According to the 'Use' section, point 6 states: 'We use it to talk about weather. e.g. It's a clear sunny day.' Therefore, 'It's raining now' follows this rule.
Class 6 › English Grammar and Composition › Chapter 7: Introductory 'There' & 'It' › Topic: Understanding the use of 'there' for existence, events, and quantities
Aligned to the NCTB national curriculum.