Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 08 2024 08:17:34 FOUS30 KWBC 080817 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 AM EDT Sun Sep 8 2024 Day 1 Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, DEEP SOUTH TEXAS, AND SOUTHEAST... ....Summary... Only a few very minor changes were made to the inherited Marginal Risk areas across the CONUS. ....Southwest... A bit more widespread convection is expected with the monsoon this afternoon across the Southwest. Much of the convection will once again be focused along the various mountain ranges from the Peninsular Ranges of southern California through the Sangre de Cristos of Colorado. As usual in the desert Southwest, somewhat limited moisture will be the limiting factor, even though areas could be 2 sigma above normal for atmospheric moisture for this time of year. Thus, the primary flood threat is for urban areas, slot canyons, and arroyos where slow moving convection in chaotic steering flow may result in long-duration localized heavy rain from the most persistent convection. Any resultant flash flooding will be isolated. ....Deep South Texas... The Marginal Risk area was trimmed closer to the coast with this update, now primarily for the urban area of Brownsville over to the South Padre Island beaches. A stalled out front will begin to focus an area of heavy rainfall associated with a slowly developing tropical low over the western Gulf. Guidance has shifted east a bit, resulting in lower forecasted rainfall totals for all of Deep South Texas, with little if any rain expected west of Brownsville through tonight. However...given the abundance of atmospheric moisture available any stronger showers and storms will be capable of very heavy rainfall rates which could cause isolated flash flooding. ....Southeast... The stalled out front that has been haunting the Southeast for well over a week now remains in place from far southern South Carolina through the northern Florida Peninsula. Besides the front itself being strong...indicative of a tight gradient between the extremely humid air mass over the Gulf and an outright autumnal air mass to its north...instability remains very lacking since this front hasn't moved in days, so diurnal heating is limited under the extensive low cloud cover. Thus, almost all of the rain will be light across this region. This is countered by the aforementioned abundant amounts of atmospheric moisture available for any convection that can form. Thus, a low-end Marginal remains in place, with the greatest threat for flooding over the Florida Peninsula well south of the front where diurnal heating can actually make a difference. Wegman Day 2 Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP SOUTH TEXAS... As on Day 1, very few changes were needed to the inherited ERO risk areas. ....Coastal Deep South Texas... Weak easterly flow north of a developing tropical low over the=20 southwestern Gulf will advect increasingly deep tropical moisture into coastal Deep South Texas on Monday. This is highly likely to result in much more rainfall for South Padre Island and over to Brownsville as compared with Day 1/Sunday. Given PWATs that could exceed 2.5 inches, or 3 sigma above normal for this time of year, any convection that moves west off the Gulf into the coast will be capable of very heavy rain rates. Thus, the real question regarding flood coverage will be how frequent storms producing those rates move over urban areas. Think right along the coast widely scattered instances of flash flooding can be reasonably expected. Any training could quickly result in significant flash flooding. That said, the system will only really just be getting organized, so overall rains will likely be more intermittent in most areas. Urban areas such as Brownsville will be at a higher likelihood for flash flooding. ....Four Corners Region... Convective coverage should shift northeastward a bit on Monday. Thus, the flooding threat shifts into the Four Corners region and away from the West Coast. Flooding concerns will be greatly=20 focused in area slot canyons and arroyos of southern Utah and=20 southwest Colorado Monday afternoon and evening with peak heating. ....Rest of the Gulf Coast... Rinse and repeat weather pattern will continue, albeit shifted just a bit south of Sunday's focus. Thus, most of the north Gulf Coast will get a relative break from the potential for steady heavy rain, as the heaviest rains shift south a bit away from the coast. The exception will be into the Florida Peninsula, where a repeat day of sea breeze showers and thunderstorms can be expected to the south, with more widespread lighter rain expected further north. Wegman Day 3 Valid 12Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TEXAS, LOUISIANA, AND MISSISSIPPI GULF COASTS... ....Summary... The inherited Slight Risk area was greatly expanded northward with this update in coordination with LIX/Slidell, LA and HGX/Houston, TX forecast offices. ....Gulf Coast... In much of the guidance, a strengthening tropical low in the Gulf=20 will push north up the far western Gulf from Mexico and into the=20 northwestern Gulf. This will both push the stalled out front that has been in this area to the north back into the South, and much better organize its associated rain. The result will be widespread heavy rainfall along the northwestern Gulf Coast on Tuesday. The bulk of the heaviest rain will be along much of the Texas Gulf Coast Tuesday and Tuesday night. Steady heavy rain directly associated with the low will hold until well into Tuesday night for the Upper Texas Coast. Meanwhile into Louisiana and points east, the rain expected in these areas will be moisture peeling off the low in the western Gulf and tracking along the stationary front separating the moisture abundant air mass over the Gulf with the fall-like air mass over much of the eastern U.S. Thus, forcing will be lesser into Louisiana and points east than in Texas. The Slight was introduced not only for the potential for long-duration light to moderate rainfall, but also due to very favorable antecedent conditions (saturated soils) from the week plus of consecutive days of rain from the same stalled front.=20 As mentioned in previous discussions, while there's good agreement that the low will be organizing in the western Gulf as it moves north by Tuesday, the fact that the low is only just beginning to get its act together as of the time of this writing has resulted in widely variable solutions as to when and where the heaviest rains will be along the Gulf Coast by Tuesday. Thus, expect potentially significant changes in the forecast rainfall footprint in the coming days as the guidance gets a much better handle on the track of the low and its associated rainfall. ....Southwest Colorado... A small Marginal Risk area was added across Southwest Colorado for lingering monsoonal moisture hitting the same mountain areas as on previous days' afternoons. Rains will also spread into the Intermountain Northwest through UT/ID/WY. With better agreement in the guidance, the Marginal may need to be expanded into this=20 region with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7sU-kAhJFSSD5cNS6v1GgP09B1yzXSl9nwRBlAQfTEBj= IppMbLFFD5m164YCVbb0s_L5KR0WrrvyMsM3GMPXUwe6ooA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7sU-kAhJFSSD5cNS6v1GgP09B1yzXSl9nwRBlAQfTEBj= IppMbLFFD5m164YCVbb0s_L5KR0WrrvyMsM3GMPX8uQpNEc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7sU-kAhJFSSD5cNS6v1GgP09B1yzXSl9nwRBlAQfTEBj= IppMbLFFD5m164YCVbb0s_L5KR0WrrvyMsM3GMPXuDBWqSY$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .