Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EDT Fri Aug 28 2020 Valid 12Z Mon Aug 31 2020 - 12Z Fri Sep 04 2020 ...Heavy rain possible across portions of the central Plains to the mid-Mississippi Valley Sunday through Tuesday... ...Overview and Guidance/Predictability Assessment... At the start of the medium range period, the circulation of Laura should be accelerating off to the east of New England on Sunday as it is forecast to become extra-tropical. Models are also in excellent agreement that the upper trough and associated low pressure system over New England will quickly exit into the Canadian Maritimes Sunday into Monday. Meanwhile, the next upper trough should be moving into the northern Plains. Previous runs from the ECMWF and the Canadian (CMC) models indicate that a shortwave will fracture from the main flow and then develop into a slow-moving upper trough/low over the western U.S. The 00Z ECMWF has switched to a much more progressive solution, with the upper trough moving steadily from the northern Rockies to the central Plains early next week. This scenario is now much more agreeable with solutions from the GFS. The latest CMC has also abandoned the slow scenario. The GFS is still ahead of the ECMWF in terms of the speed of progression of the upper trough but the discrepancy is now more typical between these models. In addition, the EC mean and the GEFS are very agreeable even into Day 7. Therefore, the morning WPC medium range forecast was based on a blend of the 00Z ECMWF and EC mean together with the 06Z GFS and 06Z GEFS, leaning toward the ensemble means for Days 6 and 7. ...Weather/Hazard Highlights... Signs of fall begins to appear in Canada where the upper-level westerlies become firmly established by next week. However, upper troughs still show resistance in digging deep into the warm air mass across the U.S. Nevertheless, a cool air mass filtering in behind a digging upper-level trough will bring fall-like temperatures across the northern Rockies into the northern Plains early next week. In fact, northwestern Wyoming should experience the first freeze of the season by Monday morning. Showers and thunderstorms are expected to sweep across the northern Plains to the upper Midwest Monday to Tuesday associated with a cold front. Farther south, guidance continues to show a signal for more widespread heavy rains for portions of the Mid-Mississippi Valley and possibly portions of the southern Plains Sun-Tue, associated with a low pressure wave developing along a lingering surface front. The front will begin to wash out toward the middle of next week but it should remain be a focus for precipitation from the central Plains to the Midwest/Ohio Valley. Meanwhile, high temperatures 10 to 15 deg F below average are expected across the northern Rockies Sun, and into the north central U.S. Mon-Tue. Cool high temperatures in the 60s and 70s will be common across the central/northern Plains and Upper Midwest Mon-Tue. Some monsoonal convective activity is expected over the central and southern Rockies early next week. Meanwhile, the heat will be confined to the Deep South, with highs expected to be 5 to 10 deg above average across much of Texas into next week, and many areas approaching or even surpassing 100 degrees. Kong/Ryan (Best of luck, Sean!) 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