Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Mon Aug 16 2021 Valid 12Z Fri Aug 20 2021 - 12Z Tue Aug 24 2021 ...Overview... Continue to expect an axis of multi-stream mean troughing aloft to extend from the Arctic through the Bering Sea/western Mainland Alaska into the northern Pacific, leading to a cool and unsettled period over a majority of the state. Recent guidance had been quite varied among each other and in consecutive runs for important embedded details but improved clustering in the latest cycle provides some hope for improved stability going forward. Once an upper trough and surface system drop southeastward after early Friday, the main features of interest will be a North Pacific system expected to track into the southeastern Bering and then the mainland along with an Arctic system that will bring a trailing front into the mainland during the weekend with portions of the front continuing southward thereafter. ...Guidance/Predictability Evaluation... The multiple streams in play and uncertainties over their interaction still temper predictability somewhat. However confidence has improved compared to previous days as latest models and means have strongly gravitated to a more common solution. With the slower CMC from recent days appearing to be the guru model at the moment, consensus now shows a fairly slow-moving closed upper low and associated surface system tracking from the North Pacific into the southeastern Bering Sea Friday-Sunday. This supports enough of a downstream shortwave ridge to allow the feature initially over the mainland on Friday to drop more southeastward as it tracks into western Canada. It still remains to be seen how upstream Siberia/Bering energy and then perhaps trailing flow may interact with the Pacific/Bering system as it most likely continues into the mainland by early next week. Meanwhile trends are to the fast side of the prior spread for the compact upper low/surface system tracking across the Arctic and pushing a front southeastward into the mainland. The improved agreement among the operational models allowed for a 12Z model blend to represent the latest consensus from Friday into early Sunday. Details still go astray to some degree by the latter half of the period as Arctic and eastern Asia/Bering energy feed into the overall mean trough aloft, which favored a transition toward a mix of models (12Z GFS/CMC and 12Z-00Z ECMWF runs) and means (12Z GEFS/00Z ECens). Total ensemble mean weight reached 50-60 percent by day 8 Tuesday. ...Weather/Threats Highlights... A broad area of precipitation across the southern three-fourths of the mainland should taper off from west to east Friday-Friday night. Pockets of enhanced totals are possible over some southern/eastern locations. The system now expected to track from the North Pacific into the southern-southeastern Bering Sea late this week into the weekend should bring organized rainfall into the Aleutians and the Alaska Peninsula. Some of this moisture will likely extend into the mainland from late weekend into next week as low pressure continues along. There are still significant detail differences that affect the coverage and intensity of the precipitation at that time. Some areas of moderate to heavy totals are possible with the best signal currently along/near the southern coast into the Panhandle. Meanwhile precipitation should accompany the cold front dropping into the northern mainland but with lighter totals. By the first part of next week there will be a question of how much moisture with the system to the south may interact with the front. The forecast continues to show below normal high temperatures over much of the state through the period but with some daily variability with system progression. Anomalies are likely to trend colder over northern areas during the latter half of the period behind the Arctic cold front. Lows should be more moderate with various areas of above/below normal readings. Best potential for above normal lows will be over some southern areas early and late in the period and over the North Slope on Saturday ahead of the Arctic front. Rausch No significant hazards are expected over Alaska during this forecast period. Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook 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