FOUS30 KWBC 261856 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 156 PM EST Fri Dec 26 2025 Day 1 Valid 16Z Fri Dec 26 2025 - 12Z Sat Dec 27 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... 1600Z Update... The Marginal Risk area has been expanded north to include much of northern CA, including the coastal ranges, Bay Area, and northern portions of the Central Valley. The arrival of a fairly strong 500/700 mb low center just offshore will be driving areas of heavy showers going through the afternoon hours. While this energy will be generally weakening with time, the addition of at least modest boundary layer heating along with some steepening of the mid-level lapse rates should favor some convective elements capable of producing localized rainfall rates of 0.50" to 0.75"/hour. Some localized additional rainfall totals of 1 to 2 inches will be possible, and especially across the northwest CA coastal range. These additional rains may foster some additional localized runoff concerns given the wet/saturated soil conditions and elevated streamflows. No changes have been made to the Slight Risk area farther south across southern CA as the threat of heavy rain will continue here into at least the early evening hours. Orrison Previous discussion... Few changes were needed for this morning's update. A positively- tilted upper level trough will push ashore and inland across the Pacific Northwest today. This will keep the flow at all levels unidirectional out of the southwest across Southern California, on the leading edge of a 140 kt jet. As the trough pushes east, less and less of it will be over the Pacific, resulting in a diminishing of the onshore flow with time. Waves of rainfall will push ashore into southern California through the day. Lingering instability will support rates that may be heavy at times. This is most likely as the storms interact with the terrain of the eastern Transverse Ranges, from L.A. north and east. It is in this area that the rainfall forecast peaks at around 3 inches at the highest elevations. Meanwhile most of the L.A. Basin picks up another 1-2 inches. 1-2 inches of rain would be problematic on a normal day, but following multiple days of multiple inches of rain per day, making the 1-2 inches expected today lighter by comparison...expect scattered instances of flash flooding to redevelop again today. While all areas of coastal Southern California and the Los Angeles basin will pick up some rain, those highest totals will be north and east of the downtown area. Thus, from downtown L.A. north and east, a higher end Slight is in effect. The flooding impacts will be the result of continued drainage of prior rainfall over Christmas Eve and Day, as well as urban effects. A Moderate Risk may need to be considered from Pasadena east through San Bernardino if rainfall amounts exceed expected totals. Elsewhere, the Slight remains in place down the coast to about San Diego, as the eastward moving bands of rain impacting the L.A. Basin should struggle to continue further south. Meanwhile the Marginal for limited shower activity and/or lowering snow levels in the foothills of the Sierra Nevada remains in place unchanged with this update. For Central and Northern California, it's probable that there will be more total precipitation in these parts of the state compared to further south, but because of lowering snow levels due to cold air influx with the aforementioned trough moving in, much of it should fall as snow at the higher elevations, reducing the flooding threat downstream. By this evening, the flow will flip around to the northwest, ushering in drier air and finally ending the rainfall threat across Southern California likely for the rest of 2025. Wegman Day 2 Valid 12Z Sat Dec 27 2025 - 12Z Sun Dec 28 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Santorelli Day 3 Valid 12Z Sun Dec 28 2025 - 12Z Mon Dec 29 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. An area of rainfall across much of the Midwest is expected with a developing low moving into the Great Lakes. The combination of prior dry soil conditions, rapid movement, poor instability, and cold air following the rain should all work to keep this event largely beneficial. Flooding concerns in this area at this time are too low for even a marginal risk. Santorelli/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