Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 30 2025 01:00:25 FOUS30 KWBC 300100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Tue Apr 29 2025 Day 1 Valid 01Z Wed Apr 30 2025 - 12Z Wed Apr 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....Southern Plains to the Mid Mississippi Valley... 01Z Update... Continued the southern adjustment, especially on the northern edge, of the outlook areas extending from western Texas northeastward across Oklahoma. Recent runs of the HRRR, as well as the 18Z HREF, indicate multiple convective rounds training northeastward across the region, producing several inches of rain, with numerous flash=20 floods likely across portions of the region. The latest guidance=20 shows the greatest threat extending from Northwest Texas across the Wichita Falls area into southwestern and south-central Oklahoma -- brushing the southern extent of the OKC metro. Deterministic runs=20 of the HRRR show accumulations of 7+ inches, while the HREF shows=20 high neighborhood probabilities for accumulations of 5 inches or=20 more through the overnight across this region. The Slight and=20 Moderate Risk areas were shifted a bit farther south across the=20 southern Texas Panhandle and western Oklahoma, where limited=20 instability in the most recent analysis supports guidance=20 indicating a lesser threat for heavy rainfall rates and flash=20 flooding. Further to the east, pulled the Marginal and the Slight Risk areas back west of the Mississippi. Relatively drier and more stable air is expected to persist until showers and storms now developing=20 back to the southwest spread into the region late in the period,=20 with the general model consensus showing relatively lighter amounts and a lesser flooding threat. ....Ohio Valley to the Central Appalachians... 01Z Update... In addition to brief heavy downpours across sensitive areas, some=20 training along the southwest flank of the storms currently moving=20 across the region may raise the threat for locally heavy rains and=20 flash flooding. But overall, storms are expected to move progressively across the area, limiting the potential for widespread heavy rainfall and flash flooding concerns.=20 Pereira Day 2 Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL AND SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN ARKANSAS... The primary change for this updated ERO for Wednesday was to recenter the Moderate Risk further south (approx. 50 mi). This was due to a corresponding shift in guidance, especially hi-res guidance, but also the UKMET, CMC, and ECMWF AIFS. It also fits with the updated expectations for the current Day 1 period, as guidance has signaled the potential for relatively strong cold pools reinforcing the effective front and gradually pushing it further south. This would center it more squarely near the Red River (OK-TX border region) or perhaps fully into N TX. There is still some uncertainty associated with the exact convective evolution over the 12-18 hours immediately prior to the start of the Day 2 period, so the Moderate Risk and Slight Risk were kept fairly broad to account for different scenarios. Nevertheless, there was a clear enough shift in the guidance to warrant an adjustment in the overall risk contours. A significant flash flooding event could be ongoing at the start of the period (~12Z Wednesday) over western North Texas and/or south- central and southwest Oklahoma. Late tonight the southward drift of the effective front and the associated zone of active convection may begin to slow or stall as the low-level jet (LLJ) begins to ramp up considerably over Texas. Where the nose of the LLJ sets up relative to ongoing convection will go a long way to determining the primary area of concern for Wednesday. Most hi-res models show the convective cluster(s) propagating east through the morning toward AR and N LA, with renewed development upstream over N TX by the afternoon and another training and backbuilding configuration. This same guidance generally supports the idea of a 24-hour rainfall maximum in excess of 7 inches somewhere in the new Moderate Risk area, but there is lingering uncertainty as to precisely where that will occur. Although the probability is currently highest in the vicinity of the Red River near where SE OK borders N TX, it's conceivable it could occur closer to or inside the DFW metro area. The metro area is now fully in the Moderate Risk area with this latest update. Some significant flash flooding will be possible as much of the rain will likely fall in 6-12 hours and the high end amounts produced by some of the guidance would approach or exceed a 100 year ARI for those durations. Neighborhood probabilities of 100 year ARI exceedance are not particularly high, reflecting both the uncertainty of the exact placement of the heaviest amounts, and that amounts well in excess of those thresholds are not currently supported by guidance. Significant and life-threatening flash flooding would be most likely in two scenarios: (1) In western North Texas or south-central Oklahoma at the beginning of the period, as a continuation from the previous night. These areas have been quite wet over the past week or so, so there is existing vulnerability to extreme rainfall. (This is more likely.) (2) In the Dallas-Ft. Worth metro area, as the rainfall in question would cause serious issues in a highly urbanized area. (This is also possible, but not certain, and highly dependent on the exact convective evolution.) Further north, a broad Marginal and Slight Risk area was maintained for the Mid Mississippi Valley and Central Plains. There is some signal for organized convection and localized heavy rainfall, including a scattering of low probabilities of FFG exceedance, but confidence in any specific scenario is lacking. Lamers Day 3 Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA FROM THE GREAT LAKES TO MID MISSISSIPPI VALLEY TO WESTERN GULF COAST... An expansive plume of instability is expected to support scattered to numerous areas of thunderstorms during the day 3 period ahead of a surface low and trailing cold front. Machine learning probabilities from Colorado State support the idea of a broad Marginal Risk; this is what was inherited and it was generally maintained. The biggest change was to extend the area southwest through portions of Louisiana and south and southeast Texas. Confidence in specifics is low, and the overall flash flood threat should be limited by the fairly progressive nature of the convection. Precipitable water values are more anomalous in the Great Lakes region (above the 90th percentile; above 1.3 inches), but that is also where thunderstorms should be moving the fastest. Meanwhile, available moisture will be less anomalous further south, in TX and LA, but thunderstorms will be moving slower. Therefore, heavy rainfall ingredients will generally be offset across the whole region. Hi-res models that extend out beyond 48 hours show more significant localized rainfall maxima in TX and LA, which makes sense given stronger instability and slower overall storm motions. This may be where a targeted upgrade to a Slight Risk is eventually needed. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!-9whLZQzN1K2RbQb4fPLyt4fk4hbu55oU5N2bNKsgCQP8mLvWERMZ9Vl-8Gv8Tn= G3WleDjuE62VsInKesiH6UDexBMw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!-9whLZQzN1K2RbQb4fPLyt4fk4hbu55oU5N2bNKsgCQP8mLvWERMZ9Vl-8Gv8Tn= G3WleDjuE62VsInKesiH6P7qS7lo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!-9whLZQzN1K2RbQb4fPLyt4fk4hbu55oU5N2bNKsgCQP8mLvWERMZ9Vl-8Gv8Tn= G3WleDjuE62VsInKesiH6jiK5zZw$=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) .