A crystal detector would work best, although I'm not sure if galena can be formed into a proper 7-inch sphere for predicting such things. Weather forecasters have been wanting to do such things for eons. About the only way you can get any lead time on that rain is by putting your sensor at the top of a very tall pole. Of course, during a storm, that invokes all kinds of other problems.
Humidity isn't always a predictor of rain. You can find either 100% humidity during a rain or 80% humidity. Just as they can use lasers to determine ceiling height, maybe one could be used to "see" approaching rain. But nothing is going to guarantee that the winds won't shift and drive a predicted rain away or an unpredicted rain toward you.
Dean