Tornado
The United States has the most tornadoes of any country, nearly four times more than estimated in all of Europe, excluding waterspouts. This is mostly due to the unique geography of the continent. North America is a large continent that extends from the tropics north into arctic areas, and has no major east-west mountain range to block air flow between these two areas. In the middle latitudes, where most tornadoes of the world occur, the Rocky Mountains block moisture and buckle the atmospheric flow, forcing drier air at mid-levels of the troposphere due to downsloped winds, and causing the formation of a low pressure area downwind to the east of the mountains. Increased westerly flow off the Rockies force the formation of a dry line when the flow aloft is strong, while the Gulf of Mexico fuels abundant low-level moisture in the southerly flow to its east. This unique topography allows for frequent collisions of warm and cold air, the conditions that breed strong, long-lived storms throughout the year. A large portion of these tornadoes form in an area of the central United States known as Tornado Alley. This area extends into Canada, particularly Ontario and the Prairie Provinces, although southeast Quebec, the interior of British Columbia, and western New Brunswick are also tornado-prone. Tornadoes also occur across northeastern Mexico.
Tornadoes of 2015
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. 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. 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. Furthermore, 2015 was the first year since 2011 to see a total of more than 1,000 tornadoes across the United States. In synopsis, 2015 was the first average or above average year in terms of the number of tornadoes since 2011.Final totals through December 31
There were 32 tornadoes reported in the U.S. in January, of which 27 were confirmed.