Since they give updates every 18 minutes, thus any time being a multiple of 18 after 2 pm will be the time they give update.
5:30 pm is 3 hours 30 minutes for 2 pm or 150 minutes from 2 pm.
In 150 minutes, there are eight 18 minutes that have passed and is remaining 6 minutes for the next multiple of 18.
Therefore, you have to wait for 6 minutes for the next weather update.