Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 20 2023 00:53:24 FOUS30 KWBC 200053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Wed Apr 19 2023 Day 1 Valid 01Z Thu Apr 20 2023 - 12Z Thu Apr 20 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR AN EASTERN PORTION OF THE CENTRAL PLAINS... The Slight risk from northeast KS into southeast NE, northwest MO and southwest IA still looks in good shape. Severe thunderstorms have rapidly increased in coverage across this corridor over the past couple hours, and are exhibiting some southwest to northeast training. The expectation is that will continue over the next few hours, before cells try to make a bit better eastward progression and thus decreasing the residence time of heavy rainfall at any one location. Rainfall of 1-2" in an hour is expected in spots, with localized totals of around 2-3" possible through 04z. This should be enough to result in some isolated runoff concerns. Attention then turns to what should be additional convective development between 06z-12z tonight into early Thursday. Increasing forcing from the west and an uptick in low level moisture transport will likely result in convective development over north central KS. This activity will then push off to the east and northeast, impacting some of the same areas that are currently seeing heavy rainfall. Still some uncertainty with whether this convection moves quickly though the area...in which case additional flash flooding should stay pretty localized...or whether this activity tends to build into the south southwest lank of the ongoing convection...in which case a period of west to east training could occur. If this latter scenario evolves then a higher coverage of flash flood impacts could result. Tough call at this point and will need to continue to monitor and update with MPDs as needed. Either way the Slight risk seems warranted and in good shape...just a question as to just how much coverage of impacts we get. Elevated convection overnight north of the warm front may pose a localized flash flood risk and remains covered by the northward extension of the Marginal risk, although not expecting any organized threat. Chenard Day 2 Valid 12Z Thu Apr 20 2023 - 12Z Fri Apr 21 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF NORTHEAST TEXAS TO THE MID-SOUTH... Deep-layer Gulf moisture streams up across Texas until a cold frontal passage early Friday. PW values of 1.75 inches (1.5 to 2 sigma) will spread over central/eastern Texas through Arkansas and the Mid-South by Thursday afternoon ahead of the frontal boundary as it approaches. Continued southwesterly deep layer flow will be nearly parallel to the front, bringing the risk for repeating and backbuilding storms. 12Z consensus for areal averages of 1 to 3.5 inches from the Heart of Texas (The I-35 corridor between Austin and Dallas) to southeast Missouri with amounts generally an inch or less for the Upper Mississippi Valley and Great Lakes, though FFG will remain lower there due to recent rains (and snowmelt in WI), thus keeping an elevated threat for flooding concerns. The Slight Risk area was expanded a bit more, further southwest into central Texas and northeast into southern Illinois. The southern bounds of the Marginal Risk ares was also expanded south and eastward to include the Houston metro and farther into western Louisiana to account for convergent activity with otherwise progressive prefrontal thunderstorm lines. Jackson Day 3 Valid 12Z Fri Apr 21 2023 - 12Z Sat Apr 22 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE LOUISIANA COAST THROUGH THE MID-SOUTH... The key excessive rainfall concern for the Friday is the timing and progression of the pre-cold frontal thunderstorms pushing through east Texas into Louisiana late Thursday night. Early CAM guidance from the 12Z NAMNest/FV3LAM and now the 18Z HRRR indicate more progressive prefrontal activity that may clear some of southwest Louisiana by 12Z Friday. Opted to make some adjustment to account for this more progressive scenario, and more adjustment to the Slight and Marginal Risk areas may be needed pending further CAM output with the 00Z HREF tonight. Ahead of this front, deep layer Gulf moisture will continue to fuel showers and thunderstorms with the strongest moisture convergences aligning from southern Louisiana to western Tennessee with perhaps the best repeating threats along the western Gulf Coast early Friday and near the Mississippi Delta later in the day. 12Z consensus is for 2 to 3 inches QPF in either of these areas and for portions of the Lower Mississippi Valley between. A Slight Risk was trimmed a bit on the west side near the Texas/Louisiana border and maintained from western Tennessee to Louisiana. The Marginal Risk was trimmed a bit in eastern Texas and expanded east a bit over the Ohio valley for greater frontal progression. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7y90yoQ-OPF8W5IX_SznS2AHW4mw8mGgLTySoiDxmtyx= TJcClzZEZyn1d_Z4d4J3V5KuKjlujuo6TuOe1C2TEMgZZYY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7y90yoQ-OPF8W5IX_SznS2AHW4mw8mGgLTySoiDxmtyx= TJcClzZEZyn1d_Z4d4J3V5KuKjlujuo6TuOe1C2Thejk2yU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7y90yoQ-OPF8W5IX_SznS2AHW4mw8mGgLTySoiDxmtyx= TJcClzZEZyn1d_Z4d4J3V5KuKjlujuo6TuOe1C2TW73OFQQ$=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) .