Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 751 PM EDT Sun May 02 2021 Valid 12Z Thu May 06 2021 - 12Z Mon May 10 2021 ...Overview and Guidance Evaluation... Aside from the occasional stray model run, the majority of guidance is consistent with the large scale pattern forecast. An upper low reaching the Aleutians/southern Bering Sea after midweek will likely persist into next Sunday-Monday though it could weaken somewhat by that time. Downstream over the mainland expect mean flow aloft to become more southerly with the possibility of ridging to develop at times. This flow may carry along a couple shortwaves into the mainland from over/south of the Gulf. Between Thursday and Saturday expect a front (with one or more embedded waves possible) anchored by initial Aleutians low pressure to progress eastward toward the northeastern Pacific. Upstream energy may support one or more additional systems that interact with or merge into the mean low over the Bering Sea/Aleutians. Meanwhile high pressure over the Arctic will drift a bit northward with time and an axis of general surface troughing/low pressure will likely prevail over parts of the southern/central mainland after Thursday. The early portion of today's forecast blend incorporated the 12Z ECMWF, 06Z GFS, and 12Z CMC/UKMET in order of most to least weight. Typical divergence of details later in the period led to increasing ensemble input (more 00Z ECMWF mean versus 12Z GEFS due to the former being deeper with southern Bering low pressure) to 40-55 percent by days 7-8 Sunday-Monday while remaining operational runs included the 12Z/old 00Z ECMWF along with the 06Z GFS. The 12Z GFS departed from consensus in multiple ways: straying weaker/northeast with the upper ridge initially just northwest of the mainland, becoming slow/southward with shortwave energy likely reaching the Alaska Peninsula and Kodiak island by around early Saturday (with corresponding issues at the surface), and wobbling south of other guidance for the Aleutians upper low during the latter half of the period. Some 06Z GFS details over the northeastern Pacific were not ideal early and late but the weight in the blend was low enough to downplay those. Meanwhile the past couple CMC runs have been most aggressive in bringing a strong western Pacific system toward the Aleutians by late in the period, so the forecast excluded that model after Saturday. Even among other guidance there is considerable spread and run-to-run variability for the details of a wave that could emerge from the western Pacific. 06Z GFS/12Z ECMWF components of the blend provided a suppressed wave that serves as a starting point while awaiting better agreement. Farther east, details continue to differ for specifics of upper low/shortwave energy initially south of the mainland and expected to lift northward while latest models appear to have diverged somewhat for details of frontal waviness that has been forecast to reach near the Alaska Peninsula/Kodiak Island by around Friday-Saturday. The model blend accounts for these uncertainties. ...Sensible Weather... The best focus for meaningful precipitation during the period should extend from the Alaska Peninsula to the Panhandle. Moisture along a possibly wavy front will likely progress eastward across at least the Peninsula late this week and should extend farther eastward to some degree. With lower confidence, another area of moisture may reach the Alaska Peninsula and nearby areas during the weekend and early next week. Through the period there is still a fair degree of uncertainty over exact totals. Shortwaves lifting northward may produce episodes of precipitation from the southern coast and Panhandle into portions of the mainland. Expect the Panhandle and portions of the southern mainland, as well as the north-central mainland, to see below normal highs for most of the period. Above normal highs should prevail between these two areas. Most of the state is likely to see above average readings for morning lows with mostly just scattered pockets of modestly negative anomalies. In addition lows may trend a bit warmer with time given increasing cloudiness promoted by the large scale pattern that becomes established. 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