Thus, most of the time, summer thunderstorms only occur west of the mountains when east winds aloft allow them to drift west into the valleys after they form.
(summer thunderstorms; only occur; T:west of the mountains; T:when east winds aloft allow them to drift west into the valleys) (they; form; ) (east winds; aloft allow; them to drift west into the valleys after they form) Context(east winds aloft allow,List([83, 105))):(them; to drift; west; into the valleys; T:after they form)
east winds aloft allow,List([83, 105))
summer thunderstorms they east winds them
only occur form aloft allow to drift
T:west of the mountains T:when east winds aloft allow them to drift west into the valleys
them to drift west into the valleys after they form
west into the valleys T:after they form