|
NSSL
Tornado Climatology
Where and when do tornadoes occur?
Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year. Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh. Both have similar topography with mountains helping catch low-level moisture from over Brazil (Argentina) or from the Indian Ocean (Bangladesh). About 1,000 tornadoes hit the U.S. yearly. Since official tornado records only date back to 1950, we do not know the actual average number of tornadoes that occur each year. Plus, tornado spotting and reporting methods have changed a lot over the last several decades.
Tornado season usually refers to the time of year where the U.S. sees the most tornadoes. The peak "tornado season" for the southern plains -- often referred to as Tornado Alley -- is during May into early June. On the Gulf coast, it is earlier during the spring. In the northern plains and upper Midwest, tornado season is in June or July. But, remember, tornadoes can happen at any time of year. Tornadoes can also happen at any time of day, but most tornadoes occur between 4-9 p.m.
Tornado Alley is a nickname for an area that consistently experiences a high frequency of tornadoes each year. The area that has the most strong and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and eastern Colorado. The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."
The dryline is a boundary separating hot, dry air to the west from warm, moist air to the east. You can see it on a weather map by looking for sharp changes in dew point temperatures. Between adjacent weather stations the differences in dew point can vary by as much as 40 degrees or more. The dryline is usually found along the western high plains. Air moving down the eastern slopes of the Rockies warms and dries as it sinks onto the plains, creating a hot, dry, cloud-free zone. During the day, it moves eastward mixing up the warm moist air ahead of it. If there is enough moisture and instability in the warm air, severe storms can form - because the dryline is the "push" the air needs to start moving up! During the evening, the dryline "retreats" and drifts back to the west. The next day the cycle can start all over again, until a larger eather system pushes through and washes it away.
Tornadoes kill an average of 60 people per year, mostly from flying or falling debris. The Tri-State Tornado of March 18, 1925 was the deadliest tornado in history, killing 695 people. It is also the longest tornado track ever known - 219 miles - across parts of Missouri, Illinois and Indiana.
Codell, KS was struck by a tornado on May 20 three years in a row: 1916, 1917, and 1918.
Understanding the threat posed by tornadoes in the United States - particularly the threat of strong and violent tornadoes - is valuable knowledge to everyone, but especially to weather forecasters and emergency management people. Knowledge about long-term patterns helps us be better prepared for natural disasters and could also help scientists detect shifting patterns in severe weather events caused by climate change.
SOURCE: NSSL
----------------------------------------------------------------------------------
Map indicating highest concentration of sig. tornadoes in the US for late April (DFW Tours)
Map indicating highest concentration of sig. tornadoes in the US for late May (OKC Tours)
Map indicating highest concentration of tornadoes in the US for June (DEN Tours)
Other Tornado Climatology Facts
More tornadoes occur in the US than any other country in the world.
More tornadoes occur in Texas than any other state.
More tornadoes have occurred in Tarrant County, Texas (home of Tempest Tours) than any US county since 2000.
The peak of tornado season for North Central Texas occurs in early May.
Dallas-Ft. Worth is the largest metropolitan area in Tornado Alley.
The highest average annual concentration of violent-class (F4-F5) tornadoes occurs between Dallas-Ft. Worth and Oklahoma City.
The peak of tornado season for Central Oklahoma occurs in mid to late May.
The peak of tornado season in Northeast Colorado occurs in late June/early July.
June is the most active tornado month of the year for Northeast Colorado, Nebraska and the Dakotas.
Northeast Colorado is home to the highest concentration of tornadoes in the US for June.
More tornadoes occur in Weld County, just north of Denver, than any other county in Colorado.
|
|
