According to the Marysville Public Works Department, the city received 2.61 inches of rain in the first 65 minutes. At its peak, the storm cell produced rainfall of 2.48 inches per hour. The total from the storm was 3.40 inches.
Public Works Director Kevin Nielsen called the storm “extremely unique.” The rainfall in those two hours was equivalent to a so-called 100-year event, which is defined by the amount of rain that falls in 24 hours, Nielsen said. A 100-year event has an estimated 1 percent chance of happening in a given year, or is expected to occur once every 100 years.
A 100-year rain event for Marysville, as calculated by the state Department of Ecology, would be 3.36 inches — in 24 hours.
The National Weather Service agrees that the Marysville deluge was particularly unique in Western Washington.
“Very few rainfalls during this time of year are above an inch,” said meteorologist Johnny Burg of weather service's Seattle office. “Usually rainfalls in May are between one-quarter to three-quarters of an inch.”
The storm sewers in Marysville are designed to accommodate a 25-year, 24-hour event, which the state estimates would be 2.75 inches of rain in 24 hours.
Evidence suggests that the trend in Western Washington is for more high-intensity, shorter-duration rains in spring and fall in recent years, Nielsen said.
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