Cold nights bring the chance of frost to these Bay Area cities

A clear night sky will create an environment where the warmth dissipates early in the day without insulating cloud cover. Residents in North Bay’s inland valleys can expect nighttime temperatures to drop into the upper 30s to lower 40s by 10 p.m.

Relatively warm water temperatures in San Francisco Bay will help offset the gradual loss of warmth and moisture, meaning San Francisco and Oakland residents can expect the mercury to drop near 50 degrees Wednesday night, rather than falling further. Weather models suggest that part of this barrier against rapid cooling will spread to the Tri-Valley area and the Santa Clara Valley, where residents in cities like San Jose, Walnut Creek and Fairfield can expect nighttime temperatures in the mid to upper 40s.

The same can’t be said for residents in Napa and Sonoma counties, where the lack of warm water insulation means the mercury will continue to drop after midnight. Residents can expect a very cold start through Thursday morning, with temperatures in the mid to upper 30s and patches of frost possible in the highlands north of Santa Rosa, Petaluma and Napa around dawn.

Plan to pack your bags, because it looks likely that similar conditions will develop again over the next couple of nights.

Wednesday collapse

San Francisco: Dry winds will promote sunny skies and daytime highs in the mid to upper 60s, while a cool breeze off the Pacific keeps temperatures closer to the 60 degree mark on the west side. The winds gradually decline after sunset. Look for a clear night and lows in the upper 40s by the water and lower to mid 40s in Twin Peaks, Diamond Heights and Noe Valley.

East Bay: The forecast calls for clear skies and light winds along the Gulf shore and delta in the afternoon. Expect daytime high temperatures to range from the upper 60s in communities along I-80, including the Oakland and Berkeley Hills, to the lower 70s east of the Caldecott Tunnel in the Tri-Valley area.

Winds will continue to decrease in the evening. Skies will remain clear throughout the night, bringing nighttime temperatures into the upper 40s on the water and lower to the 50s on Alameda Island and in Jack London Square. Residents of inland areas can look for cooler conditions as temperatures drop into the lower 40s after midnight.

North Gulf: A cool, pleasant day with clear skies and highs in the lower 70s is on offer for residents in the Santa Rosa, Napa, Petaluma and Solano County valleys. This comfortable weather will be perfect for hiking and spending time at the Gulf Beach, where residents in Tiburon, Sausalito and Vallejo can look forward to temperatures in the mid to upper 60s.