Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 636 PM EST Sun Nov 24 2019 Valid 12Z Thu Nov 28 2019 - 12Z Mon Dec 02 2019 Viewed through the perspective of the ensemble means, the large scale pattern evolution during the Thu-Mon period has fairly good agreement and continuity. Shortwave energy initially over the eastern Aleutians/Bering Sea into the North Pacific should progress eastward and weaken while a mid-latitude system to the south remains separated from most of this energy and eventually drops southeastward. Toward the end of the week and weekend the pattern will become more dominated by a deep upper low moving into or just inland from the extreme western/northern Bering. The overall trough containing the upper low should extend south into the mid-latitude Pacific, with the trough axis most likely reaching the eastern Bering around next Mon. Expect a downstream ridge aloft to cross the mainland around Fri-Sat and continue into western Canada. Not surprisingly operational model runs and individual ensemble members display enough spread and run-to-run variability to temper confidence in some important details. During Thu-Fri the most significant model discrepancy involves the handling of shortwave energy over and just south of the eastern Bering/mainland, possible interaction with the system well to the south, and a surface wave/boundary that may promote a concentrated focus for heavy precipitation along a portion of the southern coast. The majority cluster has adjusted a little west from yesterday for the surface feature but there is still good agreement that it should remain somewhat east of recent GFS runs--with a little less stream interaction aloft. Thus confidence is fairly low in the heavy GFS precip depicted over the Alaska Peninsula and southwestern mainland early in the period. The better signal for highest totals is over and/or just east of the Kenai Peninsula. Meanwhile the forecast is fairly well on track for the storm over or near the extreme western Bering Sea. There is still potential for one frontal wave to wrap into the main area of low pressure during the first half of the period, with enough uncertainty not to depict explicitly in the manual forecast. A consensus adjustment from yesterday is for faster progression of the associated front across the Bering after early Fri with introduction of a southern coast wave by Sun after the occluded front dissipates. Then there is wide model/ensemble spread (similar to yesterday) for another wave that may track northeast from the central Pacific Sat onward, with various depth and potential tracks ranging from the southeastern Bering/western mainland (GFS) to the Gulf of Alaska (ECMWF). Continuity and increasing emphasis on the ensemble means by that time lead to the forecast reflecting the northwestern part of the spread for the wave of interest, but again with low confidence. A broad area of precipitation will accompany the front and wave(s) with precise coverage and intensity determined by wave specifics. During the first half of the period the forecast started with components of the 00Z/12Z ECMWF and 12Z GFS/CMC with 30 percent total input from the 12Z GEFS/00Z ECMWF means. The blend provided better than average continuity for this sensitive evolution aloft and reflected the modest consensus adjustment for the wave near the southern coast. In light of increasingly divergent model details, the latter half of the forecast increased ensemble weight, reaching 70 percent by day 8 Mon. The 12Z ECMWF mean reinforces the ensemble consensus that suggests the 00Z ECMWF may have become too fast with the trough crossing the Bering while the 12Z run may be too slow--but at the same time fast to bring strong energy into the Gulf of Alaska by day 8 Mon. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html