Subj : MESO: Heavy Rain/Flooding To : All From : Mike Powell Date : Wed Dec 13 2023 14:30:00 AWUS01 KWNH 131457 FFGMPD FLZ000-132100- Mesoscale Precipitation Discussion 1200 NWS Weather Prediction Center College Park MD 953 AM EST Wed Dec 13 2023 Areas affected...Southeast FL and FL Keys Concerning...Heavy rainfall...Flash flooding possible Valid 131500Z - 132100Z Summary...Rainfall rates of 1-2"/hr may lead to short-term localized totals of 3-5"+ through the early afternoon. Isolated instances of flash flooding are possible. Discussion...Heavy showers and thunderstorms are slowly migrating closer to the southeast Florida coast this morning, ahead of a quasi-stationary boundary that has setup across the Florida Straits and into the Bahamas. Despite the date on the calendar, the air mass is of tropical origin (thanks to ridging nosing in from the the Atlantic and Carribean) with a wet season-like low-level easterly flow pattern transporting in ample moisture along the frontal boundary. This has resulted in highly anomalous tropospheric moisture across South Florida, with PWATs of 1.94 and 1.99 at MFL and KEY, respectively (near the max moving average per SPC sounding climatology). The mesoscale environment is also characterized by modest (but increasing) instability with SB CAPE of 500-1000 J/kg along the Gold Coast and just off the FL Keys into the Straits (per SPC mesoanalysis). In addition, the right-entrance region of a subtropical jet streak is providing synoptic scale lift and upper-divergence, along with deep layer shear of near 30 kts. The latest (12z) hi-res CAM guidance paints a very mixed picture of potential outcomes with regard to QPF. Most of the CAMs (including the ARW, ARW2, and NAM-nest) suggest almost no heavy rainfall through 20-21z across southeast FL and the FL Keys. These are likely far too dry given observational trends. The FV3, on the other hand, is a significant outlier depicting localized totals of 6-9" along the Gold Coast. Only the HRRR suggests an in-between solution with 1-3" localized totals across the region. But given the extreme solution of the FV3,, the HREF PMM and exceedance probabilities (which are still old, from 00z) are a good bit higher, suggesting a solution between the HRRR and HREF with localized totals of 3-5" (probably being most likely). It's also worth mentioning that the experimental RRFS and accompanying ensemble (also the 00z runs) depict a similar solution to the FV3 (not surprising, given they share the same model core) with a near 10" bullseye near the Miami metro in the short-term. Regardless of the resulting short-term totals, the environment will be supportive of efficient rainfall rates of 1-2"/hr. Given the uncertainty in both the realization and positioning of more extreme rainfall totals, localized/isolated instances of flash flooding are considered to be possible. East-to-west training/repeating of stronger convection would need to set-up along the Gold Coast (or Middle/Upper Keys) for 3-5" amounts to be realized, and the upper-end (6"+) solutions from the FV3-core driven CAMs appear to be unlikely (perhaps a 10-20% chance). However, concerns for locally significant impacts remain given the favorable anomalous environment and vulnerability of the greater Miami metro area. Churchill ATTN...WFO...KEY...MFL... ATTN...RFC...SERFC...NWC... LAT...LON 26557954 25637914 24618012 24248135 24568154 25668091 26478024 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .