FOUS30 KWBC 212043 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 443 PM EDT Thu May 21 2026 Day 1 Valid 16Z Thu May 21 2026 - 12Z Fri May 22 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AS WELL AS FOR THE UPPER TEXAS COAST INTO SOUTHWEST LOUISIANA... ...Southern Plains... 16Z update... A modest expansion of the western and southern portions of the boundary were made to reflect model trends and the HREF and REFS FFG exceedance of 25 to 70% for parts of Texas. A substantial westward expansion of the Marginal Risk into West Texas was made to account for a cluster of thunderstorms and a bowing segment that is progged to advance southeast through west-central Texas during the afternoon/evening hours. Rain rates of 1.5 to 2.5+ inches/hour were depicts in the CAMs and will be going over some locations that have very low FFGs thus elevating the threat for local flash flooding concerns. Campbell The combination of several ingredients coming together across eastern Oklahoma and north Texas will be the driver for rounds of training convection expected across the area. At the surface, the nose of a low level jet will advance northward over Oklahoma, spiking the PWATs over 1.5 inches this morning. Surface convergence around a trough will support low level lift and thunderstorm formation. An approaching upper level trough will increase the divergence and lift in the upper levels. The result will be a rather narrow corridor at first with mostly showers but some thunderstorms by early afternoon, as the day is approaching peak heating. Cold pools and speed convergence will focus the convection, allowing training to become more likely as the storms track northeastward. A secondary wave of thunderstorms will move northward across north Texas, which will increase the areal coverage of convection into the early evening hours. The strengthening upper level shortwave driving this secondary wave will become the primary forcing, allowing the complex of weakening storms tonight to push northward and end the rainfall threat from south to north. While the greatest flash flooding threat may be in a rather narrow corridor from north Texas north and east through Tulsa, sufficient Gulf moisture could still allow for flash flooding in other areas as well. The inherited Slight Risk was split with this update owing to better CAMs agreement that from north Texas through Oklahoma will be the best area for flash flooding risk, but less rainfall is expected across east central Texas, between Houston and the Metroplex, allowing for a downgrade to the risk in that area. ...Upper Texas Coast into Southwest Louisiana... 16Z update... Heavy rainfall was observed along the Texas and Louisiana coastline this morning with several locations reporting a few inches of accumulation already. This has helped prime soil saturation and increase sensitivity for the next round of convection expected later today. A modest northward nudge was made to the Slight Risk across southeast Texas. Campbell A large complex of storms currently over South Texas will push northeast up the Texas Coast through the day today. The area will have abundant Gulf moisture, instability, and some limited support from the upper levels. Thus, any storms will be capable of very heavy rainfall, but the time of the heavy rainfall in any one area may be limited as the complex progresses northeastward. The area was hit hard with very heavy rain yesterday, resulting in rainfall totals over 5 inches near Freeport. As this second round of storms moves through this morning, convergence into the storms, sea breeze effects, and the saturated soils from yesterday's rains could result in additional instances of flash flooding. The storms continue into the Houston Metro around midday, introducing an urban factor to the flash flooding threat. The storms then turn more northward through the afternoon, ending the flash flooding threat for the day along the coast. To summarize, the Slight Risk, albeit a low- end one, remains in place mostly for the potential for very heavy rains with PWATs approaching 2 inches and the favorably wet soils overcoming the fast movement of the storms to result in flash flooding. Wegman Day 2 Valid 12Z Fri May 22 2026 - 12Z Sat May 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY AND THE SOUTHERN Appalachians.... 21Z update... Storms will be firing along and ahead of a frontal boundary lifting northward through the Ohio Valley. With instability increasing and PW values surging up to 2-2.5+ std. dev above normal local enhancement will be expected, especially northern Kentucky and Ohio. Recent rains have lowered local FFGs to around 1-1.5 inches which may easily be surpassed. Therefore this part of the Ohio Valley was upgraded to a Slight Risk. Further south, east-southeasterly low level winds will lead to local enhanced lift along the higher terrain from northeast Georgia to far western North Carolina. During this time PWs of 2+ will be pooling over the region in proximity to the stalled front. Areal averages of 1 to 3 inches forecast for this narrow strip however very isolated higher maximums possible and a Slight Risk was raised. Campbell The inherited Marginal Risk was left largely unchanged for many areas with this update, save some trimming across both Oklahoma and Missouri due to reduced signal for heavy rainfall in those areas. Two areas in particular stand out as higher-end Marginal Risks, where a Slight may be needed with future updates: Across a portion of the Ohio Valley from northern Kentucky through southern Ohio, efficient moisture advection will spike PWATs from around 1.25 inches early in the day Friday to 1.75 inches by the afternoon. With this added moisture, so too will instability increase. Rain in the form of showers and