Complex sentence: In early spring the jet stream starts to move northwards allowing large depressions to bring strong winds and rain in from the Atlantic and in one day the weather can change from springtime sunshine to winter sleet and snow.
Split into simpler sentences: In early spring, the jet stream starts to move northwards, allowing large depressions to bring strong winds and rain in from the Atlantic. In one day the weather can change from springtime sunshine to winter sleet and snow.