Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 325 PM EDT Fri May 11 2018 Valid 12Z Tue May 15 2018 - 12Z Sat May 19 2018 At least with respect to the ensemble means there is decent agreement/continuity in the large scale pattern forecast. Some degree of mean troughing aloft should persist over the eastern Bering Sea with a possible connection to a closed Pacific system. Downstream ridging will likely build into the Gulf of Alaska and mainland by midweek, then pivot into western Canada while the Mainland Alaska part of the ridge weakens a bit. Meanwhile a western Pacific system should remain well south of the Aleutians through the period and possibly merge in some fashion with the leading upper trough/closed low. Flow over the Arctic becomes less defined with time in the means, after the pattern starts with a very high latitude ridge and modest weakness just north of the mainland. Looking at the broader array of model/ensemble solutions, the greatest challenge is for the details aloft within the mean troughing aloft over the eastern Bering/possibly western Mainland Alaska. GFS runs over the past day or so have been all over the place with respect to where the core of deepest energy aloft will be, with some runs keeping it north of 60N latitude. In contrast the ECMWF/CMC still show a significant bundle of energy dropping through the eastern Aleutians into the extreme northern Pacific but on average with a weaker Bering surface reflection early in the period and less of a tendency to pull North Pacific low pressure northward after midweek (versus yesterday's runs). An early look at the new 12z ECMWF does show a separate surface low tracking into the southeastern Bering though. The ensemble means also show the southward trend for the original Pacific system but to a lesser extent since they did not bring the system as far north as yesterday's 00z ECMWF/CMC runs. The end result of this trend is a potential reduction versus yesterday's forecast in combined duration/strength/extent of moist flow that may affect locations from the Alaska Peninsula to near the Kenai Peninsula for a portion of the period. However there is still a decent consensus toward more moisture reaching these areas than in some GFS runs. With respect to the downstream ridge aloft, the 06z/12z GFS runs in particular flatten the mainland part of the feature more than the ensemble means and ECMWF/CMC. Elsewhere, specifics of Arctic flow become increasingly ambiguous with time, as operational models expand on possible details relative to the ensemble means. Fortunately there is better agreement on a ridge of surface high pressure to the north and northeast of the mainland. Clustering is decent for the time frame involved with respect to the western Pacific system that approaches the leading North Pacific/eastern Aleutians-Alaska Panhandle evolution. Later in the period recent GFS runs do lean on the slow side of the full envelope though. Today's forecast started with a blend emphasizing operational guidance early-mid period (80 percent total 00z ECMWF/06z GFS and 00z CMC/UKMET Tue-Wed, low enough GFS weight to downplay its lower confidence details over some areas) with gradually increasing 06z GEFS/00z ECMWF mean input through Fri and 70 percent ensemble weight by Sat. The blend yielded a weaker version of continuity over the eastern Bering/Aleutians aloft Tue-Thu and reflected consensus trend to keep North Pacific low pressure farther south. Elsewhere the blend provided reasonable continuity. 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 Rausch