From Glossary of Meteorology
(Also called line storm, line gale.) Severe storms in the United Kingdom and North America that are popularly believed to accompany the vernal and autumnal equinoxes.
The belief may have originated in the mid-1700s when sailors observed the West Indian hurricanes, which are most frequent at about the autumnal equinox. Statistics of gale frequency do not show that storms are especially frequent in temperate latitudes about 22 September, though it does sometimes happen that the first severe storm of the winter half-year occurs toward the end of September. The belief has been extended by analogy to the vernal equinox, about 21 March.