Flash Flood Guidance
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Issued by NWS
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693 AWUS01 KWNH 201831 FFGMPD WIZ000-ILZ000-202330- Mesoscale Precipitation Discussion 0291 NWS Weather Prediction Center College Park MD 231 PM EDT Mon May 20 2024 Areas affected...Southern Wisconsin, extreme northern Illinois Concerning...Heavy rainfall...Flash flooding possible Valid 201830Z - 202330Z Summary...Showers and thunderstorms increasing downstream of a potent vorticity impulse will train to the N/NE through the afternoon. Rainfall rates of 1"/hr or more are likely, which could produce 1-3" of rainfall. This could result in at least isolated instances of flash flooding. Discussion...A potent shortwave noted on GOES-E WV imagery this afternoon is pivoting northeast across IA/IL/WI, producing downstream ascent through divergence and height falls towards Lake Michigan. This impressive lift is being aided by the RRQ of an upper jet streak which is gradually traversing to the east, and robust isentropic ascent as low-level flow traverses atop a warm front to the south. Together, these forcing mechanisms are driving robust lift into thermodynamics favorable for heavy rainfall characterized by GPS measured PWs as high as 1.4 inches, above the 90th percentile for the date, and SPC RAP analyzed MUCAPE of 1000-2000 J/kg. During the past 1-2 hours, convective coverage as rapidly expanded as noted via the regional radar mosaic, with radar estimated rainfall rates reaching above 1"/hr on KMKX WSR-88D. The high-res CAMs are generally under-initializing the current reflectivity observed via the radar, suggesting that the environment is more favorable than progged by models. As the potent shortwave advects northeast into the evening, pronounced S/SW flow downstream of it, with 850-300mb winds reaching 30-40 kts, will surge PWs as high as 1.6 inches and SBCAPE potentially climbing towards 2000 J/kg along and just south of the slow northward advance of the warm front. With ascent likely to remain impressive across the area, convective development is expected to continue to expand and intensify, with organization of cells likely within bulk shear reaching 35-45 kts. This suggests that rainfall rates will reach at least 1"/hr as noted by HREF exceedance probabilities reaching 30-50%, and may at times approach 2"/hr as reflected by HRRR 15-min accumulations of around 0.5". As storms continue to develop and organize, they will encounter 0-6km mean winds that are progressive around 40 kts, but aligned to the layer Corfidi vectors to support likely training training of echoes from SW to NE. Where the longest duration of training occurs, 2-3" of rainfall is possible. This portion of the Upper Midwest has been dry recently with AHPS 7-day rainfall departures generally just 25-50% of normal which has resulted in 40cm soil moisture below the 30th percentile according to NASA SPoRT. Despite that, FFG is in some places as low as 1"/1hr and 1.5"/3hrs, especially around urban corridors, which could be exceeded by these training heavy rain rates. This could result in areas of rapid runoff, with training over any urban areas producing the greatest chance for any instances of flash flooding. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ARX...DVN...GRB...LOT...MKX... ATTN...RFC...NCRFC...NWC... LAT...LON 44188865 44128810 43978771 43638762 43008768 42438773 42038786 41948836 42058894 42408951 42578987 43019011 43898957