"Our current case rate places us on the trajectory to be in the purple tier, potentially as early as Sunday," said the city's health director, Dr. Grant Colfax. "At that point we will need to abide by the state's shelter-in-place order."
Gov. Gavin Newsom has ordered a curfew in all counties in the purple tier that's set to start Saturday. As of Friday, San Francisco isn't affected -- as it's still in the red tier.
If they are moved to the purple tier, San Francisco would also have to close gyms, movie theaters, indoor museums and places of worship.
The 14-day rolling average of new coronavirus cases has spiked since Nov. 1 -- up a staggering 152%. (See how it compares to other Bay Area counties in the graph below.)
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"This surge is more widespread and dangerous than ever before," said Colfax. "We are in a surge that has the potential to overwhelm our healthcare system and force us back."
The last time the state announced tier changes on Monday, San Francisco was moved backward two tiers from yellow to red. But if recent data is any indication, they could be moved backward even further to the state's most restrictive classification.
There are three criteria that could land a county in the purple tier: a health equity metric, a testing positivity rate above 8% and having more than 7 daily new cases per 100,000 residents.
On those first two criteria, San Francisco is in the clear. But it's the third one that could land the city in trouble.
On Monday, data released by the state showed San Francisco safely in the "red" zone, with 4.7 cases per 100,000 residents. But data from the city Friday shows a much higher number: 12.1 cases per 100,000 residents. That number is far above the threshold for landing a county in the purple tier.
"Our healthcare system is bracing for an influx of patients...over the next one to two weeks," said Dr. Jahan Fahimi.
Fahimi is the medical director of UCSF's Emergency Dept. He expects the next surge of hospitalizations will be more severe.
"Within a week of their illness is when we know that people tend to get better or more sick," he said. "So as the infection gets worse...those patients will come back to see their doctor, end up in the ER, and a good number could end up hospitalized."
See the tracker below to find out how COVID-19 cases are trending in your county.
App users: For a better experience, click here to view the full trend tracker in a new window
ABC7 News' Lindsey Feingold contributed to this report.
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