Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 31 2023 08:30:29 FOUS30 KWBC 310830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 AM EST Sun Dec 31 2023 Day 1 Valid 12Z Sun Dec 31 2023 - 12Z Mon Jan 01 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Mon Jan 01 2024 - 12Z Tue Jan 02 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Tue Jan 02 2024 - 12Z Wed Jan 03 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. The Marginal Risk area in the inherited forecast from southwest Louisiana southwest through Corpus Christi, Texas was removed with this update, in collaboration with HGX/Houston-Galveston, TX forecast office. Multiple reasons were noted for this change in the forecast. First, with a 100 kt jet moving overhead, any rainfall that develops Tuesday night (as much of the heaviest rainfall will occur after 00Z Wednesday) will be fast-moving, which will greatly reduce the chances any storms can produce heavy rains in any given area for much more than a few minutes, and also reduces the chances for training, as any storms will be moving along quickly. Second, instability will be meager, generally 300 J/kg or less, with the highest values of instability over the open waters of the Gulf. Not only will this reduce the ability for any storms that develop to produce high rainfall rates, but increases the chances any storms that can produce heavy rain stay over the Gulf. Minimal instability intrudes inland. Third, trends in the forecast have been coming down, with most areas now forecast to stay generally staying under 1 inch of total rainfall through 12Z Wednesday. FFGs are quite high along the Texas and Louisiana coasts, with the 1 hour FFGs above 2.5 inches at their lowest. Thus, it appears highly unlikely that even in the most flood-sensitive areas that these high FFG values will be overcome. Fourth, atmospheric moisture will be somewhat lacking, with PWATs struggling to get higher than 1.25 inches away from the immediate coast. A lack of available moisture will limit the ability for heavy rain to develop. Fifth and finally, soil moisture levels are near average across coastal central and south Texas, becoming drier and further below average as one goes northeastward into Louisiana. These dry soils are likely to absorb most of the rainfall, especially given the likely low rainfall rates. A Marginal Risk area was also considered for the Northern Coastal Ranges of CA for the same Tuesday night time period. A front will be making its way ashore may produce rainfall rates up to 3/4 inches per hour, generally between the Bay Area and Eureka. A progressive upper level pattern will move the upper low forcing the rainfall eastward into the coast rather quickly. The fast movement will allow the rain to progress eastward and inland quickly enough that overall total rainfall should stay under 2 inches in most areas. Thus, despite some recent wet weather and wetter-than-normal soil conditions in this area, the rainfall event should be short enough to preclude any flash flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Z5LfjSkWHVuNE-hMX3lZKWuSqwI1ua49VbqxM_fzUBc= KAWh4v_YJ64hCQeXg8rzPqVcI0-0VD-vxbLoi3uNmyCj2Z8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Z5LfjSkWHVuNE-hMX3lZKWuSqwI1ua49VbqxM_fzUBc= KAWh4v_YJ64hCQeXg8rzPqVcI0-0VD-vxbLoi3uNSRlv6c0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Z5LfjSkWHVuNE-hMX3lZKWuSqwI1ua49VbqxM_fzUBc= KAWh4v_YJ64hCQeXg8rzPqVcI0-0VD-vxbLoi3uNVcZ-90g$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .