Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Fri Jul 11 2025 Day 1 Valid 01Z Sat Jul 12 2025 - 12Z Sat Jul 12 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID AND UPPER MISSISSIPPI VALLEY AS WELL AS FOR PORTIONS OF THE CENTRAL AND SOUTHERN HIGH PLAINS... 01Z Excessive Rainfall Discussion Update... The Slight Risk areas that were inherited from the day shift remained in place with few exceptions...the Slight risk area in Iowa being one such area where some territory was removed given the progression of the upper level forcing, Convection coming off the Rockies has been fairly disorganized so far...but better organization with a corresponding increase in the threat of heavy to excessive rainfall should materialize later with the development of a low level jet. Elsewhere...isolated convection should be weakening/dissipating quickly with the loss of daytime heating. Bann Previous Excessive Rainfall Discussion... ...Southern High Plains... 1852Z Update: In coordination with the local Amarillo WFO over the Texas Panhandle, a targeted SLGT risk was added for this evening as convection fires across Northeast NM and dives southeast as it develops along the leading edge of a cold front and weak shortwave reflection. Probs across the Panhandle remain relatively modest with CAMs signaling the potential for ~3" of rainfall in any area near the TX/NM border down through the Panhandle and northern Caprock of TX. The best threat aligns within the nose of weak LLJ positioned up near I-40. Despite drier soils encompassing the area, this is a case where locally higher rates and flanking storms can train and induce scattered bouts of flash flooding as the convection migrates to the southeast. Best threat will be within any urbanized settings, especially in and around Amarillo proper. The SLGT risk extends just into the northern fringes of the TX Caprock, mainly along I-87 south of Amarillo. Kleebauer ...Mid and Upper Mississippi Valley... 16Z Update: The overall evolution and anticipated flash flood concerns were slightly changed from the previous forecast, but the threat remains over much of the area outlined. There was some shift in the heavier precip focus a bit further north into WI, a lot to do with how the shortwave progression and a developing MCV are handled as CAMs indicate a east-northeast motion out of IA this evening leading to the primary vorticity exiting into southern WI overnight. The proximity of the disturbance will play a critical role in the strongest low-level convergence regime and flanking convective development, so this is something that will need to be monitored closely for near term adjustments. From a dynamical standpoint, the setup makes sense as the regional amplification expected can lead to disturbances gaining a bit more latitude, thus shifting the focus a bit further north, at least on the northern periphery of a shortwave. 12z HREF blended mean output still puts the relative QPF maxima in-of the Quad Cities area of IA with a strong consensus for heavy rain throughout much of WFO Davenport's CWA as everything materializes. This is evident with assessment of the latest HREF EAS prob fields for >2" signaling a widespread 30-60% signal within the CWA bounds, so the threat for elevated flash flooding prospects and a higher-end SLGT is certainly in play over Eastern IA. Neighborhood probabilities for >2 and >3" have increased across southern WI, especially southwest WI along the US151 corridor from the Mississippi river up near Madison. This adjustment is also reflected into the Milwaukee metro, but not as pronounced compared to area further south and west. As we move north, there's a growing consensus for any meso-low or defined SLP center to occlude by the end of the forecast cycle and shift focus for heavy precip along the northern and northwest flank of the circulation. 12z CAMs were pretty much in agreement on that type of evolution which can be a sneaky way of getting a secondary maxima to crop up in the short term. HREF prob fields are all over the threat as well with modest (20-35%) probs for exceeding 3 and 6hr FFG intervals. Compared to <10% for either probability in the 00z forecast, this is a pretty stark shift in the potential, one that has enough merit to warrant an expansion of the SLGT further north through WI where probabilities are highest. Further south into MO, trailing cold front will shift eastward with the progression likely forming a broad area of deep layer flow becoming increasingly uni-directional in-of the MO/KS border down into northeastern OK. This was very well depicted in the RAP 925-700mb wind vectors with forecasted convection likely initiating and situating southwest to northeast with cell motion along a similar path. This could manufacture another area of locally heavy rainfall capable of flash flooding. The greatest threat is likely over southeast KS, southwest MO into northeast OK where the mean wind is aligned best over an area where convection is expected overnight. HREF neighborhood probs for >3" is now upwards of 25-45% with >2" probs over 60% across the aforementioned area. This was enough to expand the previous SLGT risk further south to account for the threat. Kleebauer ..Previous Discussion.. A robust southwesterly low level jet streaming from the Plains into the Mid-Mississippi Valley will provide a steady supply of Gulf moisture into the region today. Meanwhile, a potent upper level shortwave trough will approach this area from the west. The combination of abundant moisture, instability, and forcing from the shortwave will lead to numerous thunderstorms from northern Missouri through eastern Iowa, northern Illinois, and far southern Wisconsin, including the Chicagoland area. An MCS moving across Iowa this morning will weaken in the morning daylight hours in typical behavior for MCSs. Rainfall totals yesterday in the area ranged from 1-4 inches, with the highest amounts across northern Illinois west of Chicago. This rainfall has likely contributed to saturating the soils in this area. Thus, the additional rainfall forecast for this area should result in widely scattered flash flooding. A higher-end Slight remains in place with only minor changes from inherited, namely to trim up the southern end of the Slight along the Kansas/Missouri border. Showers and thunderstorms are likely to develop over central Iowa as early as early this afternoon. As additional storms from in northern Missouri, and track northeastward along the Mississippi River, the potential for training storms will increase as the line of showers and storms moves into northern Illinois. Backbuilding may continue supporting additional thunderstorm formation as far south as St. Louis this evening. This will likely allow the potential for training storms to persist, increasing the flash flooding threat. Wegman ...Central High Plains... 16Z Update: Signal for locally heavy rainfall in-of an advancing MCS out of the Front Range still remains elevated between the triangle of Cheyenne/North Platte/Goodland with the best probabilities for >2" positioning in the middle of these 3 population centers. Rates between 1-2"/hr for multiple hrs will have the capability of localized flash flooding in the vicinity of this region within the High Plains. 3 and 6hr FFG exceedance probabilities remain steadily between 40-60% over the expected path of the complex, enough to maintain general continuity of the previous SLGT risk with only minor adjustments on the edges. Kleebauer ..Previous Discussion.. As a strong shortwave trough digs southeastward across the northern Plains, divergence at the base of the trough will increase the lift, supporting storms across northeastern Colorado this afternoon and evening. Meanwhile, typical leeside troughing will support thunderstorm formation immediately downwind of the Front Range, generally between Cheyenne and Denver along I-25. Storms may initially form as early as 22Z/4pm MDT. As the storms move east off the Front Range, they will encounter increasingly favorable lift from the shortwave, as well as some Gulf moisture tracking northward up the Plains. This will allow the storms to organize and grow upscale in coverage and heavy rain potential. It is in this region near the Colorado/Nebraska/Kansas tripoint which will have the greatest threat for flash flooding with some limited potential for training. With very low FFGs, especially across northeastern Colorado (1-1.5 in/hr), a Slight Risk upgrade was hoisted with this update. Some of the guidance suggests the storms will persist well into tonight generally across far northwestern Kansas, so the Slight Risk area includes that area as well. Wegman Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt