Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 27 2025 00:51:44 FOUS30 KWBC 270049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Wed Mar 26 2025 Day 1 Valid 01Z Thu Mar 27 2025 - 12Z Thu Mar 27 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH-CENTRAL AND SOUTH TEXAS... Only minor adjustment needed to the previous outlook across portions of south central and deep south Texas based on short term trends in satellite and radar imagery. 18Z soundings at CRP and=20 BRO depicted an increasingly moist, unstable, and uncapped=20 vertical profile which should continue to foster increasing coverage of showers and thunderstorms capable of producing torrential downpours later tonight/early Thursday morning. Meso-analysis showed diffluent upper- level flow, DPVA and ample=20 moisture transport into the southern end of the Moderate risk area=20 already underway. A deeply saturated profile with good directional shear...moderate instability with the front nearby should result=20=20 upscale growth of slow moving convective clusters throughout the upcoming night. The 18Z HREF neighborhood probabilities of 3=20 inches of rainfall in an hour peaking in the 15 to 20 percent range mainly in parts of Jim Hogg and Duval counties...with an axis of the probability of rainfall exceeding 5 inches during the period=20 from 27/00Z through 27/12Z aligned southwest to northeast across the southern parts of the Moderate Risk area. These totals seem to be within reach given the 1.9 inch precipitable water at=20 Brownsville and 1.7 inch precipitable water value at Corpus Christi from their 27/00Z soundings...especially if/where convective=20 training occurs.=20 ....Pacific Northwest... A Marginal risk remained in place across portions of western Oregon and western Washington. Convection is expected to develop over=20 portions this evening and then track off to the north. 00Z soundings from Salem and Medford OR both showed roughly 600 to 800 J per kg of CAPE and precipitable water values running between 0.75 and 1.00 inches which should support locally heavy intense=20 rainfall rates. The big limiting factor should be quick moving cells which should limit accumulations. Any rainfall rates=20 producing over 1" in an hour...however...may be enough for=20 localized flash flood concerns. Areas of terrain, urban areas, and=20 low lying areas will be most prone to localized flash flooding=20 driven by these short duration intense rainfall rates. As a=20 result...no changes were made to the on-going Marginal Risk area at this point. Bann Day 2 Valid 12Z Thu Mar 27 2025 - 12Z Fri Mar 28 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH-CENTRAL AND SOUTH TEXAS AND MUCH OF THE TEXAS GULF COAST... ....20Z Update... ....South Texas... The primary change to the Day 2 ERO includes a southward expansion of the Moderate Risk, and tightening of the Marginal and Slight Risk areas over Southeast Texas to reflect a southeastward shift in the highest QPF axis. While overall confidence in a flash flooding event is high, uncertainty remains regarding the convective evolution beyond 12z tomorrow, which will depend on outflow boundary placement originating from convection this afternoon and tonight. ....MS and OH Valley... Marginal flash flooding issues remain possible beginning tomorrow morning as showers and thunderstorms periodically train along a slowly returning warm front. The northern portion of the inherited Marginal Risk area was trimmed to reflect the corridor of higher HREF neighborhood probabilities of 2-3" rainfall exceedence. ....Pacific Northwest... Minor adjustments overall to the overnight thinking regarding the Marginal Risk area associated with another modest atmospheric river. Rainfall totals of 3-5" through Friday morning may support isolated issues owing to saturated soils across the region. Asherman ....Previous Discussion... The event described in the day 1 discussion over south TX will continue into Thursday. There is a bit more uncertainty on the details by this time as convective evolution will likely be impacted by mesoscale influences of the day 1 convection. On the synoptic scale, ingredients are likely even more favorable for heavy rainfall on day 3 then they were on day 2. Large scale forcing should reload and be even a bit stronger as the mid level trough axis and associated shortwave energy moves across the area (enhancing the upper