Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 11 2024 15:43:54 FOUS30 KWBC 111543 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1043 AM EST Sun Feb 11 2024 Day 1 Valid 12Z Sun Feb 11 2024 - 12Z Mon Feb 12 2024 ....THERE IS A SLIGHT RISK AREA COVERING EASTERN ALABAMA TO WESTERN SOUTH CAROLINA AND A MARGINAL RISK AREA EXTENDING OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY EASTWARD INTO THE SOUTHEAST U.S. TODAY AND TONIGHT... 16Z update... Latest hi-res guidance depict several cells capable of producing hourly rainfall rates of 1-2+ inches passing through Alabama, Georgia and into the Carolinas this afternoon and evening. Nearly saturated soils across this part of the region have already increased the threat for excessive rainfall and flooding concerns. Rainfall with these potential intensities will further increase the threat. An upgrade to a Slight Risk was warranted therefore placed over eastern Alabama, northern Georgia and western South Carolina. Campbell Westward expansion of the Marginal Risk areas western boundary continued as yesterdays Day 2 ERO becomes today's Day 1 ERO. The expansion covered the potential for excessive rainfall from convection working its way eastward across Texas prior of the start of the Day 1 period and its potential to overlap with areas that received rainfall on Saturday. Generally speaking...rainfall amounts on Saturday were beneficial rather than problematic and the combined amounts of rainfall from Saturday and today would not likely to need a Marginal risk area. However...the 11/00Z HREF keyed on upscale growth near the Texas/Louisiana border that pushes the probability of 1 inch of rain in one hour over 50 percent and the probability of 3-hour rainfall exceeding flash flood guidance to 20 percent later this afternoon. The dynamics are very strong across the area and the flow aloft becomes increasingly difluent between an ejecting upper low and the axis of the subtropical jet. In addition...there should be sufficient residual moisture behind Saturdays system to support some of the rainfall totals shown by the CAM guidance. Storms should be progressive enough to preclude more than ponding in low areas and in regions of poor drainage. After 12/00Z...focus shifts to the Southeast U.S. where there should be increasing coverage of heavy rainfall as low level moisture transport interacts with a quasi-stationary front. The 11/00Z run of the HREF showed a blossoming area of 20 percent probability of 1 inch of rain in 1 hour across parts of Alabama and nearby Georgia...but sub-5 percent risk of 2 inch per hour rates in the same area...and where the HRRR showed a few spots of embedded 4 inch rainfall amounts extending as far east as South Carolina. This lines up along the axis of a coupled upper jet aiding deep layer support. The previously issued ERO largely had this area covered and only minor adjustments were made. Bann Day 2 Valid 12Z Mon Feb 12 2024 - 12Z Tue Feb 13 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST U.S.... There is a continuing threat of excessive rainfall on Monday...mainly over the Carolinas...as secondary cyclogenesis begins in response to the large scale system amplifies. At the surface...one area of low pressure forms and heads northeast over the far eastern TN/OH Valleys while a second area of low pressure forms over parts of the Carolinas...supported by the deep layer vertical velocities induced strengthening divergence fields of a coupled of upper level jets. It is this component which results in the continued risk of excessive rainfall over the Southeast from Monday into the early-morning hours on Tuesday. Precipitable water values increase to values at or slightly above 1.75 inches ahead of a cold front sweeping across the region. Deterministic QPF from the GFS and UKMET seem better than the ECMWF and Canadian and tend to be aligned with the WPC deterministic amounts. Given high FFG numbers...especially where spaghetti plots of model QPF are 2 or 3 inches overlap. Bann Day 3 Valid 12Z Tue Feb 13 2024 - 12Z Wed Feb 14 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-6BHtUF0szQPWf9CSX8T2fI9xrWOoacKa8AYbbW3zSDn= RaYw8BRvvfpBJ2k7Cjp9Q0ITU373QrMT9Kcd0gA0HHgwU08$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-6BHtUF0szQPWf9CSX8T2fI9xrWOoacKa8AYbbW3zSDn= RaYw8BRvvfpBJ2k7Cjp9Q0ITU373QrMT9Kcd0gA0Ma4H7Mc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-6BHtUF0szQPWf9CSX8T2fI9xrWOoacKa8AYbbW3zSDn= RaYw8BRvvfpBJ2k7Cjp9Q0ITU373QrMT9Kcd0gA0lQ-N2vM$=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) .