A derecho, pronounced day-ray-cho, is a line of severe thunderstorms that moves in a straight line. The word derecho is Spanish for 'straight'.
Derechos come in three types: Serial, progressive and hybrid. The diagram, courtesy of the NOAA's Storm Prediction Center shows both a serial derecho and a progressive derecho.
The serial derecho forms in the warm sector ahead of a cold front with a very strong, low pressure system at its north end. The warm sector supplies the soupy tropical air that drives the thunderstorms, and the low pulls it northward.
Typically, a series of 'bow echoes' forms and joins to make a long line of severe storms. The bow, like an archer's bow, indicates strong winds behind the line and the threat of straight line winds.
It is called a 'serial' derecho because bow segments form, die out and new segments develop to replace the old ones.
There is enough rotation where the bow segments meet for occasional tornadoes from a serial derecho.
A progressive derecho forms north of a stationary front or a warm front. The derecho typically heads to the east or southeast following upper level winds. The progressive derecho looks like it follows the front like a train follows railroad tracks.
The progressive derecho usually consists of a single, large bow echo that may last for hours and progresses to the east or southeast.
Straight-line winds are the main threat from the progressive derecho, but occasionally weak tornadoes may occur.
The hybrid derecho is a mix of the progressive and serial types and can take many forms.
All derechos can produce flooding rains and most have intense, frequent lightning.