Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 04 2024 08:24:35 FOUS30 KWBC 040824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 AM EST Mon Mar 04 2024 Day 1 Valid 12Z Mon Mar 04 2024 - 12Z Tue Mar 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF EASTERN LOUISIANA AND WESTERN COASTAL MISSISSIPPI... In coordination with the LIX/New Orleans, LA forecast office, a Slight Risk upgrade was introduced with this forecast update. Southerly flow developing ahead of both a front over the Midwest and an approaching upper level shortwave trough will advect both moisture to the tune of 1.75 inches PWAT and instability with MUCAPE values over 1,000 J/kg into the central Gulf Coast on Monday. The shortwave will take a 110 kt jet streak that starts out the day Monday right over the Slight Risk area and will push it south out over the Gulf, putting Louisiana in the much more favorable left exit region of the jet Monday night. Storms will form this afternoon across southern Louisiana due to daytime heating destabilizing the atmosphere as abundant Gulf moisture advects northward. As the afternoon wears on, the storms are likely to coalesce along a boundary over southeast Louisiana that will form from a combination of merging outflows from earlier convection and the leading edge of a more significant moisture boundary. Unfortunately, CAMs guidance is in poor agreement as to where this boundary will form, with the most extreme outliers as far apart as southwest Louisiana and central Mississippi. However, there is a general consensus among the ensembles that this boundary will roughly parallel I-10 between New Orleans and Baton Rouge and southeast to the Mississippi Delta... thus on a northwest to southeast axis. Storms that form along this boundary will track east or southeastward across the Mississippi Delta, and potentially along coastal Mississippi. However, due to slow movement of the boundary and a steady influx of Gulf moisture south and west of the boundary, additional storm development is likely, resulting in an area of training convection. This has increased concerns for flash flooding impacts into southeastern Louisiana, particularly into the urban areas of New Orleans and Baton Rouge, with slowly diminishing concerns further east as storms move away from their greatest lifting mechanism. From eastern coastal Mississippi eastward, much of the guidance suggests the storms will turn southeastward into the Gulf, missing this region until Day 2/Tuesday. HREF EAS probabilities of over 2 inches of rain through 12Z are over 50% along a corridor from Baton Rouge southeastward, with the highest probabilities along the Mississippi Delta of over 80%. Neighborhood probabilities of 5 inches are 25-30% into New Orleans, and around 50% for the Delta. The PWATS of 1.75 inches are 2-2.5 sigma above normal for this time of year. Hourly rainfall rates with the strongest storms could exceed 2 inches. The surrounding Marginal Risk area was expanded west to include much of east Texas with this update. MUCAPE values Monday afternoon will be greatest in this area, approaching 2,000 J/kg. Despite less forcing, any storms that form will have extra instability which will support heavier rainfall rates, despite much more isolated coverage of storms overall. Otherwise, the Marginal largely covers the much larger area of uncertainty as to where the boundary will form, along with storms that are expected over northern Louisiana after midnight Monday night/Tuesday morning as the cold front approaches. There is reasonably good agreement that the late-night convection will quickly develop into a squall line or bow-echo, moving much too fast to pose any more than an isolated flash flooding threat. Wegman Day 2 Valid 12Z Tue Mar 05 2024 - 12Z Wed Mar 06 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEASTERN LOUISIANA AND WESTERN COASTAL MISSISSIPPI... In coordination with LIX/New Orleans, LA forecast office, a Slight Risk upgrade was introduced with this update. Ongoing convection across eastern Louisiana at the start of the Day 2 period early Tuesday morning will shift eastward along the Gulf Coast and the far northern Gulf through Tuesday morning as a potent cold front and much drier air races into Louisiana behind the front. The Slight Risk is highly dependent on flash-flooding-inducing rains having occurred overnight Monday night. By Tuesday morning, the last of the storms will move through ahead of the leading edge of cooler and drier air. Thus, the Slight Risk is very much a continuation of flooding potential into Tuesday from the bulk of rainfall on Monday, and really only is in effect for Tuesday morning. Once the drier air moves in, the flooding risk will end across southeast Louisiana. The Slight Risk area depicts the area likely to be wettest both from Monday and into Tuesday morning with straggling showers and thunderstorms, generally from New Orleans south and east. For the rest of the much larger Marginal Risk area, the storms in southeastern Louisiana in the morning will continue tracking east, with significant uncertainty as to how far north they will impact away from the immediate Gulf coast. Much of the guidance suggests the strongest storms will stay just off the coast, with limited impact for coastal communities from Mississippi to the Florida Panhandle. This was a large factor in keeping the area east of Mississippi in a Marginal Risk for now, as high FFGs should allow for effective absorption/runoff with only isolated flash flooding. Should the axis of strongest storms shift north this could change with future runs. Also worth factoring in is that most of the heaviest rains on Day 1/Monday are expected to shift south and off the coast, largely missing this area and not priming the soils for the heavier rains expected Tuesday. Overnight Tuesday night, much of the guidance suggests a surface low will form over AL/GA in the evening as a shortwave trough buckles the jet stream over the area. The low will more effectively draw the Gulf moisture along the coast inland and into Georgia. This will spike rainfall totals with more widespread 1 to 2 inch totals expected for the western two-thirds of the state and far eastern Alabama. Since most of this rain is expected overnight Tuesday night, instability will be quite lacking, though a few hundred J/kg of elevated CAPE may allow for some elevated convection. Regardless, the amounts of rain with lack of instability forecast are unlikely to result in more than isolated flash flooding impacts for this region. The topography of the southern Appalachians may locally increase flash flooding impact potential northeast of Atlanta. Wegman Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_xgV0VWGdI7Zome5jdmUZX5YuwRtO44l8KZenhNXfnIV= KKjPfEhABQdqtCkeeXkgH3NevYCrGEQ6-uj7Kn4UPJIiJfA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_xgV0VWGdI7Zome5jdmUZX5YuwRtO44l8KZenhNXfnIV= KKjPfEhABQdqtCkeeXkgH3NevYCrGEQ6-uj7Kn4UYq7GS84$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_xgV0VWGdI7Zome5jdmUZX5YuwRtO44l8KZenhNXfnIV= KKjPfEhABQdqtCkeeXkgH3NevYCrGEQ6-uj7Kn4Um8XODh0$=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) .