According to the National Weather Service, only one in ten thunderstorms that form in the U.S. get strong enough to be considered severe. In Colorado, our geography often gives us the ingredients for severe weather, even in setups that wouldn't in the rest of the country. Even fewer storms become supercells - long-duration rotating storms that can produce large hail, damaging winds, and produce the majority of significant tornadoes.
Severe thunderstorms meet one or more of three criteria: wind, hail, or a tornado. Severe winds are 58 miles per hour or greater. Hail must be the size of a quarter, an inch, or larger.
Thunderstorms form in an unstable and wet atmosphere. In Colorado, good moisture typically means we have an upslope flow out of the east pulling in moisture at the ground.
On a sunny spring day, the sun heats the ground like a stove heating the bottom of a pot of water. And just like a pot of water on a stove, that warm air bubbles upward. Warm air is less dense than cold air, and wet air is less dense than dry air. So, warm and wet air rises. It rises particularly fast if the air above it is much colder and drier.
Eventually, tall clouds condense and a storm forms.
This mechanism works under most situations. But, in itself, instability won't produce severe weather. In a highly unstable atmosphere robust updrafts can form - and in some cases these can produce "pulse" severe storms. But, instability is in general not the top criteria needed for severe weather.
The key ingredient for severe weather is wind shear. This means a change in wind speed or direction with height. While there is an upper threshold, generally the more shear the better. In most parts of the country, speed shear is the most common. This is a change in wind speed with height. Most often, this occurs with a jet streak. Upper level air is moving very fast, and lower level air is moving much slower. Directional shear occurs when the wind changes direction with height. This is what occurs most often in Colorado.
Normally, a thunderstorm forms with air going upward. As it matures, rain falls back down through the clouds. This downdraft falls straight into the rising air. So, the downdraft chokes off the storm's energy and it dies.
Wind shear tilts the storm. When it starts to rain, the rain falls away from the updraft, the storm's fuel source. Since we're not cutting off the storm's fuel, it can last longer and become much stronger.
Colorado's geography plays a key role here. In spring setups, surface winds often come from the east.
But at the height of Pikes Peak, winds are almost always out of the west.
The mountains act as a wall, giving us wind shear in weather setups that wouldn't do so in the rest of the country.
This is a double-edged sword, however. In most regions, winds from the west don't prevent thunderstorm formation. In Colorado, this set up most often produces a downslope wind - resulting in dry surface air and no storms. For Colorado to get thunderstorms, we need a mechanism that pushes surface air to the west. But, when such a system exists, the storms that do form are likely to become severe due to the inherent wind shear produced from such an extreme change in wind direction with height.
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