440 FOUS30 KWBC 201554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Mon Apr 20 2026 Day 1 Valid 12Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA ALONG WITH CENTRAL TEXAS AND SOUTH FLORIDA... 1600Z Update... The morning 12Z HREF guidance continues to support a threat for mainly elevated, but also rather efficient areas of convection impacting portions of the TX Hill Country and adjacent areas of south-central TX going through this afternoon and tonight. A combination of modest MUCAPE values of around 500 J/kg along with 850/700 mb frontogenesis and some divergent flow aloft ahead of upstream shortwave energy/troughing should continue to support areas of locally repeating rounds of moderate to occasionally heavy rainfall. The early-morning RAP data shows a weak, slow-moving 500 mb vort center over the TX Hill Country playing a role as well aside from the deeper layer modest ascent downstream of troughing over northwest Mexico and the Southwest U.S. The soil conditions are moistening up and locally some streamflows are more elevated after some of the heavy rainfall from last night, and this suggests some increasing instability that may foster a somewhat stronger concern for at least localized flash flooding going through this evening and overnight. The early-day CAMs and AI guidance suggests some locally concentrated areas of rainfall with some additional totals of as much as 2 to 3+ inches by Tuesday morning. As such, the Marginal Risk has been expanded a bit, and there is an outside chance that a targeted Slight Risk could be introduced given the relatively persistent nature of the rainfall. Only very minor mainly timing related adjustments were made to the Marginal Risk area over northern CA. A rather strong upper low offshore of the West Coast will edge east and drive a band of heavier rainfall tonight across northern CA, with generally low-end small stream and some localized urban flooding possible for the Bay Area and into the Sacramento Valley. A Marginal Risk was introduced to portions of southern Florida as a cold front will be dropping south and combining with localized seabreeze convergence for areas of heavy showers and thunderstorms. A combination of the 12Z HREF guidance along with recent HRRR and RRFS solutions suggests a localized potential for concentrated heavy rainfall along the I-95 corridor of southeast FL and also the southwest part of the state near and adjacent to the Naples area. Some pockets of 3 to 4+ inches will be possible which may favor an isolated urban flash flooding threat. Orrison Previous discussion... ...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. ...South-Central Texas... A new Marginal Risk was reintroduced for a portion of south-central 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 south-central 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 IS A MARGINAL RISK OF 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