CMC/ECMWF runs maintain lingering potential for a much more southern track so forecast confidence is not particularly high. Perhaps not surprising given recent spread/variability among individual solutions, the Arctic still shows some disagreement on specifics aloft. The ensemble means still agree on development of some high latitude troughing northwest of the mainland though with an axis a bit eastward versus yesterday--especially in the ECMWF mean. A general model/ensemble mean blend provided a reasonable starting point for the forecast. Guidance spread/day to day changes lead to low confidence in details across the western-central Pacific and Aleutians. Over the past day through the 00Z-06Z cycles there has been a trend among the ensemble means and ECMWF toward a more suppressed/disjointed appearance to low pressure (as hinted at by 06Z-12Z/16 GFS runs), with a wave tracking well south of the Aleutians and another area of low pressure hanging back over the western Pacific. However some individual runs like the 00Z CMC and 06Z parallel GFS as well as one or more members from each of the GEFS/ECMWF/CMC ensemble systems still show potential for one or more waves to reach at least as far north as the Aleutians. The 12Z GFS has also brought low pressure farther northward. Today's forecast adjusted toward the majority solution that is more suppressed but partial inclusion of the 12Z GFS at least helped to lower surface pressures a bit near the Aleutians late in the period relative to other guidance input. The model/ensemble mean blend was weighted 80 percent in favor of the operational 06Z-12Z GFS/00Z ECMWF days 4-5 Thu-Fri and then gradually increased the 06Z GEFS/00Z ECMWF means so that the forecast reached about even model/mean weight by day 8 Mon. 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 www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html