Flash Flood Guidance
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Issued by NWS
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058 AWUS01 KWNH 210524 FFGMPD MNZ000-IAZ000-NEZ000-SDZ000-211115- Mesoscale Precipitation Discussion 0477 NWS Weather Prediction Center College Park MD 123 AM EDT Fri Jun 21 2024 Areas affected...Southwest SD...Southwest & South-Central MN...Northwest IA...Ext Northeast NEB... Concerning...Heavy rainfall...Flash flooding likely Valid 210525Z - 211115Z SUMMARY...Prolonged moderate rainfall within WAA with occasional embedded bursts up to 2"/hr. Additional 3-4" Totals across saturated soils likely to continue broad area of flash flooding conditions through early morning. DISCUSSION...05z surface map depicts a meso-low along the leading outflow boundary in proximity of the Missouri River Valley along the NEB/SD line; a weakening outflow boundary extends southwest across central NEB. Eastward the warm/stationary front extends nearly due east to SUX-EBS-MIW-VTI-MXO, which generally delineates 70+ Tds to the south. Surface map also denotes a well defined surface pressure trof that extends northeast across SE SD into SW MN up to RWF-MIC/MGG; and generally appears to be a reflection of the best mid to upper level divergence ascent pocket along the southern entrance to the polar jet where ageostrophic winds are increased for favorable evacuation. As such, the MCV/shortwave continues to track eastward into that axis and the LLJ seems to be veering in response to it. This is reducing the deep layer convergence along the effective cold front/outflow boundary but increasing orthogonal convergence to the frontal boundary within increasing warm advective isentropic ascent. Instability north into MN had been slow to increase but with this veering MUCAPE values are increasing back toward the 500-1000 J/kg range. This should support broken/scattered embedded convective elements within the WAA isentropic ascent. Given the LLJ is 30+kts and orienting through the Missouri Valley where the Q-Axis resides allowing for upstream 1.75" total PWats to replenish the 1.75-2" values between the pressure trof and frontal zone downshear of the MCV into SW MN. This will allow for a broad long area of 1-1.5"/hr moderate rain-rates and where convergence/instability maximize embedded thunderstorms capable of 2"/hr rates will occur. The deep layer steering flow should allow for an long west to east WAA axis for longer duration of this moderate rainfall...mainly across SW MN with 3-4" totals in a county wide swath. Broader 1-3" totals surround for a surrounding counties in south-central MN and Northwest IA. This intersects saturated grounds where AHPS 7day anomalies are 300-400% between Sioux Falls and Minneapolis and 0-40cm saturation ratios are 70-85% and well into the 80th-90th percentile; and this is not to account for areas of SE SDAK/NW IA that had 2-4" earlier today where FFG values remain below .5" at all time periods. So with limited capacity and these totals, a broad axis of flash flooding is likely to occur through the remainder of the overnight period. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...DMX...FSD...MPX...OAX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 45189401 44989312 44369296 43459363 42619484 42279618 42539784 43239835 43849771 44509633 44899525