This plugin keeps the weather in the worlds on your server synchronized with the real-world weather in a location you specify. It does this by connecting to weather underground every few minutes (configurable; 5 minutes by default) and parsing the weather. This means if it's raining outside, it will be raining in minecraft. This is also true of thunderstorms.
You will want to configure it before you try to use it; otherwise it will synchronize your default world (named "world") with the weather in San Francisco, California. (Used for testing.)
This should work for any place you can get an RSS feed with weather in it from WeatherUnderground. See the directions below for how to find this, and if it does not work, feel free to ask for help. Please be sure to provide the URL you are using!
You can view the forecast with the /forecast command, which is available to all users. It will also show the current weather. This command can be disabled, if desired.
Download (This could be more recent than the version on this site if they have not approved the latest version yet. I strongly suggest using this link instead.)
Some people have expressed interest in having the real world time on their servers alongside this. I was going to code this myself, however there is something out there already that can do this. Check out RealTime for this!
FINDING THE RSS FEED
This plugin now asks you to provide the URL to an RSS feed to get the weather. The old method will not work past version 1.2. The method to get the URL to your RSS feed is as follows:
Navigate to http://classic.wunderground.com, and find your location.
Around the height of the title of the page on the far right is an RSS icon. You need to get the URL of this - right click it and copy the link address. It should look like:
Take this entire thing, and put it in the worlds section of your config file.
If you have problems finding this, just ask. My explanation here is not that good.