Subj : MESO: Heavy rain/flooding To : All From : Mike Powell Date : Sun Aug 06 2023 08:11:00 AWUS01 KWNH 061300 FFGMPD MOZ000-MNZ000-IAZ000-SDZ000-061900- Mesoscale Precipitation Discussion 0876 NWS Weather Prediction Center College Park MD 859 AM EDT Sun Aug 06 2023 Areas affected...Iowa and southern Minnesota Concerning...Heavy rainfall...Flash flooding possible Valid 061300Z - 061900Z Summary...Showers and thunderstorms will increase in coverage this morning and then drift slowly to the northwest across Iowa into Minnesota. Rainfall rates of 2"/hr or greater at times are likely, which through slow motions could produce 2-4" of rain. Flash flooding is possible. Discussion...The regional radar mosaic this morning shows expanding coverage of showers and isolated thunderstorms across central Iowa. This convection is blossoming downstream of a stacked low pressure moving eastward out of SD, with downstream enhanced ascent driven by height falls, impressive PVA, and modestly coupled upper jet streaks. This ascent will continue to impinge into favorable thermodynamics characterized by PWs of 1.5 to 1.7 inches measured by morning UA soundings, around the 90th percentile for the date, collocated with MUCAPE above 1000 J/kg analyzed by the SPC RAP. Within this environment, rainfall rates have been estimated via KDMX and KFSD to exceed 2"/hr, and while this may be a bit overdone, it is reflective of the possible intensity developing through this aftn. The stacked low will shift slowly eastward through the day, maintaining robust deep layer ascent across the area. At the same time, S/SE flow downstream of the wave should drive PWs to above 1.75" this aftn, coincident with MUCAPE remaining 1000-1500 J/kg. Despite significant cloud cover which may temper surface-based instability except in clearings, impressive theta-e advection lifting cyclonically into a TROWAL should supply ample elevated instability to persist heavy rain rates. This is likely manifesting as the high MUCAPE in model output, and when combined with a well-collocated overlap of the highest PW pool and warm cloud depths surging above 11,000 ft, will drive extremely efficient warm rain processes. This will result in rainfall rates which the HREF indicates could exceed 2"/hr, with the HRRR suggesting brief 3+"/hr rates noted via the 15-min rainfall accumulation product. Storms that develop will generally move slowly NW on 10-15 kt 0-6km mean winds, but increasingly anti-parallel and collapsing Corfidi vectors suggest considerable backbuilding potential to result in training and at times storm motions of just 0-5 kts. Where the most persistent training can occur, rainfall of 2-4" is possible as shown by the HREF and experimental RRFS TL ensemble probabilities. Although 7-day rainfall across northern IA and southern MN has been well below normal according to AHPS, heavy rain did impact parts of this area in the past 24-hours. This has resulted in nearly saturated top-soils according to the HRRR, and locally compromised FFG as low as 0.75-1.5"/3hrs. While the slow motions of any intense rainfall rates could result in instances of flash flooding through the aftn, the greatest risk will be where training can occur atop these most vulnerable soils. Weiss ATTN...WFO...ARX...DMX...DVN...EAX...FSD...MPX...OAX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 44479441 44219309 43479205 42409142 41139128 40439170 40349241 40859342 41439461 41799562 42379619 42889648 43299648 44069639 44379578 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .