First of all, please start to check the posts published on this forum and discussing rainfall forecast.
Example : https://community.windy.com/topic/13586/about-rain-calculate/2?_=1602329137551
There are many others, use the advanced search tool to fine them and to read them. You will find a lot of advices.
Principles are always the same :
Combining 1) and 2) should tell you the probability of getting rain or not