On average, lightning kills about 50 people every year in the United States. This year so far, 19 have died, five in Western states. Two were in Colorado’s Rocky Mountain National Park — a woman hiker and a man sightseeing at an overlook. Other deaths include a swimmer at Venice Beach, California, a man in an Arizona desert, and a cyclist in New Mexico. Lightning can strike anywhere — and does indeed hit the same place twice — but the National Weather Service is developing a “lightning potential index” that it is testing for western Colorado and eastern Utah before expanding. The index will track the conditions likely to cause strikes over a 60-hour period and be updated every three hours with computer modeling. Its developers hope that it will help protect people, but remember: Lightning by its nature is highly unpredictable, and the index but a guide.