Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 12 2024 00:14:00 FOUS30 KWBC 120013 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 813 PM EDT Fri Oct 11 2024 Day 1 Valid 01Z Sat Oct 12 2024 - 12Z Sat Oct 12 2024 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHEAST FLORIDA... A mean easterly low-level flow will bring showers and possible thunderstorms into portions of the Gold Coast overnight. Moisture is expected to build as an upper level shear line backs north and west away from South FL, with precipitable water values rising towards 1.75" by 12Z. RAP forecasts indicate some increase in instability with time, in tandem with the moisture increase, as=20 winds at 850 hPa veer from east to more southeast. While the 18z=20 HREF guidance isn't showing much overnight, the ingredients appear=20 to exist from a moisture and instability perspective. Plus,=20 effective bulk shear of near 25 kts is nearby, so there's some=20 concern for short convective bands temporarily training downwind of some of the islands of the northwest Bahamas. When combined with=20 the 200-600% of average rainfall this past week, thought it the=20 course of least regret to raise the risk level to Marginal on this=20 update. This was coordinated with the MFL/Miami FL forecast office. Roth Day 2 Valid 12Z Sat Oct 12 2024 - 12Z Sun Oct 13 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR... Moisture and instability return northward into southern Florida Saturday into Saturday night, with PWs increasing towards 2.25" and CAPE over 1000 j/kg. Meanwhile a weak wave moving into the eastern Gulf of Mexico may result in an uptick in low level flow and convergence. Thus expect we should gradually see an uptick in convective coverage this weekend over the Keys into south FL. The overall synoptic pattern still looks to favor slow moving convective cells along and near the coastal convergence axis give easterly low level flow and westerly upper level winds. Timing remains a bit uncertain. Even though the better convective coverage may end up more over the Keys this period (where flash flooding is harder to come by) the 12Z suite of model guidance still suggests the increasing moisture and instability poses at least a conditional threat of localized flash flooding Saturday into Saturday night into the more urban areas of southeast FL. If deep convection does indeed develop then urban flash flooding is possible from the upper Keys into southeast FL. Bann Day 3 Valid 12Z Sun Oct 13 2024 - 12Z Mon Oct 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR... The risk on day 3 is a continuation of the threat on day 2 over Southeast Florida. The overall thermodynamic ingredients remain the same, however there is a chance that forcing will be stronger by Sunday. Still some timing and positional differences amongst the guidance, but the general consensus is for a weak wave to move across FL Sunday, which should both result in a subtle uptick in mid/upper forcing and locally enhance lower level convergence. The 12Z suite pf numerical guidance still pointed to a somewhat greater chance of loosely organized convective clusters Sunday into Sunday night...and with wind fields still supporting slow moving cells...the risk of isolated urban flash flooding is still possible but not enough to upgrade beyond a Marginal risk area at this point. Will continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!76GSttvtjspRqYgJoKb4CCv23Deykio8Dd-o6bEIOYAw= 2AWy5MZrpBjWG6tPV5GyFnyewwQayCcpB7JJ0tcrLO3BISI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!76GSttvtjspRqYgJoKb4CCv23Deykio8Dd-o6bEIOYAw= 2AWy5MZrpBjWG6tPV5GyFnyewwQayCcpB7JJ0tcr5Mvq1hg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!76GSttvtjspRqYgJoKb4CCv23Deykio8Dd-o6bEIOYAw= 2AWy5MZrpBjWG6tPV5GyFnyewwQayCcpB7JJ0tcrjUm6HAg$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .