Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 17 2024 00:51:45 FOUS30 KWBC 170051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Sat Mar 16 2024 Day 1 Valid 01Z Sun Mar 17 2024 - 12Z Sun Mar 17 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL AND SOUTHEAST TEXAS... The Slight risk was maintained across portions of south central TX into southeast TX and far southwest LA. Model guidance indicates a mid level wave and upper jet streak will move out of Mexico and become more pronounced over TX tonight. This is expected to initiate convection over Mexico, with this activity then moving into south central TX near the stationary front. Most of the high res guidance suggest a pretty quick forward motion of this=20 convection off to the east, and mid/upper level flow does support this. However enough CAPE and shear is present for severe organized convection, and given the presence of the stationary front, there=20 is some chance activity could briefly hang up or train along this=20 boundary for a few hours later this evening into tonight. Thus=20 while most indications are for 1-2" of rain, do suspect there is a=20 threat for swaths of 3"+ totals. FFG is quite high across this=20 corridor, so we are probably looking at the lower end of the 15-40% slight risk range...nonetheless, at least a localized flash flood=20 risk could still evolve. The extension of the Slight risk into southeast TX and southwest=20 LA is more of a conditional threat driven by wetter antecedent=20 conditions and lower FFG. There is some chance organized convection tries to move onshore across these areas later tonight and towards 12z Sunday. If this occurs then flash flooding is possible by 12z. Again this is a lower confidence area, as convection may very well remain offshore through 12z. Chenard Day 2 Valid 12Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN LOUISIANA AND COASTAL MISSISSIPPI... ....2030Z Update... A low level jet with abundant Gulf moisture will run into an advancing cold front moving down the Mississippi Valley Sunday. The clash of these 2 air masses will provide a focus and lifting mechanism for the moisture, supporting strong storms capable of heavy rainfall rates to 2 inches per hour. The front will allow for the storms both to form and to track east along the frontal interface, which will extend along all of the northern Gulf Coast. In coordination with LIX/Slidell, LA forecast office, the Slight Risk has been expanded well to the east to account for the convection that will be ongoing Sunday morning. The 2 inch per hour rainfall=20 rates will be capable of rapid-onset flash flooding, particularly=20 in urban areas such as Baton Rouge and New Orleans. Recent heavy=20 rainfall over and upstream of these areas have made local streams=20 and rivers high, which will lower the threshold needed for flash=20 flooding to occur. Once the cold front moves through, conditions=20 should rapidly improve, to include rapid drying of the atmosphere. This will end the flash flooding threat caused by rainfall. Wegman ....Previous Discussion... Organized convection in the vicinity of a front, which should be=20 somewhere the southern border of TX/LA Sunday morning, is expected to lead to heavy rainfall for portions of the northern Gulf coast. Sufficient 850 hPa inflow off of the Gulf of Mexico, up to 40 kts, will lure precipitable water values of 1.5-1.75" into the region=20 and MU CAPE up to 2000 J/kg. Difluence aloft should be fostered by increasing troughing across the Great Lakes, Midwest, and=20 Mississippi Valley. There is some signal in the guidance for=20 cooling temperatures at 700 hPa, which is of concern as the=20 1000-500 hPa thickness diffluence is now very close to the LA=20 coast. This means that there is some chance that organized=20 convection could shift slightly offshore LA at some point in its=20 evolution, leading to decreasing confidence in heavy rainfall the farther east you go in LA. Both the 00z NAM and 12z-00z ECMWF are=20 on the north side of the guidance envelope (which usually isn't=20 good news for the ECMWF solution) while the remainder of the=20 guidance lies closer to the Gulf coast, including the normally=20 northward- biased 00z UKMET. The guidance shows a bit of=20 progression to the convective pattern, as an upper trough sharpens across the Ohio and Mississippi Valleys and helps act to shift the moisture plume/best 850 hPa inflow east with time. Hourly rain=20 totals to 2.5" with local amounts to 4" are possible where limited cell training occurs and/or mesocyclones happen to develop. Because of the heavy rainfall that occurred on Friday near the southern=20 TX/LA border, and per coordination with the LCH/Lake Charles LA=20 forecast office, added a targeted Slight Risk area to areas where=20 soils recently became more sensitive. Roth Day 3 Valid 12Z Mon Mar 18 2024 - 12Z Tue Mar 19 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_ui-SZ0f8WPUwm386l3FG11ixupnBJZHvmpXfGt_S9fv= 5mpFE1HQlu7qQwj94849Q1-2o9qGqBmO0z00rl3tzUDpidU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_ui-SZ0f8WPUwm386l3FG11ixupnBJZHvmpXfGt_S9fv= 5mpFE1HQlu7qQwj94849Q1-2o9qGqBmO0z00rl3trEoO93k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_ui-SZ0f8WPUwm386l3FG11ixupnBJZHvmpXfGt_S9fv= 5mpFE1HQlu7qQwj94849Q1-2o9qGqBmO0z00rl3ta7yFDnQ$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .