Never
While it would be possible for a tornado to cause major damage to a significant portion of Washington D.C. (in 2002 a large F4 tornado came within 30 miles), no tornado would be large enough to affect the entire city.
If two tornadoes came together they would merge to form a single, larger tornado. Such instances are rare, but they have happened. In most tornado mergers a large tornado absorbs a small one.
twister
Overall it would probably be Oklahoma. For its size Oklahoma probably has the most violent tornadoes and has had a few of the deadliest and most destructive. It should be noted, though that Mississippi and Missouri have had a history of exceptionally deadly tornadoes, though such violent tornadoes visit those states less frequently.
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A a radar is better: it can detect a tornado at a distance. A barometer would be of no use unless the tornado came dangerously close.
it forms from a super-cell in a huge cloud.
The Joplin, Missouri tornado of 2011 injured over 1,100 people.
Violent changes in weather are called fronts. These fronts can refer to violent temperature changes such as a cold or hot front for example.
The oldest religion of Japan is animism. Then came Shinto.
The Holy Spirit Came in the form of Tongues of Fire.
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