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<\/p>\nSan Francisco is entering what is historically its warmest three-week stretch of the year, but it appears the atmosphere didn’t get the memo this weekend.<\/p>\n
The average high temperature is at or above 70.5 degrees from Sept. 21 to Oct. 11, based on long-term climate normals. But temperatures were stuck in the upper 50s to low 60s on Sunday as thick fog squeezed out drizzle in parts of the city. The damp Sunday followed a cool Friday and Saturday, when temperatures remained in the mid-60s.<\/p>\n
The cloudy, cool weather is being spurred by a slow-moving low-pressure system over Central California. This relatively cold system is cut off from the guiding winds of the storm track, allowing the disturbance to take its time as it tracks across California.<\/p>\n
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Enhanced winds from a cut-off low-pressure system over Central and Southern California were forecast to bring clouds and a chance of showers Sunday and Monday. (Baron\/Lynx)<\/p>\n
Thick clouds associated with the cut-off low spread across the Bay Area on Sunday and prevented fog from burning off as quickly as usual in San Francisco.<\/p>\n
Significant rainfall is not expected in the Bay Area from the low-pressure system, though isolated showers popped up just off the coast. A few of those light showers could hit the Santa Cruz Mountains or South Bay on Sunday evening.<\/p>\n
The cut-off low will continue its slow journey east Monday, keeping conditions cloudy and cool across much of the state. Moisture and winds will be forced to rise upon hitting the Sierra Nevada, making mountain showers more likely. Thunderstorms are possible from Interstate 80 southward Monday afternoon, with the best chance around Yosemite, Sequoia and Kings Canyon national parks, the eastern Sierra around Mammoth.<\/p>\n
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Thunderstorms are possible over the central and southern Sierra Nevada on Monday, according to the National Weather Service. (Baron\/Lynx)<\/p>\n
A warmup is expected Tuesday as the cold system moves away from California.<\/p>\n
High temperatures Tuesday will be near to slightly below normal in the Bay Area, and in the 60s at the coast, 70s along the bay shoreline and 80s inland.<\/p>\n
By Wednesday and Thursday, 70s are likely to return to San Francisco, with low 80s possible in the warmest neighborhoods. Oakland should warm to the low to mid-80s those days, with 90-degree readings possible in San Jose.<\/p>\n
California heat tracker: Use our interactive maps and charts to see current conditions and historical data for where you live \u2794<\/a><\/p>\n Another low-pressure system is predicted to swing across California next weekend, potentially knocking temperatures back below normal.<\/p>\n Rapidly flipping temperatures from above to below normal – rather than a sustained extreme heat wave – are expected across California the next few weeks amid an unusually active early season jet stream. This region of fast-moving winds high in the atmosphere, better known as the storm track, is expected to be unusually strong this winter because of El Ni\u00f1o<\/a>. Historically, there is not a significant link between El Ni\u00f1o and California weather in early fall, though this year’s near record-strong El Ni\u00f1o may be an exception.<\/p>\n Subscribe<\/p>\n There’s more to San Francisco with the Chronicle. Subscribe today for just 25\u00a2.<\/a><\/p>\n