Tornado activity highest during late spring/summer in Minnesota, why?
MANKATO, Minn. (KEYC) -
On average 1,253 tornadoes touch down per year across the United Sates (based on data from 1991-2010). Tornadoes can develop in all months throughout the year. During the winter months, tornado activity will be highest across the Deep South. During the warmer months of Spring and Summer the threat of tornadoes begins to transition into the Great Plains and parts of the Midwest. As autumn sets in the tornado threat retreats back to the Deep South.
For Minnesota, the most active time for tornadoes is from late spring through the summer months of June, July, and August. Here’s why.
In order for a tornado to develop it needs a parent thunderstorm. There are three ingredients that must be present for thunderstorms to develop. They are moisture, instability, and lift. A fourth ingredient needs to be present for thunderstorms to have the potential of producing a tornado and that is wind shear.
Moisture is a general term referring to the water vapor content in the atmosphere. The warmer it is the more moisture that atmosphere can hold. Dew point temperature is a measurement of how much moisture or water vapor is in the air. Surface based thunderstorms require a dew point of at least 55 degrees Fahrenheit or higher. Anything less would prompt no thunderstorm activity or high based thunderstorms, which are not likely to produce tornadoes.
Instability is dependent on moisture. Higher low-level dew points and warm temperatures help increase instability. Instability causes air to accelerate in the vertical, helping the development of towering cumulus/cumulonimbus clouds. Convective Available Potential Energy or CAPE is a measurement of how much instability is available in the atmosphere and is in the units of Joules per kilogram. CAPE value of 1,500 J/kg is large with values above 2,500 J/kg being extremely large instability. Large CAPE values also indicate the potential for larger hail.
Lift is very important because without lift all you have is a very warm and humid day. Forcing allows for air in the low levels to overcome low level convective inhibition and reach a level of free convection or LFC. At this point a surface parcel will no longer need forcing and will continue to rise as long as the parcel is warmer the surrounding environment. There are many forms of lift in the atmosphere the most common are fronts, low level jet, low level warm air advection, vorticity, and jet stream.
A thunderstorm is likely to become severe and capable of producing a tornado when wind shear is added. Wind shear is the change in wind speed and/or direction with height. Wind shear is crucial in development of stronger thunderstorms for a few reasons. One, it helps tilt the storm displacing the updraft from the downdraft, as a down draft is filled with cool air which can kill a thunderstorm. Two, it helps updrafts sustain a longer life and continues thunderstorm development. Three, rotation within a thunderstorm can help in the development of a mesocyclone an area where tornadogenesis occurs.
Back to Minnesota
During the warmer months of late spring into summer, Minnesota has a perfect atmosphere suitable for the development of tornadoes. As temperatures warm up the atmosphere can hold more moisture. As the crops continue to grow, more moisture is added to the atmosphere through evapotranspiration. The average dew point for southern Minnesota (in the Mankato area) during the summer months of June, July, and August is around 60.8 degrees Fahrenheit. Enough moisture to support surface-based thunderstorms.
The warmer summer months plus higher moisture values mean higher instability. Minnesota being a higher latitude sate means that we are closer to the cooler air to the north. Any dip (trough) in the upper-level jet will cause a clash between the cool air to the north and the warm moist air in place. This interaction between air masses is the lift needed for thunderstorms to develop.
If the upper-level jet is strong and there is a low near the surface and/or strong low-level jet, wind shear will be present aiding in the development of severe thunderstorms capable of producing tornadoes.
On average, Minnesota has about 45 tornadoes a year with the month of June being the highest averaging around 18 (based on data from 1991-2010).
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