divergence/DPVA). However, the extent of the excessive rainfall risk will likely be at least partially dependent on mesoscale factors...such as the outflow location form the day 1 convection and its impact on instability. The expectation is that the outflow boundary will be near the coast by this time, and so that is where the greatest focus for training convection is expected on Thursday, and thus where the MDT risk is in place. There is some chance this is offshore by 12z Thursday, but the current consensus suggests it may struggle to clear the coast, leaving coastal areas susceptible to additional heavy rainfall and flash flooding. Even if instability is eroded by Thursday, the increase in forcing and southerly flow should be enough to destabilize portions of the area again with time, and so do think at least some flash flood risk exists even in a scenario where convection initially propagates offshore, especially Thu night with the uptick in S-SSE low-level flow. Overall confidence is high in a flash flood event, but confidence in the details by Thursday-Thu night remains low. In a worst case scenario outflow stalls near the coast allowing for repeat convection and a significant flash flood event could evolve. In a best case scenario convection propagates offshore, but redevelopment later in the day still results in some flash flood threat, albeit probably less significant. When combining day 2 and 3 it seems probable that some areas receive 5-10" of rain over south TX, and localized amounts of 10-15" are certainly possible. Confidence is increasing in the coverage of flash flooding and the possibility of at least localized significant impacts, warranting the Moderate risks on both days 2 and 3. ....MS and OH Valley... A Marginal risk continues from yesterday's Day 3 ERO across portions of northern MO, southern IA, IL and central/southern IN. A stationary front draped across this region will gradually lift north as a warm front through the period. Southerly flow will result in an uptick in instability and moisture, and at least scattered convection is expected Thursday into Thursday night. There is at least some chance for organized convective development near the front, which could briefly train from west to east. Global model QPF is locally as high as 1-3" along this corridor, and given the expected convective nature of activity, this would suggest isolated amounts over 3" are probable. A localized, and generally minor, flash flood risk could result. ....Pacific Northwest... Another modest Atmospheric River (narrow ribbon of 500+ kg/m/s IVTs) will result in pockets of 3-5+ inch 24 hour rainfall totals along the West Coast from Northern CA northward through OR-WA. Among the 00Z 3/25 models, the Regional GEM is by far the wettest, with isolated totals of 6-8+ inches. Despite the lack of instability, the wet antecedent soils along with the potential of 0.3-0.5 in/hr rainfall rates could lead to localized short-term runoff issues, particularly along small streams and over burn scars. Chenard/Hurley Day 3 Valid 12Z 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 AND SOUTH-CENTRAL LOUISIANA... ....20Z Update... Adjusted the northern extent of the Slight Risk a bit to reflect the theme of QPF focusing closer to the coastline as suggested by several of the global models, although spread remains regarding the placement and magnitude of the highest QPF amounts. Asherman ....Previous Discussion... Slow-moving mid-upper trough will push the area of most favorable deep-layer forcing (upper divergence/diffluence and DPVA) farther east along the Gulf Coast. Coincident with the favorable synoptic- scale forcing, TPWs between 1.75-2.00", along with mixed layer CAPEs around 1000-1500 J/Kg, will result in a more enhanced excessive rainfall threat (i.e. Slight) across the Upper TX Coast to Southwest and South-Central LA. 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. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_czVf942gNOFeTr_bGyMMhfcCi7nGDpD3ffRbWVd4g8H= u4i21UyHTHA8PZiMDfzGJkHIjkoAJeuumJ7KGtVBctnZ77E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_czVf942gNOFeTr_bGyMMhfcCi7nGDpD3ffRbWVd4g8H= u4i21UyHTHA8PZiMDfzGJkHIjkoAJeuumJ7KGtVBQ-320mU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_czVf942gNOFeTr_bGyMMhfcCi7nGDpD3ffRbWVd4g8H= u4i21UyHTHA8PZiMDfzGJkHIjkoAJeuumJ7KGtVBE2xH2co$=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) .