Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Mon Jul 12 2021 Valid 12Z Fri Jul 16 2021 - 12Z Tue Jul 20 2021 ...Overview... The majority of guidance shows a similar large scale evolution during the Friday-Tuesday period. Expect separate mean troughs over the Arctic/Siberia and to the south over the western Pacific/Bering Sea, with some phasing of these streams possible by late in the period. Mean ridging ahead of these troughs should align over the eastern Aleutians/southern mainland by the weekend with gradual weakening/suppression early next week. A mean trough with one or more embedded lows should persist over or a little south of the southern Panhandle/Haida Gwaii. As has been the case in recent days, there are a number of uncertainties for embedded features though some details may be starting to come into better focus. ...Guidance/Predictability Evaluation... One aspect of the forecast that has improved this cycle is the handling of initial energy from a short-range Bering Sea upper low. The 12Z ECMWF abandoned its multi-run idea of bringing the energy into the southwestern mainland this weekend (joined briefly by the 00Z UKMET before reverting back in the 12Z run). While the 12Z GFS combined this energy with a portion of flow ejecting from the upper low emerging from Kamchatka to yield a similarly-timed shortwave, the new 18Z run has trended much more toward the 12Z model consensus that is weaker/faster--keeping the leading ridge stronger as has been advertised by most ensemble mean runs. Within the expected storm track from Kamchatka/far northwestern Pacific into the western Bering Sea, there is decent agreement upon a leading system from late this week through at least the weekend (albeit with some lingering timing spread but generally trending toward an intermediate solution) and a moderate signal for a second system during the first half of next week. Details of Arctic flow continue to go astray after about the first couple days of the forecast, which has been typical lately. Of particular note in this cycle the 12Z ECMWF becomes faster than other guidance by day 6 Sunday with a compact upper low that ejects from the mean trough northwest of the state. The first part of the forecast emphasized 12Z operational guidance, though with the GFS component divided among the model and the GEFS mean to tone down the GFS handling of Bering Sea energy. Divergence for specifics later in the forecast maintained the preference of recent days to tilt the blend 70 percent toward the 12Z GEFS/00Z ECMWF means by day 8 Tuesday. This solution provides decent continuity with typical day-to-day adjustments. While the leading western Bering system dissipates by next Monday per this blend, model clustering is getting close to supporting better definition at that time. ...Weather/Threats Highlights... Northern parts of the mainland should see some precipitation late this week and weekend with the passage of an Arctic system that lifts away a leading warm front and drops a trailing front that will also stall and return back as a warm front. Low pressure tracking from the northwestern Pacific into the western Bering Sea should spread a broad area of precipitation and brisk winds across the Aleutians and Bering Sea. Moisture should begin spreading into the western mainland around Sunday with precipitation persisting and expanding gradually eastward thereafter. There could be some locally enhanced pockets of activity but it will take additional time to refine the specifics. As it is guidance is just starting to show better agreement on the timing of the overall moisture shield. Meanwhile precipitation over the Panhandle should tend to favor southern areas and trend lighter with time. The North Slope and portions of the Panhandle/southeast corner of the mainland should see the most persistent below normal highs during the period, though northern areas may see some day-to-day variability depending on frontal position. Moisture arriving from the west during the weekend into next week will bring a cooling trend from west to east, with increasing coverage of below normal highs. On the other hand low temperatures will tend to be more above normal with only smaller areas of below normal anomalies. 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 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