Extended Forecast Discussion NWS Weather Prediction Center College Park MD 126 AM EDT Thu Apr 16 2020 Valid 12Z Sun Apr 19 2020 - 12Z Thu Apr 23 2020 ...Heavy rainfall/runoff and strong thunderstorm threats on tap again for the South next week Sunday-Monday and Wednesday-Thursday... ...Overview and Guidance/Predictability Assessment and Weather/Hazard Highlights†The extended range period appears quite busy with the potential for two back to back heavy rainfall events over the Southern U.S.. The first should begin Sunday into Monday as shortwave trough energy moves from the Southern Plains on Day 3/Sunday to off the Mid-Atlantic coast by late day 4/Monday. Moist return flow out of the Gulf of Mexico will fuel widespread showers and thunderstorms from the Lower Mississippi Valley to the Southeast U.S. with increasing confidence in the potential for a heavy rainfall event. The latest extended range discussion from SPC also suggests the possibility for strong to severe storms with this system. Overall, there is very good agreement on the large scale pattern/evolution of the shortwave, with smaller scale less predictable differences in the details remaining. A general model blend was used which also maintains good WPC continuity. Upper level ridging from the West will make its way eastward following the first system, promoting a period of above normal temperatures into the Plains states. Meanwhile, another possible closed low/amplified trough should send light to moderate precipitation with a frontal boundary into the Southwest/Southern Rockies Monday into Tuesday. As the front reaches the Southern Plains on Wednesday, this sets the stage for another possible heavy rainfall/runoff/strong thunderstorm threat for parts of the Southern Plains into the Deep South. Thereâ€s decent agreement at this time frame on the large scale pattern, but plenty of run to run variability in the details of the deterministic solutions. WPCs blend for this system leaned more heavily on the GEFS/ECENS means, with some deterministic GFS/ECMWF added in for system definition. The only other thing to note in the day 3-7 period is possible shortwave energy moving into the Pacific Northwest by next Thursday. The GFS/GEFS are faster and more amplified than the ECMWF/ECENS camp though given the time frame, a general model blend (with more emphasis on the ensemble means) provides a good starting point. Expect showers and higher elevations snows to accompany this system across much of the Northwest U.S.. Santorelli 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