Dangerous early season extreme heat is on tap for parts of Texas this week in a prelude to what’s expected to be an abnormally hot summer nationwide.

Sweltering heat more commonly seen in the throes of summer than in the spring was making an unwelcome visit this week to a large portion of the U.S. – from the Dakotas to Texas and other parts of the South – and putting millions of Americans on alert for potentially dangerous temperatures.

Texans are familiar with triple-digit temperatures, but it's rare when they occur in May. Here's what makes unseasonable heat so dangerous.