FOUS30 KWBC 010029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 729 PM EST Wed Dec 31 2025 Day 1 Valid 01Z Thu Jan 01 2026 - 12Z Thu Jan 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... Generally light rain has fallen this afternoon across portions of southern California...and that is expected to persist a few more hours before there is an increase in rainfall rates with a corresponding increase in the risk of excessive rainfall. The 18Z suite of global models and ensemble runs has remained consistent with the broad idea although there has continued to be minor shifts in the timing of the arrival of instability and when the higher rates arrive. Rates in the 18Z HREF never have 1 hour rainfall exceeding flash flood guidance probabilities but do generate some 3-hour QPF exceedance of 3-hour flash flood guidance in the 06Z to 09Z time frame which corresponds with an uptick in instability and the arrival of an IVT plume. With the QPF overlapping some of the areas which are still hydrologically sensitive due to the most recent excessive rainfall event...will maintain what is effectively a high-end Slight Risk area. The changes made in the earlier Day 1 update still look good and no new changes were needed. The risk of excessive rainfall then continues beyond the end of the Day 1 period at 01/12Z. Bann ...16Z Outlook Update... The ongoing forecast is largely on track, with areas of moderate to heavy rainfall expected across the Transverse Ranges especially after 06Z tonight. The only minor change to the ongoing forecast for this update was to broaden Marginal across portions of San Luis Obispo and Santa Barbara counties, where greater rainfall totals (around/over 1 inch) could occur atop sensitive areas/burn scars during the forecast period). Isolated areas of excessive runoff cannot be ruled out in these areas today and tonight. See the previous discussion below for more forecast information. Cook ...Previous Discussion... A slow-moving upper level low centered well west of the California coast will move towards California through the Day 1/New Year's period. The upper low will remain negatively tilted as it approaches the coast. This will allow a surface low, already formed at the start of the period, to move north along the coast. Southerly winds ahead of the low will drive plentiful Pacific moisture into southern California. Upslope into the Transverse Ranges will wring out much of that moisture, resulting in multiple inches of rainfall, especially New Year's night, into the L.A. Basin and adjacent Transverse Ranges. Once the center of the surface low moves north of the Bay Area, its trailing cold front will move into southern California. This will turn the flow to out of the southwest, which will further enhance the upslope flow into the Transverse Ranges, causing heavier rainfall Wednesday night. The inherited Slight Risk for much of the L.A. Basin and higher-end Slight for the Transverse Ranges themselves was left largely unchanged with this update. The forecast weather pattern remains largely unchanged, with more parallel southeasterly flow into the Ranges supporting mostly light rain through much of New Year's Eve and into the evening, then the rain turning heavier around the stroke of midnight for the L.A. Basin. While soils across Southern California have dried out a fair bit since the Christmas rainfall event, both soils and rivers are still running above normal even several days later. Thus, the renewal of heavy rainfall Wednesday night should quickly refill any rivers, with much of the lighter rain during the day Wednesday saturating the soils in time for the heavier rains Wednesday night to largely convert to runoff. Thus, the Slight Risk looks to be in good shape, with no major changes to rainfall amounts expected requiring any further major changes to the excessive rainfall forecast. Wegman Day 2 Valid 12Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ...21Z Outlook Update... The ongoing forecast philosophy is on track and no changes are needed. While QPFs have decreased slightly across the eastern Transverse Ranges and vicinity, heavy rainfall remains likely to cause issues with excessive runoff, rock/mudslides, and flash flooding in several areas especially between 12-18Z Thursday. See the previous discussion below for more information. Cook ...Previous Discussion... Heavy rain will be ongoing at the start of the period Thursday morning across much of southern California. As the upper level shortwave moves into the coast, the winds will turn out of the west-southwest, or exactly orthogonal to the Transverse Ranges. This will maximize the upslope enhancement of the rainfall just as the maximum forcing is moving ashore. Through 18Z, this period New Year's morning will be when the heaviest rain of the event is expected into southern California. PWATs between 1 and 1.25 inches will be between 3 and 4 sigma above normal for this time of year, a sign of how much moisture will be available to convert into rainfall and runoff from the Transverse and to a lesser extent, Peninsular Ranges. Once the shortwave moves ashore and inland by Thursday evening, the combination of weakening of both upper level and surface lows, as well as rapid weakening of the onshore west- southwesterly flow behind the low, will lead to rapid diminishing of the rainfall across Southern California, such that by Thursday evening, little if any rainfall will be left. The Slight Risk inherited is largely unchanged both from the previous forecast and from the Day 1/Wednesday Slight Risk area. Both days' Slight Risks will be splitting the two halves of the same event, so little should be different. There is a higher-end Slight in effect for most of the Transverse Ranges from about Malibu east through San Bernardino. This will largely be for the morning hours, as once the rain weakens, the flooding threat associated with additional rainfall should also rapidly diminish, likely resulting in an early downgrade of the Slight Risk Thursday afternoon. Further north, along the track of the aforementioned upper level shortwave, rainfall will persist for much longer into the northern Coastal Ranges and much of the Sierra Nevada foothills, especially those along the Sacramento Valley. However, this area has more thoroughly recovered from the Christmas rainfall event and also is better equipped naturally to handle long duration rainfall. As such, only an isolated flash flooding threat, consistent with the unchanged Marginal, is expected. Wegman Day 3 Valid 12Z Fri Jan 02 2026 - 12Z Sat Jan 03 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... ...21Z Outlook Update... The ongoing forecast philosophy remains on track and no changes are needed to the outlook at this time. Models remain consistent in depicting areas of 1-3 inches of rainfall across northern California as another large mid-level wave approaches the region late in the forecast period. A few instances of flooding/flash flooding and landslides are possible. Some consideration was given to adding a Marginal Risk across portions of Alabama and northwestern Georgia as scattered convection should traverse those areas between 00Z-12Z Saturday. Models seem to hint at possible training of cells during that timeframe. Concerns about storm speeds, marginal instability/moisture, mesoscale uncertainty, and antecedent dryness precludes any risk areas/probabilities at this time. See the previous discussion below for more information. Cook ...Previous Discussion... Light rain over most of northern California from about 06Z Friday through 18Z Friday will end as another large upper level low begins to make its way towards northern California and Oregon. This upper low, while bigger, will be less amplified/elongated as compared with the last upper low. That should focus most of the rainfall into northern California. Further, the surface low will be a vertically stacked one, whose occluded front has wrapped clear around the low center. It is this occluded front making its way ashore that will restart any heavier rain into the southward facing slopes of the coastal ranges, Klamath, and the northern Sierra Nevada foothills. Rainfall totals will generally fall in the 2-3 inch range in most of the mountains, with around an inch into the Sacramento Valley. This amount of rain should be enough to raise river levels with perhaps an isolated instance or two of flash flooding in some of the flashier creeks and streams, but overall impacts should still be minimal, despite some precipitation occurring prior to this on Day 2. Much of northern California is well-suited to handle rainfall amounts such as these, so widespread impacts are not expected. The area's forecast will be monitored for any potential Slight Risk upgrades needed. 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