WRAL WeatherCenter Blog

What exactly does "30% chance of rain" mean? Is that 30% of the area will defininately get rain (if so, what area?), or is it a 30% guess that some places close by might see rainfall?

Posted March 9, 2008

MIKE MOSS SAYS:        Rick,      The probability of precipitation is an estimate of the likelihood that measurable rain (.01 inch or more) will occur at any given location during the valid period of a forecast for that location. For fairly widespread, stratiform rain systems, it amounts to an assessment of the probability that the system will actually generate rain and that its track will carry it over the location in question. In other words, how likely is the expected pressure, temperature and moisture pattern to actually produce rain?

On the other hand, in more convective situations, like the common scenario of scattered afternoon and evening showers and storms during the summertime, there is often a high degree of confidence that rain will develop, but its geographic extent of coverage may be limited such that it rains on your street, but not a mile down the road, or vice versa. In those cases, the probabiity that rain will develop may be near 100%, but because the showers may only cover a fraction of the forecast area, the probability for a given location will be less than 100%. Suppose for example that a forecaster (or a computer model algorithm) estimated that showers have an 80% likelihood of forming over a forecast area, but would only cover 50% of the area. Then the final forecast for a given location might be a 40% chance of measurable rain.

One other point about a probability like this is that it is a way of saying that the given weather pattern should produce measurable precipitation about that percentage of time that it occurs. For example, if a pattern rates an 80% probability of precipitation, you would expect that if essentially the same pattern developed on 5 different days, measurable rain would occur on four of them, a 20% probability would produce rain 2 days out of ten under similar circumstances, and so on.

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