Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 07 2025 20:28:05 FOUS30 KWBC 072027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 PM EDT Sun Sep 7 2025 Day 1 Valid 16Z Sun Sep 07 2025 - 12Z Mon Sep 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS AS WELL AS FOR THE SOUTHEAST COAST AND MUCH OF FLORIDA... ....16Z update... ....Southern Texas... Across south-central into southern Texas, low level easterly flow near a cold front containing transient axes of convergence have helped to support localized high rainfall intensities with values of 2 to 3 inches over an hour over the Edwards Plateau over the past few hours. There should not be much movement to the frontal boundary over the next 24 hours but drier air in the low levels is forecast to move into Texas from the northeast through Monday morning. This should have the net effect of shifting the flash flood threat toward the south with time and by the overnight time frame, easterly 850 mb flow of 10-20 kt should focus the threat for heavy rain to the middle and lower Texas coasts as well as interior portions of south Texas. Additional rainfall totals through Monday morning of 3 to 6 inches (locally higher possible) should be expected, but these higher values should remain rather localized. ....Southeast Coast and Much of Florida... Below reasoning remains valid with continued potential for slow moving cells with high rainfall rates within axis of moisture along the Southeast Coast containing PWAT values of approximately 1.7 to 2.0 inches. Greatest flash flood concerns will likely be with urban overlap of high rainfall rates given high flash flood guidance values for much of the outlooked region. ....Southern to Central High Plains... While no excessive rainfall area was drawn, there is a non-zero threat for flash flooding late this evening as thunderstorms move off of the higher terrain into the High Plains. There should be sufficient shear for organized storms and a highly localized concern for a cyclic cell to allow for high rainfall rates from the northern Texas Panhandle into far eastern Colorado and western Kansas. Otto ....previous discussion follows... ....Southern Texas... The combination of weak "ridge riding" upper level disturbances, a strong cold front approaching from the northeast, and abundant Gulf moisture will all poorly come together across portions of southern Texas from now through the day today and into tonight. Storms circulating around a well-defined upper level high over northeastern Mexico are being driven in part due to upsloping in the easterly flow off the Gulf. Due to the aforementioned poor merging of these various ingredients and forcings, expect widely scattered to scattered convection for the rest of the predawn hours and into much of the day today. However, these storms will really struggle to organize to result in any more than an isolated flash flooding threat. High FFGs, about as high as they get, are plentiful over much of southern Texas, while soil moisture levels are about average, which works against flash flooding. Thus, any isolated flooding in southern Texas today will require slow moving storms hitting just the right area, likely an urban one, to cause any impacts today. ....Southeast Coast and Much of Florida... An area of abundant tropical moisture with embedded weak troughs/waves will remain around the Florida Peninsula today. The strongest trough will approach the Gulf Coast side of the state today from the west. Interactions of this trough, any sea breezes, and the instability difference in the afternoon between the hotter Peninsula and the adjacent Gulf and Atlantic will be the primary forcing mechanisms for any storms. Thus, while storm coverage will likely once again be widely scattered to scattered. The various forcings likely competing with each other will work to keep any storms and clusters of storms from organizing into a more significant flash flooding threat. Further, despite days of scattered convection across the Florida Peninsula, soil moisture levels across much of the state remain below normal, meaning most of the rain that falls from the afternoon and evening storms will remain beneficial for most. Thus, the Marginal flash flooding threat is primarily focused on urban areas today, as any storms will have plenty of moisture to draw from to locally produce multiple-inch-per-hour rainfall rates. ....New England... The inherited Marginal Risk across the area was dropped with this update. All of the rainfall across New England this morning is stratiform, struggling to produce rates even to a half inch per hour. This is likely to continue today as the cold front causing the rain continues to press east