Yellowstome Lake Frozen in May?

I had a window seat in the Delta Airlines Boeing 767 enroute from Paris to Salt Lake City yesterday.  The pilot nicely informed us that Yellow Stone was off to the left side (port side) or the plane and having my digital camera handy, I snapped this pic.  It's mid May, and Yellowstone Lake is still solidly frozen?  I'd thought it would have thawed in March or April.  As Dr Desert Flower and I were planning on taking a road trip in her new convertible from Arizona to Montana sometime this Fall, I need to make damn sure we do it before the first snow falls hit in September (projected).