Extended Forecast Discussion NWS Weather Prediction Center College Park MD 139 PM EDT Sun Apr 20 2025 Valid 12Z Wed Apr 23 2025 - 12Z Sun Apr 27 2025 ...Multi-day Heavy rain threat for the South-Central U.S.... ...Guidance/Predictability Assessment... Models and ensembles continue to offer decent agreement on the overall upper flow evolution into next weekend, but considerable uncertainty in the details of individual systems. Energy progression from the southern Plains to Mississippi Valley mid to late week will take until the short range to resolve details, but will have implications on heavy rainfall distribution and amounts across this area. Big differences arise by Friday as an upper low from the Gulf of Alaska drops southward along the coast with some energy likely to split from the main low. The GFS is quicker to split this than other guidance, and the ECMWF has an open shortwave extending more south. Placement wise, the CMC was in the middle of the two, but also shows a stronger than consensus upper low centered over the California/Nevada on Day 7. Thus, it seemed necessary to transition the early period deterministic model blend to a majority ensemble mean blend mid to late period to smooth out these differences while still maintaining good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... A scattered series of generally progressive upper features and associated surface based systems are set to work over the lower 48 next week in multiple stream flows to locally focus late April weather, but there is some signal to amplify the flow from the northeast/eastern Pacific into the West heading into next weekend. WPC Excessive Rainfall broad Marginal Risk areas for portions of the south-central Plains into the Mississippi Valley for Day 4/Wednesday and Day 5/Thursday are in place, but there is still considerable uncertainty on exactly where heaviest amounts will be. Regardless, the pattern supports a moist and unstable flow for a favorable, at least local, flash flood setup. Some of this region, particularly Oklahoma into north Texas, have seen heavy rainfall this weekend and thus remains particularly susceptible to additional rains, and a slight risk upgrade may be needed in future updates. But with the model uncertainty, opted to hold off on any upgrades at this time. Heavy rain and runoff threats along with strong convective potential in this region and into the Mississippi Valley/Southeast along a lingering boundary will also be monitored for later week and next weekend given lingering support and multi- day potential. The extent of upper trough amplification and digging is not a done deal in guidance, but the prospect of surface system genesis and slow but steady translation from the northeast/eastern Pacific to the West Coast Friday and inland over the West next weekend still offers increased light to moderate widespread rain chances and mountain snows. Santorelli/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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw