FXUS02 KWNH 210707 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Mon Apr 21 2025 Valid 12Z Thu Apr 24 2025 - 12Z Mon Apr 28 2025 ...Multi-day Heavy rain threat for the South-Central U.S.... ...Guidance/Predictability Assessment... Guidance offers decent larger scale agreement on the overall upper flow evolution over much of the next week, but with considerable uncertainty in the details of individual systems. Energy progression from the southern Plains to Mississippi/Tennessee Valleys mid-late week will take until the short range to resolve details, but have implications on heavy rainfall distribution and amounts for this area. Big differences arise uncharacteristically early by Thursday/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. Latest 00 UTC model guidance remains mixed. Accordingly,the WPC forecast used a blend of GEFS/ECMWF/Canadian ensemble mean guidance through medium-range time scales. Overall, this acts to smooth the less predictable details and offers best possible WPC product continuity given lingering uncertainties. ...Weather/Hazards Highlights... An ill-defined series of generally progressive upper features and surface based systems will continue to work over the lower 48 this week to focus local weather/convection, but there does remain a varied signal to amplify the upper flow with troughing from the northeast/eastern Pacific into the West heading into next weekend. The WPC Excessive Rainfall Outlook (ERO) offers Marginal Risk areas from parts of the south-central Plains into the Mississippi and Tennessee Valleys for Day 4/Thursday and Day 5/Friday, 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. Recent and ongoing heavy rainfall over portions of these regions will make some particularly susceptible to additional rains, so an ERO Slight Risk upgrade may be needed in future updates. But with the model uncertainty, held off on any upgrades at this time. Uncertain central to eastern U.S. heavy rain and runoff threats with strong convective potential near lingering boundaries with slow system translations through the weekend will be monitored. Meanwhile, the extent of upper trough amplifications and digging off the West Coast this week is not a done deal in guidance, but the prospect of surface system genesis and slow but steady system 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. 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 $$