Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 PM 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... The 00Z/06Z suite of model guidance remains in reasonably good agreement regarding the evolution of the large scale pattern, including a slow-moving upper trough and closed low over the Intermountain West and Four Corners, and a northern stream shortwave/closed low north of the Great Lakes and the Northeast through the early part of the weekend. Model differences become more apparent and impactful by Sunday, however, as the closed low reverts back to an open wave before ejecting eastward into the Plains. Aside from the 00Z CMC, which remained quite fast and more negatively tilted, the 00Z ECMWF, 06Z GFS, and 00Z UKMET were all in a similar cluster of keeping the wave slower and farther south across southern New Mexico and far western Texas. Other than the generally minor differences in timing/strength typically seen at these extended ranges, the 500 mb and surface evolutions through the remainder of the period were handled roughly the same overall by the deterministic guidance, with the wave and frontal system consolidating and gaining latitude from the southern Plains to the Upper Midwest. More drastic changes were seen with some of the new 12Z guidance, especially the 12Z GFS and 12Z ECMWF, which were both much faster and farther north compared to their earlier runs. The 12Z CMC on the other hand was slower and farther south, while the 12Z UKMET was similar to 00Z albeit slightly farther north. Given the above, it's clear that the synoptic features in play still remain quite sensitive to even small changes from run-to-run, which thus leads to lower than average confidence overall for days 5-7. Later in the period, another trough is expected to dig southward across the Pacific Northwest, with the 12Z CMC and 12Z UKMET more aggressive compared to their previous runs and the new 12Z GFS and 12Z ECMWF (the latter which was more agressive at 00Z). A general blend depicted broad troughing for days 6 and 7, which is a fair compromise for now. The WPC forecast was based on a blend of the deterministic guidance using the 00Z ECMWF, 06Z GFS and to a lesser degree the 00Z UKMET and 00Z CMC. Gradually included some ensemble mean guidance and increased the means to half by the end of the period to temper the individual model differences. Overall, this resulted in a slightly slower and farther south trend in the progs compared to continuity. ...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. Maintained a Marginal Risk for 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 remains 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. Miller/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