Flash Flood Guidance
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Issued by NWS
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008 AWUS01 KWNH 121751 FFGMPD FLZ000-122330- Mesoscale Precipitation Discussion 0428 NWS Weather Prediction Center College Park MD 151 PM EDT Wed Jun 12 2024 Areas affected...South FL into the Upper FL Keys Concerning...Heavy rainfall...Flash flooding likely Valid 121750Z - 122330Z Summary...Rainfall rates of 1-3"/hr to continue across already soaked regions of South FL, producing additional localized totals of 5"+. Isolated to scattered instances of flash flooding likely (some locally significant). Discussion...A convective complex has gradually organized over the past several hours over South FL, most evidently identified via GOES-East with the emergence and expansion of very cold IR cloud tops (reaching -80deg C). This deep convective activity (with 30 dBZ echo tops indicated to be as high as 50k ft with the strongest updrafts) has produced impressive estimated hourly totals of 3-4" (per MRMS), though the highest totals are likely closer to 3" (given that MRMS has demonstrated a bit of a high bias over the past several hours, compared to local gauges). Even still, convection is expected to remain organized given ample instability (1000-3000 J/kg of ML CAPE), highly anomalous tropospheric moisture (with PWATs 2.4-2.8", near all-time records at the top of the range), and impressive effective bulk shear of 25-35 kts (near the 90th percentile, per MFL sounding climatology). Strong low-level moisture transport and convergence from the southwest should allow for continued convective development and maintenance with training/repeating of 1-3"/hr rates most favored from near Naples eastward along Alligator Alley/Tamiami Trail to the greater Miami metro. Hi-res CAMs have done reasonably well over the past 6 hours, given that estimated totals reached 6"+ over portions of South FL (though a bit farther inland than many CAMs anticipated, a result of deep convective activity growing into a more organized complex). Concerns for the next 6 hours exist across many of the same metro areas along the southwest FL coast, but increasingly concerns are also growing over southeast FL (i.e. greater Miami metro area), as some of the most impressive convection is poised to propagate towards the SSE (following the upwind propagation vector, eventually shifting into the Upper FL Keys as well). The CAM signals for additional significant accumulations remain strong, with the 12z HREF probability matched mean (PMM) indicating localized totals of 6"+, and 40-km neighborhood probabilities for 5" exceedance of 30-60%. Additional isolated to scattered instances of flash flooding are likely (especially given relatively widespread prior 24-hr totals of 3-7"). Some instances of flash flooding (particularly in poor drainage, urbanized terrain) may be locally significant. Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...KEY...MFL...MLB...TBW... ATTN...RFC...SERFC...NWC... LAT...LON 27418054 27017986 26057975 25078029 25008118 25628197 26808239