How rare was yesterday's flood in Greeley?
Greeley picked up about a quarter of its typical annual rainfall in about an hour on Thursday. DENVER  There are afternoon thunderstorms, and then there are truly incredible storms like what Greeley saw on Thursday afternoon. Based on several weather station observations from Thursday afternoon, at least three different stations in Greeley saw over three inches of rain in just a few hours on Thursday, leading to widespread flooding. While Greeley's official weather observation site at the University of Northern Colorado (UNC) saw only slightly over an inch of rain yesterday, three different volunteer-run sites on the west side of the city each reported at least 3.3 inches of rainfall. Three-plus inches of rain in a single day (or, in this case, about an hour and a half) is a rare event in Colorado. In fact, it's exceptionally rare. Consider the fact that Greeley averages only about 14 inches of rain a year. That means Greeley picked up about a quarter of its average annual precipitation in about 90 minutes early on Thursday afternoon. That's also more than double Greeley's full average July rainfall of 1.5 inches. In fact, the last time that Greeley picked up 3.3 inches of rain or more on a single day was on July 13, 2001, almost exactly 20 years ago to the date. That's based on data from Greeley's climate site on the UNC campus, which again, did not see 3.3 inches of rain yesterday. But the hyper-local nature of yesterday's rain meant that totals varied widely over extremely short distances, including from one side of a city to another. And again, it was the west side of Greeley that had three reports of 3.3 inches or more of rainfall. Compare that to other similar, nearby climate sites. Based on observations from Denver's old Stapleton Airport site, Denver's only seen 3.3 inches or more of rain in a day four times since 1948, making it a roughly 1-in-18 year event. Cheyenne, Wyoming has only seen three days with 3.3 inches of rain or more in its 150-year climatological history, amounting to a roughly 1-in-50 year type of event. Denver and Cheyenne both average about 14 inches of precipitation a year and have nearly identical climates to Greeley.