The polar vortex is splitting — an exceedingly rare phenomenon for this time of year that will be an unusual driver of warmer temperatures, as the calendar flips to March and meteorological spring.

DC swings from a warm 50 weekend to a cold start to the week, with highs in the 30s and a wintry mix as the polar vortex and jet stream bring Arctic air.

When the polar vortex stumbles, the weather at ground level can pivot from mild to brutal in a matter of days. A circulation that usually spins quietly above the pole can suddenly send lobes of Arctic ...