thunderstorms following a warm front, the same wave that will impact the Slight Risk areas on D1/Thu will increase in coverage at peak heating Friday afternoon. Mostly stratiform rain will lead the convection in the morning, helping saturate the soils that in some areas of the Ohio Valley remain nearly saturated from prior days' rains. Then during the afternoon as instability and moisture rapidly increase and the cloud cover and relative stability push north into northern Ohio and Pennsylvania, areas of showers and storms will impact northern Kentucky into southern Ohio. There remains considerable uncertainty as to how much instability can develop given the short time frame between the morning's stabilizing rains with the warm front and the following convection. Further, there is uncertainty as to how the storms will organize, with some training needed to produce anything more than isolated instances of flash flooding. Thus, for now, the Marginal was maintained, but is likely the higher of the two areas for a potential Slight risk upgrade with future updates. The other area of the large Marginal that is drawing additional scrutiny is the area from north Georgia into the Carolinas. Here moisture and instability will be plentiful, supporting clusters of heavy-rain-producing thunderstorms. Winds are generally unidirectional which may support training. However, lack of forcing with upper level ridging in place will act as a counter to allowing the storms that form to organize. Thus, clusters of disorganized convection will be unlikely to produce much in the way of flash flooding. Further, much of this area is in severe drought, and soils are extremely dry. Sandy soils would work against flooding as well into the Piedmont, so that highly unfavorable hydrology should also work to effectively keep any flooding threat in check. Wegman Day 3 Valid 12Z Sat May 23 2026 - 12Z Sun May 24 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF EASTERN TEXAS AND WESTERN LOUISIANA... 21Z update... The latest guidance had small increases in the QPF for counties near the Rio Grande in South Texas therefore the southern boundary of the Slight Risk was extended to the border with Mexico. This also required a small nudge of the Marginal Risk boundary near the border as well. Much of the coastline had received 4 to 6 inches of rain recently and soils are saturated. This particular period has the potential for additional few inches to fall over these sensitive areas and are fairly confident that this will fall on the higher end of the Slight Risk. There is enough spread in the placement of the highest QPF totals (offshore or not) that confidence was not there to introduce a Moderate Risk for much of the Texas Coastline. This part of the state will continue to be monitored with future updates on whether there is a need for an upgrade. Campbell Perhaps the most active day of the Days 1-3 period will be on Saturday across eastern Texas into Louisiana. Plentiful moisture and instability will be drawn north off the Gulf, spiking PWATs to around 2 inches in some areas, and instability could peak over 4,000 J/kg right at convective initiation midday/early afternoon Saturday. Lines of intense thunderstorms will develop in this extremely favorable air mass during the afternoon. The storms will generally track northeastward. However, as they move north, additional clusters are likely to form as both moisture and instability rapidly recover. In the upper levels a series of small shortwaves, but nonetheless potent ones given the highly favorable atmosphere will allow for new clusters of storms to form across southeast Texas soon after the prior rounds move off to the north. Once daytime heating wanes in the evening, it appears likely that additional storms that form will predominantly move southeastward towards the Gulf as they follow the instability. This could lead to additional periods of heavy rainfall over many of the same areas hit with heavy rains Saturday afternoon. Further, these storms moving against the prevailing windflow could slow down the storm movement, increasing both interaction potential between cells as well as the duration of the heavy rainfall, this threat appears particularly probable in the Houston metro area. With CAMs input over the next day or so, a Moderate Risk may need to be considered in and around the Houston metro, perhaps extending southwest along the Texas coast with future updates. Elsewhere, the Marginal across portions of the Northeast was removed with this update as the warm front over the area will have zero instability to work with, resulting in a long duration stratiform rain. Some elevated convective elements may sneak their way that far north, but the otherwise light rainfall should allow any rain to soak into the soils with no flash flooding expected. Conversely, a Marginal Risk was introduced for a portion of north Georgia into the Carolinas. Following likely rain from storms on D2/Friday, additional widespread storms are expected again Saturday afternoon into the evening over many of the same areas. As on Friday, forcing in this area will be very limited. The storms will generally track east-southeast towards the coast, with enough movement that interactions are generally unlikely to initiate additional convection. Any lines of storms should align perpendicular to the flow reducing the training risk. Finally, despite any rains on Friday, the ongoing severe drought should still greatly mitigate most of the flash flooding threat. Given the plentiful moisture and instability the storms will have to work with, they will still likely produce very heavy rain in their cores, raising resultant flooding potential into the Marginal category. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt