下大雪(xià dà xuě) - to snow heavily in Mandarin

下大雪 (xià dà xuě) is to snow heavily, built by slotting 大 (dà, big, heavy) into the middle of 下雪 (xiàxuě, to snow): 下 (xià, to fall) plus 大雪 (dàxuě, heavy snow, itself 大 dà, big, and 雪 xuě, snow).

The pattern is productive: 下小雨 (xià xiǎo yǔ) is to drizzle, light rain, by the same logic with 小 (xiǎo, small) instead of 大 (dà). You can also hear 下大雨 (xià dà yǔ), to rain heavily, for the rain equivalent.

Watch the word order: the size word goes between 下 (xià) and the weather noun, not in front of the whole phrase. 大下雪 is not correct Chinese; 下大雪 (xià dà xuě) is the only order that works.