Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 28 2025 19:11:42 FOUS30 KWBC 281911 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 311 PM EDT Fri Mar 28 2025 Day 1 Valid 16Z Fri Mar 28 2025 - 12Z Sat Mar 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER TEXAS COAST THROUGH SOUTHWEST LOUISIANA... 16Z Update: Maintained the Slight Risk for the Day 1 ERO Update with minimal adjustments. It's still rather uncertain how today/tonight will play out but given the environmental setup (mid/upper level trough approaching and favorable moisture transport) and the most recent model guidance still pointing towards some scattered higher end QPF, saw no reason to dramatically change the messaging and ERO risk level. Finer details and mesoscale interactions will be driving how this will evolve, but the latest HREF probs point toward upper TX coast and coastal LA as the primary focus area for repeating/training convection. Neighborhood probs for 5" approach 40-50% along the coastal areas with a low-end (10-15%) signal for 8" accumulations through 12Z Saturday. Overall, this lines up with the previous forecast and ERO, so saw no reason to significantly alter what was inherited. Previous discussion follows... An inherited Slight risk area was maintained (and adjusted) once again for this update, as a slow-moving mid-upper trough will push the area of most favorable deep-layer forcing (upper divergence and DPVA) farther east along the Gulf Coast. However, considerably uncertainty remains (especially for an initial Day 1 outlook) as much is dependent on the evolution of convection this morning, as convection continues to consolidate farther south than virtually all of the guidance indicated at this time yesterday (resulting in the heaviest rainfall totals of 6"+ over the past 24-48 hours occurring over far South TX). Should a similar error occur today, a Slight risk may well not verify (as the consolidating MCS at the time of writing over far South TX will propagate offshore and may prevent critically necessary moisture flux transport northward into the Gulf coast, as best depicted by the 00z HRRR). In addition, attributing to a lower risk (Slight) compared to the previous days (Moderate) will be the weakening low-level flow, which given the more limited deep-layer shear, would more likely result in more effective convective outflows and thus shorter duration of the more intense updrafts and heavier rainfall cores (as well as generally drier antecedent conditions). However, should more intense convection initiate and sustain itself farther north (as much of the rest of the 00z HREF indicates), then localized totals of 3-6"+ totals will be possible from Southeast TX and the Upper TX coast through much of southeastern LA (per 00z HREF PMM QPF and associated 40-km neighborhood probabilities for 3" and 5" exceedance of ~30-60% and ~20-40%, respectively). Meteorological factors supporting this potential include favorable synoptic-scale forcing (particularly the continued impressive divergence aloft), precipitable water values between 1.75-2.00", and CAPE building to 1000-1500 J/kg. These same factors may still fuel intense convection offshore which ultimately fail to materialize further inland, thus the Slight risk is considered to be conditional. On that note, the Marginal risk was also expanded southwestward to include South TX, given the ongoing flooding and low-end potential for convective redevelopment once again with daytime heating (though this convection should be much less intense with the bulk of the forcing shifting eastward). Churchill/Taylor Day 2 Valid 12Z Sat Mar 29 2025 - 12Z Sun Mar 30 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST... A continuation of the decreasing flash flood risk is expected from Day 1 into Day 2, as ensemble guidance indicates the potential for localized totals of 1-3" (mostly concentrated near the Gulf Coast where FFGs are rather high). The limited inherited Marginal risk area was maintained for this update, as forcing and instability continue to look rather minimal (and most deterministic guidance at this point indicates localized totals of less than 1" for the most part, including the new WPC forecast, but decided to maintain the Marginal. Any localized flash flood threat should mostly be=20 confined closer to the Gulf Coast (and limited to more sensitive=20 urban areas). Churchill/Taylor Day 3 Valid 12Z Sun Mar 30 2025 - 12Z Mon Mar 31 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS... 20Z Update: A broad area of convection is expected Sunday into Sunday night with a pair of shortwaves moving into the Central U.S. during the=20 period. The forcing for ascent and associated moisture transport=20 suggests several areas of heavier precipitation from the Lower MS=20 Valley northward into the Ohio Valley. The overlap of the best=20 instability and forcing right now points toward portions of the TN=20 Valley into the Mid South region and this coincides where the=20 ensembles show the greatest potential for some higher rainfall=20 totals (2-4"). Ultimately, a Slight Risk may be needed in future=20 updates if confidence increases but for now, confidence in exact placement is low as well as the forward progression of convection that may be just fast enough to limit duration of heavy rainfall at any location for too long. Previous discussion follows... A much broader inherited Marginal risk area (relative to Day 2) has been largely maintained (and expanded a bit), as multiple shortwave troughs emerging from the Intermountain West and phase with the polar jet and an associated longwave trough. This complex interaction is expected to result in a broad area of convection across much of the eastern CONUS, including portions of the MS, TN, and OH Valleys. Despite the complexity of the synoptic and mesoscale details, global models are in fairly good agreement in indicating a convection breaking out across a broad warm sector (characterized by PWATs of 1.5-2.0", SB CAPE of 1000-2000 J/kg, and effective bulk shear of 40-60 kts). Most deterministic solutions depict 1-3" localized totals (with 2-3" amounts most commonly located farther south across the Lower MS Valley and Mid-South, where better instability and moisture tend to be located) with both the GEFS and ECENS indicating 2" exceedance potential (with the GEFS favoring portions of the Mid-South from north MS into West and Middle TN, while the ECENS favors locations farther southwest into the MS Delta). These types of probabilities from the ensemble guidance suggest that CAMs may well depict 2-4" localized totals, and a targeted Slight risk introduction may be necessary in subsequent outlooks (once better ensemble agreement and/or a strengthening trend is more evident in the global guidance). Churchill/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7yxIo2W8AU8LBDwp3YW8gnXnTJQ8izwlY3clRUSsy8Ml= fzcuvOoDg5JicQVckJLvAM_yDf7V-6PGDb-DkenzYPS0qW4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7yxIo2W8AU8LBDwp3YW8gnXnTJQ8izwlY3clRUSsy8Ml= fzcuvOoDg5JicQVckJLvAM_yDf7V-6PGDb-DkenzP2Tyclw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7yxIo2W8AU8LBDwp3YW8gnXnTJQ8izwlY3clRUSsy8Ml= fzcuvOoDg5JicQVckJLvAM_yDf7V-6PGDb-DkenzGVLxfQA$=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) .