The biggest issue is that the ice tends to form on the upper surface of the wing in the areas of greatest curvature. * (near the front) This causes airflow disruption which increases drag and reduces lift. At best it decreases performance and increases stall speed. At worst the airplane literally becomes unable to fly. Propellers would have the same issue, but are spinning fast enough to sling the ice off.
*If an airplane has de-icing, this is the area where you will always find de-icing systems. Some aircraft will have additional de-icing systems in other areas.
So, when it happens in CA, is it in areas where people would really be driving? As fachverwirrt notes, it happens on Mt. Washington, but a. I’ve never heard of it happening anywhere but there in this state b. in the winter the people at the weather observatory are pretty much the only ones at the top of the mountain during the winter, so motorists aren’t likely to encounter it.
