Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Thu Apr 24 2025 Valid 12Z Sun Apr 27 2025 - 12Z Thu May 01 2025 ...Potent West/Rockies to Central U.S. Spring Storm expected Sunday into Monday... 19Z Update: The 12Z model guidance suite features above average model agreement across the Continental U.S. through most of the forecast period, especially so on Sunday and into Monday. Therefore, a general deterministic model blend was utilized as a starting point in the forecast process. Minor timing differences emerge with the storm system crossing the north-central U.S. early in the week, but all are close enough to the ensemble means and AIFS guidance to be considered good solutions. Towards the end of the forecast period Wednesday/Thursday, there are some differences on how far south the cold front gets across the Southeast and Mid- Atlantic region after the June-like warmth ahead of the front. There are also some differences with how a potential weak upper low develops over the Intermountain West by Thursday, and at the time of fronts/pressures preparation, a blend a little closer to the CMC/ECMWF was favored, with the GFS a little weaker and favoring more of an open trough aloft. Still, model agreement is fairly decent by that time and the ensemble means were only increased to about 30%, along with about 10% of the previous WPC forecast. The previous forecast discussion follows below. /Hamrick ---------------------- ...Overview... Wrap-back rains lingering over northern New England will depart Sunday as the supporting deepened low and frontal system exits offshore. Meanwhile, an unsettling and cooling upper troughing will move from across the West and produce widespread moderate precipitation. Expect enhanced snow for the higher elevations from the Cascades down to the Sierra and inland to the north-central Intermountain West/Rockies this weekend, with Rockies activity enhanced by lee cyclogenesis and frontogenesis into early next week. Deep moist inflow ahead of the upper trough and developing storm system will significantly increase the coverage of rains and strong to severe thunderstorms across much of the central and into east central U.S. early-mid next week. Temperatures will mostly remain above normal east of the Continental Divide while trending near to below normal in the West. ...Guidance/Predictability Assessment... Model and ensemble forecast clustering has improved through medium range time scales, bolstering forecast confidence. Smaller scale guidance differences remain, but seem somewhat less of an issue given a pattern evolution trend toward amplification and with better system organization that tends to increase predictability. Accordingly, the WPC medium range product suite was mainly derived from a composite blend of guidance from the 18 UTC GFS/GEFS mean and the 12 UTC ECMWF/UKMET/Canadian/ECMWF ensemble mean along with compatible 01 UTC National Blend of Models and WPC continuity. ...Weather/Hazards Highlights... An amplified upper trough and surface system digging across the West this weekend offers widespread rains and enhanced snows for the higher elevations from the Cascades down to the Sierra and inland over the north-central Intermountain West. Mountain snows then expand into the central and northern Rockies early next week. Downstream, rich moisture streaming in ahead of a strengthening storm system over the Plains will fuel rounds of heavy rain and thunderstorms across portions of the central U.S. from the weekend into next week. A WPC Marginal Risk Excessive Rainfall Outlook was reduced, but remains in place for Day 4/Sunday and Day 5/Monday from portions of Montana and the Dakotas into the Upper Midwest given an increasing signal for anomalous moisture and an expanding convective QPF footprint. Activity will be in the process of transforming into an emerging comma-shaped rainfall area with potential for emerging convection down over the south-central Plains and Mississippi Valley. Thunderstorms will likely become more organized and robust with training runoff threats into Tuesday/Wednesday with strong frontal proximity, instability and focus under favorable upper trough/jet support. SPC has already issued a medium range threat for severe weather to further monitor. Focus may also eventually work into some moisture laden soils of the east-central U.S. into next midweek given further surface low and wavy frontal structure, evolution and downstream punch. Temperatures along and east of the Continental Divide will largely remain at or above normal through much of the medium range period as upper level ridging builds. Meanwhile, temperatures across the West will be near to below normal given unsettled conditions underneath an upper level trough. 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