Extended Forecast Discussion NWS Weather Prediction Center College Park MD 437 PM 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... The latest guidance and ensembles are maintaining a good grasp on the synoptic pattern, the magnitude and evolution over the CONUS through the extended periods. The ECWMF has trended closer toward the GFS solution from its previous cycle. The WPC medium range product suite was primarily derived from a a combination of the ECWMF/GFS/CMC/UKMET through days 3-5 (Sunday-Tuesday) with increasing weights of the EC ensemble and GEFS mean through the end of the forecast period. ...Pattern Overview and Weather/Hazards Highlights... The widespread heavy to extreme rainfall across the Southwest and south-central Great Basin/Rockies is expected to relax after the weekend.; while there will be lingering moisture and instability over the region, the threat for excessive rainfall will lessen. The focus for substantial heavy rain will shift to the Southern Plains into the Lower Mississippi Valley for several days 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. There is general consensus for the heaviest QPF to align over eastern portions of Texas and southern Oklahoma, with the axis spanning from the Texas panhandle, southward to Texas hill Country and east toward central Mississippi. Once again the latest solutions continued to prefer the southern placement of the QPF. 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; small adjustments were made to expand the Slight Risk areas to reflect the latest WPC QPF and model trends. Further east along this wavy, slow-moving front showers and thunderstorms will linger across the Deep South, Southeast and the Mid-Atlantic through the end of the extended period. There may be local areas that receive moderate to heavy downpours. Across the Great Lakes and Ohio Valley showers and thunderstorms will gingerly spread east/northeast as the the upper trough and surface frontal weakens and decays. Slow cell motions/instability near the closed feature offer potential for local downpours with runoff issues. An energetic Pacific upper trough and associated frontal system with limited summer rainfall potential is forecast to move inland across the Pacific Northwest early next week, but activity may become more organized into favored terrain downstream over the northern Great Basin/Rockies through next midweek. Campbell 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 Hazards: - Heavy rain across portions of the Central/Southern Plains, the Middle/Lower Mississippi Valley, and the Tennessee/Ohio Valley, Sun-Tue, Aug 21-Aug 23. - Heavy rain across portions of the Southern Rockies/Plains, and the Southwest, Sat, Aug 20. - Flooding occurring or imminent across portions of the Southern Plains. - Flooding likely across portions of the Southern Plains. - Excessive heat across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat, Aug 20. - Heavy rain across portions of mainland Alaska, Sat-Mon, Aug 20-Aug 22. 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