Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 08 2025 08:06:02 FOUS30 KWBC 080805 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 AM EDT Mon Sep 8 2025 Day 1 Valid 12Z Mon Sep 08 2025 - 12Z Tue Sep 09 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTION OF OREGON, NORTHERN CALIFORNIA, THE SOUTHERN AND CENTRAL PLAINS, THE UPPER MISSISSIPPI RIVER VALLEY, AND FLORIDA PENINSULA... ....Texas Panhandle into the Central Plains... An ongoing convective complex is expected to produce areas of locally heavy rainfall across central/eastern Kansas toward the beginning of the forecast period (early Monday morning). Convergence on the nose of strong low-level flow amid modestly steep mid-level lapse rates aloft are likely to sustain the storms for a few hours (between 12-18Z Monday) before veering and weakening low-level flow causes weakening. Local backbuilding/training could result in rates exceeding 1 inch/hr locally, which may result in a few issues with excessive runoff. Isolated thunderstorm activity is expected from southwestern Kansas through the Texas Panhandle during the afternoon and evening hours. The Marginal Risk is being maintained for consistence with prior forecasts, although newest models depict modest convective coverage. Nevertheless, supercellular wind profiles and steep lapse rates aloft may result in intense convection and local rain rates exceeding FFG for a few hours during the afternoon and early evening. This Marginal is conditional on sufficient convective coverage for localized prolonging of heavier rainfall. Lastly, models depict a resurgence of elevated convection on the nose of strong low-level flow. The best convergence (and resultant convective coverage) should extend from southeastern Nebraska south-southeastward along the Kansas/Missouri border region. Localized backbuilding and training could result in spots of 2-4 inch rainfall totals, and flash flooding is possible on an isolated basis. Soil moistures are relatively dry though, further suggesting only an isolated flash flood threat. ....Oregon, northern California... A substantial mid-level system will approach the region from the northeastern Pacific during the afternoon and evening. Ascent/cooling aloft will be well aligned with peak heating, resulting in sufficient instability for widespread showers and occasional thunderstorms. Recent CAMs suggest an area of focus for convection across higher terrain of southwestern Oregon (near the Medford area) and into terrain-favored areas of northern California. Appreciable mid-level lapse rates and PW values approaching 1 inch suggest potential for several areas of wetting convection, and areas of flash flooding are possible especially near burn scars and low-lying, flood prone areas. Deeper convection may also extend toward northern Oregon along the Columbia River Basin, with isolated flash flood potential expected. ....Upper Mississippi Valley... Low-level warm advection across a warm frontal zone from the Dakotas into Iowa should result in areas of showers and thunderstorms by 00Z. The orientation of this convection favors some training/backbuilding that could enhance rain rates locally. 2-3 inch totals are expected, with locally higher amounts possible particularly across northeastern Minnesota. ....Florida... A nearly stationary front will remain across northern Florida. Abundant moisture (2+ inch PW values) should reside south of this front. A weak low should also reside just west of the state over Gulf of America waters. Convection is expected to develop over much of the Peninsula in tandem with daytime heating and destabilization. Cells should remain relatively slow-moving, with areas of 2+ inch/hr rain rates (and 3 inch totals) expected throughout the forecast period. Isolated flash flooding is possible especially where these heavier rates can materialize over urbanized/sensitive areas. Cook Day 2 Valid 12Z Tue Sep 09 2025 - 12Z Wed Sep 10 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE=20 FLORIDA PENINSULA... Few changes were made from the inherited Marginal Risk area=20 inherited from yesterday's Day 3 ERO. The quasi-stationary surface boundary over north Florida early Tuesday will nudge slowly into=20 central FL by Wednesday morning as southern periphery of the mid- upper level trough remains parked across the central Gulf Coast=20 region. Continued large-scale forcing downstream of the upper=20 trough -- weak upper level difluence and subtle shortwave=20 energy/periodic DPVA -- will again favor more widespread/organized=20 convection during the peak heating hours from late morning into=20 early evening on Tuesday. Low-level convergence over the eastern=20 peninsula will likely get a boost from the sea- breeze component,=20 as the southwesterly low-level flow weakens and backs more=20 southerly near the east coast. As a result, could see more=20 widespread convective clusters and heavier rain farther east toward the I-95 corridor, as supported by the higher HREF and RRFS=20 probabilities of 2-3 inch/hr rainfall rates across this area after=20 17Z Tuesday. Thermodynamic parameters will be more than supportive=20 for highly efficient warm rain processes, with mixed-layer CAPEs at least 1000-2000 J/Kg, TPWs of 2.2-2.3", and wet bulb zero levels=20 above 14KFT (optimal warm layer depth). Moreover, 0-6km bulk shear=20 values around 20-25 kts should allow for more convective=20 organization, though the degree of downwind propagation=20 (southwesterly Corfidi vectors around 10-15kts) should inhibit the=20 potential for cell training.=20 As a result, any excessive short-term runoff on Tuesday will be=20 localized and likely tied to urban and/or poor drainage areas. Hurley Day 3 Valid 12Z Wed Sep 10 2025 - 12Z Thu Sep 11 2025 ....Discussion forthcoming... Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_42JAo4ryEO4ohJm1lcxO13DMaBhYuj2ehJ-5cbywAq9= T3co4g5PGM6-7iqKsTremDSPkeo9hTU4kl0iX58tB19EwkE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_42JAo4ryEO4ohJm1lcxO13DMaBhYuj2ehJ-5cbywAq9= T3co4g5PGM6-7iqKsTremDSPkeo9hTU4kl0iX58txE1FF6M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_42JAo4ryEO4ohJm1lcxO13DMaBhYuj2ehJ-5cbywAq9= T3co4g5PGM6-7iqKsTremDSPkeo9hTU4kl0iX58tC42Fwuw$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .