Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 02 2023 00:10:21 FOUS30 KWBC 020010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 809 PM EDT Sat Apr 01 2023 Day 1 Valid 01Z Sun Apr 02 2023 - 12Z Sun Apr 02 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z 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... The closed low over southern CA (as of 18z Sat) will track into the Southern Plains on Sunday and be responsible for an area of organized convection Sunday afternoon and night. Model agreement in the overall synoptic setup is pretty high. The aforementioned mid level shortwave combined with an upper jet to the north will provide the large scale ascent. This forcing will interact with a lifting warm front, with upwards of 40 kts of 850mb flow resulting in pretty good moisture transport/convergence and PWs around the climatological 90th percentile. QPF differences amongst the models are most likely mainly driven by different handling of the convective evolution. Cells will form Sunday afternoon over portions of northeast TX into southeast OK, and should evolve into an organized area of convection fairly quickly. Mean wind fields would suggest an east northeastward progression of this convection. However as convection organizes it should tend to build into the low level jet and track along the instability gradient. This would support more of a east southeast and eventual southeast progression of the most intense convection. Corfidi vectors and thickness lines also support this east southeast to southeast movement. For this reason we continue to lean away from the guidance that has more of a east to northeast progression of the heaviest rainfall. Guidance such as the 12z HRRR, ARW and Gem Reg seem to have the more realistic evolution based on these thoughts. The system as a whole will be progressive, limiting the higher end QPF/flood potential. However some east to west training seems reasonable given the broad moisture transport along the warm front, and this scenario is supported by some of the 12z HREF members. It seems likely that we will see a swath of 2-4" of rain somewhere from northeast TX towards the AR/LA border and into central MS, although at the moment 5"+ probs from the HREF are zero (indicative of the upper bound cap due to the progressiveness of the system). Still think this supports an isolated to scattered flash flood risk, so the Slight risk will be maintained. The area was adjusted a bit north over northeast TX, and then south over MS to account for the aforementioned expected east southeast movement of stronger convection. Chenard Day 3 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 FLORIDA PANHANDLE... 19z Update: No changes made to the Marginal risk. Organized convection should be ongoing at 12z Monday across the area. What happens the rest of the day remains a bit uncertain, but some chance we see continued convective activity with potential training/backbuilding. The better forcing leaves the area, but 850mb moisture transport persists and some guidance does have a training signal. The GFS remains too dry, with something closer to the 12z GEM Reg, FV3CAM and ECMWF preferred...which all depict 2-3"+ totals in spots. We may eventually need to give consideration to a Slight risk upgrade across portions of southern AL into southwest GA, but given relatively high FFG and some lingering uncertainty with convective details, thought leaving the Marginal was good for now and we can continue to reassess with future updates. Chenard ....Previous Discussion... While there are differences amongst the guidance mass fields, there is a general signal for a moisture plume with precipitable water values at or above 1.5" to become stuck vaguely near the FL border with AL and GA. Instability, low-level inflow, and effective bulk shear appear sufficient for convective organization. The flow should be fairly unidirectional with height from the west-southwest, leading to concerns for cell training. With low-level inflow slightly convergent, cell mergers can't be ruled out either. The low- to mid-level frontogenesis seen on Sunday should be exiting to the east and northeast. The guidance spread in amounts is fairly wide, with the 00z NAM the driest and the 00z ECMWF the wettest, showing local amounts of 5". Soil moisture across portions of southern AL and the FL Panhandle and Big Bend is elevated, and rainfall during the day Saturday (and possibly Sunday) could lead to further saturation. The model guidance has a general signal for 1-3" in this area, which could fall as quickly as an hour or two. Considering the above, a Marginal Risk area was added for this region. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8WNXhH8E5hYjPWen5q0uyCLfm4U7TuC6Jh_M76FiX-IY= Zxz8qX82f0X43ecVA_LiJP_90rNH-1FiAFNvaSYEwUi3uOk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8WNXhH8E5hYjPWen5q0uyCLfm4U7TuC6Jh_M76FiX-IY= Zxz8qX82f0X43ecVA_LiJP_90rNH-1FiAFNvaSYEtMw4VT0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8WNXhH8E5hYjPWen5q0uyCLfm4U7TuC6Jh_M76FiX-IY= Zxz8qX82f0X43ecVA_LiJP_90rNH-1FiAFNvaSYEjy3wTQ8$=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) .