A blast of cold air grips the Bay Area ahead of a big storm

A cold, dry air mass has moved into the San Francisco Bay Area and the coldest temperatures of the week are expected to grip the area Friday night through Saturday, the National Weather Service said.

The cold conditions precede a large storm, a so-called atmospheric river, which is expected to cross the region Saturday evening, Sunday and Monday. Rainfall totals are expected to range from 1 inch to 3.5 inches in urban areas and up to 6 inches in the coastal mountains, the weather service said.

Temperatures dropped into the 30s in the interior valleys and into the 40s near the coast on Friday morning.

“Most of North Bay was frozen or nearly frozen last night,” Weather Service meteorologist Drew Peterson said. “There are a few pockets of freezing temperatures in the East Bay, especially near Walnut Creek and Mount Diablo, and in the San Jose Hills.”

Even colder temperatures are expected Saturday morning, and the weather service has issued a frost watch for late tonight through Saturday morning in North Bay. South of the Bay Area, the Salinas Valley could see temperatures drop into the low 20s.

“Sensitive vegetation should be covered or brought indoors,” the weather service said. “Pets must be protected.”

The weather will change Saturday afternoon as the atmospheric river approaches the area, bringing a significant risk of widespread heavy rain.

“Chances of rain are expected to spread north to south across the region, with chances of starting as early as Saturday night for North Bay,” the weather service said in its report. “However, in general, the best window for significant rainfall appears to be Sunday midday through Monday afternoon. Chances of showers may then persist in southern parts of the region into Tuesday.”

Atmospheric rivers are long, narrow bands of water vapor — about 300 to 400 miles wide and 500 to 1,000 miles long — that form over an ocean and cross the sky. They occur worldwide but are particularly significant on the West Coast of the United States, where they create 30% to 50% of annual precipitation and are linked to water supply and issues such as floods and storms. landslides, according to the National Oceanic and Atmospheric Administration. Climate models suggest that in the future, California will have more severe atmospheric rivers and longer dry periods between them.

Atmospheric rivers most often cause flooding when they stagnate, dumping large amounts of rain over an area.

Peterson said weather models suggest the system could stall over the San Mateo Coast and Santa Cruz Mountains on Sunday evening, but that forecast could change in the coming days.

“We probably need another 12 to 24 hours to be sure,” Peterson said. “At the moment, it’s in the realm of possibility. With the atmospheric rivers, we see them stagnating quite frequently in our area. They get caught in our mountains, either in the Marin Headlands or in the Santa Cruz Mountains. .”

The weather service classifies atmospheric rivers as weak, strong or moderate. Peterson said he classifies the forecast rainfall totals for this storm as moderate, but the storm has a low impact because no major flooding is expected at this time.

“Six inches of rain…that’s a lot of rain,” Peterson said. “There are going to be some localized impacts, but it will almost entirely be beneficial rain. They should be able to capture all that moisture because it’s been dry for so long. If we had just had multiple systems, we could have had more. problems.”

Dale D. Schrum