Typically, a tornado watch is issued when there are strong winds, which rapidly change direction near the earth's surface, that combine with high heat and humidity.
(a tornado watch; is issued; T:when there are strong winds; T:Typically) (strong winds, which rapidly change direction near the earth's surface; combine; with high heat and humidity)
a tornado watch strong winds, which rapidly change direction near the earth's surface
is issued combine
T:when there are strong winds T:Typically
with high heat and humidity