Question: My co worker has stated that when the weather person says there is a 40% chance of rain for the viewing area, that 40% of the viewing area will get rain.
I say that when they say there is a 40% chance of rain that they are saying that over the recorded history of the date that it rained 40 times out of 100 on that day.
Answer: Actually, both of you are wrong. A probability of precipitation or POP of 40% doesn't mean that it will rain 40% of the day, or that rain will fall on 40% of your area or that it rained 40 times out of 100 on that particular day in the past (that is a climatological forecast). It also says nothing about how much rain or snow will occur.
A POP of 40% means that the forecasters have calculated that in a 100 similar weather situations, rain has fallen 40 times in the forecast area. POP is for any point in your forecast area, not the whole area. So, for instance, a POP of 90% for rain means that 9 times out of 10 when this weather situation is predicted, you ought to get rain somewhere in the forecast area, e.g., at your home, playground or at the airport.
Precipitation is often hard to forecast accurately because it can be hit and miss due to showers. One part of town can be drenched while in others people are out sunbathing. This is particularly true when showery weather occurs. Thus one part of the area gets rain or snow and another doesn't. On the other had a wide spread area of rain such as occurs with large storm systems often has a very high probability 90 to 100 percent because it is almost certain some rain will fall in the forecast area. But it may not fall at your particular location. Those are the quirks of weather that makes it so interesting. And forecasting so frustrating at times.
Learn More From These Books Chosen by The Weather Doctor
Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
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