Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 01 2025 00:51:19 FOUS30 KWBC 010050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Sat May 31 2025 Day 1 Valid 01Z Sun Jun 01 2025 - 12Z Sun Jun 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....01Z Update... With rainfall rates gradually diminishing over the Northeast, the Slight Risk and Marginal Risk over northern New England have been taken out. While some localized ponding or run-off along complex terrain in the Green/White Mountains and across northern Maine=20 cannot be ruled out, rainfall rates have diminished to the point=20 where the flash flood potential is effectively done. In the Central Plains, the Marginal Risk was only tweaked after viewing 18Z HREF=20 guidance and new CAMs guidance. Mullinax --Previous Discussion--- ....Southern Plains... Broad NW flow on the upwind side of an expansive trough centered over Ontario will persist over the Plains as an embedded but potent shortwave tracks SE tonight. This shortwave will push a wave of low pressure and weakening cold front south into Oklahoma tonight, where it will interact with a slowly advancing warm front out of Texas on the intensifying LLJ. This LLJ, progged to reach at least a modest 20-25 kts from the SW, will transport more robust thermodynamics (PWs to 1.5 inches and MUCAPE above 1000 J/kg), to support expanding convection along this decaying cold front. With bulk shear expected to be in the 30-40 kt range, convection will likely grow upscale into clusters or an MCS tonight as it races southeast on 20-25 kts of 0-6km mean winds. This this will limit the duration of heavy rain, rates that will likely reach 1-2"/hr could result in isolated instances of flash flooding, especially during any short-term training that may develop on the periphery of this MCS. The guidance, as expected, features considerable longitudinal spread of the MCS path, which makes the cosmetic adjustments to the inherited MRGL risk the best approach for the risk area tonight. Weiss Day 2 Valid 12Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST... Impressive closed low off the northern Baja coast (NAEFS 500mb heights near -2 sigma at 12Z Sunday) will gradually fill and shift northeast through D2, reaching Arizona by Monday morning. Downstream of this low, southerly flow will become more intense, drawing increased moisture and instability northward into the Southwest. This will support impressive thermodynamics as PWs surge to 1.25-1.5 inches, above the 99th percentile according to NAEFS (and in some areas above anything measured in the 06Z CFSR climatology), which will overlap with MUCAPE climbing to 250-750 J/kg, an increase from the past few model cycles. As ascent increases through height falls and PVA, convection is expected to blossom rapidly and then organize through 25-35 kts of bulk shear. This organization within the favorable thermodynamics will support at least briefly heavy rain rates that may exceed 1"/hr (10-20% chance). With storm motions slowing as the upper low pivots overhead late D2, this could produce pockets of 1-2" of rain, especially in the low deserts of AZ. Farther east, aligned Corfidi vectors with the mean flow will allow for repeating rounds of convection to lift northward, producing other areas of 1-2" of rain despite the faster progression. Despite dry soils from a lack of recent rain, this could produce instances of flash flooding, including atop sensitive terrain or burn scars, and after coordination with WFOs TWC and PSR, a small SLGT risk was added for south-central AZ. Weiss Day 3 Valid 12Z Mon Jun 02 2025 - 12Z Tue Jun 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL HIGH PLAINS AND CENTRAL PLAINS... ....Central High Plains and Central Plains... Baja mid-level low which will be opening over Arizona to start the period will continue to weaken and shear out as it becomes embedded within pinched westerlies south of a trough diving out of Montana. This will result in expanding deep layer lift through height falls and modest deformation expanding from the Four Corners through the Northern Plains, with additional ascent occurring as the northern stream trough drives a cold front southeastward and a jet streak pivots meridionally into Canada leaving favorable RRQ diffluence overhead, and a surface low is also forecast to develop along this boundary. This synoptic setup will produce an expansive area of lift into increasingly robust thermodynamics, and it is likely that a large area of showers and thunderstorms will develop much of D3. Increasing low-level flow as much as 40-50 kts at 850mb will emerge from the Gulf, pushing PWs to 0.75 to 1.25 inches which is extremely anomalous and above the 97th, or even 99th percentile, in some areas. This will overlap with MUCAPE rising to 250-1000 J/kg, sufficient to produce rainfall rates of 0.5"-1"/hr. While there is still some uncertainty in the placement of the heaviest rainfall, the guidance is in general good agreement in a maximum of 2-3", or locally higher as reflected by <10% probability for 3"/24 hrs from the ECENS/GEFS, from eastern CO and WY through eastern NE, which is where the SLGT risk was drawn. Some of this area is the Sand Hills of NE which do not typically flood and have high FFG, but for potential and after coordination with the affected WFOs, the inherited SLGT risk was adjusted just slightly. Farther to the north from SD through MN, fast moving storms on 20-30 kts of 0-6km mean winds will have the potential to train where Corfidi vectors are aligned to the mean wind and the cold front. This will offset, at least marginally, the faster motion, allowing for an isolated flash flood risk across this region. Farther SW, the MRGL risk was expanded into western CO and the southern half of UT where impressive PWs and slower moving storms could organize to produce areas of heavy rainfall exceeding 1 inch. This region is sensitive due to the variety of slot canyons and other sensitive terrain, which feature FFG that is as low as 0.5"/1hr. After coordination with WFO SLC and GJT, the MRGL risk was expanded across these areas. ....South Florida... The inherited MRGL risk was maintained, but trimmed to just the SE portions of the peninsula to focus on the more urbanized regions of the state. As an upper trough digs south along the FL Panhandle and into the peninsula, it will push a weakening cold front and shortwave southward, enhancing ascent across the area. With PWs approaching 2" overlapped with MUCAPE above 2000 J/kg, this will support scattered to widespread heavy-rain producing convection on Monday. Mean storm motions will be generally west to east, aligned to the front, and this should focus storms along the SE coast where sea breeze interaction occurs. With rain rates expected to exceed 2"/hr at times, this could produce above 3" of rain as reflected by 5-20% probabilities from the SREF and ECENS. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9gCWgyaVv093rvH44NDy-XCPc0pHGlgT9Pb3QVlTetSY= THpqM7qm9-IVG7We62H7rSdME3d-BUjRdRnmbZHG5rQC_Sw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9gCWgyaVv093rvH44NDy-XCPc0pHGlgT9Pb3QVlTetSY= THpqM7qm9-IVG7We62H7rSdME3d-BUjRdRnmbZHGjtGR_wk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9gCWgyaVv093rvH44NDy-XCPc0pHGlgT9Pb3QVlTetSY= THpqM7qm9-IVG7We62H7rSdME3d-BUjRdRnmbZHGJhoBmPw$=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) .