The much-anticipated mark of spring is almost here: Ice-out is expected to be declared on Lake Minnetonka this weekend — about three weeks earlier than last year.

Hennepin County sheriff’s deputies with the Water Patrol checked the west metro lake Friday afternoon, but said there’s still a large ice mass in the middle of the lake that’s more than 1 inch thick.

Ice-out is determined not when it’s completely ice-free, but when a boat can navigate through the lake’s channels to bays. It’s the unofficial start to spring for thousands of boaters on the Twin Cities’ busiest lake.

“It’s the most wonderful time of the year,” said Steve Woods, executive director of the Freshwater Society, a nonprofit that’s tracked ice-out on Minnetonka since 1857.

There’s no universal definition for the springtime ritual in Minnesota; decades ago, residents would put a car on the lake to determine ice-out when it fell through. Starting last year, on April 24, the nonprofit and Hennepin County started jointly declaring ice-out after years of disputes over the criteria. Usually ice-out on the 14,500-acre lake happens in mid-April. The earliest date: March 11, 1878. The latest date ever recorded is May 5, 1857 — a record 2013 nearly hit with ice-out on May 2.

Whenever it happens, it’s a big deal for boaters and lake businesses alike. An estimated 10,000 boats dock on the lake and 61,000 boats use its launches.

Other lakes had ice-out as early as mid-March. Go to dnr.state.mn.us/ice_out for a map of ice-outs across Minnesota.

KELLY SMITH