I use Google Analytics to track readership and other characteristics of my blog. One of the features of the program allows me to view search terms that are used frequently on a daily, weekly or monthly basis. One of the top search terms, to my surprise, this past week was "a tornado cannot be ruled out..." After realizing that I have been using that phrase somewhat frequently lately (particularly on the near-daily severe weather outlooks), I thought I'd give a brief explanation.
While I can't speak for other meteorologists, I can say that when I use that phrase, I'm indicating that tornadoes are not likely within a particular area, but there is a "non-zero" chance of one (hence the term - "a tornado cannot be ruled out.") Often at this time of the year conditions are more favorable for damaging winds and/or large hail rather than any widespread tornado development. With that said, conditions sometimes become favorable for isolated and/or brief tornado production, often during the first couple of hours of thunderstorm initiation (when storms are more isolated in nature and may become better organized). The main ingredient necessary for such a possibility is the turning of wind direction with height (i.e. South or Southeast winds at the surface shifting Westerly as you rise up into the middle-levels of the atmosphere).
So, the next time you see or hear the phrase "a tornado cannot be ruled out", that means that while organized and/or widespread tornado development is not likely, you should remain alert if threatening weather approaches your area - because a tornado "cannot be ruled out..."
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