After a reprieve from the dry weather that has plagued January and most of February, rain has returned to the forecast for the Bay Area this week, spelling good news for the below-average rainfall amounts since the beginning of the new year.
An atmospheric river that began soaking the Pacific Northwest early Monday is expected to dip down into the Bay Area, but it’ll be much weaker as it makes it way down south, according to the National Weather Service. It’s expected to reach North Bay Wednesday afternoon before shifting southward and inland Thursday toward the Central Coast. Rainfall amounts are looking on the lighter side, with at least a tenth of an inch or so widespread across the Bay Area. But some isolated areas could get anywhere from a quarter to a half an inch of rain.
High pressure, which has resulted this week in unseasonably high temperatures in the upper 60s and 70s, has prevented rain storms from moving down into the region. The weather pattern is typical of a La Niña occurrence in the Pacific Ocean, in which cold water rises and leads to droughts in western parts of the country. After this week’s storm system glances and passes through the area, temperatures are expected to drop Wednesday into the upper 50s and low 60s during the day while overnight lows could slip back into the 30s and 40s.
A secondary trough could then move into the region after the first system exits, bringing wet conditions Thursday and Friday.
“There is the potential for unsettled weather to continue late into the week and potentially into the upcoming weekend,” said NWS forecaster Roger Gass. “We’re not expecting widespread rainfall but more in the form of rain showers in the colder air mass after the passing of the Thursday system.”
Another storm clipped the North Bay from Saturday night into Sunday morning, but only dropped a few hundredths of an inch of storm, according to the weather service. Last week marked the first measurable rain to hit the Bay Area since early January, with most of the region getting around a tenth of an inch of precipitation. Hail was also reported across the Bay Area, with a dusting of snow gracing the tops of the highest peaks.
“Overall, this is a relatively weak storm system moving through and the rainfall amounts are going to be as such as well,” Gass said. “It’s more similar to the last rainfall event where the rainfall numbers and spots were spotty across the region. Some areas got a half inch while most areas got a tenth of an inch or less.”
Going into its third year of drought, California has seen below-average rainfall amounts compared to historic averages since the beginning of the new year in what would usually be the state’s wettest time period, posing a concern for year-round wildfire risk and the state’s water supply. Atmospheric river storms in October and December filled the statewide Sierra Nevada snowpack to 168% of normal on New Year’s Day and marked the 21st wettest December on record for San Francisco. By Monday, the Sierra snowpack had dipped down to 64% of normal, according to the California Department of Water Resources.
“Since January 1st, we’re still sitting anywhere from five to 10% of our year-to-date total,” Gass said. “It’s been a very dry January and February and as many folks can see across the region, what would normally be green hillsides are already drying up. That does rain concerns for fires. There is still opportunity to get beneficial rain this month and even in April but as far as significant rainfall that will help us in the long-term drought, that’s looking less and less likely.”
Climate scientist Daniel Swain on Twitter that the Bay Area should expect “drier than average” conditions for the next few weeks, into the middle of March.
Despite occasional precipitation along North Coast and Sierra (with maybe some very light showers south to Bay Area), there remains pretty strong multi-model ensemble agreement that drier than average conditions will persist over next 2+ weeks into mid-March. #CAwx #CAwater pic.twitter.com/tOv2R7p4c5
— Dr. Daniel Swain (@Weather_West) February 27, 2022
But the Bay Area has still received more precipitation this water year compared to previous ones. San Francisco reached 100% of historic rainfall averages Sunday, receiving 16.88 inches of rain since Oct. 1, the beginning of the water year, according to the weather service. During the last water year, the city got 7.10 inches of rain in that same time period, accounting for 42% of historic averages.
“Putting in perspective, it could’ve been much worse, but unfortunately we missed out on the really good rains that January and February would typically bring.” Gass said. “We have to be thankful for the rain we got earlier in the season. Otherwise, we’d be in a much more dire situation.
