In a by-now familiar development in San Francisco’s ongoing wet winter, the National Weather Service (NWS) has issued a flash flood watch and a high-wind warning for the entire Bay Area as new storms brew off the coast.
A powerful cold front will bring heavy rain to the region. Rain rates will approach one inch per hour around the time of frontal passage late Friday night into Saturday morning. Heavy rainfall over a short amount of time will likely overwhelm storm drains and result in flooding of low lying urban areas.
Also, expect rapid rises on small streams and creeks, some of which may reach or exceed bankfull. [...] Travel through low lying areas on Friday night and Saturday morning may be hazardous. Do NOT drive through flooded roads.
NWS defines a flash flood as “[f]looding that begins within six hours, and often within three hours, of the heavy rainfall (or other cause).”
Although it often adds up to little more than an inconvenience, flash flooding can be extremely dangerous.
“If people are at their homes or businesses, the water may rise quickly and trap them, or cause damage to the property without them having a chance” to take counter measures, NWS warns.
A flash flood watch is not as serious as a flash flood warning. However, today’s advisory notes that conditions for a warning could well develop during Friday’s storm and that Bay Area residents should pay attention for one, particularly those who have to drive during the storm.
A searchable map created by the San Francisco Public Utilities Commission here identifies the blocks and streets in San Francisco most likely to fall victim to flooding.
NWS also put out a high-wind warning that will last from 3 p.m. Friday through 10 p.m. Saturday, anticipating “gusts 50 to 65 miles per hour” and noting that “power outages are expected” and that some property damage is possible.