Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 13 2025 22:37:37 FOUS30 KWBC 132237 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 637 PM EDT Thu Mar 13 2025 Day 1 Valid 01Z Fri Mar 14 2025 - 12Z Fri Mar 14 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 2 Valid 12Z Fri Mar 14 2025 - 12Z Sat Mar 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TO MID MISSISSIPPI VALLEY INTO PARTS OF THE OHIO AND TENNESSEE VALLEYS... ....2030Z Update... Prior discussion reviewing the synoptic setup and caveats on the more limited potential for a flash flooding threat compared to a severe threat remain on track. Expect a line of convection to develop along the cold front by early evening, first for more northern areas across the Middle Mississippi Valley with additional storms extending southward through the Lower Ohio and Tennessee Valleys through the night. Updated 12Z hi-res guidance indicates the combination of strong dynamics, moist southerly return flow, and sufficient instability could lead to some locally heavier rainfall amounts around 2" supported by quick downpours with rain rates of 1-1.5" per hour. However, this potential appears to remain rather limited given the quick progression of the storms/cold front, and thus only a few isolated instances of flash flooding mainly for urban areas is expected. Putnam A strong mid-to-upper level shortwave crossing the Southwest on Day 1 is expected to move east of the Rockies, with a negatively- tilted low developing over the central Plains early in the period. Powerful forcing is forecast to interact with what is expected to be a relatively modest plume of moisture advecting north ahead of the system. Storms are forecast to develop initially over the lower to mid Mississippi Valley Friday afternoon before moving east into the Tennessee and Ohio valleys during the evening and overnight. Cell motions will likely be quite progressive given the very strong wind fields in place. Therefore, this convection will likely pose more of a severe risk than a flash flood threat. However, given the strong dynamics, along with sufficient instability for deep convection, expect that rainfall rates will be briefly intense enough to produce localized amounts that exceed current model forecasts (maybe locally over 2 inches). Still, this appears to be a lower-end flood threat, and probably confined to localized impacts over more sensitive urban or low lying areas that are impacted by a quick burst of intense rainfall. Pereira Day 3 Valid 12Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE TENNESSEE AND OHIO VALLEYS... Even as the previously noted low lifts to the north through the Great Lakes into Ontario, the upper pattern will remain amplified across the central U.S. as an upstream shortwave dives southeast into the southern Plains on Saturday morning and spreads eastward over the Lower Mississippi Valley into Saturday afternoon/evening. A surface wave developing along the leading system's trailing cold front will help focus deeper moisture tapped by strengthening southerly winds and with increasing upper-level dynamical support as the southern shortwave approaches. This will help lead to the continued development and maintenance of an intense line of thunderstorms pushing eastward along the front. The general consensus of the most recent 12Z guidance indicates that widespread accumulations of 2-4 inches centered over parts of the Tennessee and lower Ohio Valleys are likely. A large Slight Risk was maintained across the region, reflecting the higher probabilities for 2 inches or more shown by the GEFS/ECENS where this favorable combination of deeper moisture, interacting with strong forcing, along a relatively slow moving front, is expected to raise the potential for heavy amounts late Saturday into early Sunday. This remains a higher-end Slight Risk for parts of the area, and a more focused corridor with the potential for an upgrade to a Moderate Risk is becoming apparent where the potential exists of pre- frontal/warm air advection forced convection on the leading edge of returning higher theta-e air. The latest 12Z guidance indicates this potential is currently highest from northeast Mississippi/northwest Alabama northeast through central Tennessee into south-central Kentucky where locally higher totals of 5-6" could be realized. However, have opted to remain at a higher-end Slight Risk for now given that the placement of similar corridors in these setups often initially has a northern bias in the guidance, and that once the period moves within the hi-res window the placement of the heaviest rain rates/amounts will become more clear. Putnam/Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-T97mmqCzW1TC5b1Ne2gNZtGs6Cmi6CZfjmXiYhIu0OD= eEcyRvCdZHA7FvBLDK24v11PcZyV0n7lMWcurSG66eROt2I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-T97mmqCzW1TC5b1Ne2gNZtGs6Cmi6CZfjmXiYhIu0OD= eEcyRvCdZHA7FvBLDK24v11PcZyV0n7lMWcurSG6JNGcFO4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-T97mmqCzW1TC5b1Ne2gNZtGs6Cmi6CZfjmXiYhIu0OD= eEcyRvCdZHA7FvBLDK24v11PcZyV0n7lMWcurSG6JQf0LdM$=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) .