Flash Flood Guidance
Issued by NWS
Issued by NWS
671 AWUS01 KWNH 211202 FFGMPD MNZ000-SDZ000-NDZ000-211800- Mesoscale Precipitation Discussion 0296 NWS Weather Prediction Center College Park MD 802 AM EDT Tue May 21 2024 Areas affected...Eastern SD...West Central MN...Far Southeast ND Concerning...Heavy rainfall...Flash flooding possible Valid 211200Z - 211800Z SUMMARY...Several hours of heavy and efficient rainfall rates that can approach 1"/hr may result in flash flooding this morning, especially low lying spots and in areas with sensitive soils. DISCUSSION...A potent 500mb trough over eastern CO is taking on a negative tilt this morning, resulting in a blossoming baroclinic leaf signature over the Midwest on GOES-16 water vapor imagery. This is further enhanced by the region lying beneath a dual-jet structure at 250mb with the divergent right-entrance region of a 130kt jet streak over south-central Canada paired with the divergent left-exit region of a 110kt jet streak in the southern High Plains. Exceptional synoptic scale forcing will work in tandem with 925-850mb frontogenesis that is oriented from SW-NE from southeast SD to central MN and becoming more and more pronounced by the hour according to RAP mesoanalysis. The frontogenesis is a byproduct of not only strengthening WAA within that layer, but also a quickly strengthening IVT over the Nation`s Heartland. NAEFS shows IVT values in southern MN and eastern SD that are as high as 500 kg/m/s around 12Z this morning. By 18Z, the IVT values in central MN will surpass 750 kg/m/s and be well above the 99.5 climatological percentile. As the morning unfolds, the moisture out ahead of the developing 850mb low will eventually wrap around the northern and western flanks of this low, eventually leading to a robust deformation axis that will envelope most of the highlighted region. Instability will generally be elevated and not in abundance given the thick cloud cover and the precipitation shield occurring on the opposite side of the where the warm sector will be. That being said, model soundings from the RAP/HRRR/3kmNAM all suggest warm cloud layers will be 9,000ft deep or even deeper in some cases with low-mid level RH values >90%. This will support highly efficient rainfall rates despite the absence of deep convection this morning and through midday. The 06Z HREF`s 6-hr LPMM between 12-18Z that depicted >2" amounts near the border of northeast SD and western MN. Note that for some areas, especially the area just referenced, the pivoting axis of the deformation zone will likely continue beyond 18Z and may require a follow up MPD depending upon how much rainfall will have occurred this morning. FFG`s may not be surpassed in the 1-hr timescale, but 3-hr FFGs in some areas are 1.5-2.0" and hourly rainfall rates could range between 0.75-1.00"/hr this morning. Storms in central MN have already produced as much as 0.60"/hr rainfall rates and the best moisture advection has yet to occur. With all that said, there is the potential for flash flooding in the more flood prone areas where soils are more sensitive and in lower lying areas. Mullinax ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ABR...DLH...FGF...FSD...MPX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 46759453 46269378 45679409 45089490 44519578 43539665 43029715 43099811 43769828 45029781 45979676 46609548