That's what the locals say, right?
Seems this phrase is one of those that is imagined to be a bit of "local folklore" all over the world. I've spotted it appearing in Scotland, Alaska, Australia, Vermont, and Texas, and I've no doubt I'd've found it in more places if I'd had occasion to ask people in more places.
I've encountered this phrase on the home page for a town in Texas which was bragging about having the highest number of sunny days per year in the area. This sense takes it as meaning, we have so little bad weather that if we're having some right now, it probably won't last. In other words, their weather is remarkably consistent.
But most of the places I've heard it cited, the interpretation is almost the opposite, that weather is so volatile, that whatever it is now, it'll be something else in a few minutes. That's certainly how it's used in Vermont, even though to me, Vermont's weather seems about middling between consistent and volatile. The most volatile weather I've lived in is Long Island, where, amusingly, I've never heard the phrase used.
Neither interpretation really explains why it's used in Juneau, Alaska, which is where I first heard it. Most of the time, the weather in Juneau is extremely consistent over time; the temperature might not move more than a few degrees, day and night, for a week or more at a stretch. (Though it'll be different right up the road.) And not only is it consistent, but it's certainly not what you'd call good; Juneau has a level of rainfall that makes Seattle seem sunny.
Most amusing is how it's a bit of "local folklore" in so many places around the world. What makes people get convinced something like this is uniquely local? And how does that illusion of unique locality survive in this era of globality? Maybe this blog post will be one pebble in the avalanche of its eventual demise.