One of the first things a soldier or a hiker in the Negev desert learns is not to sleep in a wadi (dry river bed). It can rain dozens of miles away, but if water flows down from the mountains into the wadi, the wadi becomes a raging river that can flow as fast as 80 MPH. That means that if you're hiking in the area of Shivta or Avdat, and it rains on the Jordan border 40 miles away, a torrent of water will reach you in a mere 30 minutes. The flow of water can be strong enough to wash away a tank.
Thank G-d, we've enjoyed rain the past few days in the south of Israel. Here's a clip from last year that shows what we're talking about: