Week of rain starts Sunday, forecasters say — good time to batten down hatches

It won’t rain for 40 days and nights next week but it might seem like it. Wet weather is expected in the Bay Area every day through next weekend.

But even the coming deluge won’t bring rainfall totals to anywhere near normal, forecasters said.

Saturday is expected to be the last dry day for a while, the National Weather Service said. On Sunday, forecasters put the chance of rain at 100%, which is as close to a sure thing as there is in the meteorology trade.

Light to moderate rain will fall on Sunday and Monday, then heavier rain after that.

“A much stronger and wetter system will then impact the region with potentially heavy rain and strong winds from Tuesday through Thursday,” according to the weather service forecast.

The wet weather on Sunday is coming from the Gulf of Alaska and the wet weather on Tuesday looks to be an atmospheric river from the west, according to weather service meteorologist Brayden Murdock.

“It’s going to be quite a reversal from the weather we’ve been having,” Murdock said.

So far this season, San Francisco has received 3.22 inches of rain, compared to 8.51 inches at the same time last year and the average amount of 12.02 inches.

“This is one of the driest years ever,” Murdock said. “We’re in such a deficit. Will we catch up this week? No, it won’t happen.”

So far, the 2020-21 season is among the five driest years since the Weather Service began keeping track in 1850, Murdock said.

The Sunday showers could drop a half inch of rain in most areas and perhaps an inch in the South Bay mountains. Isolated showers will continue Monday. On Tuesday, temperatures will drop and a “much stronger and wetter system will arrive from the northwest, bringing potentially heavy rain and strong winds through Thursday,” forecasters said.

The midweek deluge could drop 2 to 4 inches of rain on low-lying areas and up to 7 inches in the hills, creating the risk of mudslides and flash flooding.

A gale watch was issued for the Northern California coast for Sunday afternoon through Monday, with wind gusts forecast for up to 45 miles an hour and swells up to 19 feet. The Weather Service advised mariners to “stay in port.” Heavy winds will continue through the week.

High temperatures for most areas will be in the 50s all week, with lows in the 40s. Inland areas could approach freezing.

Air quality for most areas was expected to be good this weekend and into next week, the AirNow air monitoring service said.

Heavy snow was heading to the Sierra. Squaw Valley ski resort at the north side of Lake Tahoe said it was expecting up to 3 feet of snow by Wednesday, perhaps another 2 feet on Thursday, with wind gusts up to 70 miles an hour. On Saturday, chains were required on Highway 50 over Echo Summit — and that requirement is expected to be in effect for much of the week.

Steve Rubenstein is a San Francisco Chronicle staff writer. Email: srubenstein@sfchronicle.com Twitter: @SteveRubeSF