Explosive spread F was first described by Woodman and LaHoz [1976>. In such events the 50 MHz radar signal rises at Jicamarca to 10-20 db above the noise level within a few milliseconds but then disappears within 100 ms or less. Woodman has suggested that electric fields from thunderstorms might drive a plasma instability which could cause these echoes, and recent observations [Woodman and Kudeki, 1984> provide convincing evidence for this triggering. Here we discuss the instability mechanism and show that transient thunderstorm electric fields comparable to those recently observed by rockets in the midlatitude ionosphere could excite a rapidly growing two-stream plasma instability that is driven by the E¿B drift of the F region electrons in the short period (less than the ion gyroperiod) before the ions reach the same velocity.