towards the coast. While a few areas of embedded convection from mid-level instability can't be ruled out, fast motion will also cut into the rainfall amount any one area picks up today. Wegman 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=20 PENINSULA... ....Texas Panhandle into Central Plains... An area of convection across Kansas will likely be ongoing at the=20 start of the period (early Monday morning). The potential early=20 morning storms would be within a low level jet (LLJ) of 30-40 kt=20 which is expected to maintain above average strength into the=20 afternoon, though some weakening beyond the typical early morning=20 peak is expected for mid-day Monday. Within the axis of the LLJ,=20 moisture anomalies are forecast by the 12Z NAM/GFS to be in the +1=20 to +2 range and CAPE of at least 1000 to 2000 J/kg, supportive of=20 higher rates (1-2 in/hr) where cell training is able to set up.=20 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=20 farther offshore away from the southern Mid-Atlantic coast.=20 However, the front is expected to remain quasi-stationary across=20 the north-central Florida Peninsula with a repeat threat for=20 localized flash flooding as has been the case for several days now=20 across the region. In addition, a slow moving surface low in the=20 eastern Gulf will maintain a draw of additional moisture and storms towards the eastern Gulf Coast region. Individual cells and=20 multiple clusters of storms are expected across much of the Florida Peninsula with added forcing along sea breeze and outflow=20 boundaries. Once again, an isolated flash flooding threat will be=20 where cell mergers or slow moving storms happen to stall out over=20 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=20 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=20 anticipated in the vicinity of Duluth, MN, with movement toward the east. Upstream development to the west appears likely overnight=20 along the elevated portion of the warm frontal boundary as it is=20 overrun by the LLJ beneath increasing right-entrance jet ascent=20 from a 70-80 kt speed max north of Lake Superior. Current thinking=20 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=20 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=20 the coast of the Carolinas. The trough, while it has been in place=20 for an extended period of time, still will demarcate a strong=20 boundary between a cool, autumn-like air mass over much of the=20 East Coast, and a typical summer-like airmass characterized by hot and very humid conditions for the coast. Thus, the trough will=20 still be plenty able to provide the forcing for strong=20 thunderstorms capable of heavy rain. As the heavy rain approaches=20 the coast Tuesday, any flash flooding for coastal communities will=20 result from that heavy rain, urbanization, and coincident high=20 tide cycles. Uncertainty with exactly where that axis of heaviest=20 rain will set up will preclude any possible upgrades to the ERO at=20 this time, but a greater westward shift of the heaviest rain into=20 the coast will support possible upgrades in future updates. Meanwhile over Florida, the primary forcings will be the=20 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=20 place on Tuesday as in previous days, supporting at least an=20 isolated flash flood threat across the Southeast coastline into the Florida Peninsula.=20 ....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=20 convection across the region Tuesday afternoon during peak heating. The cutoff low will be slow moving, allowing for an extended=20 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=20 potential negatives for flash flooding. These negative factors=20 include progressive storm motions, uncertain instability and storm=20 coverage. Nonetheless, portions of the highlighted region have=20 burn scar regions that are highly susceptible to flash flooding=20 from rain rates as low as 0.3 to 0.5 inches in a short period of=20 time which will be possible. The flash flood threat will coincide=20 within the diurnal peak of thunderstorm coverage from mid/late=20 afternoon into the early overnight.=20 Otto/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9_n_QINkpHnxEXDMrG9lauG0Xy3P7pQUsjmTXEVSFSvi= jZU4H4OjVVXEqY-DVoPRvrTu1vzW_rsDuBGo6X39fpT_64o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9_n_QINkpHnxEXDMrG9lauG0Xy3P7pQUsjmTXEVSFSvi= jZU4H4OjVVXEqY-DVoPRvrTu1vzW_rsDuBGo6X39Qptm-BY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9_n_QINkpHnxEXDMrG9lauG0Xy3P7pQUsjmTXEVSFSvi= jZU4H4OjVVXEqY-DVoPRvrTu1vzW_rsDuBGo6X39sejb0lY$=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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