661 FOUS30 KWBC 200804 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 AM EDT Mon Apr 20 2026 Day 1 Valid 12Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026 ...THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND SOUTHCENTRAL TEXAS... ...Northern California... A large upper level cutoff low located off the coast will drift eastward into northern California today. The cutoff low will not have a connection to the tropics, which will limit both amount of moisture and any instability that can be transported into northern California. Despite that, as is typical of cutoff lows, the movement will be slow, which could allow for an extended duration of light to moderate rainfall. That rain will impact an area that has burn scars, urban areas (Bay Area), and steep terrain which favors localized flash flooding. Thus, the Marginal Risk is left unchanged with this update. ...Southcentral Texas... A new Marginal Risk was reintroduced for a portion of Southcentral Texas with this update in coordination with EWX/San Antonio, TX forecast office. A subtle shortwave in the upper levels will add forcing to an already highly moist environment with relatively slow storm motions. Instability will be the primary limiting factor for heavy rainfall rates, but the shortwave is expected to overcome that instability a bit with the increased forcing. This solution is depicted in several of the CAMs, which show a series of training storms developing well after sundown, and tracking slowly eastward across Southcentral Texas. Many of those CAMs highlight the storms tracking right over the San Antonio metro area. The primary flash flooding threat involves urban concerns in the San Antonio metro Monday night. Wegman Day 2 Valid 12Z Tue Apr 21 2026 - 12Z Wed Apr 22 2026 ...THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA AND SOUTHEAST TEXAS... ...California... A cutoff large upper level low over the coast Tuesday morning will progress east/inland through Tuesday night. This will direct the southwesterly onshore flow further south and east across central California. A pocket of instability with MUCAPE values as high as 500 J/kg may move into central California during the day Tuesday as well. The primary forcing however will remain the steep slopes of the Sierras, where the greatest rainfall rates and amounts are expected. Due to the instability however, the more populous valley locations may also see periods of heavier rains, including across the Bay area and Sacramento. Burn scars and heavier rains into some of the coastal ranges and western Transverse Ranges could also result in localized flash flooding. The forecast remains on track across California, and no changes were made. However, increases in forecast rainfall along the Sierras in the Central Valley may require a Slight Risk upgrade with future updates. ...Southeast Texas... The Marginal Risk across southeast Texas remains unchanged with this update. Onshore flow of deep Gulf moisture will advect across southeast Texas along the periphery of a large area of high pressure across the Southeast. MUCAPE values are expected to rise above 1,000 J/kg across much of the MRGL risk area. Slow-moving and training thunderstorms could cause isolated instances of flash flooding, especially in urban areas such as Houston. Wegman Day 3 Valid 12Z Wed Apr 22 2026 - 12Z Thu Apr 23 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA... Persistent onshore flow of deep Gulf moisture and instability will continue into southeast Texas and southwest Louisiana on Wednesday. This is a continuation of the moist, unstable flow off the Gulf from prior days, just drifted a bit to the east. Instability across this region continues to increase compared to prior days, with areas of over 2,000 J/kg in MUCAPE. Showers and thunderstorms will continue to move northwestward amidst the southeasterly flow off the Gulf. Urban areas such as Houston and Beaumont will have a higher threat of isolated flash flooding, which will occur where the storms with the heaviest rainfall rates are most persistent. 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