Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 AM EDT Mon Apr 14 2025 Valid 12Z Thu Apr 17 2025 - 12Z Mon Apr 21 2025 ...Heavy rainfall and flooding possible in parts of the central U.S. by the weekend... ...Overview... As the medium range period begins Thursday, a potent upper trough will be diving into the Northwest while southern stream troughing reaches the Southwest. These features will combine and develop an upper low atop the Great Basin late week that should gradually pivot south and eastward over the weekend. Cooler temperatures and increased precipitation chances are expected with this developing low, including the potential for heavy snow in the Northern/Central Rockies Thursday-Friday. Meanwhile a northern stream trough is forecast to move through the north-central and northeastern U.S. accompanied by a low pressure system, as an elongated frontal system stretches 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 combining with frontal systems atop the south-central U.S. will lead to some heavy rainfall and flooding potential there, perhaps beginning Friday but certainly over the weekend. ...Guidance/Predictability Assessment... Model guidance has become more stable with the pattern described above. Minor differences in the combination of northern and southern stream troughing late week could be taken care of with a multi-model blend. Meanwhile farther east, there are some model differences in the timing of the northern stream trough and the associated surface low. The 12Z GFS was the main outlier with its timing as it was slower than the guidance consensus. Then into early next week, the Southwest upper low should shift east and then northeast as the workweek begins, while a surface low consolidates and deepens. Once again the 12Z GFS was slower with this upper low, and there is a fair amount of model spread with the exact position and depth of the surface low, including in the 00Z models, so this will continue to be refined. 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. Since the large scale pattern was reasonably agreeable, maintained a majority of deterministic models through the period while blending in some ensemble means to temper the model differences. ...Weather/Hazards Highlights... Later this week, strong upper-level energy will provide support for moderate to heavy precipitation across parts of the West, stretching from the northern/central Rockies and High Plains across the Intermountain West into the Sierra Nevada. With the cold trough aloft, accumulating snow in higher elevations is likely. The heaviest snow expected in the higher elevations of Wyoming and southern Montana while some snow is likely in the Wasatch and Sierra. Some precipitation is forecast to continue in the Four Corners states into the weekend, with possibly multiple days of snow in the Colorado Rockies. Farther east, the northern stream upper trough and a surface low pressure system will traverse the north-central Plains and Midwest on Thursday, supporting shower and thunderstorm chances. The Storm Prediction Center indicates a severe threat in parts of these areas. Locally heavy rain rates may be possible in thunderstorms, but storms should be moving quickly with the progressive low pressure system, limiting flooding concerns. There is also a fair amount of uncertainty in thunderstorm placement. Thus there may be a nonzero flash flooding risk but likely less than the 5 percent required for any Marginal Risk in the Excessive Rainfall Outlook. Then on Friday, rain/thunderstorm chances will continue east into the Great Lakes region and Upper Ohio Valley. Once again there may be some locally heavy rain. Individual thunderstorms should move quickly but there may be repeated rounds moving west to east somewhere in the vicinity of Lower Michigan and/or Indiana/Ohio. With antecedent dry conditions and uncertainty with the axis of potential heavy rain, will indicate a less than 5 percent risk in the ERO, but will continue to monitor flooding potential. Farther south, the tail end of the frontal system in the southern Plains should help to focus moisture advection ahead of the Southwest upper low. Some rain and storms become likely by Friday evening or night over parts of Texas and Oklahoma, with some likely lesser amounts over Arkansas/Missouri. Models vary in how much rain could fall through Friday night, that is through the Day 5 ERO period. Since it is the beginning of the heavy rain event, have no ERO areas outlooked through 12Z Saturday, but future forecasts will continue to assess the risk. By Saturday heavy rain is likely to continue in the southern Plains, and this should warrant ERO areas, but this is beyond the Day 5 ERO period. By Sunday as the surface low and frontal system shift northeast, rain and thunderstorms will spread into the central/southern Plains and the Middle/Lower Mississippi Valley. Heaviest rainfall amounts are 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. The forecast details could change over the next few days and should be closely monitored to assess any possible impacts. Initial upper ridging across the Plains will allow for above normal temperatures there on Thursday. Above normal highs by 20-25 degrees will raise highs into the 90s from Texas to Kansas. Meanwhile cooler than normal temperatures, especially for highs, are likely to spread across the Interior West Thursday and Friday under the upper trough. Warmer than average temperatures are forecast to moderate and shift east into late week and focus across the southeastern U.S. by Sunday. 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