Flash Flood Guidance
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Issued by NWS
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071 AWUS01 KWNH 261853 FFGMPD FLZ000-GAZ000-ALZ000-270000- Mesoscale Precipitation Discussion 1068 NWS Weather Prediction Center College Park MD 253 PM EDT Thu Sep 26 2024 Areas affected...Central FL Panhandle into central GA Concerning...Heavy rainfall...Flash flooding likely Valid 261852Z - 270000Z Summary...Nearly continuous rainfall with rates of 1-3"/hr will persist into this evening ahead of Hurricane Helene. Training of these rates will result in widespread additional rainfall of 2-3" by this evening, with local maxima around 5" possible. This will expand and enhance ongoing flash flooding, with significant impacts likely in the central FL Panhandle. Discussion...The GOES-E visible imagery this afternoon shows a well defined and intensifying Hurricane Helene across the Gulf of Mexico blifting north towards the Florida Panhandle. Downstream of Helene, impressive moisture advection is persisting on strengthening SE flow around the hurricane, driving PWs to as high as 2.5" as measured by GPS, which is above the daily record for the central FL Panhandle and into southern GA. This exceptionally moist air is pivoting northward into an area of intense deep layer ascent driven via strong mid-level divergence overlapping the diffluent RRQ of a northward arcing jet streak. Additionally, a cold front draped from northern GA into the Gulf of Mexico is helping to produce focused moisture convergence along which heavy rain rates will train. 24-hr MRMS rainfall across this region has been generally 4-8" with locally as much as 12", and ongoing rainfall rates of 1-2"/hr are producing MRMS FLASH unit streamflow responses as high as 300-500 cfs/smi. Flash flooding is currently ongoing. During the next several hours, moisture north of Helene will continue to surge northward and impinge into a region of focused ascent from the central FL Panhandle to as far north as Atlanta, GA. This will likely result in both an expansion and intensification of rainfall, with rates potentially peaking around 3"/hr in stronger convection, although will more commonly be 1-2"/hr. The strengthening 850mb inflow will not only enhance moisture transport, resulting in stronger moisture convergence onshore and into the front, but also cause Corfidi vectors to become anti-parallel to the flow near the Gulf Coast, merging towards parallel along the front. This suggests increased backbuilding and training of echoes within spiraling and convergent convective clusters, and the HREF neighborhood probabilities indicate a 30-60% chance for 3" from the Forgotten Coast of FL northward along the front into central GA, while the WoFS confines the highest probabilities to the FL Panhandle where locally 4-5" is possible. This rain will be occurring atop soils that are fully saturated from heavy 24-hr rainfall,. This is reflected by NASA SPoRT 0-10cm RSM that is above 85%, and resultant 3-hr FFG as low as 0.75-1.5". This will likely be exceeded in many areas, and where the heaviest rates train across the most vulnerable soils, especially in the vicinity of Apalachicola, resultant impacts will become significant even before Helene`s landfall later this evening. Weiss ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...BMX...FFC...MOB...TAE... ATTN...RFC...SERFC...NWC... LAT...LON 34078422 33438374 32898324 32088278 31858354 31248398 30698390 30068403 29838433 29528491 29678553 30278618 31508624 33388549 34058495