Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sun Apr 13 2025 Valid 12Z Wed Apr 16 2025 - 12Z Sun Apr 20 2025 ...Overview... As the medium range period begins Wednesday, an upper low will be moving across southeastern Canada with troughing extending south across the Eastern U.S. and upper ridging will be in position across the Plains. Upstream, an energetic trough over western Canada is expected to begin nudging into the Northwest while a Pacific upper low meanders east towards the Coast of California. The trough is forecast to dive southwards across the West during the second half of next week and form a closed low over the Southwest. Cooler temperatures and increased precipitation chances are expected with this trough/low, including the potential for heavy snow in the Northern/Central Rockies Wednesday and Thursday. Meanwhile, a northern stream trough is forecast to swing across north-Central and northeastern U.S., accompanied by a strengthening surface low pressure system and widespread precipitation chances from the Midwest to the Northeast. By next weekend, the frontal system will become elongated and settle across the Southern Plains and Middle/Lower Mississippi Valley. This set up could result in heavy rainfall, and will continue to monitor this potential. ...Guidance/Predictability Assessment... After a significant pattern change over the past couple of days, models have now stabilized on the overall pattern described above for mid to late next week. All available model guidance over the past 24 hours has consistently shown upper level energy diving south across the West and forming a closed low over the Southwest. Overall, model agreement is good, but there are some differences in the details, such as how the low moving south across the West will interact with energy moving east from the Pacific and timing/position of the late week northern stream trough. For the western low, the 00Z CMC and UKMET solutions fall east of where the 00Z ECMWF and 06Z GFS have the low, but the eastern position is closer to the ensemble mean positions from the latest GEFS/ECENS/CMCE. The 00Z ECMWF is on the southern edge of model spread, but the 12Z GFS trended south, closer to the ECMWF solution. Overall, a solution in the middle of the east/west spread seemed reasonable with a slightly southward bias towards the ECMWF solution. Once the trough/low reaches the Southwest it should interact with Pacific energy from the Pacific low. Models are showing potential for some Pacific energy to be ejected northeast across the Central U.S. towards the Midwest, which could enhance precipitation Friday into Saturday. For the northern stream trough, there is quite a bit of spread among the different model solutions. The 06Z GFS seemed to be the farthest outlier from the general consensus, with a slower and more southwards track. The 00Z GFS was closer to the ECMWF solutions, which falls in line with the ensemble mean solutions as well. However, the 12Z GFS held on to the slower and southern trend. For now, a solution closer the the means was favored, but there is potential for this to change in future iterations of the forecast. WPC's afternoon forecast blend consisted of purely deterministic guidance from the 06Z GFS and 00Z ECMWF/CMC/UKMET for the first two days (Wednesday and Thursday). For the rest of the period, the 06Z GFS was phased out and the 00Z GFS was phased in, and ensemble means from the GEFS and ECENS were added in increasing amounts. ...Weather/Hazards Highlights... There will likely be some lingering precipitation in the Northeast on Wednesday under the upper trough axis, and a shortwave could produce some modest rain across the Mid-Mississippi Valley. However, a rainfall totals should stay well below excessive rainfall thresholds, so there are no risk areas on the Day 4 Excessive Rainfall Outlook. More significant precipitation is expected across the West with the combining energies from the upper lows. Precipitation chances will spread from the Northern Rockies Wednesday-Thursday to the Central Rockies and Southwest Thursday-Friday. With the cold upper trough, accumulating snow in higher elevations is likely, with the heaviest snow expected in the higher elevations of Wyoming and southern Montana. Precipitation amounts have trended up slightly in the Sierra Nevada and across the Great Basin, including some higher elevation snow in the Sierra and Wasatch. Some precipitation is forecast to continue in the Four Corners states into next weekend. As the upper low dives south in the West, a smaller portion of the northern stream energy will continue east, trailing along the northern tier of the nation with an accompanying surface low pressure system. This will support shower and thunderstorm chances from the Northern Plains to the Midwest on Thursday. Locally heavy rain rates may be possible in thunderstorms, but storms should be moving quickly with the progressive low pressure system which would limit any flooding concerns. For this reason, there are no risk areas on the Day 5 Excessive Rainfall Outlook. Rain chances will continue east into the Great Lakes region on Friday and the Northeast and Mid-Atlantic on Saturday. Meanwhile, the tail end of this frontal system combining with a front ahead of the slow moving Southwest upper low could support a focus for heavy rain from the Southern Plains to the Mid-Mississippi Valley, with modest rainfall chances extending into the Ohio Valley as well. Some areas could overlap with areas of the Ohio and Mississippi Valleys that may still be sensitive from recent flooding, but fortunately the axis of the heaviest amounts through Saturday is expected to be to the west. Even so, runoff from these areas could flow into the Mississippi River, could exacerbate ongoing flooding issues in some areas. The forecast details could change over the next few days and should be closely monitored to assess any possible impacts. Initial upper ridging across portions of the Interior West and Plains will allow for above normal temperatures there. Temperatures are generally forecast to be 10-20 degrees above average, and even up to 25 degrees above normal in the Central and Southern Plains on Thursday with highs in the 90s from Texas to Kansas. Cooler than average temperatures are likely in the East on Wednesday and Thursday. Lows should be 5-10 degrees below average and highs 10-20 degrees below average. After that, below normal temperatures are expected to develop southward across the West underneath the upper low while above normal temperatures gradually shift towards the East, becoming more limited to the Southeast next weekend. Dolan/Tate 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