Is your rain gauge wired or wireless? A while back I analyzed a wireless (433Mhz) rain gauge and was able to use an Arduino or Rpi to listen to the sensor and decode the signal:
The raspberry pi code is provided at the end of the blog post.
As the main point of the posts, I described the procedure I used to reverse engineer these sensors. I am all tempted to follow the same procedure to analyze a more comprehensive weather station, but I haven’t got time to do so.