Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 AM EDT Fri Aug 06 2021 Valid 12Z Mon Aug 09 2021 - 12Z Fri Aug 13 2021 ...Overview... A typical late summer pattern is expected to evolve over the CONUS during the middle of next week with building heights aloft. The main storm tracks will skirt the northern Rockies/Plains into the Upper Midwest/Great Lakes regions with the rest of the country likely to see a drier weather regime. Temperatures will side toward above normal for much of the country with some bouts of excessive heat likely. ...Guidance/Predictability Evaluation... The biggest concerns with the latest suite of 12z/18z guidance was regarding a compact upper low lifting from the northern Rockies to south central Canada in the beginning of the period, which eventually gets absorbed into a shortwave dropping south through Canada mid-week. The 18z GFS remains noticeably faster with the remainder of the guidance, while the 12z ECMWF and CMC are slowest, with the 12z UKMET in the middle. There does seem to be a faster trend with this feature amongst most of the guidance, but not ready to go as fast as the 18z GFS yet. Opted for a blend of the 12z UKMET/12z ECMWF and the 12z GFS (which was a bit slower than its 18z run) which offered a nice middle ground solution and also worked well with WPC continuity. While it was reasonable early, the 12z CMC left the pack later in the period to bring a closed low into the northern Plains so it was excluded from the blend entirely. Elsewhere, the deterministic guidance showed fairly good consistency and agreement with the ensembles, although the typical late period details remain fairly uncertain. For this reason, WPCs forecast trended towards a 50/50 blend of the deterministic ECMWF and GFS (12z run) with the ECENS and GEFS ensemble means by day 7. ...Weather Highlights/Hazards... A leading shortwave into the Great Lakes will continue rainfall chances across the northern Ohio Valley into the Great Lakes on Monday, but should wane some by the middle of the week as the system lifts into southern Ontario/Quebec. Daily rounds of convection will likely fire along the trailing cold front (and another Canadian front following) mid to late week from parts of the Northeast to the Tennessee and Ohio Valleys. The shortwave/compact upper low across the Northern Rockies-southern Canada could also bring rain to locations in the far northern Plains to Upper Midwest Monday and Tuesday. Temperatures will be about 5-15 degrees cooler than normal in the Pacific Northwest and northern Rockies behind the lead cold front this weekend into Monday with a moderation thereafter. Ahead of this cold front, temperatures will be 5 to 15 degrees above normal over the Plains and expanding to the East Coast next week. Heat and humidity combined will bring heat indices into the low 100s for many. With little relief expected overnight, excessive heat is possible most notably from parts of the central/southern Plains/Mississippi Valley and also parts of the Mid-Atlantic later in the week. Santorelli 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 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