Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Wed Sep 02 2020 Valid 12Z Sat Sep 05 2020 - 12Z Wed Sep 09 2020 ...High heat rebuilds over the West late this week while much cooler air arrives for the north-central U.S. early next week... ...Weather/Hazard Highlights and Guidance/Predictability Assessment... With an amplifying upper level pattern expected to evolve over the course of the medium range period across the CONUS, temperature anomalies are the main sensible weather headline. The amplified pattern will open up the northern portions of the country for periodic intrusions to Canadian high pressure, the first of which is expected to arrive late in the weekend or early next week. The latest model guidance shows good agreement through about Sunday but for Monday/Tuesday next week, the ECMWF digs the trough axis further west into the Rockies while the GFS is a bit more progressive and is centered over the central Plains. For now, a compromise of the two using the ECENS/GEFS means was used. But both global models agree that there will be another cool air surge into the northern High Plains by next Tuesday, when high temperatures of 15 to more than 20 degrees below normal across the northern Plains will plunge toward the central Plains. Conversely, the heat will remain in place over the Southwest U.S. and could strengthen into the weekend and surge northward into the Pacific Northwest. High temperatures well into the 90s to near 100 are also forecast for the hottest locations over the interior Pacific Northwest through early next week. By Tuesday, the heat may begin to ease a bit as the cool air mass over the Rockies edges closer toward the region. Excessive precipitation is not expected across the U.S. although a lingering front will keep a good chance of thunderstorms for Florida through the medium range period. By next Tuesday, models are indicating the possibility of heavy rain across the central Plains toward the upper Midwest as the cold front associated with the first surge of cool air slows down and moisture return from the Gulf lifts north and interacts with the slowing/stalling boundary. The latest ensemble probabilities for exceeding 2 inches of rain in the period are up to 30-40 percent from near Kansas City northeast toward the western Great Lakes. Mountain snows will be possible across the highest elevations of the central/northern Rockies early next week. The latest winter weather outlook shows slight probabilities of exceeding 0.25 liquid equivalent as the anomalously strong upper trough digs into the area. Given the above average agreement and run to run continuity in the latest model guidance, the WPC blend was composed primarily from the 12Z ECMWF/CMC/UKMET and 18Z GFS with inclusion of the 12Z ECENS and 18Z GEFS for day 5/7. Kong/Taylor 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