Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 06 2025 00:52:35 FOUS30 KWBC 060052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Sat Jul 5 2025 Day 1 Valid 01Z Sun Jul 06 2025 - 12Z Sun Jul 06 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A PORTION OF CENTRAL TEXAS... ....Central Texas... A new Moderate Risk area is out for a portion of central Texas. The remains of Barry aloft and resultant "new" 850 hPa cyclone are parting ways in a sheared environment, with the 850 hPa low moving west while the mid-level trough moves east. Normally the night after this happens is the last night/subsequent morning of heavy rain potential. Effective bulk shear due to the opposing low-level and upper-level flow is sufficient for organized convection and cells being stuck in place. There's an instability gradient near 29.5N latitude, seen on SPC mesoanalyses, that's creeping north and likely to be the better focus for heavy rainfall this period as low-level inflow intercepts it -- used this as the southern bound of the risk areas. The retrograding upper low is also a possible spot for such...used its track as the northern bound of the risk area. Some retrograding convection is possible on its west side as outflow boundaries try to instigate new convection on that side of the 850 hPa circulation/trough. Precipitable water values remain near and above 2.25". ML CAPE south of the instability gradient is 2000+ J/kg. Convective environments with this much moisture in northeast shear since 2016 (August 2016 convective low in Louisiana and Imelda in 2019) have favored the wetter mesoscale guidance, so leaned a bit on the high- resolution NAM, HRRR, and RRFS. Hourly rain amounts to 4", with local amounts to 8", appear the best bet. This on its own is problematic, but the heavy rains could also fall on already saturated ground. Coordination with the TX forecast offices -- SJT/San Angelo, EWX/New Braunfels, and FWD/Fort Worth -- as well as the National Water Center led to the new Moderate Risk area. As a side note, the remains of Barry are the 20th tropical cyclone or remnant since 1913 to produce 15"+ across interior=20 portions of Texas. ....Carolinas/T.S. Chantal... Much of the high resolution guidance for Chantal keeps the focus of heavy rainfall into the early morning hours Sunday near the NC/SC border. The guidance has had a westward bias thus far, and radar trends show that what passes as a CDO/Central Dense Overcast keeps moving north more towards NC. Both the HREF/RRFS insist on a northwest turn with an occluded look overnight, which would be something and not befit a tropical cyclone that's supposed to be in a less sheared environment. Tightened up the western gradient of the risk areas as a precaution -- the SC portion of the risk areas remains the most uncertain. ....Midwest... Trimmed the area per radar reflectivity trends and HREF output. Activity should forward propagate and fade overnight. ....Northern High Plains... A well-defined shortwave moving into the northern High Plains is leading to widely scattered thunderstorms. Forecast PWs are expected of 1 to 1.25 inches (1 to 1.5 std dev above normal) are high enough to imply saturation in the High Plains, implying rainfall efficiency. While a widespread heavy rainfall event is not expected, heavy rainfall rates and some potential for redeveloping storms may produce locally heavy amounts (greater than an inch) and isolated runoff concerns. ....Southeast Arizona... See Mesoscale Precipitation Discussion #598 for more details. ....Western Florida... Pared back the Marginal Risk area to western FL per radar reflectivity trends and 18z HREF output. Convection should fade and/or move westward into the Gulf. Roth Day 2 Valid 12Z Sun Jul 06 2025 - 12Z Mon Jul 07 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL TEXAS AS WELL AS FOR PORTIONS OF THE CAROLINAS... ....2030Z Update... ....Central Texas... In coordination with SJT/San Angelo, TX and FWD/Fort Worth, TX forecast offices, a Slight Risk upgrade was introduced with this update for portions of central Texas. While moisture amounts and advection will both be waning by Sunday, especially as compared with previous days, there will certainly still be enough moisture and forcing to allow for a renewed round of slow-moving showers and thunderstorms to affect this area. The biggest contributing factor to the upgrade is the ongoing flooding and its associated impacts. Thus, even lesser amounts of rain, as forecast on Sunday, will have outsized impacts given the vulnerabilities in that area. The storms should taper off by sunset across central Texas, ushering in a prolonged period of much drier weather. ....Carolinas/TS