Heavy rains to hit Bay Area starting Tuesday
Bay Area residents may have spent Sunday watching the skies and waiting for promised showers to start, but there shouldn’t be any doubt about rainfall starting Tuesday.
The cold weather system that was supposed to bring some light to moderate showers Sunday afternoon wasn’t expected to be nearly as heavy as the “atmospheric river” expected to arrive on Tuesday. The plume of moisture being pulled toward the California coast by a low-pressure system should bring heavy rain and a fair amount of snow, experts said.
National Weather Service meteorologist Brayden Murdock said that the strongest showers should arrive Tuesday night through Thursday.
Both the San Jose and San Francisco areas will see between 3 to 4 inches of rain, while higher elevations such as the Santa Cruz Mountains could see as much as 8 inches, a number that could be met in Big Sur.
“We’ve had some stronger ones (storms), but in terms of the water year, this is the most significant rain chances we’ve seen since October, when the water year starts,” he said.
Even if San Francisco received the heavy rainfall that’s projected, it would only put the city close to half of where its rainfall should be for the year, Murdock said.
When the atmospheric rivers move over the mountains, the water vapor rises and cools to create rain, experts say. Although some are weak systems that sim
ply provide beneficial rain or snow, more powerful ones can cause extreme rainfall and floods. A strong atmospheric river can transport an amount of water vapor equivalent to 7.5 to 15 times the average flow of water at the mouth of the Mississippi River, according to the weather service.
Temperatures are not expected to vary much in the next several days, Murdoch said. In both the Peninsula and Santa Clara Valley, mid-50s had been expected in the lower 40s overnight Sunday. In the East Bay it was to be a little colder overnight Sunday into today, with temperatures as low as the mid-30s; highs today are expected in the mid-50s.
The Sunday rain had been expected to bring around a tenth of an inch to a quarter of inch of rain in San Francisco, San Jose and the East Bay, with higher elevations such as Mount Diablo getting as much a quarter of an inch, according to the National Weather Service. Some areas with elevations above 3,000 feet were to see some snow and a light dusting as low as 1,500 feet.
A wind advisory was issued for most of the coast, including San Francisco, the Peninsula coast, North Bay and as far south as Monterey Bay and Big Sur, beginning at 7 p.m. Sunday and lasting through 10 a.m. today, with 15 to 30 mph winds and gusts ranging from 40 to 50 mph in the forecast. This is caused by the cool air filtering into the area.
The Sonoma County area was already seeing rain Sunday morning, but much of the rest of the Bay Area waited for rain to begin in the afternoon, with a few scattered showers.