Persistent flow in the upper atmosphere form the west combines with topography to establish a surface lee-trough just west of Oklahoma. The lee- trough is a common location for development of the dryline and thunderstorms that develop along it. Moisture from the Gulf of Mexico that feeds the thunderstorms flows northward east of the lee-trough. In addition to the oceans, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, form which moisture evaporates into the atmosphere. Convective storms, which generally move individually from southwest to northeast, move eastward as complexes across the State and provide most spring and early summer rainfall.
Flooding caused by convective storms tends to localized unless the storms fail to move quickly.
If intense thunderstorms repeatedly develop over the same terrain for several hours or days, localized flooding can be massive. A second large-scale feature in Oklahoma's precipitation delivery system is the Bermuda High. Clockwise circulation around this airmass controls most of the surface-moisture supply received by Oklahoma from the Gulf of Mexico. The location of the Bermuda high-pressure system substantially affects late summer and fall rainfall. If the system is south of its normal location, polar airmasses can move southward into Oklahoma.