FOUS30 KWBC 050025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 725 PM EST Sun Jan 4 2026 Day 1 Valid 01Z Mon Jan 05 2026 - 12Z Mon Jan 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SIERRA AND FOOTHILLS... 01Z Update: Few changes made to the previously updated Excessive Rainfall Outlook based on the neighborhood probabilities and exceedance guidance from the 18Z HREF. Even though the probability of rainfall exceeding flash flood guidance overnight tended to persist for only one or two hours at any spot...they still tended to fit within the Marginal risk areas and the Slight Risk area still captured the area where broad, persistent light to occasional moderate rainfall is expected to persist the longest. Bann 16z Update: Locally heavy rainfall rates into coastal Monterey County will continue to result in an isolated flash flood and mudslide risk through ~00z. Hourly rainfall between 0.5"-1" will continue to locally occur, with a slow southward shift expected in these heavier rainfall rates through the day. Rainfall totals of 2-4" are expected along the coastal portion of Monterey County. Another focus for heavy rainfall will be across the eastern valley into the foothills of the central Sierra. Southerly flow up the valley, combined with westerly flow behind an approaching cold front, will locally enhance convergence and support some periodic low topped convection capable of rainfall rates over 0.5"/hr. The combination of upslope enhanced rainfall and these locally higher rates will result in a continued isolated to scattered flash flood risk through today. Showers into southern CA through today may periodically drop ~0.5" of rain within an hour...but the coverage and magnitude of rainfall is not expected to match what occurred yesterday over Santa Barbara and Ventura counties. By tonight the approaching cold front should increase shower coverage, but again hourly rainfall should generally peak around 0.5" or less. HREF FFG exceedance probabilities stay below 15%, and while there is a risk of isolated flash flooding and mudslides, the magnitude of the threat is lower than yesterday and also mainly focused farther south. Chenard ...Previous Discussion... Precipitation is expected to increase in coverage and intensity Sunday morning across northern to central California as the next batch of shortwave energy rotates through the mean west coast trof and inland across central CA. PW values and 850-700 mb moisture flux are expected to increase Sunday associated with this next batch of shortwave energy. This will support locally heavy precip totals along much of the California coast Ranges and into the upslope of the Sierra. Hourly rainfall rates in the .25-.50" range possible along the central CA coast and into the upslope of the northern Sierra. A small slight risk area was maintained in the upslope of the northern Sierra where model consensus is for potential of additional 1 to 2" of rain. The HREF probabilities of 3 hr precip exceeding 3 hr FFG values shows an area of 15-30% probabilities sinking southward along the upslope of the northern Sierra Sunday where the slight risk area is drawn. Overall, no changes of note made to the previous outlook for the upcoming day 1 period. Oravec Day 2 Valid 12Z Mon Jan 05 2026 - 12Z Tue Jan 06 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE CENTRAL TO NORTHERN CALIFORNIA COAST AND NORTHERN CALIFORNIA... 19Z Update: Our model preference is shifting towards the closer to the coast low track and thus wetter scenario of the ECMWF/GEM/UKMET/AIFS. Still not a high confidence forecast, but this scenario seems to have more support than the farther offshore GFS/NAM solution. The upward trend in QPF supports a solid Marginal risk of excessive rainfall, with amounts of 1-3" and some flood impacts likely across central/northern CA. We gave some consideration to a Slight risk upgrade, but it remains unclear whether rainfall rates will get high enough for that level of flash flood risk. So for now we will keep things at the Marginal level and continue to monitor. It's worth noting that most 12z HREF members appear too far offshore with the closed low, with this poor synoptic scale forecast negatively impacting their QPF forecast. Thus we were not really able to rely on the high res hourly rainfall forecasts this cycle...so hopefully the 00z HREF will trend more in line with the global model consensus, which should in turn result in more useful hourly rainfall rate data to diagnose the flash flood risk. Chenard ...Previous Discussion... There are large scale differences developing day 2 with the amplification of a closed low in the base of the mean trof off the Central to Southern CA coast. The NAM and GFS are showing a more southwest solution compared to the EC, CMC and UKMET. This farther southwest solution is resulting in a trend of less precip along the central CA coast to the south of San Francisco and into the Northern Sierra in the GFS and NAM as the low level flow has a much weaker onshore component compared to the EC, CMC and UKMET. Models often struggle with closed formation downstream of flattening upper ridges with overall low confidence at the moment. The latest WPC qpf did trend towards less precip across the central CA Coast Range and over the Northern Sierra, but not as dry as the GFS and NAM. Subsequently, the previous marginal risk area was decreased in size. There may be additional changes needed to the risk area with future model cycles, depending how the evolution of the closing off mid to upper level center evolves. Oravec Day 3 Valid 12Z Tue Jan 06 2026 - 12Z Wed Jan 07 2026 ...The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard 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