I was reminded by the comment I wrote about a trip from Lethbridge to Cutbank, MT. Forecast by Canadian weather guessers was for CAVU all the way to Cutbank so I left fat, dumb and happy thinking about a nice juicy MT beef burger when I landed. About to cross the border and made all appropriate radio calls and I had been fooled by that forecast. About 10 miles into US airspace two thunderstorms popped up, one on each side of my path...evil black things with lightening. I called Flight Service and asked if I could cross back into Canada and explained my situation. NOPE.. gotta file 24 hours in advance. At that point, I was flying a Cessna 185 which is a good "off field" plane so I landed on a ranch road and sat for a couple hours until the evil things passed to the East...got to Cutbank and no burger to be had. Spent the night under the wing in my sleeping bag and fueled up in the morning. Thunderstorms SUCK!!!!!