The wind will come first with rain following behind it. Remember, that the main threat with any squall line is severe damaging winds associated with the shelf cloud, although brief spin-up tornadoes can occur. Shelf clouds most often form just ahead of intense lines of thunderstorms.
A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. A wall cloud that may produce a tornado can exist for 10–20 minutes before a tornado appears, but not always. Cool, sinking air from a storm cloud's downdraft spreads out across the land surface, with the leading edge called a gust front. A shelf cloud is attached to the base of the parent cloud, which is usually a thunderstorm cumulonimbus, but could form on any type of convective clouds.
The wall cloud is usually to the rear of the visible precipitation area. A wall cloud is an isolated cloud lowering attached to the rain-free base of the thunderstorm. This outflow cuts under warm air … A shelf cloud will usually be associated with a solid line of storms. While menacing in appearance, shelf clouds are not tornadoes or wall clouds… A shelf cloud is a low, horizontal wedge-shaped cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Often times, these tornadoes are rain-wrapped and short-lived. Rising cloud motion can often be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Shelf clouds are typically seen at the leading edge of a thunderstorm or squall line of thunderstorms. It may appear to rotate on a horizontal axis.