Flash Flood Guidance
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Issued by NWS
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157 AWUS01 KWNH 250805 FFGMPD TNZ000-KYZ000-251300- Mesoscale Precipitation Discussion 0321 NWS Weather Prediction Center College Park MD 404 AM EDT Sat May 25 2024 Areas affected...south-central KY into Middle TN Concerning...Heavy rainfall...Flash flooding possible Valid 250800Z - 251300Z Summary...Locally heavy rainfall of 1-2"/hr may lead to short totals of 2-4". Isolated to scattered instances of flash flooding are possible. Discussion...Convection has been gradually growing upscale into a small MCS over portions of central KY into Middle TN early this morning, after initial multicell clusters collided and merged into a more linear single bow echo near the KY/IN/IL border region several hours ago. As convection grows downstream ahead of this bow, localized training of 1-2"/hr rates are beginning to occur along the southwest flank (just as storms reach more vulnerable terrain with 3-hr FFGs of 1.0-2.0"). The mesoscale environment ahead of this compact MCS is characterized by MUCAPE of 1000-2000 J/kg, PWATs of 1.3-1.6" (near the 90th percentile, per BNA sounding climatology), and effective bulk shear of 25-35 kts. In addition, strengthening of the LLJ (at 850 mb) to 25-35 kts just upstream (southwest) of the MCS may favor additional backbuilding of convection perpendicular to the bow following the initial passage of storms, occurring above the cold pool as warm air isentropically ascends (assisted by directional and speed convergence of the LLJ, fitting the conceptual model of a `bow and arrow` MCS, which was also apparent with the MCS over AR earlier in the night). To boot, the aforementioned downstream growth of convection (along an effective warm front aloft preceding the MCS) is oriented NNE to SSW, near parallel to the upwind propagation vector (directed SSW at 10-20 kts). This may allow for additional localized training (from the NNW to SSE, but in a more limited fashion) ahead of the main bow echo. This complex orientation of meso-beta storm structures was finally depicted reasonably well by the latest (06z) HRRR, which suggests a swath of 1.0-2.5" totals across much of central KY into Middle TN (from NW to SE), generally just north and east of the Nashville metro (but backbuilding along the western flank may directly impact the metro). The orientation of the 06z HRRR QPF also matches better with the other 00z HREF members, increasing confidence in the general outcome. That said, these totals from the HRRR (and HREF PMM for that matter) may be a bit underdone, as suggested by 00z HREF 40-km neighborhood probabilities of 20-40% and 10-20% for 2" and 3" thresholds, respectively. But even so, the short-term totals from the HRRR are enough to meet or possibly exceed the aforementioned FFGs locally, suggesting that isolated to scattered instances of flash flooding are possible. Should a more `worst case` scenario occur with the backbuilding convection, the max totals may end up quite close to the Nashville metro itself (as indicated by the HREF 3" exceedance probabilities of 10-20% located very near the metro), conditionally leading to locally significant flash flooding. Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...JKL...LMK...MRX...OHX...PAH... ATTN...RFC...LMRFC...OHRFC...NWC... LAT...LON 37438545 36928485 36428452 35788456 35498520 35808641 36578703 37178709 37418653