As a lifelong resident of the Pacific Northwest, I was always confused by the road signs that say “Slippery When Wet“. It rains just about year-round. But there are definitely times and that warning should be minded. The first rain after a dry spell comes immediately to mind. If it seems like there are a lot of car crashes on those days, there's a good reason.
In general, traction is the function of how well the tire, and therefore the car, can stay and maneuver on a roadway. The friction between the rubber and the road surface is key to keeping cars safe and on the roadway.
Over time, normal usage on the road allows the buildup of oil, grease, and other materials. Not only are some of those substances slippery in themselves, but they also create a physical barrier between the road surface and the tire, reducing friction, and in turn, traction.
Independently, water on the roadway reduces friction between the tire and the pavement. However, a steady supply of rain also keeps the roadways relatively clean of buildup. When there is a dry spell, the buildup has not been washed away for a period.
When it rains under these conditions, it is truly oil and water mixing. This compounds the effect of the normal reduction in friction in the reduction in friction due to buildup on the roadway. This causes the roads to be incredibly slick.
A few things to keep in mind on the first day of rain after a dry spell. The roads are slippery. You may need to drive slower in general. You will need longer distance to brake and stop. Be mindful of losing traction around corners and curves. Be on the lookout for other drivers who are not minding these tips. Above all, stay in control of your car and drive safely.