Throughout much of the first half of 2015, tornado activity was near record low mostly due to a continuous pattern of a trough in the east, which brought colder than average temperatures there, and a ridge in the west, which brought warmer than average temperatures in the west. The pattern changed, slightly, in late March and early April to allow for some severe weather.[8] During May, a few large tornado outbreaks occurred, which led it to be the most active tornado month since April 2011, and the most active May since 2008.[9] Due to a significant El Niño event, November and December were extremely active months with over 20 fatalities in December alone. 93 tornadoes were reported in December, well above average for that month.[10] Furthermore, 2015 was the first year since 2011 to see a total of more than 1,000 tornadoes across the United States.