S.L.I.M – The Ingredients Needed For Severe Weather

Or M.I.L.S If You'd Like... Or Even L.I.M.S (Without The B)

Winter weather is now coming to an end, minus the occasion spurt of ice or mixed precipitation as the jetstream continues to move both north and south. The end of winter weather marks the beginning of my favorite season, personally, which is severe weather. From hail and tornadoes to flash floods and wind, severe weather can come in many different forms but severe weather needs, generally, four key ingredients.

Wallcloud over Indianapolis, Summer 2014

Meteorologists have come up with a simple acronym to remember what is needed to create a severe thunderstorm – S.L.I.M. We’ll take a look at each one in this weather blog and explain why each one is just as important as the next and why some of them are connected with another, and we’ll start with the most important, in my opinion, M.

                                                Surface Dewpoints on Thursday Afternoon

Clouds, rain, hail and snow are all formed by the most important ingredient needed to create severe weather – moisture. Whether it’s being brought up from the Gulf of Mexico, the Pacific Ocean or transported over the Rockies, moisture is all around us… even in the winter when our dew points are very low. From the surface all the way up to the thermosphere (that’s on the edge of space) there’s moisture however, the farther up you go the less amount of moisture the atmosphere can hold. On a clear day, if you see some thin, whispy clouds, those are very high in the atmosphere and are basically ice but still moisture nonetheless.
Moisture is crucial to cloud development. Moisture particles develop on tiny particles like dust and, over time, as more and more particles form, clouds develop. When those particles get too heavy… they fall as rain, snow, hail or mixed precipitation. When plenty of moisture is brought in from the Gulf of Mexico our next ingredient is also brought up as well.

                                                     CAPE and CIN (blue) over Midwest on Thursday 

Like you and I, storms need energy… not only form but to also maintain themselves over time. Called CAPE, convective available potential energy, and measured in joules per kilogram (j/kg), the energy that storms need can be found in unstable airmasses and can be produced by the sun as well. Whether it’s a few 100 j/kg or thousands, a relatively small amount of CAPE is needed to produce thunderstorms as well as tornadoes however, the more CAPE that is available to storms the stronger they can be. That’s if, however, they can overcome a cap or CIN, convective inhibition.

Think of CIN like a soda bottle cap, trapping in the carbonation. That carbonation builds up over time, especially if you shake it, and once the CAP is broken all that energy is then free to be used. A cap can be both good and bad when it comes to severe weather; a strong cap can potentially stop severe weather from forming, if the air can not break it, or it can bottle all the energy up to be unleashed later. One way that the CAP can be broken through the next ingredient in our acronym.

Whether it’s S.L.I.M or M.I.L.S or even L.I.M.S our next ingredient takes the moisture and the CAPE and produces a thunderstorm. Rising air is how the atmosphere produces clouds and severe weather while sinking air, typically found with high pressure systems, suppresses cloud development. A source of lift is crucial for severe weather and, interestingly enough, that source of lift can range from fronts, like a cold front, warm front or even a dry line, to vorticity and topography! Large enough hills and mountain ranges can force air to rise over it.
When a source of lift, like a cold front (which is actually a very strong forcing mechanism) interacts with moist, unstable air, it’s forced to rise through the atmosphere. As the moist, unstable air lifts higher and higher the moisture condenses and forms clouds while the CAPE is used to produce lightning and tornadoes. But before we get tornadoes, we need our last ingredient. We need our atmosphere to turn.

That’s where shear comes in (not the tool used to trim sheeps wool). Defined as the turning of winds in the atmosphere, that turning of the winds will take an ordinary thunderstorm and turn it into a severe thunderstorm.
Each storm has an updraft, where new, untouched air, is fed into the storm while the downdraft is where the used air is discarded (usually on the backside of the cell). When an ordinary thunderstorm enters into an environment where there’s a good amount of shear, or rotating winds, throughout the entire atmosphere that storms has a good chance of becoming a severe thunderstorm. Now that ordinary thunderstorm has a rotating updraft that is able to suspend rain particles, hail (strong updrafts have been credited for some very large hail) and can sometimes extend to the ground and produce a tornado. Shear in the lower levels helps provide the turning needed for the tornadoes while the shear in the mid and upper levels help support the rotating updraft. In some of the stronger storms you can even see that rotation by striations seen on the clouds.

                             Supercell Thunderstorm in Oklahoma, via Wunderground.com

It’s here that it should be noted that during severe weather events, tornadoes like to form in thunderstorm that are alone. An individual cell does not have to worry about other storms updrafts and downdrafts instead, the individual cell will have untouched air instead of having to “fight” for it from other cells. While a tornado warnings sometimes occur when there is a line of thunderstorms, they’re sometimes called gustnadoes and only last a few seconds versus long lived tornadoes that are found in individual cells. That’s not to say that you shouldn’t take shelter when a tornado warning is issued with a line of storms (because you should) rather, I explained that so you have a little bit more information when it comes to severe weather in this upcoming season.

There are the main four ingredients needed when it comes to producing a severe thunderstorm and those four ingredients form many different parameters meteorologists look at when forecasting and covering a severe weather event. As we continue to make our way into and through the 2017 severe weather season, make sure you have liked the KDLT Weather page on both Facebook as well as Twitter and followed the individual meteorologists for weather updates. You’ll also want to tune into KDLT News during the last week of April during Severe Weather Awareness Week when we go over all topics like lightning, tornadoes, floods as well as the difference between a watch and a warning.

Categories: Weather, Weather Blog