Mesoscale Precipitation Discussion 0489 NWS Weather Prediction Center College Park MD 200 PM EDT Sat Jun 22 2024 Areas affected...Northern and Eastern IA...Southern MN...Southern WI...Northern IL Concerning...Heavy rainfall...Flash flooding likely Valid 221800Z - 230000Z SUMMARY...Additional rounds of heavy showers and thunderstorms are expected to develop and expand in coverage this afternoon and going into the evening hours. Sensitive antecedent conditions with high streamflows and wet/saturated soils will favor a likelihood of flash flooding. DISCUSSION...GOES-E WV suite shows a strong shortwave trough advancing off to the east across the northern Plains which will proceed downstream toward the Upper Midwest this afternoon. Favorable DPVA associated with this coupled with interaction with a quasi-stationary front draped from far eastern NE through northern IA and southwest WI will set the stage for additional rounds of heavy showers and thunderstorms going through the afternoon hours. Radar imagery already depicts a broken area of elevated showers and thunderstorms advancing into southwest MN, and as the approaching energy begins to interact with an increasingly unstable airmass reloading along the front just down to the south and east, there will be the initiation of new rounds of convection. MLCAPE values across northern IA in close proximity to the front have risen to 1500+ J/kg over the last couple of hours, and additional boundary layer destabilization is expected with more diabatic heating over the next few hours. This coupled with a belt of stronger effective bulk shear values of 40 to 50 kts and southwest low-level flow overrunning the front should help to initiate new rounds of convection in a general west to east fashion through the mid to late-afternoon hours across northern IA and eventually spreading into southern WI and northern IL. Proximity of a pre-existing outflow boundary over northeast IA through northern IL may also act as a catalyst for convection to initiate and focus by late this afternoon. High PWs of 1.75 to locally a little over 2 inches are pooled across the region based on the latest GPS-derived data, and the CIRA-ALPW data shows highly concentrated moisture in the warm 850/500 mb portion of the cloud-bearing layer. This will yield very efficient rainfall processes as convection initiates and expands in coverage with rainfall rates that may reach 2.5+ inches/hour. The 12Z HREF guidance suggests as much as 3 to 5 inches of rain may focus by early this evening across areas of northeast IA into southwest WI where there will also be concerns for some cell-training. Given the high rainfall rates, isolated heavier totals cannot be ruled out. Lesser amounts should generally be noted across southern MN, but with the elevated convection here, some localized additional 1 to 3 inch amounts will be possible. Given the sensitive wet/saturated soil conditions and high streamflows in general, these additional rains are likely to result in areas of flash flooding. Urban flash flooding will also become a concern by this evening at least locally. Orrison ATTN...WFO...ARX...DMX...DVN...FSD...GRB...LOT...MKX...MPX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 45319425 45169288 44729158 44309041 43798817 43118740 42168753 41738823 41568977 41729136 42219341 43249504 44739534Download in GIS format: Shapefile | KML
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