A weather advisory has been issued by the National Weather Service for the Bay Area, indicating the impending arrival of a major storm system that is predicted to inundate the area with heavy rains, powerful gusts, and possibly flooding. Meteorologists predict that starting late Tuesday, a deep upper low will enter the region, driven by a strong Pacific jet that will create a trough to the southeast and a downstream ridge over the United States to the northeast.
The Bay Area and Central Coast are expected to be hit on Wednesday by a surface trough that originates from a deep parent surface low close to British Columbia, according to ensemble guidance from the National Weather Service. There is a high degree of confidence in the forecast that Wednesday’s frontal passage will bring with it spells of moderate to heavy rainfall. Still, there’s not much chance of a halting system right now.
The most significant rainfall is predicted to arrive with the first frontal passage, which is predicted to happen within a day. Instead than concentrating on the total amount of rainfall for the week, forecasters advise paying attention to exceedance probability during this period. For the North Bay, the heaviest rates of precipitation are expected to start Wednesday morning and move southward throughout the day.
There is a very high degree of confidence (95%) among meteorologists that higher altitudes in the North Bay and adjacent coastal areas will see at least 2 inches of rain in a 24-hour period. The Santa Cruz Mountains, Big Sur coast, and Sonoma coast range all have a high degree of confidence (70–90%) that they will receive at least 3 inches of rain in the same period of time. With the exception of rainshadowed places like the Santa Clara and Salinas valleys, lower elevation and urban areas are likely to receive at least one inch of rain in a 24-hour period; the North Bay valleys and Santa Cruz proper should expect at least two inches.
More rounds of lighter, dispersed showers are predicted as the storm system persists over the weekend. The most significant phase, however, is anticipated to be Wednesday morning through Thursday early.
Regarding possible effects, the area’s soils are already almost saturated from the last storm cycle, which increases the chance of runoff onto roads, creeks, and streams. According to the update, there’s a greater likelihood that main stem rivers will rise over the flood stage. On Thursday and Friday, there’s a 10–30% risk that these rivers could reach minor flood stage, especially in the North Bay rivers.
Strong winds during the major frontal passage on Wednesday, many small landslides along steep slopes, floods in streams and cities, and other possible effects are expected. There is a slight possibility of isolated gusts up to 50 mph over the Big Sur coast and the Santa Cruz Mountains’ crest. Winds are predicted to gust over 40 mph along exposed coastal sites and ridgetops. There is a chance that gusts of 30 to 40 mph will occur elsewhere in the area, which raises the possibility of isolated powerlines and tree collapses.