Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 07 2024 23:02:50 FOUS30 KWBC 072302 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 702 PM EDT Sun Apr 7 2024 Day 1 Valid 01Z Mon Apr 08 2024 - 12Z Mon Apr 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Mid-South... Widely scattered showers and thunderstorms have formed west of Memphis near within a pool of 1000+ ML CAPE. The 18z HREF indicates that there is some chance for repeat convection as it bores a hole/erodes the instability field and shifts slowly southward. Its chances of 3"+ are ~25%, which could be a problem if such rainfall were to fall upon an urban area quickly enough. With precipitable=20 water values at or above 1.3" and effective bulk shear near 50=20 kts, mesocyclones are developing. Since the air mass is starting=20 off dry-ish, they'll likely contain decent sized hail to start.=20 However, with time, should a complex develop and have some staying=20 power, heavy rain could materialize at amounts up to 2" in an hour. The storms are expected to be just mobile enough to avoid=20 significant flash flood issues, but isolated issues can't be ruled=20 out. The threat of such is non-zero, but does not appear to rise to the 5% threshold of a Marginal Risk. Roth Day 2 Valid 12Z Mon Apr 08 2024 - 12Z Tue Apr 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AND ADJACENT SOUTHERN MISSISSIPPI VALLEY... 21Z update... The overall trend with the QPF footprint over the south-central U.S. was for an east-west elongation from northeast Texas to northern Alabama with areal averages of 2 to 4 inches. The highest accumulations will likely be near the Arkansas and Louisiana border. The Marginal Risk was expanded west across northern Texas and east into northwest Alabama. The Slight Risk was expanded eastward across northern Louisiana, southeast Arkansas and far western Mississippi. Campbell The potential for heavy to excessive rainfall looks to develop over portions of the southern U.S. as shortwave energy diving southward over the Intermountain region helps carve out a deep layer southern stream trough. Flow aloft over the Slight Risk area backs and begins to accelerate Monday evening and Monday night...with increasingly divergent flow aloft and increasing low level moisture transport helping to set the stage for convection capable of producing downpours. Model guidance has been showing run to run differences in placement and rainfall amounts...with the GFS pretty much on its own scooting one area of heavy rainfall eastward quickly away from Texas/Arkansas/Louisiana into the Tennessee Valley overnight while other guidance tended to be less progressive with the rain. Spaghetti plots from successive runs of both the GEFS and SREF showed some consistency with 2- or 3-inch amounts that was in general agreement with earlier thinking that 2 to 4 inches (with isolated higher amounts certainly possible) could fall. With maximum precipitable water values over southeast Texas being in excess of 1.75 inches resulting in an axis of coherent moisture transport that could interact with an old cold front boundary...the idea of heavy rainfall capable of producing flooding is reasonable although some uncertainty remains. Bann Day 3 Valid 12Z Tue Apr 09 2024 - 12Z Wed Apr 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN MISSISSIPPI VALLEY... 21Z update... General consensus of the latest guidance suggested an elongation of the southwest-northeast QPF axis across eastern Texas to central Kentucky. WPC went this direction with the forecast. The area of concentration for the highest accumulations appear to span from northern Louisiana to western Tennessee which necessitated adjusting the Moderate Risk to the north and east across eastern Arkansas and western Mississippi along with nudging further south in Louisiana. The Slight Risk was adjusted further into southwest Kentucky and the Marginal was expanded eastward into east-central Kentucky where QPF trended up. Campbell Heavy to potentially excessive rainfall should be on-going at the beginning of the outlook period on Tuesday morning that persists into the daytime hours. Flow aloft should be increasingly diffluent east of an approaching deep-layer trough and closed low while moisture from the Gulf continues to be drawn northward. How long it takes for convection to become progressive is one of the main forecast concerns. The GFS continues to be some 12 hours faster than other guidance in when it shunts precipitation south and east. The 07/00Z runs of the ECMWF and Canadian tended to push some isolated maximum totals of 4 to 6 inches well south into Louisiana while the more progressive GFS tended to have smaller maximum values pushing into Tennessee by the end of the outlook period. The real concern is in the 48-hour rainfall totals...with 5 to 7 inch amounts possible embedded within a broader 3 to 5 inch area. With those amounts...hoisted a Moderate Risk area that generally extended across southern Arkansas into northern Louisiana where there was the best overlap of the various guidance. Additional adjustments can be expected...assuming the short-term numerical guidance sorts out timing and placement details. Bann Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9b9v6E0pdtH4SSeNYzoqwDrhVzi1w5V4I0P-Ua-rnlIN= 38ntCqU1MGTObgjfsUjzN-YVm_vHzovPppQqaKhFBH7UCtY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9b9v6E0pdtH4SSeNYzoqwDrhVzi1w5V4I0P-Ua-rnlIN= 38ntCqU1MGTObgjfsUjzN-YVm_vHzovPppQqaKhFXcY-fMA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9b9v6E0pdtH4SSeNYzoqwDrhVzi1w5V4I0P-Ua-rnlIN= 38ntCqU1MGTObgjfsUjzN-YVm_vHzovPppQqaKhFt6VEuT8$=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. 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