Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 25 2022 08:07:39 FOUS30 KWBC 250807 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 306 AM EST Fri Nov 25 2022 Day 1 Valid 12Z Fri Nov 25 2022 - 12Z Sat Nov 26 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FAR UPPER TEXAS COAST INTO SOUTHERN LOUISIANA/MISSISSIPPI AND PORTIONS OF SOUTHWESTERN ALABAMA... ....01z Update... The Slight Risk area was trimmed additionally for this update, primarily across much of eastern TX and northern LA (where instability has largely been tapped and rainfall rates have subsequently decreased). The Marginal Risk area was also reduced significantly on the northern and western fringes of the outlook area (with largely stratiform rainfall now prevailing). Farther east into portions of the Lower Mississippi Valley and Central Gulf Coast, the Slight and Marginal risk areas were actually expanded a bit north and eastward given observational trends (strengthening low-level jet and increase in MU CAPE) and resulting hourly trends in the HRRR (which have come significantly south and eastward with QPF maxima relative to the 12z HREF suite). Localized totals through 25/12z are expected to run as high as 2-4+ inches, with the bulk of that occurring in a less than 3-hr period (per 18z HREF neighborhood probabilities as high as 20-40% for 2" exceedance over 3-hr).=20 Churchill ....16Z Update... The Slight Risk area across TX has been trimmed a bit towards the southeast with this update. The convection ongoing across northeast TX from north of San Antonio through to south and east of Dallas has thus far only been producing isolated instances of 1-1.5 inch/hour rainfall rates. Both the isolated and transient nature of these higher rainfall rates have thusfar not been enough to produce flash flooding. Therefore, the threat in these areas has been assessed to be better defined in the Marginal category. A little convection has been developing towards Houston and the Gulf Coast. There is good consensus in the HiRes guidance that this developing convection will merge with the ongoing convection further inland during peak heating with better PWATs closer to the Gulf Coast. Therefore, the reasoning for the ongoing Slight Risk in these areas remains unchanged. The higher rainfall rates described in the previous discussion below are far more likely to occur in the southern areas of the Slight Risk. Overall, however, the higher FFGs in this area would suggest that even here, the flash flooding risk is on the lower end of the risk category. Wegman ....Previous Discussion... A multi-day heavy rainfall event is expected to develop over the next 48 hours across portions of Texas and the Lower Mississippi Valley. Aloft, an active subtropical jet will remain positioned across eastern Texas to the Lower MS Valley, positioning the coastal regions in the favorable right entrance region of the 120-130 kt jet today. Meanwhile, southeasterly to southerly flow on the backside of the eastern U.S. high will allow anomalously high moisture to be lifted into the region as a warm front slowly moves northward. PWs between 1.7-1.9" with some isolated signal for 2"+ values will be near 3 standard deviations above normal for this time of year while upwards of 1500 J/kg of MUCAPE is likely along the coast and areas immediately inland (though will likely drop off quickly further north, limiting the intensity of the heavy rainfall). Convection is expected to develop later this morning or early afternoon across portions of the mid/upper Texas coast and track northeast/easterly across southern Louisiana into southern Mississippi later this evening and tonight. Some of this convection will have the potential to train and repeat over the same areas with the mean flow ending up near parallel to the expected storm motions. The environmental ingredients (sustained lift, anomalous moisture, and instability) supports intense rain rates approaching 2"/hr at times with an isolated signal for 2-3" hourly totals based on the 00Z HREF probabilities. An axis of 3-5" is likely across the mid/upper TX coast through southern LA and southern MS with some potential for isolated 5-6"+ totals where repeating rounds occur. This could lead to isolated to scattered instances of flash flooding today, particularly highly urbanized corridors and other sensitive locations. Further north across northeast Texas, moderate to locally heavy rainfall will likely repeat over the same area through early afternoon with some potential for hourly totals greater than 1" at times and totals of 2-3".=20 Taylor Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 Valid 12Z Sat Nov 26 2022 - 12Z Sun Nov 27 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9K08QirHSH5M6oNsjft1q3ZzgXxlGnzfuy0wRkg-upkl= HqucLM6U2NIHpo-vS9W98PyqnYKhOEutqNfHJZWRPW6V1P4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9K08QirHSH5M6oNsjft1q3ZzgXxlGnzfuy0wRkg-upkl= HqucLM6U2NIHpo-vS9W98PyqnYKhOEutqNfHJZWR7NnAT0I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9K08QirHSH5M6oNsjft1q3ZzgXxlGnzfuy0wRkg-upkl= HqucLM6U2NIHpo-vS9W98PyqnYKhOEutqNfHJZWRaYZuA_Q$=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) .