Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sun Jul 22 2018 Valid 12Z Thu Jul 26 2018 - 12Z Mon Jul 30 2018 Aside from subtle adjustments most guidance looks remarkably similar to yesterday's consensus. Multi-day means valid near the end of the extended forecast time frame continue to be very agreeable across much of the Northern Hemisphere, suggesting above average confidence in the large scale pattern. The majority cluster shows eastern Asia energy capturing a leading upper low ejecting from southwest of Kamchatka in the short range period. The overall trough should then amplify into the Bering Sea/Aleutians and likely interact with the leading (well-behaved in recent forecasts) upper low just south-southwest of the Alaska Peninsula as of early day 4 Thu. Specifics of this interaction will likely remain an unanswered question for a while. By the weekend there is reasonable agreement that energy aloft over the Bering should form a closed low with a corresponding moderately strong surface low, with this system lingering into early next week. Over the past day the GEFS/ECMWF means have converged quite well after some strength differences yesterday. The past three ECMWF runs have been a little more consistent than the GFS with the strength/track of the system mid-late period but as a whole latest models/means are well within typical guidance error for forecasts 6-8 days out in time. The primary adjustment from yesterday is that the system seems to take a little longer to evolve. Moisture focus will transition from the southern coast ahead of the system just off the Alaska Peninsula at the start of the period toward a persistent flow into the extreme eastern Aleutians and Peninsula late in the forecast in response to the Bering system. As for other isolated solutions that appear to have low probability but require monitoring: The 00Z CMC strays from consensus fairly early in the period by digging upstream energy well westward while keeping the leading upper low stranded over the North Pacific. Note that the CMC mean conforms to the majority scenario. Another potential complication could come from Tropical Storm 14W in the western Pacific. Thus far the 12Z GFS and 06Z parallel (FV3) GFS are the only solutions that are so fast with northward progression that the feature tracks into the southern Bering ahead of or coinciding with the upper trough that amplifies over the Bering. Consensus/JTWC forecasts keep this system sufficiently slow/westward that it does not get incorporated into the Bering Sea evolution. To the north and east there is reasonable continuity with a strong and progressive short range/early extended Arctic storm leaving a trailing front draped over northern Mainland Alaska as of early Thu. Eastward movement of a trailing ridge aloft will help to push this boundary northeastward as a warm front with time. 06Z/12Z GFS runs are still somewhat faster than remaining models with the Arctic storm but at least slower than the 00Z GFS. Consensus persists in merging the high latitude ridge with the initial ridge extending from the Panhandle/western Canada into the southeastern mainland, leading to a broad ridge extending from the northeastern Pacific/western Canada through the eastern/northern mainland late in the period. Today's forecast started a blend of the 00Z ECMWF/06Z GFS/00Z UKMET (in order from higher to lower weight) for day 4 Thu into day 5 Fri. As was the case yesterday clustering has continued to be sufficiently good to allow for 70 percent model input days 6-7 Sat-Sun and 50 percent on day 8 Mon. Ensemble input came from the 06Z GEFS/00Z ECMWF means, while departure of the Arctic system permitted introduction of the 00Z GFS as part of the overall blend mid-late period. 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