FOUS30 KWBC 290802 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 AM EDT Wed Apr 29 2026 Day 1 Valid 12Z Wed Apr 29 2026 - 12Z Thu Apr 30 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND SOUTHEAST TEXAS TO THE CENTRAL GULF COAST... The Marginal Risk area was maintained with very few changes. A threat of isolated flash flooding from heavy rainfall is still expected, largely due to an environment that will favor very high instantaneous rain rates. Strong instability with highly anomalous PWs (above the 99th percentile for late April) would support rain rates in excess of 2 inches per hour. Hi-res guidance does, in fact, show rain rates that high, particularly in far eastern Texas, central Louisiana, and south-central Mississippi. However, two key limiting factors may keep any higher rainfall totals and flash flooding impacts relatively isolated. First, much of the Marginal Risk area is in a severe to extreme drought with 1-hour flash flood guidance in the 3 to 4 inch range; so flash flooding would tend to require high rain rates sustaining for multiple hours or intersecting urban areas. Second, most hi-res guidance indicates a steady progression to convective clusters and lines with limited backbuilding or training, which would tend to reduce the potential for sustained heavy rain for multiple hours. We will continue to monitor for areas in which more concentrated heavy rainfall may occur, given the overall environment. However, confidence is not currently high enough to increase probabilities in any specific area. Lamers Day 2 Valid 12Z Thu Apr 30 2026 - 12Z Fri May 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN CENTRAL TEXAS... The risk areas for Thursday and Thursday Night were generally maintained from previous outlooks with minimal changes, refining for better consistency with the latest rainfall guidance. Increasing mid-upper level height falls, due to a strong shortwave pushing through northern Mexico, should lead to a low-level mass field response with increasing south to southeast inflow across southern and central Texas. In general, the arrival of the better synoptic forcing should be later in the period, which should concentrate the main heavy rain and flash flood threat into Thursday Night across the region. By Thursday Night, strong low-level convergence and frontogenesis is expected near the nose of the developing low-level jet. This is depicted consistently across model guidance, despite some variability in the positioning. This will coincide with anomalous, deep moisture with PWs of 1.5 to 1.8 inches (around the 99th percentile for late April) and narrowing CAPE profiles with lower equilibrium levels. This fits a conceptual model for low-topped, convective rain bands that have a more efficient warm rain process, and could train and backbuild along the strong convergence axis. The result could be several hours of persistent heavy rainfall in a region of Texas that is less affected by the broader drought conditions across the region, and with much more achievable flash flood guidance benchmarks (around 3.5 inches in 3 hours). We are concerned that the overall setup could merit a further risk upgrade, but that may be more likely to occur the following day (12Z Friday to 12Z Saturday) as the convective rain bands may not get organized until very late in the forecast period, especially after 06Z Friday. Therefore, while the event will get underway on Thursday Night, impacts would be more likely to be realized in the following forecast period, and the risk was held at the Slight level for now in Central Texas. Lamers Day 3 Valid 12Z Fri May 01 2026 - 12Z Sat May 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN CENTRAL AND SOUTHEAST TEXAS AND THE CENTRAL GULF COAST... Organized heavy rain bands may be ongoing at the start of the forecast period (Friday morning) across Central Texas, described in the Day 2 portion of this discussion. The Slight Risk area was pulled back to the west to account for the possibility of ongoing impacts; the rainfall may be split between the two forecast periods, so the QPF for 12Z Friday to 12Z Saturday, viewed in isolation, would tend to undersell the flash flood risk on Friday morning in central Texas. That was the main adjustment made to the outlook at this time. We continue to see a strong signal for heavy rainfall from Texas into the central Gulf Coast region. The rainfall forecast has increased a bit with the major axis shifting south about 80 miles over the past four forecast cycles. Models show a very strong low-level convergence signal with a strong frontogenetic circulation shifting east along the Gulf Coast during the period. This will be situated in an environment with anomalously high PWs (above the 90th percentile for late April; generally 1.5 to 1.8 inches) and narrow CAPE profiles. The strong convergence should help focus efficient convective rain bands and could lead to training of heavy rain rates for multiple hours. We will continue to monitor guidance trends for possible increases in probabilities (and risk levels) if confidence grows in the placement of the heaviest rainfall. Training rain bands in that environment could easily lead to several inches of rain in just a few hours. While much of the Slight Risk area is experiencing drought conditions, that can be overcome for flash flooding impacts if heavy rain rates are prolonged enough in a given location. Therefore, the key to any increases in ERO probabilities would be much greater confidence in placement of training rain bands. For now, the Slight Risk highlights the general area where this is possible, given available guidance and the overall parameter space. Lamers 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