Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 08 2023 08:20:20 FOUS30 KWBC 080820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 AM EST Wed Nov 08 2023 Day 1 Valid 12Z Wed Nov 08 2023 - 12Z Thu Nov 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Thu Nov 09 2023 - 12Z Fri Nov 10 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Still not planning on reintroducing any Marginal risk areas at this time from TX into the lower MS Valley. There will be a moisture connection to the Eastern Pacific in the mid/upper levels, and some Gulf of Mexico moisture in the lower levels...so PWs will be elevated, with values starting to approach early November max values. So while there will be plenty of moisture in place, low level moisture transport magnitudes and instability should remain limiting factors. There has been a modest upward trend in expected instability near the front Thursday afternoon, with expectations of 500-1000 J/KG across portions of eastern TX into northwest LA and southwest AR. This instability is forecast to erode fairly quickly...so it's really the 18z Thursday to 00z Friday 6hr window where any more robust convection is possible. Thereafter rainfall should transition to more of a post-frontal rain with lower rates. During this afternoon period HREF data supports localized hourly rainfall over 1", but not seeing much, if any, signal of 2"+ within an hour....likely due to the only modest instability and moisture transport. Even with these embedded higher rates, not really seeing any FFG exceedance in the high res guidance, with only localized areas getting to even 50% of FFG. Thus while a swath of 1-4" of rain appears likely from portions of eastern TX into northwest LA and southwest AR, flash flooding still appears unlikely at this time. Far south TX still looks to have a threat of localized flash flooding, but even here think the risk remains below 5%. Some chance the surface front hangs up over the area Thursday night, bringing the potential for training convection. However this is dependent on the speed of the cold front, and models differ with this evolution. A quicker southward surge of the front would reduce instability and limit the duration of any heavier convection. Thus rainfall rates and intensity would likely stay below FFG thresholds in this scenario, which appears most likely at this time. Thus while the flood threat is non-zero, in the end just not seeing enough support for a Marginal risk at this time...especially given the relatively dry conditions going into this event and continued question with regards to instability and rainfall rates. Chenard Day 3 Valid 12Z Fri Nov 10 2023 - 12Z Sat Nov 11 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. The threat of moderate to heavy rainfall continues across portions of south TX and along the TX coast Friday into Friday night, but not confident enough in there being a flash flood threat to warrant reintroducing any risk areas at this time. The 00z ECWMF is the slowest with the frontal passage and thus keeps portions of south TX and the TX coast within the more unstable airmass. If the ECMWF were to verify then some flash flood risk would probably exist over coastal TX. However at this time the ECMWF appears to be an outlier, with the better model clustering (including the ECMWF ML model) supporting a stronger southward push of the cold front and clearing TX before stalling just offshore. At the moment this appears to be the more likely evolution, which would result in a lower flood risk. The 00z NAM and GEM reg show as much as 2-5" of post-frontal rain over portions of south TX. However this is falling in a stable boundary layer behind the front, with only minimal elevated instability above the boundary layer noted in area forecast soundings. Upper support and low/mid level overrunning of this front is impressive, which could certainly drive areas of heavy rainfall...but the lack of instability suggests rainfall rates will struggle to exceed 1" in an hour. If rainfall totals from day 2 into this day 3 period start piling up and get over 3-5" then even sub 1" an hour rain may begin to cause some runoff issues as soil conditions saturate. This is a possibility, but also a low confidence forecast. Thus when considering the frontal timing uncertainty and the likely primarily post-frontal nature of rainfall during this time frame within an environment of little instability...think holding off on any Marginal risk is prudent at this time, however will continue to monitor. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6pSYnj6PEgDV3yGap3b9_s-dkginuHsELnVTyfIn03v5= mj4x2i8F79hAJKhd6cwqN3XnNHf0ZzXupVWiFVMOtTTyfaM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6pSYnj6PEgDV3yGap3b9_s-dkginuHsELnVTyfIn03v5= mj4x2i8F79hAJKhd6cwqN3XnNHf0ZzXupVWiFVMO6vI6wxI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6pSYnj6PEgDV3yGap3b9_s-dkginuHsELnVTyfIn03v5= mj4x2i8F79hAJKhd6cwqN3XnNHf0ZzXupVWiFVMOFzTcr-M$=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) .