aborrascarse

A meteorological verb meaning for the weather or the sea to turn stormy (borrasca is a storm system) — mostly found in weather reports and nautical or literary descriptions of worsening weather.

• El cielo se aborrascó de repente. — The sky suddenly turned stormy.
• El mar se aborrascaba conforme avanzaba la tarde. — The sea grew stormier as the afternoon wore on.
• Se aborrascó el horizonte antes de la tormenta. — The horizon turned stormy before the storm hit.

This is specialized weather vocabulary — for everyday "it's getting stormy," learners are more likely to hear se está poniendo tormentoso or simply va a llover fuerte.