WRAL WeatherCenter Blog

(1) I often hear of a Tornado Watch or a Tornado Warning. Can you describe the difference between a Watch and a Warning? (2) When it is stated that there may be a 50 % chance of rain on a particular day, isn't that like saying " It may rain today or not, let's flip a coin and see ? " If it has a 50 % chance of rain, then it is equally likely that it will not rain also ?

Posted September 3, 2008

MIKE MOSS SAYS:       K.,    Regarding (1), a tornado watch is issued when the general atmospheric conditions over an area (wind speeds, wind shear, instability, vertical distribution of moisture, etc) are such that there is a better than average chance that tornadoes may form. They usually cover a sizable geaographic area and a valid period of several hours duration. A warning, on the other hand, means that either a tornado has been sighted and is on the ground, or that radar interoggation of a storm cell has yielded signatures that correspond to a high likelihood the cell is either producing a tornado or that one is imminent. In this case, the warning is issued for a much smaller area that is expected to be in the path of the parent thunderstorm cell. Warnings may cover a single county, a portion of a county, or portions of multiple counties and usually expire or are cancelled after about thirty minutes to an hour.

As for (2), it isn't quite like flipping a coin in that there is a contiuum of precipitation probablities that runs from zero to 100 percent, so while a 50% chance of rain does indeed mean it may rain and it may not (with near-equal probability, so that given 10 days with similar conditions, a location in the forecast area would likely receive measurable rain on about 5 of those days), it still provides the information that the chance of rain is much better than on a day with a 10 percent chance, and not nearly as good as on a day with a 90% chance. Since many areas in the U.S. experience measurable rain about one day in four on average (for a climatological probability of about 25% on any given day), a 50% chance could be looked at as an elevated chance at rain for that given day, compared to normal. Likewise, to put it in coin terms, if you were given a fair coin and four "fixed" coins that are known to produce heads 10, 25, 75 and 90 percent of the time, the 50 percent chance associated with the fair coin would certainly provide useful information about how likely a "head" is on a given flip compared to the other coins.


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