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Flash Flood Guidance
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Issued by NWS
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759 AWUS01 KWNH 152132 FFGMPD MOZ000-MNZ000-IAZ000-KSZ000-NEZ000-SDZ000-160300- Mesoscale Precipitation Discussion 0443 NWS Weather Prediction Center College Park MD 531 PM EDT Sat Jun 15 2024 Areas affected...far northeast KS, eastern NE, western IA, far southeast SD Concerning...Heavy rainfall...Flash flooding possible Valid 152130Z - 160300Z Summary...Showers and thunderstorms will increase along a northward advancing warm front through the evening. Rainfall rates within developing convection could reach 2"/hr at times, which through short-term training could produce 2-3" of rain with locally higher amounts. Flash flooding is possible. Discussion...The GOES-E IR imagery this afternoon indicates expansion of rapidly cooling cloud tops associated with deepening convection from central KS through eastern NE. These thunderstorms are developing immediately ahead of a warm front which is slowly lifting northward, with significant additional ascent provided via an impressive shortwave ejecting out of KS noted on the GOES-E WV imagery, and within the RRQ of a modest jet streak pivoting into IA. This deep layer lift is occurring into intensifying thermodynamics as the atmosphere recovers from this mornings outflow boundary as reflected by increasing mid-level lapse rates noted by the SPC 3-hr lapse rate change, with PWs reaching 1.5 to 1.8 inches overlapping a ribbon of MLCAPE as high as 3000 J/kg according to the SPC RAP analysis. As the warm front gradually lifts north and the shortwave pivots into the region, ascent is forecast to intensify into the evening while thermodynamics become even more impressive noted by PWs progged to reach 2 inches, which would be a daily record for the area, combined with the strong MLCAPE. Together these should result in a rapid expansion of convective development, especially where low-level convergence and isentropic ascent merge in the vicinity of the warm front. Additionally, the isentropic lift atop the warm front should rapidly intensify as the 850mb LLJ surges to around 40 kts, reaching nearly double the mean 0-6km wind to produce intense moisture convergence into eastern NE and western IA this evening. This surging LLJ will also drive a veering of the propagation vectors to become increasingly aligned to the right of the mean wind, indicating a higher training potential in the next few hours. With HREF neighborhood probabilities indicating a 40-50% chance of 1"/hr rates (and 10-20% for 2"/hr), where training or multiple rounds of heavy rain occur, total rainfall will likely reach 2-3", and there is a low potential (5-10%) for isolated amounts reaching 5". FFG across this region is generally 1.5-2"/3 hrs, although 0-40cm soil moisture from NASA SPoRT is generally around normal. This could have at least a somewhat moderating effect on the flash flood potential, but the HREF 3-hr FFG exceedance probabilities still reach 20-30%. This suggests an increasing flash flood potential into the evening, with the greatest risk likely beneath any training to enhance the duration of these rain rates. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...DMX...EAX...FSD...GID...LBF...OAX...TOP... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 43769653 43659545 42849479 41459451 39759528 39249622 39129731 39359796 40049853 42149825 43429740