Flash Flood Guidance
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720 AWUS01 KWNH 180159 FFGMPD SDZ000-NDZ000-180730- Mesoscale Precipitation Discussion 0457 NWS Weather Prediction Center College Park MD 958 PM EDT Mon Jun 17 2024 Areas affected...northern South Dakota, central/eastern North Dakota Concerning...Heavy rainfall...Flash flooding possible Valid 180157Z - 180730Z Summary...Showers and thunderstorms moving across western ND will expand in coverage to the east while intensifying tonight. Rainfall rates of 1-2"/hr are likely, which through training could produce 2-4" of rain with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this evening depicts widespread showers moving out of western ND, and in the past 1-2 hours an intensification of reflectivity has been noted. This area of precipitation is being driven by forcing ahead of a potent shortwave embedded within an approaching trough axis, with height falls and upper diffluence in the RRQ of a departing jet streak contributing to the impressive synoptic ascent. At the same time a stationary front is draped across ND, with an intense overlap of PWs of 1.2-1.4 inches and MUCAPE nearing 1000 J/kg aligned south of this boundary. Regional VWPs indicate the 850mb LLJ has begun to intensify, reaching 40 kts from the SSW, which is impinging favorably into the front for additional ascent, while also drawing the more robust thermodynamics northward. Activity is blossoming more rapidly than the high res CAMS suggest, but the overall trend in the models is still supported. During the next few hours, the 850mb LLJ is progged to reach an incredible 50-60 kts across NE/SD, above the all-time max climatological percentile according to NAEFS. This will surge low-level moisture flux to above +5 sigma, resulting in an extreme convergence of moisture transport vectors into eastern ND. With increasing MUCAPE also occurring across that area, it is extremely likely that convective coverage will expand and intensify, with both the HREF and REFS probabilities suggesting a 50-70% chance of rainfall rates reaching 1-2"/hr. Mean 0-6km winds will continue to be elevated at 30-40 kts, but aligned Corfidi vectors to the stationary front and the mean wind indicates that training is expected, with some additional development S/SW into the greater instability supporting even more prolonged training even as the entire cluster pivots east. This could produce stripes of 2-4" of rain with locally higher amounts. 7-day rainfall according to AHPS has been well below normal across much of the area, although exceptions exist, especially in northern ND, generally north of the front. Fortunately, the heaviest rainfall footprint from the CAMS, which is supported by the ingredients, is generally expected to fall across drier soils where FFG is 2-3"/3hrs. The intensity of the rainfall could still produce rapid runoff across these areas however, so instances of flash flooding are possible. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ABR...BIS...FGF...UNR... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 48369853 48349741 48119717 47559696 46799704 46389769 45599978 45240059 44900132 44730195 44930247 46720156 47800038