Chantal... With the guidance better handling TS Chantal and its track through the Carolinas, and a bit stronger storm forecast to make landfall in South Carolina tonight, by Sunday the storm will be rapidly weakening. However, its associated rainfall and abundant moisture will persist well inland from the landfall point. Most of the guidance is in very good agreement on where the swath of heaviest rainfall will occur on Sunday, namely from north-central South Carolina through central North Carolina. For now...the consensus keeps the rainfall shield just east of Charlotte, but may impact Greensboro, and perhaps Raleigh and Winston-Salem. Forecast rainfall has increased markedly, with the swath of rainfall now featuring amounts exceeding 3 inches in 24 hours (of course local rainfall amounts will be higher). Despite antecedent dry soil conditions, the potential for very heavy rainfall with Chantal's remnants are very likely overcome the initially dry soils in that area, resulting in flash flooding. However, the dry antecedent conditions should at least delay any flooding, and of course take a little off the top of any flooding's severity. Thus, have introduced a Slight Risk for north-central South Carolina and portions of central North Carolina, with an area from Greensboro south into northern South Carolina in a higher-end Slight Risk. Any slowing of the storm or a better signal for heavy rainfall over a bigger urban center would require a targeted Moderate Risk upgrade with future updates. ....Central Gulf Coast... Widely scattered showers and thunderstorms across southern Louisiana will have ample Gulf moisture to feed upon Sunday afternoon. With the potential for any of those storms moving over a sensitive urban area such as New Orleans or Baton Rouge, a Marginal Risk was introduced with this update. Wegman Day 3 Valid 12Z Mon Jul 07 2025 - 12Z Tue Jul 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE UPPER MIDWEST TO THE SOUTHWEST, THE NORTHEAST TO THE OZARK REGION, PORTIONS OF CENTRAL TEXAS, AND EASTERN NORTH CAROLINA TO THE DELMARVA... ....2030Z Update... ....Eastern North Carolina and Southeastern Virginia... The threat for heavy rainfall will continue to decrease, however some additional heavy amounts are possible as Tropical Storm Chantal moves over eastern North Carolina and southeast Virginia. The inherited Marginal Risk was expanded north across much of the Delmarva and the DC and Baltimore metros with this update for the potential for widely scattered storms capable of very heavy rain. While the heavy rain threat will be greater towards the Delmarva, given the sensitivity of the DC and Baltimore metros to heavy rain, opted to include them for that limited heavy rain potential. ....Northeast to the Ozark Region... The previously noted cold front will push across the Northeast while lingering back across the Ohio into the mid Mississippi Valley. Overnight models indicate a greater threat for heavy amounts will focus along with some right-entrance region upper jet forcing and greater moisture anomalies across northern New England. However, sufficient moisture is expected to remain in place ahead of the boundary to support at least localized heavy amounts and an isolated flooding threat further southeast along and ahead of the boundary. ....Northern Plains and Upper Midwest... Models show a well-defined shortwave moving across the region this period. With plenty of differences in the details, models indicate some potential for organized heavy rains developing ahead of this wave as it interacts with an influx of deeper moisture. ....Central Plains to the Southern High Plains/Rockies... Similar to the previous day, the models are presenting a notable signal for showers and storms, supported by moist low level inflow, to develop over High Plains, with some potential for locally heavy amounts to occur as they move east. Besides minor model differences, the only big changes was to carve out the Nebraska Sand Hills from the Marginal Risk with this update. Meanwhile, the coverage of storms and potential flooding concerns is expected to expand across New Mexico as the monsoon pattern builds, with moisture increasing across the region. Pereira/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-ptENKAKyVf9YdT_ArSYP6KRaC4pwSDghjva0SnqJ6ts= kW8Yrpo4bznREHMjX_nYcLj2DxP7kMbgqnM0eJ-I-FjVM6c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ptENKAKyVf9YdT_ArSYP6KRaC4pwSDghjva0SnqJ6ts= kW8Yrpo4bznREHMjX_nYcLj2DxP7kMbgqnM0eJ-IQN3HpQ0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ptENKAKyVf9YdT_ArSYP6KRaC4pwSDghjva0SnqJ6ts= kW8Yrpo4bznREHMjX_nYcLj2DxP7kMbgqnM0eJ-IjJ6-QCk$=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) .