The first real rain always finds something
By late November or December, most Los Angeles houses have gone six or seven months without a serious rain. Then the first storm rolls in off the Pacific, dumps an inch or two in a day, and suddenly you notice things you hadn't thought about since last winter. A stain creeping across a ceiling corner. A musty smell in the hall closet that wasn't there in October. Water sitting against the foundation where the yard slopes the wrong way. None of this is random bad luck. It's the house telling you where it's been quietly failing all summer, and the first storm just made it visible.
Why older LA houses take this hard
A lot of the housing stock here, the 1920s Spanish bungalows, the 1950s ranch houses in the Valley, the hillside places in Silver Lake and Eagle Rock, was built for a climate that's dry most of the year. Roofs, flashing, and grading got sized for occasional rain, not sustained wet weeks. Decades of dry summers let small problems sit: a cracked roof tile, a gap in stucco, a downspout that dumps water two feet from the foundation instead of away from it. None of it mattered in July. In a wet January, it does.
Many of these houses also have crawl spaces or raised foundations with minimal ventilation, and older additions where the roofline doesn't quite match up with the original structure. Those seams are where water gets in without anyone noticing for weeks.
More on this from What The First Winter Storm Finds In An Older Los Angeles House.
What to check yourself after the first storm
Walk the exterior while it's still damp or soon after. Look at the base of exterior walls for dark staining, efflorescence, or soft stucco. Check under sinks, around tubs, and in any hall or garage that backs up to a hillside. Open closets on exterior walls, especially ones that don't get much air circulation, and smell for must rather than looking for stains. Mold often announces itself by smell before it shows itself visually.
Check your attic if you can get up there safely. A flashlight scan for new water trails along rafters, especially near chimneys, skylights, or vents, tells you a lot. In the crawl space, if you have one, look for standing water or damp soil that stays wet for more than a day or two after rain stops.
Outside, trace where water actually goes when it rains hard. Stand at a window during the next storm and watch your own gutters and downspouts work, or fail to. A lot of mold problems trace back to one downspout quietly aiming water at a wall for years.
When it stops being a weekend project
Surface mold on a bathroom tile or a windowsill, something you can wipe down with a damp cloth and it doesn't come back, is normal household upkeep. That's not a crisis.
It's a different situation when the smell persists after things dry out, when a stain keeps reappearing in the same spot after every rain, or when you can see or smell it but can't find where the water is getting in. Mold behind drywall or under flooring doesn't show itself the way surface mold does, and in an older house with questionable insulation and odd wall cavities from past remodels, water can travel sideways and show up on a wall ten feet from where it actually entered. At that point, poking around yourself usually just disturbs the problem without solving it, and it's worth having someone test the air and check behind walls properly before you start any repair.
The pattern that matters most in LA
Because so much of our rain comes in a handful of storms rather than steady weeks of drizzle, a lot of homeowners treat each one as an isolated event. The house tells a different story. Water that got in during the first December storm often doesn't dry out before the next one arrives in January or February, especially in parts of the house that don't get sun or airflow. That's the stretch where small leaks turn into real mold growth, quietly, over a season rather than overnight.
If the first storm showed you a stain or a smell, don't wait for it to prove itself with a second and third storm. By March you'll have had several wet weeks to make a small problem much larger, and by then it's a bigger job than it needed to be.