Everything

It’s always easier to work with something that’s already there. And so the forces (and Forces) involved in manipulating existing weather patterns are simpler than the ones that enact drastic changes upon the local climate. Even so, your mage needs Forces 4 or higher in order to alter the weather in any meaningful way.

System

-wise, your mage could alter the existing weather in several different ways:

• Correspondence 3/ Forces 3/ Entropy 2 might let the mage make minor tweaks to the climate by affecting the probability that a given weather-pattern will form. Correspondence helps the mage reach further than Forces 3 alone would allow him to reach. This sort of weather-witching takes time, and it follows its own course, not the mage’s demands – after all, he’s merely suggesting what the weather might do, not bending it to do what he wants it to do.

• Forces 4/ Entropy 3 gives a stronger “suggestion,” altering probability so that the storm-front comes together quickly and follows the mage’s design.

• Forces 4 or 5 simply reaches out and yanks the elements into alignment.

In all cases, the weather follows the natural tendencies of the existing climate. You could make it snow on a cold November evening, but not on a bright and sunny day in July.