156 FOUS30 KWBC 300100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Mon Dec 29 2025 Day 1 Valid 01Z Tue Dec 30 2025 - 12Z Tue Dec 30 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 2 Valid 12Z Tue Dec 30 2025 - 12Z Wed Dec 31 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Cook/Wegman Day 3 Valid 12Z Wed Dec 31 2025 - 12Z Thu Jan 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ...21Z Outlook Update... Most 12Z guidance has increased QPF totals across portions of the central and eastern Transverse Ranges compared to prior forecast runs. This increased expected rainfall totals into the 3-4 inch range, with most of that rainfall occurring in the 06-12Z January 1 timeframe. Some consideration to a Moderate Risk upgrade was given, but with several model cycles left to go before the landfalling AR arrives (and attendant uncertainty), decided to hold at Slight for now. See the previous discussion below for more details. Cook ...Previous Discussion... A potent upper level shortwave trough will move east through the day Wednesday and approach the southern California coast New Year's night. As it approaches, the attendant surface low will tap into a large plume of subtropical moisture that will advect north on the southerly flow ahead of the trough. The trough itself will be negatively tilted, supporting cyclogenesis. The surface low will elongate thanks to the negatively tilted trough, allowing the southerly flow to spread rainfall north along essentially the entire California coast. However, with southerly flow, the east- west oriented Transverse Ranges will be particularly well set up to capture that moisture and wring it out in the form of upslope rainfall, in much the same way as the Christmas rainfall event a week ago. The difference between the Christmas and New Year's rainfall events in California will be the depth of moisture (less of it this time around), and lack of instability this time. This combined with the first part of the event having a southeasterly flow in the lower levels should help to mitigate the total rainfall impacting the L.A. Basin and the Transverse Ranges relative to the Christmas rainfall event. Forecasted PWATs will be around 1.25 inches near the coast. This is still 4 sigma above normal for this time of year, and therefore still represents a highly unusual amount of available moisture. The latest forecast has up to an inch of rain overspreading much of the lower elevations of the L.A. Basin, while 1-3 inches of rain are forecast for the south- facing slopes of the Transverse Ranges. Full reservoirs from the Christmas event, some draining still ongoing due to wetter than normal soils for this time of year, burn scars, and urban effects should all work to worsen any flooding effects from the expected rainfall. Thus, while forecasted rainfall amounts are lower, expect that there will still be widely scattered instances of flash flooding, especially in and around burn scars. A Slight Risk upgrade was introduced with this update, in coordination with LOX/Oxnard, CA forecast office. Fortunately, unlike the Christmas rainfall event, for the aforementioned reasons, do not expect to have additional upgrades in this area. Short of some much larger forecast change that results in much higher forecasted rainfall amounts into SoCal, a possibility with a low probability of occurring, this should be the final risk category upgrade. Nonetheless the forecast should continue to be monitored for the latest updates. 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