Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Aug 18 2022 Valid 12Z Sun Aug 21 2022 - 12Z Thu Aug 25 2022 ...Monsoonal rainfall threat for the Southwest and south-central Great Basin/Rockies to relax early next week as a significant excessive rainfall/flash flooding threat settles into the Southern Plains/Lower Mississippi Valley... ...Guidance Evaluation/Predictability Assessment... Overall, the latest models continue to handle the mid-larger scale features and generally pattern evolution over the lower 48 and vicinity fairly consistently. The WPC medium range product suite was primarily derived from a composite model blend days 3-5 (Sunday-Tuesday) before switching by mid-later next week for Day 6/7 into a blend of the GEFS/ECMWF ensemble means along with best clustering model guidance from the GFS/ECMWF along with the National Blend of Models and some WPC continuity to ease run to run variances, but much of recent trends seem pretty reasonable. ...Pattern Overview and Weather/Hazards Highlights... A weekend pattern with ample monsoonal thunderstorms and rainfall over the Southwest and south-central Great Basin/Rockies will relax after the weekend, but lingering moisture and instability along with terrain forcing will continue to fuel scattered activity. However, the substantial heavy rain and flooding threat will shift to the Southern Plains and Lower Mississippi Valley for much of next week. The multi-day threat seems formidable as moisture and instability pool with return flow into slow moving/wavy fronts in favorable upper flow as upper trough energies eject out from the West/Rockies. Latest guidance trends continue to shift the main QPF axis southward from previous runs which is often the case with convective episodes. WPC has experimental day 4/5 Slight Risks for excessive rainfall spanning from eastern New Mexico through Oklahoma and Texas into the Lower Mississippi Valley from Sunday into Tuesday so far. Showers and thunderstorms are expected to linger all next week along a slow-moving, wavy/pesky frontal boundary over the South/Southeast/Mid-Atlantic. There may be local areas that have heavy downpours thanks for moist pooled air. Meanwhile farther north, convection will gradually spread across the Great Lakes/Ohio Valley states then Northeast through the period with slow progression and demise of a closed upper trough/low and surface frontal system. Slow cell motions/instability near the closed feature offer potential for local downpours with runoff issues. There is also now a stronger signal to bring an upper trough and associated frontal system with limited summer rainfall potential inland from the Pacific across the Northwest. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml