Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 PM EDT Sat Aug 29 2020 Valid 12Z Tue Sep 01 2020 - 12Z Sat Sep 05 2020 ...Heavy Rain Threat South-Central Plains to Mid-MS River Valley Early/Mid Next Week... ...High heat rebuilds over the West next week... ...Overview and Guidance/Predictability Assessment... Guidance shows above average agreement and run to run continuity that suggests above average medium range forecast confidence for much of the CONUS and vicinity. The 06/12 UTC GFS, 00 ECMWF/Canadian and GEFS/ECMWF ensembles are well clustered overall and are the primary WPC forecast components through day 4/Wednesday. Modest later period timing differences and continuity led to more reliance on the ECMWF/Canadian and GEFS/ECMWF ensembles days 5-7. This includes a signal to monitor for tropical wave activity to affect the Caribbean and Florida/Gulf of Mexico. ...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 southern stream shortwave trough slowly works over the area and interacts with a wavy front. Several 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. This could produce runoff issues in the wake of Laura and short range rainfall. The weather pattern generally features a renewed upper level ridge over the western U.S. and troughing over the central/eastern U.S.. Above normal heat will rebuild over much of the West and in particular, the desert Southwest will have daily highs upwards to 110-120F. Conversely, the passage of a strong cold front mid-late next week will usher in cool/drier Canadian high pressure and an early taste of autumn weather for much of the central to eastern states. This is supported by the passage of a series of amplified northern stream upper troughs that dig southward from Canada. There is also an unusually strong guidance signal for additional flow amplification across the Pacific/Alaska that works downstream into Canada and the lower 48 states late period/next weekend. This seems related to a downstream ripple effect from the concurrent forecast track of now developing Typhoon Maysak from the west tropical Pacific robustly northward into northeast Asia. Schichtel 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