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
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it forms from a super-cell in a huge cloud.
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.
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.
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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