Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 24 2025 08:31:27 FOUS30 KWBC 240831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 431 AM EDT Wed Sep 24 2025 Day 1 Valid 12Z Wed Sep 24 2025 - 12Z Thu Sep 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE TENNESSEE AND OHIO VALLEY REGIONS... ....Southern Plains to the Central and Southern Appalachians... Overall, no large-scale changes to the previous outlook areas, maintaining a broad Marginal Risk that extends from portions of central and southern Texas to the southern and central=20 Appalachians, with an embedded Slight Risk centered over parts of=20 the Ohio and Tennessee valleys.=20 Composite radar this morning shows widespread showers and storms=20 extending from the Ohio Valley back into the southern Plains. The=20 overnight CAMs indicate that ongoing storms along the far eastern=20 Oklahoma-Texas border into southern Arkansas will remain organized into the late morning as the drift slowly south, with training=20 storms likely to produce heavy amounts across portions of=20 northeastern Texas and northwestern Louisiana. Supported by=20 shortwave energy moving through the base of a broader-scale=20 positively-tilted upper trough and PWs at or above 2 inches, HREF=20 probabilities indicate locally heavy amounts exceeding 3 inches are likely. However, dry soil conditions, reflected by high FFGs, are=20 expected to limit the threat for widespread flooding concerns. As energy continues to move through the base of the upper trough, an area of low pressure is expected to become better organized and track northeast through the Ohio Valley beginning later this afternoon and continuing into the overnight. Deep southwesterly flow ahead of the wave will support increasing moisture, with PW anomalies increasing to over 2 std dev above normal per the GFS. Given the ongoing rains and relatively wetter soil conditions (lower FFGs), a Slight Risk extending from far northern Alabama to far southern Ohio and western West Virginia was maintained. HREF=20 probabilities indicate that amounts of 1-2 inches will be common,=20 with locally heavier amounts over 3 inches possible within the=20 Slight Risk area. In the wake of the low, showers and storms producing heavier rates are expected along the trailing front as it sinks south through=20 the lower Mississippi Valley and along the upper Texas coast back=20 into South Texas. Here too, the HREF is indicating locally heavy=20 amounts over 3 inches are likely. But similar to areas to the=20 north, higher FFGs suggest flooding issues will be mostly contained to vulnerable areas, including urbanized centers. ....Northeast... The general consensus of the CAMs shows an area of showers and storms moving northeast well in advance of the system over the Ohio Valley. Supported by deepening moisture along a slow-moving boundary and increasing mid-to-upper level forcing, showers and storms will advance northeast across Pennsylvania, with repeating storms potentially resulting in locally heavy amounts from northeastern Pennsylvania into the lower Hudson Valley/NYC area. A A Marginal Risk was maintained for areas where the HREF shows=20 higher probabilities for amounts over 2 inches and where FFGs are=20 relatively lower. ....Southeastern Lower Michigan... An upper low closing off this morning is expected to remain centered over Lower Michigan into Thursday morning. Showers and storms are likely to develop east of the center, with relatively deeper moisture wrapping into the system raising the potential for locally heavy rainfall rates. A small Marginal Risk was=20 introduced, highlighting the area where the HREF shows high=20 probabilities for accumulations exceeding 2 inches. ....Central California... A slow-moving upper low currently offshore will drift north and begin moving inland Thursday morning. With anomalous moisture remaining in place (2.5-3 std dev above normal), locally heavy amounts will remain possible. Reflecting the high HREF probabilities for amounts over an inch, a Marginal Risk was maintained along the central California Coast from the Monterey=20 Bay south and along the adjacent mountains.=20 Pereira 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=20 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=20 was maintained across this area where PWs are forecast to increase=20 to 1-2 standard deviations above normal by the afternoon. ....Northeast... Unsettled weather developing Day 1 is forecast to continue into=20 Thursday as showers and storms continue to develop ahead of the=20 system lifting out of the Ohio Valley into the Northeast.=20 Strengthening southwesterly flow along the Mid Atlantic coast into=20 the Northeast will support deepening moisture (PWs increasing to ~2 inches) and an increasing threat for heavy rainfall rates. A=20 Marginal Risk was maintained from northern New Jersey into southern New England, where HREF probabilities indicate that localized=20 amounts over 3 inches are possible late Thursday morning into the=20 afternoon. ....Southern Appalachians... The consensus of the latest guidance indicates that showers and storms developing along and ahead of the cold front trailing the=20 low moving through the Northeast will be generally progressive.=20 With widespread heavy amounts not expected and FFGs high, the=20 previous Marginal Risk was removed from much of the Mid Atlantic.=20 However, some models do show the potential for training storms to=20 produce some localized heavy amounts (2+ inches) across parts of=20 the southern Appalachians, where a small Marginal Risk was=20 maintained. 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=20 and evening hours. A Slight Risk, with a footprint similar to the=20 Day 2 area, was maintained for Day 3 across parts of Arizona and=20 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!5vA739x67VpnfVraAik3IwQLSuWXLa9vminnKBPVMd88= _4ip2yq-4cSfT_dtRHVGrOOe9KqZkZf6iCg6bJT_HUWuYcM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5vA739x67VpnfVraAik3IwQLSuWXLa9vminnKBPVMd88= _4ip2yq-4cSfT_dtRHVGrOOe9KqZkZf6iCg6bJT_2G4rUvQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5vA739x67VpnfVraAik3IwQLSuWXLa9vminnKBPVMd88= _4ip2yq-4cSfT_dtRHVGrOOe9KqZkZf6iCg6bJT_5YHAnno$=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) .