Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 08 2025 01:07:59 FOUS30 KWBC 080107 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 907 PM EDT Sun Sep 7 2025 Day 1 Valid 01Z Mon Sep 08 2025 - 12Z Mon Sep 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH TEXAS, THE SOUTHEAST COAST, AND PARTS OF NORTHERN AND CENTRAL=20 FLORIDA... 01z Update... ....South Texas... As the front across the region continues to drift slowly south and=20 more stable and relatively drier air filter in from the=20 north, removed much of the northern extent of the previous=20 Marginal Risk outlook from southern Texas. A small Marginal Risk=20 was maintained across Deep South Texas, where deep moisture (PWs=20 ~2.25 inches) and ample instability (MUCAPE 2000-3000 J/kg) may=20 support heavy rainfall rates as storms continue to move south=20 across the region this evening. ....Southeast Coast/Florida... Reduced the footprint of the previous outlooks across these areas as well. Along the Southeast Coast, expect the greater threat for heavy rainfall rates and potential runoff concerns to be mostly limited to a pool of deeper moisture centered along the South Carolina Coast. Mesoanalysis indicates that the present low level flow has been sufficient to maintain a stripe of 2 inch PWs, while an axis of greater instability (MUCAPEs 1000-2000 J/kg) extends=20 west from the coast along a surface-to-low level boundary. Back to the south, rainfall rates have begun to decrease across North Florida. However, some heavier rates continue, including=20 across those areas where heavy rains have been falling into the=20 evening. Plenty of moisture remains across the region (PWs at or=20 above 2.25 inches). However, SPC mesoanalysis shows increasing=20 CIN, with the trend expected to continue with the loss of daytime=20 heating. Pereira Day 2 Valid 12Z Mon Sep 08 2025 - 12Z Tue Sep 09 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS AS WELL AS FOR THE FLORIDA PENINSULA... ....Texas Panhandle into Central Plains... An area of convection across Kansas will likely be ongoing at the start of the period (early Monday morning). The potential early morning storms would be within a low level jet (LLJ) of 30-40 kt which is expected to maintain above average strength into the afternoon, though some weakening beyond the typical early morning peak is expected for mid-day Monday. Within the axis of the LLJ, moisture anomalies are forecast by the 12Z NAM/GFS to be in the +1 to +2 range and CAPE of at least 1000 to 2000 J/kg, supportive of higher rates (1-2 in/hr) where cell training is able to set up. By Monday evening, a second round of thunderstorms is expected over the Texas/Oklahoma Panhandle region into western Oklahoma with another diurnal increase of the LLJ into the 30-40 kt range just after 00Z. Similar, localized potential for 1-2 in/hr rain rates and flash flooding will exist. ....Florida Peninsula... The slow moving cold front will drift east on Monday/Monday night, taking tropical moisture and the probabilities of high rain rates farther offshore away from the southern Mid-Atlantic coast. However, the front is expected to remain quasi-stationary across the north-central Florida Peninsula with a repeat threat for localized flash flooding as has been the case for several days now across the region. In addition, a slow moving surface low in the eastern Gulf will maintain a draw of additional moisture and storms towards the eastern Gulf Coast region. Individual cells and multiple clusters of storms are expected across much of the Florida Peninsula with added forcing along sea breeze and outflow boundaries. Once again, an isolated flash flooding threat will be where cell mergers or slow moving storms happen to stall out over urban or other flood prone areas. ....Upper Mississippi Valley... Low level warm advection across a warm frontal zone draped southeastward from the Dakotas into Iowa is expected to lead to the development of showers and thunderstorms by 00Z Tuesday. A 30 to 40+ kt LLJ should be in place from the central Plains into the upper Mississippi Valley along with a zone of +1 to +2 standardized precipitable water anomalies. Initial storm development is anticipated in the vicinity of Duluth, MN, with movement toward the east. Upstream development to the west appears likely overnight along the elevated portion of the warm frontal boundary as it is overrun by the LLJ beneath increasing right-entrance jet ascent from a 70-80 kt speed max north of Lake Superior. Current thinking is for some 2-4 inch totals but 12Z HREF probabilities for 5 inches (12 hour accumulation) are spotty 10 to 20 percent from northern Minnesota to the far western U.P. of Michigan. Otto/Wegman Day 3 Valid 12Z Tue Sep 09 2025 - 12Z Wed Sep 10 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHEASTERN COAST AND THE FLORIDA PENINSULA AS WELL AS FOR PORTIONS OF NORTHERN IDAHO AND WESTERN MONTANA... ....Southeast... A trough that has been parked across the Florida Peninsula for well over a week and associated boundary that has been off the coast of the Carolinas for the past several days will begin to move northwest back towards the coast as a warm front on Tuesday. A surface low may also form along the front which may also help move the heavy rain associated with the boundary northwestward towards the coast of the Carolinas. The trough, while it has been in place for an extended period of time, still will demarcate a strong boundary between a cool, autumn-like air mass over much of the East Coast, and a typical summer-like airmass characterized by hot and very humid conditions for the coast. Thus, the trough will still be plenty able to provide the forcing for strong thunderstorms capable of heavy rain. As the heavy rain approaches the coast Tuesday, any flash flooding for coastal communities will result from that heavy rain, urbanization, and coincident high tide cycles. Uncertainty with exactly where that axis of heaviest rain will set up will preclude any possible upgrades to the ERO at this time, but a greater westward shift of the heaviest rain into the coast will support possible upgrades in future updates. Meanwhile over Florida, the primary forcings will be the front/trough centered off the coast, a slow and erratic moving low over the Gulf and upper level divergence within the right-entrance region of an anti-cyclonically curved jet max over the Mid-Atlantic states. While confidence in thunderstorm coverage is lower at the Day 3 time frame, antecedent rainfall will continue to be a factor with overlap of heavy rain atop near saturated soils a concern for localized flash flooding. Many of the same ingredients will be in place on Tuesday as in previous days, supporting at least an isolated flash flood threat across the Southeast coastline into the Florida Peninsula. ....Pacific Northwest... A large cutoff low parked over Washington State and Oregon will pinwheel several pieces of shortwave energy northward along its eastern periphery across Idaho and Montana on Tuesday. Abundant forcing and upslope support will coincide with roughly +1 to +2 standardized precipitable water anomalies, resulting in scattered convection across the region Tuesday afternoon during peak heating. The cutoff low will be slow moving, allowing for an extended period of favorable widespread lift out ahead. While forcing for showers and thunderstorms looks good, including notable diffluence and divergence aloft, there are some concerns which will be potential negatives for flash flooding. These negative factors include progressive storm motions, uncertain instability and storm coverage. Nonetheless, portions of the highlighted region have burn scar regions that are highly susceptible to flash flooding from rain rates as low as 0.3 to 0.5 inches in a short period of time which will be possible. The flash flood threat will coincide within the diurnal peak of thunderstorm coverage from mid/late afternoon into the early overnight. Otto/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9nT1m3n94FIi8CHAxyximMZOzFH8dcWwCflnKGB0W5lj= LlgnvJhRZCPcsKkm9BCbya9sGZZtCn53XehehBvbs1kAUK4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9nT1m3n94FIi8CHAxyximMZOzFH8dcWwCflnKGB0W5lj= LlgnvJhRZCPcsKkm9BCbya9sGZZtCn53XehehBvbWw3RAc8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9nT1m3n94FIi8CHAxyximMZOzFH8dcWwCflnKGB0W5lj= LlgnvJhRZCPcsKkm9BCbya9sGZZtCn53XehehBvbDS3mzxc$=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) .