This isn't inherent to voice assistants though (which is what this subthread is about). Siri is a known, particularly poor implementation of a voice assistant. I don't have an Alexa, but I just moved to a city where rainy days come out of nowhere. My Google Home is concise and precise about the chance that I'll need an umbrella, including what time the chance of rain starts.

I don’t know if it’s related but the weather app on iOS frequently takes minutes to update the weather for my location even when looking directly at the app. It’s incredibly annoying and has led me to believe the weather was going to be completely different more than once.