Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1213 PM EDT Wed Aug 05 2020 Valid 12Z Sat Aug 08 2020 - 12Z Wed Aug 12 2020 1615 UTC Update... Overall, changes to the ongoing forecast during this update were minimal. The 00Z ECMWF/06Z GFS remained the preferred deterministic solutions, and a blend of these was heavily used during days 3-4 (Sat-Sun). A gradual trend toward heaver weighting of ECENS/NAEFS ensemble means was shown through time, with a majority of the forecast blend based on the means by days 6-7 (Tue-Wed). Overall, consensus among guidance was average to slightly above average, particularly at large scales. Typical detail differences on timing/amplitude of features emerged later in the forecast period, but the increased weight on ensemble means should resolve these differences toward the current consensus, while maintaining good forecast continuity. Ryan Previous Discussion (issued at 653 UTC)... ...Overview and Guidance/Predictability Assessment... North Pacific/western North American flow that continues across the northern U.S. and southern Canada during the period will bring a sequence of two fronts into the Northern Plains during the weekend. The leading front should ultimately dissipate with the second front continuing into the Great Lakes/southeastern Canada, possibly nearing the Northeast around the middle of next week. Also by midweek guidance is agreeable and consistent with an upper trough settling over the Pacific Northwest and western Canada as a leading cold front moves into the Northwest. The upper trough has good teleconnection support relative to the core of positive height anomalies/ridging which D+8 multi-day means show to the west--providing added confidence in this aspect of the forecast. Around the end of the period this trough should eventually gain enough amplitude to interact with a weak upper trough/low near California. Near the East Coast a weak upper trough departing this weekend will push along a western Atlantic wave whose trailing front will linger over the Southeast for a time. Upper ridging will prevail over most of the southern tier U.S. with embedded highs likely to be strongest over the southern Rockies/Plains and Florida/eastern Gulf of Mexico. Some timing and amplitude differences remain for the northern tier flow but generally faster GFS/GEFS trends and slower ECMWF trends over the past day have led to progress toward an intermediate solution for specifics aloft and southward/eastward extent of the second cold front. Meanwhile guidance agreement for the upper trough reaching the Northwest late in the period is better than average relative to typical days 6-7 forecasts. These guidance comparisons supported a blend of the 18Z GFS and 12Z runs of other operational models early, trending to an even blend of the 18Z GFS/12Z ECMWF and their ensemble means later in the period as some operational model details diverge. ...Weather/Hazard Highlights... The active northern stream aloft and associated fronts, along with an advancing Plains/Mississippi Valley warm front during the first half of the period, will support episodes of showers and thunderstorms from the Northern Plains into the Great Lakes and Ohio Valley. Some of this activity may extend farther eastward late in the period. Guidance continues to signal the potential for some areas of heavy rainfall but it will likely take into the short range time frame to resolve with greater detail the most favored regions for highest rainfall totals. The Southeast/Florida will see diurnal convection of varying intensity during the period. The front lingering over the Southeast should provide an added focus for rainfall during the weekend. Ahead of the northern tier fronts there will be an area of temperatures 5-12F above normal progressing from the Northern Plains to the Northeast. The southern High Plains into Arizona will be persistently above normal by up to 5-10F through the period. Expect coverage of modestly below normal temperatures over the West Coast states and northern Rockies to shrink after Sat. Some areas along the California coast may remain a little below normal though, and the Northwest should see a cooling trend by the middle of next week as an upper trough reaches the area. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml