And Just To Really Cheer Up Your Day

Here's a puzzle for you: What is this chart about? I'll leave it up for a while just in case anyone can figure it out. One clue is that you'll care about it especially if you live in Wyoming -- or anywhere in the northern / central Rockies for that matter. Another clue is that event #1 hasn't happened yet. (And yes, if you look at the HTML for this post -- or just hold your mouse over the chart -- you will figure it out also.)