Flash Flood Guidance
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Issued by NWS
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338 AWUS01 KWNH 141523 FFGMPD FLZ000-142100- Mesoscale Precipitation Discussion 0436 NWS Weather Prediction Center College Park MD 1123 AM EDT Fri Jun 14 2024 Areas affected...South Florida and the Florida Keys Concerning...Heavy rainfall...Flash flooding possible Valid 141522Z - 142100Z Summary...Showers and thunderstorms are expected to redevelop across South Florida and train to the east through the afternoon. Rainfall rates of 2-3"/hr are expected, producing 1-3" of additional rain with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this morning indicates widespread showers and thunderstorms bracketing the southern Florida Peninsula, but so far activity has struggled to develop onshore. During the past hour, a few showers have begun to shift northeast onto the southern Lee Island Coast and the Everglades in response to increasing instability over the land. Recent local VWPs indicate a subtle veering to the 850mb winds, and although wind speeds remain light, this is sufficient to draw the higher instability and moisture northward to erode the morning CIN. Rainfall rates offshore have been estimated via local radars to be 1.5-2.5"/hr. This convection is developing within the increasing thermodynamics thanks to strengthening ascent through modest low-level convergence along a trough extending from a wave of low pressure off the NC coast, enhanced by convergence at the nose of this modest LLJ and modest upper level diffluence. The morning U/A soundings from both KEY and MFL had PWs that were much lower than 24-hrs ago, primarily due to drier air noted above 700mb, but still above the 90th percentile for the date. Additionally, freezing levels and warm cloud depths were still around 16,000 ft and 14,000 ft, respectively, supporting efficient warm-rain collision processes. This will support heavy rainfall rates in any convection that develops today, for which HREF neighborhood probabilities indicate a 40-50% chance for exceeding 2"/hr, and the 15-min HRRR accumulations support 3"/hr at times. Although there remains uncertainty in the coverage of thunderstorms this aftn, the CAMs are in relatively good agreement in a stripe of heavier rainfall from SW FL through Biscayne Bay and points south, where 1-3" of rainfall is likely and the HREF neighborhood probabilities indicate a 10-30% chance for an additional 5 inches in some areas. These higher rainfall amounts appear most likely in portions of SW FL where propagation vectors align anti-parallel to the mean wind and collapse to less than 5 kts, suggesting more prolonged backbuilding into the higher instability just offshore. The greatest risk of these higher rainfall totals appear to focus in SW FL and across the Everglades, somewhat limiting the flash flood risk, despite soils that are fully saturated from recent heavy rainfall. The NASA SPoRT 0-10cm RSM is above 80% across this region, producing an environment more susceptible to rapid runoff due to limited infiltration potential. Where any of these impressive rates can train, especially atop the most sensitive soils or in any urban areas, renewed instances of flash flooding will be possible. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...KEY...MFL... ATTN...RFC...SERFC...NWC... LAT...LON 26348053 26348007 26008014 25768011 25428017 25078033 24838062 24578122 24468162 24448198 24468219 24518231 24658208 24758173 24938147 25078135 25198130 25498133 25728154 25848171 25998166 26188123