Flood — San Mateo, California

2021-12-13 · near Belmont, San Mateo, California

Event narrative

Roadway flooding caused hard closure of SR 92 between SR 35 AND SR1.

Wider weather episode

A low pressure system descended from the Gulf of Alaska southward along the entire Pacific Coast and tapped into sub-tropical moisture originating from the Central Pacific to drop heavy rain throughout the state. A stalled frontal boundary over the Santa Cruz Mountains along with strong southerly winds from a coastal low-level jet, enhanced orographic rainfall over this region, leading to rainfall totals over a 2 day period (Dec 12th-13th) of up to ten inches in the coastal mountains. The stalled boundary was reinvigorated by a secondary boundary bringing in a cold unstable airmass that set off convective showers and lowered snow levels below 2000 ft. A wind advisory, later upgraded to a high wind warning was issued for the southern half of the region from the SF Peninsula down to the Big Sur Coast including the inland zones due to the coastal jet bringing sustained winds up to 40 mph with gusts up to 55 mph. A high surf advisory was issued for the entire coast due to the arrival of a strong NW swell train, along with Gale Warning throughout the coastal waters. A flash flood watch was issued for the Santa Lucia Mountains and Dolan Burn Scar area, where up to 12 inches of rain fell.

View location on OpenStreetMap → (37.5100, -122.3500)

Source: NOAA Storm Events Database, event_id 1000077. Narrative written by the NWS forecast office that issued the report.