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Flash Flood Guidance
Issued by NWS
Issued by NWS
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509 AWUS01 KWNH 141455 FFGMPD MNZ000-SDZ000-NDZ000-142045- Mesoscale Precipitation Discussion 0882...Corrected NWS Weather Prediction Center College Park MD 1054 AM EDT Wed Aug 14 2024 Corrected for Concerning tag to Flash Flooding Likely Areas affected...North Dakota...Ext Northern South Dakota... Concerning...Heavy rainfall...Flash flooding likely Valid 141450Z - 142045Z SUMMARY...Strong dynamics/moisture flux into confluence zone should support heavy rain rates and localized totals of 3-5" are possible resulting in possible flash flooding through late afternoon. DISCUSSION...GOES-E WV suite depicts the leading shortwave/height-falls of broad large scale Pacific Northwest trough is starting to press eastward across the Northern High Plains, with NNW to SSE ridge axis across northern NDAK back through the Lower Missouri River Valley where embedded MCV/southern stream shortwaves are moving through with the most potent crossing out of NEB into SW MN at this time. The polar stream shortwave appears to be negatively tilted across WY with a favorable south to north right entrance 60kt jet entrance across E ND at this time. The combination of strong dPVA and divergence aloft is spurring deepening sfc to 850 mb low along/north of the Black Hills, while backing low level flow and effective warm conveyor belt into eastern SD and to central ND. 20-30kts of 850mb LLJ continues to flux increasing moisture aloft with values of 1.5 total PWat, increasing to 1.75" over the next few hours. The orientation of the pattern is also strengthening FGEN/confluence along and downstream of the 850mb through central ND as westerly flow impinges and strengthening convergence, flux of the amplified moisture will overcome more limited instability/lapse rates (1000-1250 J/kg MUCAPE) to generate stronger thunderstorms with enhanced rainfall over the next 3-6hrs. Already a few clusters have developed along and west of the boundary in SW ND and along the boundary exiting Perkins, SD into Grant county, ND. Given strength of height-falls/dPVA, cells will move northward along the boundary with reducing forward propagation vectors the further north going along the deformation zone. This should allow for back-building and favorable training/merging profile for 3-4 hrs. Rates of 1.5"/hr occasionally reaching 2"/hr, seem probable to induce a spot or two of 3-5" totals along the line likely resulting in localized flash flooding into early evening, confidence is enhanced by 3"/6hr probability from the 06z HREF of 45-50% with a spot of 20% of 5". While 12z HREF is still coming in, the 12z components still continue to further support this potential if a shade further north than the 06z signal. Downstream across E ND, NE SD; warm advection in the q-axis suggests moderate rainfall with perhaps an embedded convective element may increase rainfall totals to be at or just below lower FFG values in the area. As such, the area is incorporated into this MPD, as there is a non-zero potential (therefore lower confidence) of FFG exceedance resulting in a possible incident of low-end flash flooding as well. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ABR...BIS...FGF... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 49140116 48939935 47689882 46339696 45579656 45229704 45319817 45769944 45590123 46220261 46630387 48010383 48770276