Detailed Answer
Sacramento experiences atmospheric river events that can deliver 2–4 inches of rain in a single day — far more than most gutter systems are designed for. Whether guards work during these events depends almost entirely on guard type and installation quality.
Micro-mesh guards are the strongest performer during heavy rain. High-quality systems are tested to handle rainfall rates of 22+ inches per hour — well beyond anything Sacramento records. The mesh allows water to flow through continuously while the open gutter channel below carries it to downspouts without obstruction. Guards keep the channel clear of debris, which is actually the biggest advantage during high-intensity rain: a debris-free gutter handles surge flow better than a partially clogged one.
Reverse-curve (surface tension) guards are the most commonly cited failure type during heavy rain. Water traveling along the curved surface is supposed to follow the curve into the gutter opening, but at high flow rates, water momentum carries it past the opening and over the edge. This "waterfall effect" causes overflow even on clean gutters during significant rain events. Several Sacramento homeowners have reported this behavior during atmospheric rivers, particularly on homes with moderate to steep roof pitches.
Cheap screen guards create their own problem. If debris has accumulated on top, heavy rain pushes water through the debris mat rather than through the screen opening, causing overflow well before the gutter itself is full.
Proper downspout capacity matters regardless of guard type. Even a perfectly functioning guard can't compensate for an undersized downspout or one with a partial clog. Before any major storm, confirm that downspouts flow freely and that discharge points are clear.