When Does Thunder Lake Freeze? Typical Ice-In Dec 8, Ice-Out Apr 21
Thunder Lake in Cass County, Minnesota usually freezes over around Dec 8, and usually has ice-out around Apr 21, based on Minnesota DNR ice observations recorded between 2002-2015. That works out to roughly 231 days of open water a year. These are typical dates across many winters, not a forecast — individual years swing by weeks in either direction.
Ice record for Thunder Lake
| Typical freeze-up (ice-in) | Dec 8 |
| Typical ice-out | Apr 21 |
| Earliest freeze-up on record | Nov 29, 2002 |
| Latest freeze-up on record | Dec 14, 2004 |
| Earliest ice-out on record | Apr 13, 2015 |
| Latest ice-out on record | May 14, 2013 |
| Open-water season | ~231 days |
| Winters observed | 13 |
| Location | Cass County, Minnesota |
These are dates, not an ice-safety reading
A typical freeze-up date tells you when Thunder Lake has usually iced over — it does not tell you the ice is safe, and we hold no thickness measurement for this or any lake. The Minnesota DNR is explicit that no ice is ever completely safe: thickness varies across a single lake, and current, springs, aerators, and insulating snow all weaken it. Ice that held a truck last week may not hold a person today.
Read the ice-thickness guidelines before going out, and check current conditions with the Minnesota DNR ice-safety page and local bait shops, which track conditions lake by lake.
How much the timing varies
Ice-out on Thunder Lake has landed as early as Apr 13, 2015 and as late as May 14, 2013 — a spread of weeks around the Apr 21 median. Freeze-up has ranged from Nov 29, 2002 to Dec 14, 2004. That year-to-year swing is why a median date is a planning tool, not a schedule. Freeze-up is driven by accumulated cold after the lake turns over in fall; ice-out by late-winter air temperature, sun angle, and how much snow sits on top of the ice reflecting it. Thunder Lake carries an overall water-quality grade of A — the full breakdown is on the lake report.
Source: Minnesota DNR Climate Office ice-in / ice-out records, observations from 2002-2015.