It takes a thunderstorm to produce a tornado but a relatively small number of thunderstorms have all the ingredients to make a tornado.
There are four main factors that must be present for a thunderstorm to produce a tornado and these are shear, lift, instability and moisture. Meteorologists have come up with a simple acronym to remember these ingredients and that is S.L.I.M.
S in S.L.I.M. stands for shear so let’s start there. For severe thunderstorm that produce tornadoes we look for vertical speed and vertical directional shear. Vertical speed shear is a significant increase of wind speed with height. Vertical directional shear is a significant change of wind direction with height.
Vertical speed shear is the change in wind speed with height and will tilt a storm’s updraft and enforce the strength of the updraft. It also will create a horizontal rolling in the atmosphere that can become vertical and increase rotation in mesocyclones.
Vertical directional shear is the change in wind direction with height. In the example above the wind near the surface is blowing from the south to the north.
As the elevation increases the direction veers (changes direction in a clockwise motion) becoming southwest, west to northwest. These two forms of shear are key for tornado formations in thunderstorms.
Lift is the next component in the S.L.I.M. acronym and is very important to get the thunderstorms started. Sources of lift in the atmosphere can include dry lines, cold fronts, warm fronts, sea breezes and changes in terrain elevation. Gust fronts from ongoing thunderstorms can also provide lift and help to create new storms in its wake. Differential heating of the earth’s surface and convergence at the lower levels of the atmosphere are two more examples of situations that can provide lift.
Instability through a deep layer of the atmosphere is next in our ingredient list. Supercell thunderstorms can form when the atmosphere is warm and humid near the ground with increasingly cooler temperatures with height. In an unstable atmosphere a parcel of air put in motion in the vertical will continue to rise through latent heat release of condensation. Condensation heat release keeps the parcel of air warmer than the environment around it keeps the updraft of a thunderstorm strong forming the familiar tall cumulonimbus cloud that is the thunderstorm.
M is for Moisture. To produce an abundance of condensation that drives the thunderstorms you need plenty of moisture which of course is the M in our acronym S.L.I.M. This may be the most important ingredient. One of the main sources for this moisture that feeds thunderstorms is the Gulf of Mexico. Many times and especially during the spring months strong low pressure systems will move west to east pulling this key ingredient in off the Gulf. This source can provide an abundance of moisture and synoptic scale storms can spread this moisture north well into Canada.
Shear: The changing of wind with height, direction or speed.
Lift: Components that lift an airmass.
Instability: Warm at the surface and cooling above.
Moisture: Higher amounts of humidity. The fuel of thunderstorms.
These four ingredients (S.L.I.M.) are key when predicting thunderstorms and tornadoes and are main parameters we start to look at when forecasting severe weather.
For Weather Nation: Meteorologist Mike Morrison