You can make rain, slow, sleet, hail...basically anything that falls in a similar pattern, you can make it rain blood if you so desire, you might be able to make it rain lava with a bit of custom work. You can have fog of any colour.
All the above can be done on CS 1.6 and Spirit of Half-Life, so it CAN be done in ESF, they just need to code it in, or copy the code from Spirit depending on the teams views on plagerism.
Other things like meteor storms are not possible, nothing with multiple breakables is, and the bigger the breakable the more it'll lag. Things like a few panes of glass is fine in multiplayer, and the tree models in outdoors maps, but not huge meteors, smashing mountains, etc
The nearest thing you'll get to a fully functional erupting volcano (lag free) is a picture of an erupting volcano as part of the skybox, and then have it rain ash and lava (in the same way CS 1.6 can rain or snow)