Heavy Rainfall Trends

http://www.wunderground.com/blog/weatherhistorian/comment.html?entrynum=298

By: Christopher C. Burt , 7:46 PM GMT on August 22, 2014

Yet another phenomenally intense rainfall event has occurred in the U.S. this morning (August 22nd) when 3.95” of rain in one hour was measured by a COOP observer at a site 3 miles southwest of Chicago’s Midway Airport. The return period for such at Midway Airport (according to NOAA’s ‘Precipitation Frequency Data Server’) is once in 500 years. This is similar to the Baltimore, Detroit, and Islip, New York events last week (although the Islip event was probably more in the range of once in a 1000 years). Brian Brettschneider of Borealis Scientific LLC has kindly offered this guest blog today featuring research he has done on heavy rainfall trends for 207 sites across the U.S. for a homogenous POR of 1949-2013.

Significant research has been conducted in recent years regarding changes in precipitation amounts and patterns in a warming climate. From a theoretical perspective, warmer air holds more moisture so increases in temperature should lead to increases in precipitation. On the flip side, increased temperatures may dry out soils and lakes (sources of moisture), cause air currents to change, or lead to other situations that counter-balance the increase in atmospheric moisture.

A chapter from the recently released National Climate Assessment discusses the trends in long-term heavy precipitation events for the entire U.S. during the last century. In particular, they note how the proportion of annual precipitation from extreme events has increased since the 1950's. The map below shows Figure 2.18 from that report. The map shows that large increases in very heavy precipitation events have been observed in the eastern half of the country.

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Using a threshold of 0.50", patterns begin to emerge. Many stations from northern Texas to the Dakotas and then eastward to include the entirely of New England saw a statistically significant increase in the number of days with at least 0.50" of precipitation. Much of the West consistently recorded a decrease in the number of days with 0.50" of precipitation but only a few stations were statistically significant.

The statistical significance pattern is even more apparent when looking at days with at least 1.00" of precipitation. Nearly 90% of stations east of the Rocky Mountains saw an increase in the number of 1.00" precipitation days and approximately half of those stations met the 95% statistical significance threshold.

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At the 2.00" threshold, the trend direction (positive or negative) and the significance levels are not nearly as distinct as they were for the 0.50" and 1.00" events. Nevertheless, a clear pattern exists in the northeastern portion of the country and a strong majority of stations east of the Rocky Mountains saw an increase in the number of days with at least 2.00" of precipitation

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We showed that the rate of small precipitation events has not changed much in the last 64 years (see Figure 5). However, when the precipitation intensity rises, so does the strength of the statistical significance. Most of the eastern half of the U.S. has experienced an increase in the number of days with at least 0.50", 1.00", and 2.00" of precipitation. The western half of the country has, on average, seen a slight decline in the rate of those precipitation thresholds when enough observations are available for analysis– but not at a statistically significant level.

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