We learn from the passage that a tornado warning _______.

When a tornado watch is announced, it means that
tornadoes are expected in or near your area. After
such an announcement, it is best to listen for advice
on the radio or television. Watch the sky, especially
to the south and Southwest. When a tornado watch is
announced during the approach of a hurricane,
however, keep watching the sky to the east. If you
see any revolving, funnel-shaped clouds, report them
by telephone to your local police or weather bureau.
Do not use the telephone to ask for additional
information when a tornado warning is announced,
take shelter immediately. The warning means that a
tornado has actually been sighted, and may strike in
your area. Your best protection is an underground
shelter, or a substantial steel-framed or reinforced-
concrete building.