Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 28 2025 18:30:44 FOUS30 KWBC 281830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 230 PM EDT Wed May 28 2025 Day 1 Valid 16Z Wed May 28 2025 - 12Z Thu May 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND COASTAL TEXAS AS WELL AS CENTRAL AND SOUTHERN KANSAS... ....16Z Update... ....Kansas... A new Slight Risk upgrade was introduced with this update for portions of Kansas and far northwestern Oklahoma. This evening, a southeastward moving vigorous shortwave trough will force thunderstorms to form, likely in a line based on CAMs guidance, but could evolve as multiple line segments. Meanwhile, an LLJ, though not a particularly robust one, will stream Gulf moisture into the region from the south. This LLJ will support backbuilding and stronger storms capable of heavy rainfall. While individual storms should be quite progressive as they track southeastward, any backbuilding along the southern end of the line as well as potential for new segments to form could increase the flash flooding instances to more widely scattered frequency. Further, NASA Sport soil moisture maps show that soils in the Slight Risk area are at or above normal for moisture, which will convert more rainfall to runoff. ....Texas... The Slight Risk area inherited from the previous forecast remains in place from the upper Texas Gulf Coast and Houston northeastward into portions of central Texas. While the complex of storms further south as moved into the Gulf, a remnant boundary on the eastern edge of the return flow/LLJ off the Gulf will support renewed widely scattered thunderstorm activity across this region this afternoon into this evening. Soils here, like in Kansas, are also at or above normal for moisture levels due to recent rainfall activity. Thus, expect the storms, especially where they train, to pose a widely scattered flash flooding risk. Elsewhere, the Slight Risk was removed from the lower and much of the middle Texas Gulf Coast as the complex of storms that caused flash flooding in these areas earlier this morning has moved east into the Gulf and no longer pose a flash flooding risk. Any further convective development this afternoon should be isolated enough to support only a Marginal Risk in this area. ....Mid-Atlantic... The inherited Marginal Risk area was trimmed out of most of Virginia with the exception of the Hampton Roads area. Instability north of Hampton Roads is at or near zero, and therefore does not support strong convective development. However, further south where the Marginal remains for the Outer Banks and eastern North Carolina, afternoon instability is expected to increase enough to support convective development along a stalled stationary frontal boundary draped over eastern North Carolina. ....New Mexico and West Texas... The Marginal Risk area was removed with this update for this region. CAMs guidance is showing only a mediocre signal in this area, suggesting most areas see only convective showers at most. A lack of moisture should preclude flash flooding. A stray instance or two can't be ruled out, but was determined to be sub-Marginal. Wegman Day 2 Valid 12Z Thu May 29 2025 - 12Z Fri May 30 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHEAST... ....19Z Update... The inherited Slight Risk area was removed with this update. The latest CAMs guidance for the period shows that the shortwave driving expected convection through the period will weaken, and struggle to organize the convection ahead of the surface cold front in such a way as to promote a greater than isolated flash flooding threat. FFGs across much of the Southeast, while slightly depressed from earlier rainfall, have largely recovered and while soils are a bit wetter than normal, the lack of organization should hold any flash flooding to isolated instances. Thus, the combination of high FFGs and no consistent signal of heavy rainfall should preclude more widespread Slight Risk impacts. For the Gulf Coast, the urban centers in the area will have the greatest threat for flash flooding, but that assumes the storms linger over those urban areas long enough to cause flash flooding. Conversely, the surrounding Marginal Risk was expanded to include more of the harder hit areas of Texas and the portions of the Mid- Atlantic that got a good soaking rain this morning. The disorganized nature of the afternoon convection could cause=20 isolated flash flooding in these areas should the storms form over particularly sensitive areas. A secondary area of convection that begins its life in Kansas tonight will move across Missouri and eventually move into Kentucky and Indiana Thursday night. This more organized convection will locally increase the flash flooding threat compared to many other parts of the Marginal Risk area. However, these storms will be=20 quite progressive, so defining where a potential Slight Risk with=20 these storms is also not possible at this time, but the area will=20 be monitored. Instability on the northern side of the complex will likely further hamper the storms' ability to produce rainfall amounts needed to cause flash flooding. Throughout the Marginal Risk area, it's probable, if not likely that locally storms will organize enough to cause Slight Risk impacts, but where that will occur is highly uncertain at this time. Wegman Day 3 Valid 12Z Fri May 30 2025 - 12Z Sat May 31 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC, OHIO VALLEY, AND SOUTHEAST... ....19Z Update... No major changes were made to the inherited Marginal Risk area=20 along the Mid-Atlantic and Southeast. The Marginal was expanded a bit due to convective uncertainty across the Southeast, while in the Mid-Atlantic the complex of storms from the Ohio Valley will continue across the area through the period. Once again, the storms should be moving fast enough to preclude anything other than isolated flash flooding, with the urban centers at a bit higher risk for flooding. Wegman ....Previous Discussion... A potent mid-latitude trough will pivot through the Ohio Valley on Friday morning with a broad axis of diffluence downstream of the mean trough leading to a large scale forcing regime that will enhance regional precip field across the Mid Atlantic. At the surface, low pressure will navigate into the Mid Atlantic with a trailing cold front migrating eastward through much of the Eastern CONUS, interacting with a wedge of low to mid-level instability situated over the Central/Southern Appalachians and points east. Greatest thermodynamic alignment will likely be closer to the coastal plain, but elevated MUCAPE signatures can be found within the Mid Atlantic Piedmont and under the primary trough. The tandem of increased sfc-500mb forcing from the trough/front combo will lead to a scattered to widespread convective pattern that will induce locally heavy rainfall and potential for flash flooding. Timing of the ascent pattern and frontal progression will be key in determining the logistics of where the main axis of convection will occur. As of now, the best chances for convectively driven heavy rain are within the Central and Southern Appalachians, Southern VA through the NC Piedmont, and the Eastern Carolinas up to the VA Tidewater. These are the areas conducive for flash flooding at this time with localized amounts reaching 2-3" in the places hit the hardest. 1-2" is more likely out in the terrain of the Apps, but that is a signal sufficient for flashier responses due to the complexity of the topography. Despite the questions of timing, the threat remains leading to a broad MRGL risk over much of the Mid Atlantic with a southern inflection down towards FL as the trailing front will move through the region and interact with a deeper moisture layer and plentiful instability to induce some heavier convective cores capable of flash flooding in more urbanized zones. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6flt1bmIKk-S7n7HFuE1GRZ8d_YMeZwjFdSVtlsCKnOJ= gVAO2x7DXjk_Pa5OU7-qBBV7QMDOW9unw2Qu7oG8TIM2IEU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6flt1bmIKk-S7n7HFuE1GRZ8d_YMeZwjFdSVtlsCKnOJ= gVAO2x7DXjk_Pa5OU7-qBBV7QMDOW9unw2Qu7oG8Dbfgn-8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6flt1bmIKk-S7n7HFuE1GRZ8d_YMeZwjFdSVtlsCKnOJ= gVAO2x7DXjk_Pa5OU7-qBBV7QMDOW9unw2Qu7oG8andGaZo$=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) .