"There's a typhoon headed toward us" means, simply, "A typhoon is headed toward us." But the first sentence is better for telling someone about the situation of preparing for a storm. In English, we often describe a situation using "There's a ____." For example, if you hear someone's phone ringing, you say:
There's someone calling you.
If a delivery person brought a package for someone at your office, you can say:
Hey, there's a package for you.