Tornadoes most frequently occur during the middle and late afternoon.
http://en.wikipedia.org/wiki/Tornado:
"Tornado occurrence is highly dependent on
the time of day, because of solar heating.[53]
Worldwide, most tornadoes occur in the late
afternoon, between 3 pm and 7 pm local time,
with a peak near 5 pm.[54][55][56][57][58]
However, destructive tornadoes can occur at
any time of day. The Gainesville Tornado of
1936, one of the deadliest tornadoes in history,
occurred at 8:30 am local time.[10]"
The numbers in brackets refer to references on
the Wikipedia page.
The later in the afternoon, the more convective
activity (warm air rising from the Earth's surface)
can occur from solar heating of the ground,
resulting in more frequent, larger, and
intense thunderstorms that can potentially result
in more frequent and intense tornadoes.