Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Sun Apr 08 2018 Valid 12Z Thu Apr 12 2018 - 12Z Mon Apr 16 2018 Mainland/Arctic flow aloft has been exhibiting a fairly high degree of spread and run to run variability in the guidance over recent days. This behavior keeps confidence in the lower half of the spectrum as notable trends have developed over the past day. Across the North Pacific/Bering Sea today's guidance continues to show general agreement on overall areas of low pressure but some detail disagreement/adjustments from continuity. Meaningful timing differences become apparent across the western/northern Pacific into the Bering during the weekend and early next week. Mainland details aloft have ranged over a wide spectrum among ridges/troughs/compact closed lows with the full guidance average tending to show the area under a col region between ridges to the northwest/southeast, and cyclonic flow over/north of the Canadian archipelago and North Pacific systems. Latest operational model runs show a more pronounced tendency toward northern/eastern mainland and Yukon troughing aloft while the ensemble means are more reserved about trough amplitude. At the very least this recommends leaning toward a guidance average more in line with the 00z ECMWF versus more amplified GFS runs--including the 12z version whose trough amplitude is sufficient to affect the surface pattern over/near the Gulf of Alaska. At the leading edge of this trough expect a cold front to push southward over the course of the period. An area of low pressure tracking well south of the Alaska Peninsula on Thu should reach near 140W longitude by early Sat (farther west in the 00z CMC/CMC mean). This overall area of low pressure may contain multiple low centers with detail specifics being sufficiently small in scale to have low predictability until closer to the short range time frame. This would favor a model/ensemble composite to represent the most agreeable themes of guidance. By Sat there is a general trend toward low pressure reaching somewhat closer to the southern Alaska Panhandle than in previous forecasts before the previously advertised southeastward motion or weakening in place occurs. A more amplified trough aloft at higher latitudes could pull low pressure even farther northward but at this time it is a minority scenario. Continuity is fairly good in principle for low pressure tracking into the western Aleutians/Bering during next weekend. Operational model runs continue to show differences in frontal wave development but compared to yesterday there is a noticeable consolidation in models and ensemble members toward primary emphasis on low pressure over the western Bering by late Sat-Sun, whether from the parent low or frontal wave(s) tracking back into its position. Thus far the ensemble means have provided the best signal of this scenario. As was the case yesterday, timing differences arise with upstream flow by late weekend/early next week, with influence of these differences extending downstream to the weekend Bering system. GFS/GEFS runs have tended to be on the faster side of the envelope so far. ECMWF/ECMWF mean runs have been waffling with the 00z model/ensemble run slower than in most previous cycles. The 00z CMC and CMC mean offer varying degrees of compromise. This spread and ECMWF inconsistency favor an intermediate timing for the manual forecast. Adding up the considerations for areas/features of interest, the forecast starts with primary emphasis on the 00z ECMWF/00z-06z GFS early in the period with increasing 06z GEFS/00z ECMWF ensemble mean input leading to 60 percent ensemble weight by days 7-8 Sun-Mon. Rausch