Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 28 2025 15:52:19 FOUS30 KWBC 281552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM 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.=20 ....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.=20 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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....Southeast U.S... The combination of shortwaves ejecting eastward out of the Southern Plains and the approach of a cold front from the north will lead to ample ascent between sfc-500mb along a stretch of elevated moisture between +1 to +2 deviations in PWAT anomalies. Consensus has increased in a broad, scattered convective regime from the Lower Mississippi Valley into the Southeastern U.S. leading a potpourri of QPF maxima within areas of convective impact. Signals at the end of the 00z HREF were quite bullish in the 12-hr window from 12-00z Thu/Fri with neighborhood probabilities for >3" between 35-60% across several small areas over the Central Gulf Coast, including some "bullseye" patterns within a few urban centers between Baton Rouge to Tallahassee. Thunderstorm concerns will continue until the front passes through any specific area south of I-40 in the period, so the prospects for local maxima capable of flash flooding are elevated when you add the lower FFG indices currently in place between MS/AL. This will come after seemingly 3 straight days of heavy rainfall at any point in time for the region, so the FFG recovery is pretty unlikely at this juncture. The previous SLGT risk inherited was generally maintained, but did expand west given the timing of the approaching shortwave out of TX and the accompanying higher QPF signatures over LA between 12-18z Thursday. ....Ohio Valley... A broad ULL positioned over the Midwest will pivot southeast with a positioning across the Mid-Mississippi and Western Ohio Valley through D2. Large scale forcing under presence of the ULL will generate periods of convection across the above areas with a focus on MO/IL/KY/IN. QPF maxima based on the ensemble bias corrected output was between 1.5-2.5 over Central Ohio Valley with highest signature over the Ohio River basin. The current outlook is still within the MRGL risk threshold due to the lower instability presence at the surface as much of the ascent is rooted in the mid to upper levels given the ULL/trough influence. The previous MRGL risk was relatively unchanged with only minor tweaks around the periphery to account for the latest heavy QPF distribution. ....Texas... More convection is expected over the Lone Star State on Thursday afternoon through the end of the period. West TX will be the initiation point with convection firing in-of the theta_E ridge positioned over the Big Bend up through the Pecos River Valley. A cold front will be pressing in from the north with a sharp cutoff of any instability presence as MUCAPE will flat-line with fropa as drier air advects behind the front. The best threat for convection will likely occur over Southwest TX with some question as to whether the cold front will subdue convective concerns for areas north of I-20 by the second half of the forecast. Guidance is split on how it wants to handle the shallow cold push with the NAM Nest the most aggressive in its latitudinal push and globals lagging enough to warrant storm threats as far north as the Caprock. For now, left a broad MRGL over the region, but there will likely be some fluctuations in the northern periphery of the MRGL risk area in place. Will be monitoring over the next succession of updates. Kleebauer 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... 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!_wjHWH19J1jrzUUrv-16PXPtgOwBARIj3gdleUKELGXy= QsohHhTnXVzcMQHts4lNSyfwcPCpRvLPc4A-s7Bo2pVcciQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_wjHWH19J1jrzUUrv-16PXPtgOwBARIj3gdleUKELGXy= QsohHhTnXVzcMQHts4lNSyfwcPCpRvLPc4A-s7Bok91-kOg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_wjHWH19J1jrzUUrv-16PXPtgOwBARIj3gdleUKELGXy= QsohHhTnXVzcMQHts4lNSyfwcPCpRvLPc4A-s7Bo2VavSJ4$=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) .