Cool May Breaks San Diego’s Long Warm Spell

San Diego’s warm spell is broken. When the clock struck midnight on Thursday, a 54-month run without a cooler-than-normal month ended. The last time a month was cooler than normal in town was October 2013 — 4½ years ago. It was a remarkable, rare run of warmth. There have been other extended warm spells in the city’s past, most notably in the early 1980s, but none were quite like the one that just ended. What caused the extended warm spell, and what does it mean?