FOUS30 KWBC 161559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat May 16 2026 Day 1 Valid 16Z Sat May 16 2026 - 12Z Sun May 17 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS AND MID-MISSISSIPPI VALLEY... ...Central Plains to Mid-Mississippi Valley... Upgrade to Slight Risk for northern MO into IA back along KS/Neb border into western IL with note on this being a dynamic situation that may require adjustments heading into tonight. Warm and moist advection ahead of a low over western KS will continue to converge along a stationary front and additional surface troughs through tonight. Convection on outflow from earlier activity continues to grow over northern MO into western IL with 1500 J/kg MLCAPE and 1.5" PW. 12Z HREF and recent HRRRs have struggled with this morning activity, though the 12Z RRFS is decent. This ongoing activity should wane through midday, but help define the boundary for redevelopment along and farther west into KS/Neb this afternoon/evening. PW should be 1.75" by then which is 2 sigma above normal and instability will be ample, 2000-3000 J/kg of MLCAPE promoting heavy thunderstorm development. Little synoptically forced movement is expected for this activity as the surface low remains in the southern High Plains/retrogrades west as the next shortwave trough approaches from the northern Rockies. Despite parched conditions, the risk for 3-5" is great enough (12Z HREF has 60% probability for 5" 18Z to 06Z along the IA/MO border) that a Slight Risk upgrade was coordinated with OAX/EAX/DMX/DVN. Thunderstorm development near the western KS/Neb border should increase through this evening. Backbuilding could cause a localized flash flood threat, so the Marginal Risk was expanded farther west. ...Lower to Mid Ohio Valley... Low level convergence of southerly flow continues over southern IL through the OH/KY border today with PW up to 1.5" (1.5 sigma above normal) and MLCAPE increasing to around 1500 J/kg. General thunderstorms with some repeating risk allows maintenance of the Marginal that was connected to the central Plains per morning radar trends. Jackson Day 2 Valid 12Z Sun May 17 2026 - 12Z Mon May 18 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... A robust low level jet feeding plentiful moisture ahead of a developing low over the Plains will provide the moisture and instability needed for numerous thunderstorms across a large portion of the upper Midwest from far eastern Nebraska northeast through the Arrowhead of Minnesota. The storms are likely to be their strongest Sunday night. This will coincide with the diurnal strengthening of the low level jet. Clusters of showers and storms will develop with increasing forcing ahead of a slow moving upper level trough over the High Plains. The storms are likely to train as several rounds of upper level energy support several rounds of storm clusters. Much of Iowa saw beneficial rains overnight tonight, such that by Sunday night, the soils will still have some moisture left over. Urban concerns, especially in and around the Twin Cities could also locally increase the flash flooding threat. Significant dry time in between the storms should limit the flooding threat such that the inherited Marginal still looks good, and few changes were needed. Wegman Day 3 Valid 12Z Mon May 18 2026 - 12Z Tue May 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER MIDWEST... An upper level trough that has been "stuck" over the northern High Plains for a few days will finally begin to eject across the northern Plains on Monday. This will occur because a strong shortwave on its southern edge near the 4 Corners region will help to nudge the trough eastward. This strong shortwave will run into plentiful Gulf moisture that will continue streaming northward across the middle of the country, as it has been doing for the past several days. A cold front on the northwestern boundary of that low level jet will provide ample surface forcing for the storms. The initial meeting of the warm, moist air mass to the east and the cooler, drier air mass over the northern Plains will be along the Missouri River from far northeastern Kansas north across much of western Iowa. This will be the corridor where the heaviest rainfall totals are expected on Monday through Monday night. This is the same region that will have been hit by multiple days of scattered heavy rainfall. By Day 3/Monday, soils should be much closer to saturation. Thus, with the greatest rainfall amounts areally expected on Monday, the flooding threat increases into the Slight Risk category. Any storms that form will continue to rapidly progress northeastward into the upper Mississippi Valley. This area too will have seen multiple days of storms by Monday. Thus, expecting wetter than normal soil conditions in most areas by Monday, the Slight Risk area was expanded northward to cover the upper Mississippi Valley to the western tip of Lake Superior. There remains some uncertainty as to where the axis of heaviest rainfall will set up. Thus, the surrounding Marginal covers extensive real estate east of the Slight from northern Illinois north through much of the U.P. of Michigan. While expected rainfall totals are lower in this region, greater urban concerns are present in this area, which supports the Marginal Risk. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt