Long cable run for a flip-flop sensor
If you put the flipflop near the rain gauge, resistance of the wire will become problematic. The voltage drop from resistance and inductance of the wire could create noise and voltage drops. One could put the flip flop on the rain gauge, but it would also be wise to put a voltage regulator there also, to ensure that the voltage rail of the flip flop is constant. It might also be advantageous to use a Schmidt trigger to detect the voltage from the relay before the flip flop. 

Since the gauge is simply a relay and open or closed, it would be sufficent to put the flip flop at the computer. There are several ways to prevent magnetic noise, one being twisted pair (which CAT 3 is) but it might also be better to use shielded twisted pair.

With outdoor wires, lighting and other electric fields can induce high voltages that could be too high for a digital input. Here are some ideas/example circuits to prevent high voltage from accessing digital electronics. 

Either isolate the input with an optoisolator:



Source: https://www.avrfreaks.net/forum/level-protection-interfacing-24v-residential-electrical-installation

Or use voltage transient suppression (with a smaller fuse):



Source: https://dmohankumar.wordpress.com/tag/tvs-diode/