Bay Area weather: Storm winds down, more rain looms later this week

Low-pressure systems off the coast of Hawaii are forecast by the American weather model to gradually intensify by Thursday and Friday. While these systems are forecast to stay well out to sea for the rest of the week, some of their moisture will accelerate toward California, raising slight chances for rain in parts of the Bay Area.

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The strong storm that slammed the Bay Area on Tuesday with serious winds and heavy showers is on its way out this morning, leaving a few light showers and south winds in its wake. Conditions will gradually continue to improve in the Bay Area and across California as drier air rolls onto the West Coast. This air will keep most of California from being washed out by another round of heavy rainfall over the next couple of days.

But two low-pressure systems are slated to approach Northern California by Thursday and Friday. These systems will be relatively weak but have the potential to raise spotty showers in parts of the state, including the Bay Area. Depending on the duration of the dry air overhead, some of this rainfall could be on the moderate end.

A rain door that won't fully shut

The available energy in the atmosphere left behind by Tuesday's storm will settle over Southern California, the Central Coast and patches of the North Bay and East Bay. This energy is what rain bands use to become more widespread and stronger, often becoming thunderstorms in the process.

Wednesday's air mass over California will hold some lingering energy from Tuesday's storm, but most of it will be on its way out of the state. Some of that energy could produce a few more showers in the Bay Area before it all clears out.

Winds will gradually shift in the Bay Area and will begin to blow out of the northwest by Wednesday afternoon. This shift will help draw some moisture from the Pacific Ocean, raising a light sea breeze and a few pockets of fog along the coast.

The amount of moisture reeled into the Bay Area is then forecast to increase by Thursday and Friday. That's because the leading bands of precipitation on the two low-pressure systems off the coast of Hawaii are going to accelerate toward California. Those bands will raise the chances for showers in some of the North Bay highlands — the Marin Headlands, Mayacamas Mountains — and the East Bay's hillsides and mountains. For now, chances for rain are forecast to be between 15% to 30%, with most of those showers light and spotty.

The rest of the Bay Area is forecast to enjoy a comfortable Thursday and Friday with modest temperatures in the 50s and plenty of sunshine. But these upcoming light showers in the mountains are a precursor to the unsettled weather thousands of miles away in Hawaii.

What this means is that the forecast could call for yet more widespread precipitation across California early next week. For now, we will have to assess the evolution of the two low-pressure systems off the coast.

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Wednesday breakdown

San Francisco: East winds will blow into downtown this morning as residual showers from yesterday's storm fizzle away. These winds will generally range between 10 to 15 mph, briefly gusting up to 25 mph in some of the east-facing hillsides in Bernal Heights, Twin Peaks and Mount Diablo. These east winds and rain showers will gradually peter out after 1 p.m. as drier air filters into the west side from the Pacific.

This afternoon's drier air will usher in a shift in the direction of the wind and will lead to winds from the west after 4 p.m. These will mostly be light, but they’ll help clear out some clouds while lowering humidity across the city. Look for a comfortable afternoon with daytime highs in the lower 50s across the Sunset and Richmond districts, Lake Merced and the Presidio. The higher humidity east of Sutro Tower will result in upper 50s for daytime highs in the Mission, Castro and SoMa.

After sunset, temperatures in the city will steadily fall to the mid-40s, while a few pockets of West Portal and Glen Park could fall to the lower 40s as cool air seeps in.

Pacific Coast and Peninsula: Debris from Tuesday's storm along Highway 1 between Half Moon Bay and Pacifica may cause delays for morning commuters, but breezy winds and showers on the coast will steadily drop off. The same can be said for residents in the San Bruno Gap, where rain will clear out by noon, making way for sunnier skies in Daly City, Millbrae and around San Francisco International Airport. Less rain and wind is also slated for residents south of Millbrae along the 101 and 280 corridors, though isolated pockets around Lake San Andreas, Redwood City and Foster City could see showers through 1 p.m.

Once skies begin to clear, daytime temperatures across the Peninsula will climb to the upper 50s. An exception will be right along the coastline facing the Pacific Ocean, where a light west wind will stream into Pacifica and Half Moon Bay and temperatures will be lower. These breezes will also reduce humidity levels, meaning the coast will likely see nighttime temperatures in the lower 40s. As for nighttime temperatures across the rest of the Peninsula, expect mid-40s as humidity levels stay a little higher along San Francisco Bay. The exception will be along the 280 corridor, where cool, dry air from the coast will draw some of those lower 40s inland.

North Bay: Light sprinkles are slated for most of the Marin Headlands and parts of the bayside Wednesday morning, including San Rafael, Novato and Tiburon, as moist winds blow in from the east. These will gradually subside over the morning as light showers fizzle away. A similar drop-off in rain is on tap for the rest of the North Bay, including Solano, Napa and Sonoma counties, where sunnier skies will return after 1 p.m. Winds will shift after 3 p.m., becoming more westerly in the afternoon and pulling in drier air to the Santa Rosa, Petaluma and Napa valleys.

The sunnier skies and drier air will lead to daytime temperatures reaching the upper 50s across the region, though spots closer to the Pacific coastline, like Point Reyes and Bodega Bay, may stay closer to the lower 50s thanks to the cool afternoon breeze from the west. By nighttime temperatures will steadily drop to the 40s across most of the North Bay, with lower 40s on tap for the inland valleys.

East Bay: Aside from a few light showers in the Oakland and Berkeley hills, and a handful of showers just east of the Caldecott Tunnel along the foothills of the San Ramon and Livermore valleys, most of Contra Costa and Alameda counties will experience light easterly winds and a gradual drop-off in both winds and rain over the morning. By afternoon, a light breeze from the west will filter into Oakland and most of the I-80 and I-880 corridor, cooling the region with temperatures in the mid-50s. Farther inland east of I-680, residents in Walnut Creek, Dublin and Pleasanton will see sunnier skies and highs in the upper 50s. By nighttime, the drier air will pull temperatures back down to the 40s across the region, with lower 40s possible in the inland valleys away from the delta.

South Bay and Santa Cruz Mountains: After rounds of heavy rain and winds, residents in Scotts Valley, Ben Lomond, Felton and across the Santa Cruz Mountains will wake to calmer winds and a few light showers. South winds will fizzle as the atmospheric faucet closes. Conditions across northern Monterey Bay, the 101 corridor between Morgan Hill and Gilroy and the Santa Clara Valley will improve over the course of the day, with partly cloudy skies returning across the region this afternoon.

Look for daytime highs in the upper 50s along the coast and in most of the inland valleys, including San Jose. Residents in the Santa Cruz Mountains will get to enjoy sun and daytime highs in the mid- to upper 50s. After sunset, look for a steady fall in temperatures as cool, dry air promotes nighttime lows in the lower 40s.

Reach Gerry Díaz: [email protected]; Twitter: @geravitywave