Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Apr 15 2025 Valid 12Z Fri Apr 18 2025 - 12Z Tue Apr 22 2025 ...Heavy rainfall and flooding possible in parts of the central U.S. by the weekend... ...Overview... A potent upper trough developing an embedded upper low will be in place over the Interior West Friday and pivot east over the weekend. Cooler temperatures and increased precipitation chances are forecast with this developing low, including snow in the central Rockies into Friday and Saturday. Meanwhile a northern stream upper trough will advance through the Midwest to Northeast late week, along with a surface low and an elongated front that is forecast to stretch from the northern tier southwestward through the south-central U.S. late week into the weekend. This will lead to rain and thunderstorm chances, including possible severe weather. Moisture streaming in ahead of the Southwest upper low interacting with frontal systems atop the south-central U.S. will lead to some heavy rainfall and flooding potential there, beginning Friday but with rising potential over the weekend. Farther east, warm upper ridging will take hold over the Southeast. ...Guidance/Predictability Assessment... Model guidance begins in good agreement with the overall pattern as upper troughing in the West closes off an upper low over the Great Basin and Four Corners while upper ridging dominates over the East, other than a trough poking into the northern tier. Some impactful model differences start to arise by around Sunday as the upper low pivots east. Models vary with the timing of the low's ejection and the upper trough possibly becoming negatively tilted. The 12Z/18Z GFS and 12Z ECMWF and CMC were reasonably agreeable with a somewhat fast track of the upper low moving east and then northeast, along with its surface low and QPF. Meanwhile the 18Z AIFS was slower to tilt and move the upper and surface lows northeast. Other AI models were in between, and ensemble members showed some timing spread, so the good agreement of the deterministic models was a little suspicious. The newer 00Z ECMWF and especially the 00Z GFS are somewhat farther south and slower than their previous runs, though the 00Z CMC is fast. The AIFS, which was already particularly slow, now shows an even slower track in its 00Z run. While these model differences are not the most stark, they do cause some notable sensible weather concerns like surface low and QPF placement. So the details will continue to be refined in future forecasts, including a potentially slower trend of the surface low and frontal system. Did think the new 00Z GFS was a slow outlier with moving the feature northeast into the Great Lakes or so early next week. Upstream, another trough that is not as deep looks to move into the Northwest early next week. The 12Z ECMWF seemed to be a little on the aggressive side with the depth of the trough (and showing an embedded upper low) compared to the other dynamical and AI models, but could still be used as part of a blend. The WPC forecast was based on a blend of the deterministic guidance using the 12Z ECMWF, 18Z GFS, 12Z CMC, and 12Z UKMET from most to least at the beginning of the period. Gradually included some ensemble mean guidance and increased the means to half by the end of the period to temper the individual model differences. ...Weather/Hazards Highlights... Precipitation is likely across the Four Corners states and nearby on Friday-Saturday, with support from the trough/low aloft. Some snow is possible in the Wasatch and the Mogollon Rim on Friday before the trough moves east, and the central Rockies could see snow pile up both Friday and Saturday. Then another trough pushing into the Northwest by early next week could spread some modest rain and higher elevation snow particularly to the northern Rockies/High Plains. Farther east, a surface low pressure system will traverse the Midwest/Great Lakes on Friday, supporting shower and thunderstorm activity in that area and southwest along the trailing front. The Storm Prediction Center indicates a severe threat from the Upper Ohio Valley through the Mid-Mississippi Valley and Southern Plains. There may be some locally heavy rain on Friday and Friday night with this convection. There may be some flooding concerns from the south-central Plains into the Mid-Mississippi Valley, where storms may train by Friday night as the front starts to become more west- east oriented. Have added a Marginal Risk to the Day 4/Friday ERO for this potential. By Saturday, the slow pivot of the upper low, the consolidating frontal system, and precipitable water anomalies over the 90th percentile are likely to cause widespread heavy rain of 3+ inches, with locally higher amounts. A Slight Risk of excessive rainfall stretches from north-central Texas across Oklahoma and into southwestern Missouri (clipping southeast Kansas and northwest Arkansas) for Day 5/Saturday. A broader Marginal Risk surrounds the Slight and is drawn across much of the Ohio River where streamflows remain high. By Sunday there is some model spread in the timing and placement of the upper and surface lows and thus where rainfall will focus, but broadly rain and thunderstorms chances are likely from the southern and central Plains into much of the Mississippi Valley. The heaviest rainfall is likely to be west of the most sensitive areas of the Mississippi Valley and Ohio Valley from recent flooding. However, some rain atop recently flooded areas, and runoff from other areas into these rivers, could exacerbate ongoing flooding issues in some areas, so continue to monitor. Rain chances spread farther east by Monday. Late this week, temperatures will be below average especially in terms of highs in the Interior West underneath the upper trough. These should moderate toward normal by early next week, though temperatures in the Northwest may end up below normal by a few degrees. Meanwhile farther east, above average temperatures by 10 to 20 degrees are likely from the southern Plains into much of the eastern half of the U.S. on Friday. The primary cold front will push above normal temperatures toward the Northeast/Mid-Atlantic on Saturday but remaining over the Southeast as an upper ridge sets up. The Southeast should remain warm into early next week, with expansions of above average temperatures northward at times. 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