Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 AM EDT Sat Mar 15 2025 Valid 12Z Tue Mar 18 2025 - 12Z Sat Mar 22 2025 ...Another Rockies to Great Lakes Snowstorm Threat next week... ...Guidance/Predictability Assessment... Recent GFS/ECMWF/UKMET models and GEFS/ECMWF/Canadian ensemble mean solutions through the current latest 00 UTC guidance cycle seem best clustered with main systems and the mid-larger scale pattern evolution in active flow through much of next week. However, lingering smaller scale timing differences, without resolution, seem to favor preference for a composite guidance forecast plan. Leaned blend weights somewhat more on the models for early-mid next week for details before shading later next week toward compatible ensemble means amid forecast spread growth. This plan seems generally in line with the National Blend of Models guidance and acts to maintain good WPC product continuity. ...Weather/Hazards Highlights... The weather story for next week will highlight an emerging signal for a main winter storm set to spread organized precipitation including terrain enhanced heavy snows across the Interior West to the Rockies Tuesday. Downstream cyclogenesis over the Plains Tuesday will follow with a windy low track over the Midwest/Great Lakes midweek and to eastern Canada by Thursday. This will combine with secondary cold frontal cooling to enhance upslope in an emerging wrap-around heavy snow threat from the north-central Rockies/Plains to the Upper Midwest and northern Great Lakes/Northeast Tuesday-Thursday. A wavy trailing cold front will have less moisture than an earlier next week system, but will align and sweep an axis of showers and thunderstorms through the central to eastern U.S. Thursday/Friday. Most activity exits the East Coast by next weekend. Meanwhile well upstream, steadily progressive and moderately amplified Pacific systems will pool moisture to spread precipitation inland from the West/Northwest through the Rockies. Moderately heavy rains and terrain focusing snows are most likely via channeled moisture into the Pacific Northwest in this pattern. Subsequent downstream rain chances may emerge next weekend with uncertain system details and return flow developments. 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