Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Thu Apr 26 2018 Valid 12Z Mon Apr 30 2018 - 12Z Fri May 04 2018 Models and means show a transition to a more amplified regime compared to the progressive pattern in place during the short range time frame, as a northeastern Pacific ridge builds over eastern Mainland Alaska and western Canada and an eastern Siberia/Bering Sea trough extends into the northern Pacific. Depending on the exact orientation of the trough, the downstream ridge axis may begin to tilt back into the mainland late in the period. Higher latitude flow appears to remain fairly blocky at least for the first half of the week with an upper low meandering in the vicinity of the Chukchi Sea and modest ridging over the Arctic. As the aforementioned pattern develops, the first half of the period (early Mon into early Wed) generally reflects a refinement of details/strength compared to yesterday's forecast. A blend among the 12z GFS/00z ECMWF and to a lesser degree 00z GFS/00z CMC helps to mitigate what guidance spread still exists for individual features while keeping systems more defined than the means. A progressive Bering system should reach the northwestern mainland as of 12z Mon while modest waviness reaches the Gulf of Alaska. The next in the series of Bering systems is most likely to reach the southwestern mainland/Bristol Bay by 12z Wed. Between early Tue and early Wed the models/ensemble members diverge considerably for latitude of best defined low pressure but are more agreeable for timing, with the favored blend providing an intermediate solution. By the latter half of the week the primary focus will be on another system tracking out of the northwestern Pacific. Over recent days there have been decent signals for its existence but considerable model/ensemble spread for track and timing. Ensemble means and some operational runs have been favoring a track into the Bering in recent days but the last three ECMWF runs through 00z/26 have been closer to the Aleutians with the 00z run actually slightly south of the Aleutians. GFS runs are trending to this southern track as well, and in fact drop south of the ECMWF after next Thu. The 00z ECMWF mean holds onto a southern Bering track but has a notable southeastward trend versus prior runs. Given the spread/trending in guidance thus far, prefer a conservative approach by way of adjusting the forecast more toward the ensemble means (more 00z ECMWF mean relative to 00z NAEFS) by days 7-8 Thu-Fri but still keeping a minority of operational input. This provides some account for recent trends while lowering the risk of having to make a significant reversal. Ahead of this system during Wed-Fri, northern Pacific troughing aloft along 150-160W may interact with energy/moisture from low-mid latitudes of the east-central Pacific. This aspect of the forecast will require monitoring as model/ensemble guidance is suggesting potential for significant precip totals over parts of Alaska's southern coast and panhandle. 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