Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 02 2023 19:41:08 FOUS30 KWBC 021941 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 PM EDT Sun Apr 02 2023 Day 1 Valid 16Z Sun Apr 02 2023 - 12Z Mon Apr 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... 16z Update: Previous outlook still looks in good shape, thus just made some minor adjustments (mainly tweaking the northern edge of the risk areas a bit further south). Still looks good for organized convective development and some west to east training by this evening into the overnight hours. The heaviest convection will move west to east, and eventually take on a bit of a southerly component as it follows the instability gradient and propagates into the strong low level inflow. 12z HREF neighborhood probabilities for 3"+ are 50-80% from northeast TX into northern LA and far southern AR, with HREF EAS probabilities of 3"+ showing 10-20% and 2" 40-60%. The neighborhood probabilities indicate the high end potential, while the EAS probabilities are more indicative of coverage. Thus this data would support good coverage of 2" totals over the area, with high confidence in low coverage of 3"+ totals. Where these localized 3"+ totals do occur an isolated to scattered flash flood risk could evolve, especially if it overlaps with any more flood prone areas. The probabilities of exceeding 5" remain very low, likely due to the overall progressive nature of the convective complex. Tend to think it would take some 5"+ amounts to start getting more numerous and/or higher end flood impacts, and with totals this high appearing unlikely, at this time do not think anything more than a Slight risk is needed. Chenard ....Previous Discussion... A small but potent shortwave disturbance starting out the day over southern NM will race eastward across TX, reaching southeastern OK by the evening. Meanwhile, a leeside low will develop over southeastern CO and move southeast into OK. This low will help draw substantial Gulf moisture northward on a 30-40 kt LLJ. PWATs feeding the convection could get as high as 1.75 inches, which would be 2.5 sigma above normal for this time of year. For early April that is quite notable. The final piece of the puzzle will be a stalled out trough set up in southern AR, that will further help (along with the Ozarks) with uplift of that moisture into the developing convection. As far as the timetable goes, convection is likely to break out starting during the mid-afternoon across north central TX, then quickly race E to ESE along the aforementioned boundary across northeast TX and into southern AR/northern LA through the evening. Once the initial round of storms moves through, potentially as many as 3 more additional lines of storms is likely to quickly develop behind the leading line, which all will also quickly race eastward, but also congeal into a more solid area of moderate to heavy rain as they move. Thus, the possibility of many distinct rounds of storms following one another in quick succession is being portrayed in much of the guidance with storm total rainfall as high as 4 inches, though locally amounts could certainly be higher. This approaches the values for FFGs in the area. In coordination with SHV/Shreveport, LA and JAN/Jackson, MS forecast offices, largely kept the Slight as is, generally aligned east-west covering an area roughly 50-100 miles on either side of the LA/AR border. Multiple CAMs are also suggesting the heavy rain associated with the storms gets going further west than previous guidance indicated. While the storms won't have time to congeal like further east, they still have the potential of increasing in intensity to include rain rates as high as 2 inches per hour, even if that intensity of rainfall only lasts for an hour or so. Thus, in coordination with FWD/Dallas-Fort Worth, TX forecast office, expanded the Slight risk westward to include the city of Dallas, as 2 inches in an hour, should that occur over the city, would result in localized flash flooding concerns. Wegman Day 2 Valid 12Z Mon Apr 03 2023 - 12Z Tue Apr 04 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN AND AROUND SOUTHERN AL, SOUTHWEST GA AND THE FL PANHANDLE... Only minimal changes were made to the inherited Marginal risk. Organized convection should be ongoing at 12z Monday across the region. This activity will likely still be producing heavy rainfall Monday morning, however it should be progressing off to the southeast at a decent clip keeping any flood risk localized in nature. The bigger question concerning the flood risk is what happens for the remainder of the day. The better forcing from the shortwave departs the area, but we still have some dynamics in the left exit of the southern stream jet, and lingering 850mb moisture convergence. There continues to be a signal in some of the guidance for backbuilding convective cells behind the early day main convective complex. If this were to occur then an increased flash flood risk could result. The 12z HREF neighborhood probabilities of exceeding 3" are over 50% and 5" probabilities are even 15-30%. However HREF EAS probabilities (a better indicator of potential coverage) are much lower. This would indicate that the potential is certainly there for some higher end 3-5"+ totals, however at the moment the coverage of these higher amounts (if they occur) is expected to be pretty small. Thus while some flash flooding could certainly evolve from this setup, the coverage is currently expected to stay below Slight risk levels. So while a Slight risk upgrade was contemplated, we opted to stick with the Marginal for now and we will continue to monitor trends. Chenard 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!_TP1428UQkEWemjbCSe8p7zWsXiKmC2aD9C3_DHQkhw9= XgtjrWaJFgGSJOan1itkhioEzAspDwNWOJulLij-uz9cRj8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_TP1428UQkEWemjbCSe8p7zWsXiKmC2aD9C3_DHQkhw9= XgtjrWaJFgGSJOan1itkhioEzAspDwNWOJulLij-Zn3R-WQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_TP1428UQkEWemjbCSe8p7zWsXiKmC2aD9C3_DHQkhw9= XgtjrWaJFgGSJOan1itkhioEzAspDwNWOJulLij-rif9NAw$=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) .