The city of Santa Clara is transforming its convention center into a temporary hospital for up to 250 COVID-19 patients with non-emergency symptoms who may need it. The space is stocked with beds and enough medical supplies and medicine for at least three days.

On Friday, members of the California National Guard placed white blankets and towels on dozens of new beds at the Santa Clara Convention Center, adding the final touches on what is now a federal medical station operated by federal, state and local authorities.

Statewide and in the Bay Area, counties are preparing for a surge of patients stricken with coronvirus that could come within days or weeks.

In Contra Costa County, health officials are eyeing Alhambra High School in Martinez — which is close to the county hospital — as a possible site for a makeshift pandemic hospital. Marin County officials say they expect to establish such sites, and Alameda and San Mateo counties are working with state and federal officials to identify places for that purpose, as well.

“We have a legal obligation as well as a moral and ethical obligation to provide the highest level of cooperation during this time,” Contra Costa County officials said in statement this week.

The U.S. now has more cases of COVID-19, the illness caused by the coronavirus, than any other country, Johns Hopkins University reported Thursday. That number exceeds 86,000, with at least 1,300 deaths.

California had 4,208 positive cases as Friday morning, including 1,525 in the Bay Area. Statewide, 85 people have died.

Experts say early measures in California, especially in the Bay Area, discouraging people from gathering in crowds and then ordering them to shelter in place could soften or even prevent a surge. Nonetheless, officials and health-care providers are preparing for an increase in patients, particularly as more widely available testing reveals more cases of infection.

Santa Clara County is the epicenter of the coronavirus crisis with 574 confirmed cases — more than in any Bay Area county.

Nearly half of the patients at Kaiser Permanente San Jose Medical Center have or are believed to have COVID-19, a Kaiser executive said Wednesday.

When it comes to preparing for a surge, the three essential factors are supply, space and staff, said Jim Morrissey, the Alameda County tactical medical program director.

The county is working with state and federal officials to identify locations for “alternative care sites, mobile field hospital and surge capacity medical facilities,” he added. No locations have yet been selected.

“We are preparing for at least certainly the supplies,” Morrissey said. “There are a number of plans in place for increasing the surge capacity of hospitals and clinics.”

Morrissey said the county had a nearly 30% reduction in 911 emergency calls since Alameda County’s shelter-in-place order on March 16. The reduction in emergencies — more people staying at home, the less chance for an emergency — keeps hospital beds open, and allows for more fire and police officers to be available, he added.

Currently, the county has 421 available hospital beds, 86 isolation beds, 39 ICU beds and 141 ventilators.

“Time will tell if we actually flatten this curve,” Morrissey said. “We all realize that this could get bad at some point and hopefully it doesn’t. But we will do our best to be ready if it does.”

Similarly, San Mateo County officials are working with the state and federal officials on plans for additional care facilities and surge protocols, but declined to give specifics.

Laine Hendricks, a spokeswoman for Marin County, said the county works all year with hospitals and medical facilities to prepare for disaster responses. Using the county’s flu pandemic plan as a baseline, officials are expanding its response to COVID-19 as they learn more about the virus and how it spreads, she said.

Local hospitals are preparing to create field hospitals or skilled nursing facilities for an influx of coronavirus patients, but not specific details were immediately available, Hendricks said.

“We have been in contact with our hospitals,” she said. “They worked really hard to increase capacity within their facilities and we are looking at how we can expand beyond that as need be.”

Elsewhere in the state, Los Angeles is also preparing for a surge. The Navy’s Mercy hospital ship, with room for 1,000 beds, has arrived at the Port of Los Angeles to help.

Sarah Ravani is a San Francisco Chronicle staff writer. Email: sravani@sfchronicle.com Twitter: @SarRavani