Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 29 2023 19:41:10 FOUS30 KWBC 291941 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 240 PM EST Wed Nov 29 2023 Day 1 Valid 16Z Wed Nov 29 2023 - 12Z Thu Nov 30 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell/Kleebauer Day 2 Valid 12Z Thu Nov 30 2023 - 12Z Fri Dec 01 2023 ....THERE IS A MARGINAL RISK AREA FROM THE CENTRAL GULF COAST TO THE MID-MISSISSIPPI/OHIO RIVER VALLEY... ....20Z Update... Primary change to the previous D2 issuance was trimming back on the northern extent of the MRGL risk across central and eastern MO into southern IL. Consensus on warm front approach late in the period will allow for some shallow convection to develop, but the lack of appreciable rates worthy of flash flood concerns will be a deterant with the latest 12z HREF 1"/hr probabilities totally void over the aforementioned area. Totals will be generally within the 0.75-1.5" territory, and likely over an extended period of time. Thus, brought the northern extent further south into southern MO where there's a slightly better risk for convection and locally higher rates. The trend continues to have a focal point of convection and flash flood concerns across east TX into the western half of LA where instability maximum for the period will be co-located within the best ascent from our passing shortwave trough to the north and surface convergence within the confines of a propagating cold front. Rates will be generally between 1-2"/hr with perhaps a few isolated storms capable of up to 2.5"/hr, mainly within the southwest LA coastal areas where instability is forecasted to be highest. The antecedent conditions present across the Lower Mississippi Valley will deter more widespread flood prospects and much of this rain will be beneficial. However, the prospect for supercell generation across east TX given the deep shear profiles and backing low-level wind field off the Gulf will yield some enhanced rainfall within the SPC Enhanced/Slight severe storm risks respectively. Later in the period, convegent signature along the leading edge of an advancing cold front will act as a focal point for a line of heavy thunderstorms to move eastward across the LA/TX border into the central portion of LA. CIPS analogs were very much in line with the current forecast with multiple analog from the 90s and 80s during stronger el ninos depicting a very similar setup with an outcomes well within the means of current guidance. Those events yielded more localized flash flood impacts with the primary concerns aligned with the urban settings around east TX into LA. This was also another reason to maintain the MRGL risk with less widespread focus and more localized impacts anticipated. Kleebauer ....Previous Discussion... Surface low over the plains will evolve with a developing cold front pushing eastward within the time frame. The latest guidance continues to show a swath of QPF of 1 to 3 inches lifting from the Gulf Coast to the Ohio River Valley. Moisture return from the Gulf will have the PW values upwards of 2 to 2.5 standard deviations above normal over a large portion of the Lower and Middle Mississippi Valley. One this deep moisture plume meets the greatest instability hourly rates will likely max out near 2 inches/hour, leading to local maximums near 4 or more inches during this period across Louisiana and the immediate vicinity. The Marginal Risk that was already in effect from the Central Gulf to southwest Indiana remains in effect. Minor adjustments were made along the eastern boundary to trim it back to the west reflecting the latest model trends and WPC forecast. Campbell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5eL55FdCftuRYzdT5Q8VEHmlr8xHHjwoJman5RmvDhb7= nr0ktKPHEGUAjRBcdBUOs4P5SQWMEOQOSMRhowdGlwd4BY8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5eL55FdCftuRYzdT5Q8VEHmlr8xHHjwoJman5RmvDhb7= nr0ktKPHEGUAjRBcdBUOs4P5SQWMEOQOSMRhowdGFudJdS8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5eL55FdCftuRYzdT5Q8VEHmlr8xHHjwoJman5RmvDhb7= nr0ktKPHEGUAjRBcdBUOs4P5SQWMEOQOSMRhowdGaqTzKN4$=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) .