Complex sentence: As the town's tornado sirens go off, a panicked man with a bloody nose, Dan Miller (Jeffrey DeMunn), runs into the store warning of something dangerous in the oncoming mist, which quickly envelops the store, making it impossible to see outside.
Split into simpler sentences: As the town's tornado sirens go off, a panicked man with a bloody nose, Dan Miller (Jeffrey DeMunn), runs into the store warning of something dangerous in the oncoming mist. Shortly after, the mist envelops the store, making it impossible to see outside.