FOUS30 KWBC 211556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Tue Apr 21 2026 Day 1 Valid 16Z Tue Apr 21 2026 - 12Z Wed Apr 22 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA AND TEXAS... ...California... 16Z update... Steady onshore will maintain rainfall across the region during this period with rates intermittently pulsing up to 1.25 inches/hour as it nears the Sierra Nevada Range. A couple of the CAM guidance is depicting isolated maximums up to 5.5 inches. No changes were made to the Marginal Risk area. Campbell A cutoff large upper level low moving into California this morning will continue to spread low-elevation rain and high elevation snow into much of the West Coast states through tonight. Southwesterly onshore flow ahead of this low is driving a plume of rainfall northward up the coast. Some instability will allow for convective elements to support cells with heavier rainfall within the broader rain plume. This instability may allow the heavier rains to impact urban areas such as San Francisco, Sacramento on up towards Seattle. In addition to these urban concerns, steep upslope against the Sierra Nevada range will support multiple inches of precipitation into the mountains. While the highest elevations could see as much as 2 feet of snow, much of the lower elevations could pick up 2-4 inches of new rainfall, which will quickly flow into the Central Valley. The inherited Marginal captures the isolated potential for flash flooding due to both urban concerns and heavy rainfall being quickly directed into steep valleys draining the Sierra Nevada. No changes were needed to the inherited Marginal Risk area. ...Texas... 16Z update... The Marginal Risk remains in good standing as it covers the locations with potential for excessive rainfall. Hourly rates up to 1 inch/hour expected with the weak disturbances as they pass through; with areal averages of 1 to 3 inches. Neighborhood probabilities do suggest an isolated potential (1-5 percent) for 5 inches to occur. Campbell A plume of rain moving across eastern Texas is the result of deep Gulf moisture advecting northward off the western Gulf ahead of a series of weak upper level disturbances moving across Texas. These disturbances are supporting the broader plume of light rain and where the forcing is greatest, supporting locally heavier rainfall rates. Guidance has come into enough agreement about the potential for locally heavy rain associated with slow-moving cells impacting portions of eastern Texas as far north as the Metroplex. Thus, given the lower FFGs associated with the large urban area constituting the Metroplex, the Marginal has been expanded north to cover the Triangle to include the Dallas-Fort Worth metro. Nonetheless, with more moisture and a bit more instability, the threat for heavy rain capable of producing isolated instances of flash flooding should hold closer to I-10. Wegman Day 2 Valid 12Z Wed Apr 22 2026 - 12Z Thu Apr 23 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER TEXAS COAST AND SOUTHWESTERN LOUISIANA... Only minor tweaks were made to the inherited Marginal Risk area across far southeastern Texas (upper Texas Coast) and southwestern Louisiana. Steady southerly flow off the Gulf will continue to advect a plume of Gulf moisture northward across the Marginal Risk area. The flow will not be much to write home about, generally between 5-15 kts at 850 mb. In addition to the rather weak advection, it's notable that unlike Day 1/Tuesday, the upper levels will become decidedly less favorable for widespread rain across the region as ridging and associated subsidence builds in. Thus, there is good agreement that any heavy rains will be disorganized and widely scattered in nature, as compared with the rains further west across Texas the past couple days. Urban concerns remain a driving factor for maintaining the Marginal, which appears to be a lower- end risk. Wegman Day 3 Valid 12Z Thu Apr 23 2026 - 12Z Fri Apr 24 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSOURI AND MID-MISSISSIPPI VALLEYS... A plume of moisture originating from the Gulf will advect moisture and instability northward into the Missouri and Mississippi Valleys on Thursday. Meanwhile, a strong cold front emanating from a deep low over the northern Plains will sweep eastward across the same region. Where these two competing air masses meet, most likely in the Marginal Risk area, expect widespread development of showers and thunderstorms. The greatest clashes are expected after sunset Thursday night, as the low level jet advecting the moisture and instability strengthens with the normal diurnal cycle. As that warm, moist air collides with the cold front, it will stall the latter's southward progress, creating an east-west boundary, likely starting around the Kansas/Missouri border well south of Kansas City, then expanding eastward with the front. Ozarks topography will likely also support locally heavier rainfall on the south- facing slopes as well. Guidance is in very poor agreement as to exactly where and when this clash of air masses will occur, with the above-described scenario seemingly the most likely solution based on an ensemble of the guidance. Should the guidance come into better agreement with strength and timing of the heaviest rainfall with future updates, a Slight Risk may need to be considered. The still full rivers and saturated soils across southeast Kansas and southwest Missouri from recent heavy rainfall events will contribute to more widely scattered instances of flash flooding should the heaviest rainfall occur over this region. 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