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Cooler temperatures, but fire hazard in these places

Northwest winds with gusts of 20 to 40 mph will gust across much of the Bay Area Sunday afternoon.

Baron/Lynx

A changeable weather pattern will bring cooler temperatures and gusty winds to the Bay Area this weekend. California is located between a strengthening North Pacific High to the west and a retreating ridge of high pressure to the east, allowing for strong winds to develop during the afternoons Friday through Sunday and triggering a fire warning for parts of the Sacramento Valley.

Winds will pick up from the west-northwest on Friday afternoon, driving gusts of 15 to 30 mph through coastal and inland waters and valleys. Gusty winds will continue overnight, enhanced by a strengthening pressure gradient along the coast, dramatically reducing cloud and fog formation.

The strongest winds will be Saturday and especially Sunday afternoon. A fast but powerful storm system in the Pacific Northwest will pass close enough to increase the pressure gradient along the coast and produce some gusty northwest winds.

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The wind direction will allow for some topographical improvements in the Petaluma Valley and San Bruno Gap, where isolated gusts of 40 to 50 mph are possible. The strong winds will be a particular threat Sunday afternoon and will likely cause some travel disruptions at San Francisco International Airport.

The strongest winds will remain offshore and over the ocean, but they will be impressively strong and will whip up large surf. Over the weekend, winds over the water will be northwesterly at 45 to 60 mph at times. Marine warnings are already in place, and may be intensified as winds increase and waves reach up to 15 feet.

Offshore, waves will reach heights of up to 15 feet, but they will not be as high on the immediate coast as the geography provides some natural protection.

Offshore, waves will reach heights of up to 15 feet, but they will not be as high on the immediate coast as the geography provides some natural protection.

Baron/Lynx

The strong winds will pose a different threat to inland areas in the Sacramento Valley, where strong winds from the north will meet a much drier air mass, increasing the fire weather risk Saturday into Monday. The favorable conditions have led the National Weather Service in Sacramento to issue the first fire weather warning of the year. Relative humidity will remain high enough in the Bay Area to eliminate the fire weather threat.

Weekend breakdown

San Francisco: Morning clouds and fog will quickly dissipate with increasing winds from the west. Highs citywide will range from around 60 degrees in Ocean Beach to the upper 60s downtown. Gusty west-northwest winds at 15 to 25 mph. Nighttime lows will be near 50 degrees.

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The script will change little over the weekend. Gusty winds, now from the northwest, will limit the impact of the morning clouds and fog, and temperatures will be cool for the season. Highs on both days will range from just under 15 degrees near the water to just under 15 degrees downtown, with northwesterly wind gusts reaching 15 to 25 mph, even more in some areas on Saturday and Sunday afternoons.

North Bay: Friday will be sunny and warm across the North Bay, with increasing westerly winds that will have a noticeable impact through the weekend. Highs on Friday will be just under 90 degrees in Fairfield and Santa Rosa, just over 80 degrees from Petaluma to Sonoma, and just over 70 degrees in Mill Valley and Sausalito. Strong west-northwest winds will be present in the afternoon, reaching gusts of up to 35 mph in coastal areas and the Napa and Sonoma valleys.

Temperatures will cool off a bit over the weekend, but winds will pick up Saturday and Sunday afternoon. Wind gusts may reach 45 mph at times, especially along the coast, in the coastal valleys of inland Marin County, and in the Petaluma Valley. Highs both days will be between 77 and 80 degrees Fahrenheit in Napa and Sonoma counties, and between 65 and 75 degrees Fahrenheit in Marin County. Overnight will be clear with lows between 45 and 50 degrees Fahrenheit.

East Bay: Friday will be sunny and temperatures will be pleasantly warm throughout the East Bay. Highs along I-880 will be between 70 and 77 degrees Fahrenheit, and inland between 77 and 84 degrees Fahrenheit. Winds will increase from the west-northwest in the afternoon, reaching gusts of 10 to 25 mph, especially in inland areas. Clear overnight with lows in the low 50s.

Temperatures will cool a few degrees over the weekend, with highs between 25 and 27 degrees in Walnut Creek and Livermore and highs between 25 and 27 degrees in Oakland and Fremont. Winds will be felt more strongly Saturday and Sunday afternoon, with northwest winds of 10 to 20 mph and gusts of 25 to 30 mph. The air will be drier in eastern Alameda County and near the intersection of Interstate 5 and Interstate 580, bringing a slightly increased fire danger.

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Pacific coast and peninsula: The highest wind speeds will likely be in the San Bruno Gap Friday through Sunday. Northwest winds will reach 15 to 30 mph Friday afternoon and remain strong overnight. Highs on Friday will be between 65 and 70 degrees Fahrenheit between Daly City and Millbrae and between 77 and 80 degrees Fahrenheit in San Mateo and Redwood City. Highs along the coast will be around 65 degrees Fahrenheit.

Sunny skies and even stronger winds are expected on Saturday and Sunday. Northwest winds could reach gusts of 40 to 50 mph at times during the afternoon, likely causing delays at SFO. Similar gusts will occur along the coast, affecting places like Pacifica and Half Moon Bay. Highs on both days will be in the 60s in Daly City and the mid-70s in Redwood City.

South Bay and Santa Cruz: It will be a windy weekend in the Santa Clara Valley. Temperatures will rise to just above 80 degrees on Friday, but northwest winds will increase to 10 to 20 mph by the afternoon and remain strong overnight, with lows in the low 10s. Temperatures will remain similar on Saturday and Sunday, but northwest winds will be strong at 15 to 25 mph, with stronger gusts at times..

On Friday, Santa Cruz and the coast will have clouds and fog in the morning, but most of it should clear by the afternoon as winds pick up. Highs will be around 70 degrees Fahrenheit in Santa Cruz and the mountains, with west-northwest winds of 10 to 20 mph. Sunny skies and highs around 70 degrees Fahrenheit are expected over the weekend, but with stronger winds. Northwest winds will be present both weekend days, gusting 15 to 25 mph, with even stronger gusts near the coast..

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Reach Greg Porter: greg.porter@sfchronicle.com

Anna Harden

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