Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 25 2025 00:46:41 FOUS30 KWBC 250046 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 846 PM EDT Wed Sep 24 2025 Day 1 Valid 01Z Thu Sep 25 2025 - 12Z Thu Sep 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE OHIO=20 VALLEY AND PORTIONS OF SOUTH TEXAS... ....01Z Update... ....Ohio and Tennessee Valleys... Rain associated with a pair of disturbances is developing across the lower Ohio Valley this evening. The locally heavy rain associated with embedded convection will spread northeast up the Ohio Valley, where rain from recent days has moistened the soils somewhat. Thus, the Slight Risk was adjusted north, but remains in place for the potential for widely scattered instances of flash flooding. Further south into the Tennessee Valley the rain is generally more progressive, less widespread, and there are large areas with no rain occurring. Those areas have been dropped down to a Marginal with this update. ....South Texas... Developing convection along a cold frontal interface is expected to become more widespread overnight, though model agreement on how that convection behaves once formed is poor. The frontal interface will likely be where the most persistent convection forms and remains before likely pushing south into the heat and instability in place across Deep South Texas. Thus, the rather narrow corridor from Corpus Christi on the coast to Laredo on the Rio Grande is=20 likely where the highest chances for flash flooding will be through tonight. Wegman Day 2 Valid 12Z Thu Sep 25 2025 - 12Z Fri Sep 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST ARIZONA AND FAR SOUTHWEST NEW MEXICO... ....Southwest to Sierra Nevada... After moving inland early Thursday, the closed low over central California is forecast to drop south into southern California by early Friday. With ample moisture remaining in place (2-3 std dev above normal) near and east of the upper center, locally heavy amounts are possible from the Sierra Nevada east into southern Nevada and southwestern Utah. As the center drops south, increasing southerly flow east of the low will draw deeper moisture up from Mexico, fueling widespread shower and thunderstorm coverage, especially across southeastern and central Arizona into southwestern New Mexico. A Slight Risk was maintained across this area where PWs are forecast to increase to 1-2 standard deviations above normal by the afternoon. ....Northeast... Unsettled weather developing Day 1 is forecast to continue into Thursday as showers and storms continue to develop ahead of the system lifting out of the Ohio Valley into the Northeast. Strengthening southwesterly flow along the Mid Atlantic coast into the Northeast will support deepening moisture (PWs increasing to ~2 inches) and an increasing threat for heavy rainfall rates. A Marginal Risk was maintained from northern New Jersey into southern New England, where HREF probabilities indicate that localized amounts over 3 inches are possible late Thursday morning into the afternoon. ....Southern Appalachians into parts of the Southeast and Eastern Gulf Coast... ....2000 UTC Update... Amplifying mid-upper trough digging into the northwest and north- central Gulf Thu-Thu night will slow the eastward progression of the surface cold front and thus favorable pre-frontal buoyant airmass (ribbon of higher TPW and deep-layer instability). More protracted period of favorable low-level moisture transport ahead of the upper trough/surface front. 12Z HREF probabilities depict this idea well, with several hours of spotty areas with enhanced probs for 1-2+ in/hr rainfall rates. Therefore, supported by the latest CSU UFVS-verified ERO First-Guess field, have expanded the Marginal Risk south to the MS-AL-western FL Panhandle Gulf Coast. ....Previous Discussion... The consensus of the latest guidance indicates that showers and storms developing along and ahead of the cold front trailing the low moving through the Northeast will be generally progressive. With widespread heavy amounts not expected and FFGs high, the previous Marginal Risk was removed from much of the Mid Atlantic. However, some models do show the potential for training storms to produce some localized heavy amounts (2+ inches) across parts of the southern Appalachians, where a small Marginal Risk was maintained. Hurley/Pereira Day 3 Valid 12Z Fri Sep 26 2025 - 12Z Sat Sep 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO... ....Southwest... A second day of numerous showers and storms is expected across the Southwest, especially for much of southeastern Arizona and parts of southwestern New Mexico. The upper low setting into southern California the previous period is expected to move little this day, remaining centered near the California/Mexico border. This will help to maintain anomalous moisture across the region, fueling widespread shower and thunderstorm coverage during the afternoon and evening hours. A Slight Risk, with a footprint similar to the Day 2 area, was maintained for Day 3 across parts of Arizona and New Mexico. ....Southeast... A boundary will remain in place across the Southeast as an upper trough evolves into a closed low that is forecast to linger beyond the period. Moisture and energy lifting ahead of the closing system will support periods of shower and storms. While the guidance at this point is not signaling any widespread heavy amounts, the pattern does lend itself to at least the threat for locally heavy amounts, which may raise at least isolated flooding concerns. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4QDYoBFeNMOd1Ld9bCQPgYzlRQX5emLDwLwCBoyFY48R= Clcn8tnCyqBzBMMpEAOMrOQUBya-8ZfpL6lqWvLjXgO07DA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4QDYoBFeNMOd1Ld9bCQPgYzlRQX5emLDwLwCBoyFY48R= Clcn8tnCyqBzBMMpEAOMrOQUBya-8ZfpL6lqWvLjv9146VM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4QDYoBFeNMOd1Ld9bCQPgYzlRQX5emLDwLwCBoyFY48R= Clcn8tnCyqBzBMMpEAOMrOQUBya-8ZfpL6lqWvLjPA8nWGM$=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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