Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Sun Aug 30 2020 Valid 12Z Wed Sep 02 2020 - 12Z Sun Sep 06 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... The latest model guidance continues to show above average agreement and run to run continuity resulting in above average forecast confidence for much of the medium range period (Sept 2-6). To start, the 12Z ECMWF/CMC/UKMET and 18Z GFS showed fairly tight clustering with respect to a weakening shortwave trough over the central U.S. while northern stream troughing persists from the northern Plains to Great Lakes. Finally, all the models depict upper level ridging building and expanding over the Southwest to western U.S., particularly by late in the forecast period. The model differences seen in the later portions of the period were generally timing related and a higher inclusion of the ECENS/GEFS were used to help drive the WPC blend toward a consensus approach by day 6/7. ...Weather/Hazard Highlights... Heavy rain potential continues at the start of the forecast period from portions of northern Texas through the south-central Plains and Mid-MS River Valley as a southern stream shortwave trough slowly works over the area and then interacts with a wavy/stalled front. The ensemble probabilities for excessive rainfall are still present and some runoff issues are possible, particularly 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. 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