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Name: Donald W. J.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 7/31/2004

In August of 2000 heavy rains of up to 14 inches hit the area around Sparta New Jersey. The was the result of an apparently unusual storm that hung over the area and continued to dump rain. The average monthly precipitation for the area is about 4.5 inches. My question is this: Is there any way to determine the probability that such a rain event will occur in 1 year, 5 years, 10 years, 20 years? And if so,what is that probability?

There is no way of "calculating" the probability of such an event. You can arrive at a sort of probability by looking back at the historical record of rainfall and see how often a certain rainfall occurred, but that is not a predictive tool.

Vince Calder


Commonly, the frequency of extreme weather events can be determined simply by seeing how many events have occurred in a certain period during the past. This requires digging back through the weather data of the past 100 years or so. These statistics have been compiled for most of the National Weather Service (NWS) observation stations. I recommend calling your local NWS office to see if they have this information.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

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