247 FOUS30 KWBC 210039 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 839 PM EDT Mon Apr 20 2026 Day 1 Valid 01Z Tue Apr 21 2026 - 12Z Tue Apr 21 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS INCLUDING PARTS OF THE HILL COUNTRY... 01Z Update: Maintained the SLGT risk across south-central TX with emphasis on the I-35 corridor between Austin and San Antonio. Regional mesonet networks and several PWS stations across the area between New Braunfels and San Antonio came in with a 3-6" marker due to persistent, training convection that materialized this afternoon. The overall pattern is conducive for one more round of heavy precip potential, mainly due to the approach of a shortwave ejection out of Coahuila that will propagate across the central Rio Grande and migrate through Hill Country and points east through the overnight. The confluent area situated across central TX was a key component of the flash flood threat this afternoon and that axis of low-level convergence will stick around for at least another 6-8 hrs. before the pattern shifts further to the east- northeast as the surface ridge across the Southeast CONUS breaks down enough to warrant a re-positioning of the western flank of the surface high. The latest HRRR has been handling the shortwave ejection the best of all the prevalent CAMs this evening, albeit the magnitudes of rainfall have been less pronounced compared to observation. This allows for some confidence in the spatial coverage anticipated this evening for convection, but the intensity is likely under-done, to a degree. Areas of additional 1-2", locally higher totals are anticipated across the area extending from I-10 west of San Antonio up through New Braunfels this evening with the highest flash flood threat overlapping the area between KEWX down to San Antonio proper due to the ongoing flooding in the region, and the urbanization factors along the I-35 corridor that are prone to run off. This allowed for a continuation of the SLGT risk from earlier, but with a smaller areal coverage of the risk itself. Across CA, rainfall continues with the highest IVT advection signature likely occurring overnight along the coastal areas from Monterrey up through the Bay area. The prospects for flash flooding remain pretty low, generally 5%, but with the best IVT pulse forecast tonight, didn't want to deviate too much from the previous forecast, so maintained continuity. Kleebauer Day 2 Valid 12Z Tue Apr 21 2026 - 12Z Wed Apr 22 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA AND SOUTHEAST TEXAS... 1900Z Update... Generally no changes to the previous thinking with the deep closed low/trough coming into CA. Locally heavy rains are expected for the coastal ranges and especially into the Sierra Nevada foothills where locally a few inches of rain will be likely for this period. Minimal changes were made to the Marginal Risk area here where localized urban flooding concerns and an isolated threat for burn scar flash flooding will exist. Meanwhile, the mid-level shortwave energy/troughing over central TX that will be gradually shifting eastward will not have much in the way of instability to work with for most of the period. However, the environment will be rather moist which coupled with low-level convergence near an inverted surface trough and at least modest DPVA aloft will support at least broken areas of heavy showers and thunderstorms. Relatively efficient rainfall processes may foster some heavy rainfall totals of locally 2 to 4 inches across southeast TX where the convection becomes more concentrated and slow-moving. Therefore, an isolated concern for runoff problems/flash flooding will exist which will mainly be urban in nature. Orrison Previous discussion... ...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 Sierra Nevada, 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 Sierra Nevada 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... 1900Z Update... Some modest eastward adjustment to the Marginal Risk area was accommodated for this update given some slightly more progressive trends (especially with the AI guidance) concerning the gradually ejecting shortwave energy across the region. Interaction with a fairly well-defined plume/nose of moisture and instability off the Gulf may favor some locally concentrated areas of heavy showers and thunderstorms across area near the Upper TX Coast and southwest LA. FFG values are generally pretty high, but there will be pockets of slow-moving storms capable of producing high rainfall rates that may exceed 2 inches/hour. This would pose a concern for the more urbanized locations for some runoff problems. Orrison Previous discussion... 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