When forecasting severe weather, meteorologists across the country will often refer to the acronym S.L.I.M. Analyzing these variables helps to predict the severity and kind of thunderstorms expected.
Shear
This refers to wind shear, or how winds change in speed and direction. When winds turn clockwise with height, this can allow for rotating supercell thunderstorms. A supercell is the first step in tornado formation. When winds are increasing in speed with height, this will help to sustain a thunderstorm updraft for longer. This can aid in forming large hail.
Lift
Often, air needs a bit of a push before convection begins and air rises freely. This can happen from a boundary, like a cold or warm front or dryline. The jet stream or incoming low-pressure system can also supply lift. And even more common in Colorado is winds being forced upslope by terrain. Upslope winds are the little push that warm, humid air needs to bring near-daily thunderstorms to the front range in late spring and early summer.
Instability
Unstable air will rise freely, condensing into clouds. This can also be thought of as buoyancy, like letting go of a volleyball at the bottom of a pool. Very unstable air can create explosive thunderstorm development, seen visually as bubbly cauliflower-like cumulus clouds.
Moisture
Colorado is a high elevation semi-arid climate that generally needs moisture transported in from somewhere else. SE winds will bring in humid air from the Gulf of Mexico. In addition, late summer can bring in Pacific moisture from the North American Monsoon pattern.
Colorado magic
Colorado is a unique place to forecast severe weather, with terrain and elevation being huge factors in storm development. The high elevation of the state means the freezing level in the atmosphere is relatively low, allowing for hail development even in the weakest of thunderstorms. And the complex terrain of the state manipulates air by forcing it up or down, depending on wind direction. Upward motion tends to trigger condensation and cloud formation. Downward motion tends to compress and dry out the air.
