Rain...

simple answer would be "you can't" but there are some back alley methods you can use

1) use the weather plugin by @Miwarre and set it up for the rain event to occur more frequently

2) when it stops to rain just type in the console "weather rain" (it might also be setweather not 100% sure)

3) use my servertools plugin, you can ignore most of the features except for the "/st weather rain" (in your case) command

4) you can disable weather in SP through the config.properties file "game_weather_enabled=true" option by setting to to false (instead of true), then in game use either the console or my plugin to set the weather to rain and it shall stay raining forever. (I also now saw there is an option "game_weather_preset=default" wonder if @red51 could enlighten us to what that does and if there are other options for harsher, calmer weather)

5) I can write a new plugin to elongate rain events

6) I will think of more options in time XD

As @Minotorious mentioned, there are various ways to have an influence on the weather About the game_weather_preset setting: You can change it to "unsettled", then there will be less sun and more rain in general. Apart from that, you could also type "sunny" there to have a calmer weather (less rain, more sun)