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occur over a long period, can have significant impacts where the contributing drainage area to a lake or <br />pond is large and the outlet is small. <br />Table 2-2 Selected Rainfall Events Used for Design Purposes <br />0000000000000000000000000000000000000000000000000000000000000000000 <br />IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII <br />00000000000000000000000000000000000000000000000000000000000000000000uIIIIIIII <br />�IIIIIIIIIIIIIIIIIIII <br />„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„„,„„„„„„„„„„„„„„„„„„„„„„, Donvoio 9,i000000001,1,1000000000000000000000000000000000000 <br />2-year <br />0000l00000u00000000u000l00000t0000 <br />111111111III 00 100 <br />111111111111111111111111111111111 <br />111111111111111111111111111111110000000000000000000 <br />24 hour <br />Source: NOAA Atlas 14 — Volume 8. Station: Saint Paul — Station 21-7377. <br />These depths reflect the 50% exceedance limit. <br />There are typically wide variations in climate conditions in the District. However, climatologists have found <br />four significant recent climate trends in the Upper Midwest (NOAA, 2013): <br />• Warmer winters decline in severity and frequency of severe cold; warming periods leading to <br />mid -winter snowmelt <br />• Higher minimum temperatures <br />• Higher dew points <br />• Changes in precipitation trends — more rainfall is coming from heavy thunderstorm events and <br />increased snowfall <br />According to NOAA's 2013 assessment of climate trends for the Midwest, annual and summer <br />precipitation amounts in the Midwest are trending upward, as is the frequency of high intensity storms. <br />Annual precipitation at the University of Minnesota -Saint Paul averaged 35.9 inches from 2010-2019, a <br />3.7 inch increase over the 1981-2010 climate normal. Annual precipitation exceeded the 1981-2010 <br />climate normal average (32.2 inches) in 7 of 8 years since 2010. <br />Higher intensity precipitation events typically produce more runoff than lower intensity events with similar <br />total precipitation amounts; higher rainfall intensities are more likely to overwhelm the capacity of the <br />land surface to infiltrate and attenuate runoff. Climate normal data available from the Minnesota <br />Department of Natural Resources indicates annual precipitation is increasing within the Minneapolis -Saint <br />Paul metropolitan area by approximately 0.31 inches per decade (see Figure 2-1). <br />2-3 <br />