FXUS02 KWBC 091830 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 PM EDT Fri May 9 2025 Valid 12Z Mon May 12 2025 - 12Z Fri May 16 2025 ...Multi-day Southeast U.S. Heavy Rainfall/Flooding Threat... ...General Overview... A slow moving and well defined upper low developing over the interior Southeast U.S. going into early next week will make weather headlines with periods of heavy rainfall from Florida to the Mid-Atlantic through Wednesday. Out West, a potent trough with embedded closed upper low will move inland and affect the West Coast and Intermountain West with colder temperatures and mountain snow inland to the Rockies. Very warm and dry conditions are expected across the Plains through midweek with an upper ridge extending north across the Upper Midwest, with a cold front slowly approaching from the west and bringing a return to more seasonal conditions by the end of the week. ...Guidance/Predictability Assessment... Recent model and ensemble guidance has been in overall very good synoptic scale agreement through medium range time scales, albeit with lingering smaller scale system specific variances that have varied from run to run. A multi-deterministic model blend coupled with WPC continuity and the National Blend of Models seems to offer a solid forecast and threat messaging starting point, with the composite tending to mitigate differences as consistent with individual predictability. ...Weather/Hazards Highlights... The slow moving occluded surface low and associated upper low will result in a strong influx of deep moisture from the Gulf and western Atlantic and result in multiple sustained rounds of moderate to heavy rainfall extending from northern Florida to the Mid-Atlantic region. In particular, enhanced moisture flux with upslope component across the southern Appalachians on Monday (Day 4) will likely result in higher rainfall totals across western SC/NC, and there is the potential that a Moderate Risk area may be needed as this event enters the short range forecast period. However, for the time being, a Slight Risk area still remains valid from southwestern Virginia to the northern Florida Peninsula, and a Marginal Risk area across much of North Carolina into the Mid- Atlantic for Tuesday (day 5), lingering into Wednesday, as the moisture plume slowly works its way north while slowly weakening. Out West, rain and higher elevation snows will become more widespread into next week with the amplified upper low passing through. This especially holds true for the Sierra early and into the northern Rockies going into the middle of next week with some of the higher ranges picking up enhanced late season snowfall. As the system progresses east, moderate to locally heavy rainfall may develop on the north and west side of a surface low over the north- central Plains, but for the most part not expected to be heavy enough to warrant any excessive rainfall risk areas at this time, but will be monitored. An early season heatwave is likely across the Dakotas and into Minnesota early in the week as this region will be in the warm sector of a developing surface low. Temperatures will be the warmest of the season thus far with highs reaching the upper 80s to potentially middle 90s, and easily setting some daily record highs. It will also be hot across much of Texas with some 100+ degree readings expected near the Rio Grande. Warmer temperatures are also coming to much of the Eastern U.S. by the end of the week, with humidity levels also increasing. In contrast, unseasonably cool and unsettled conditions are forecast for the Intermountain West with the amplified upper trough moving in, with highs running 10-20 degrees below mid-May averages in some cases. Hamrick/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 $$