Nexon proved something with the visitor patch.

Have the weather triggered by the item give you a buff which lasts say 10 seconds every 10 seconds so you only get the buff for 10 seconds max if you leave, like an underwater or an El Nath map.

Or have the entire system's time status cycle through different weather effects every 6 hours depending on Central Standard Time.

Rain/snow could last for about 3 hours on the system clock, then it cycles though to another weather condition. Seasons also could play a factor in this, say if your playing in the spring/fall:

You might have a 40% chance to witness rain/sunshine, or a 15% chance of a lightning storm , and a 5% chance to witness snow.

Dawn/Afternoon/Dusk/Nightfall could occure every 6 hours, so it leaves flexability even for the people who play in different parts of the world.