Extended Forecast Discussion NWS Weather Prediction Center College Park MD 222 AM EDT Sat Aug 29 2020 Valid 12Z Tue Sep 01 2020 - 12Z Sat Sep 05 2020 ...Heavy Rain Potential for South-Central Plains to Mid-MS River Valley Early/Mid Week... ...Overview and Guidance/Predictability Assessment... For the medium range forecast period (Sept 1-5), the latest deterministic guidance showed average to above average agreement and run to run continuity resulting in an above average forecast confidence through day 5/6 for much of the CONUS. A strong, progressive shortwave trough is expected at the beginning of the period over the central Plains and is the primary feature for potential heavy rainfall for portions of the south-central Plains to mid-MS River Valley early in the period. The 12Z ECMWF, 18Z GFS, and 12Z CMC showed the best agreement and were the primary components of the WPC blend used this cycle through day 5. By the end of the forecast period, the models all somewhat agree that a strong push of Canadian high pressure will drop from the northern Plains toward the Great Lakes and Northeast, though its timing by day 7 is in question. The 18Z GFS was a bit slower and holds the front back along the east coast, meanwhile the 12Z ECMWF is faster/progressive with the front marching off the east coast and only confined to the Gulf Coast region. Given some of these timing differences, a compromise of the two combined with higher weights of the ECENS/GEFS was used. For the western/southwest U.S. there are signals for a building ridge anchoring over the Desert Southwest, which would extend/intensify the excessive heat. ...Weather/Hazard Highlights... Forecast guidance continues to suggest a heavy rain signal for portions of the south-central Plains toward the mid-Mississippi River Valley Tuesday/Wednesday as a shortwave trough approaches and interacts with a stationary/wavy front draped in the area. A couple rounds of heavier showers and thunderstorms could produce some flooding and the most recent model guidance shows the highest probabilities for excessive rainfall to be from northeast Texas, southeast/eastern Oklahoma, and Arkansas. As the upper level energy picks up speed and moves toward the eastern U.S. mid to late week, the surface front is expected to wash out and drier conditions to prevail. With the weather pattern generally featuring upper level ridging over the western U.S. and troughing over the central/eastern U.S., the above normal temperatures are likely to persist for the Southwest where daily highs could reach 110-115F over the Desert Southwest and above normal readings of 5-15F for the Rockies and west coast. Conversely, the passage of a strong cold front mid to late week will usher in cool, Canadian high pressure and an early taste of autumn weather for much of the northern areas of the central U.S., Great Lakes, and Northeast/New England. Daytime highs in the 70s/80s and lows in the 40s to low 60s will be common. These type of values will be 5-10F below normal for early September. The stronger Canadian high pressure is also forecast to push much of the humidity/moisture to the coastal areas from Texas to the Mid-Atlantic, where the front could stall/wash out. Lastly, models are also trending toward a stronger upper high over the Southwest by the end of next week, thereby increasing the chance of heat to build across the Desert Southwest. 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