Along the sun-baked corridors of Southern California, heat and humidity have arrived together — an uncommon pairing for a region long accustomed to dry warmth. What began as a sweltering Tuesday is forecast to deepen into a sustained trial of endurance, as moisture amplifies the burden of already punishing temperatures. Forecasters offer no quick comfort: the discomfort will worsen before it relents, asking residents to reckon with a climate that is, at least for now, behaving like somewhere else entirely.