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Severe Weather in Colorado

The spring months of May and June typically bring most of the severe weather to Colorado’s Front Range.  We have already witnessed with many hailstorms so far this year and an unforgettable tornado that tore through the town of Windsor.  Why does most of the severe weather occur in the spring versus the summer?

There are a couple reasons why the spring is more active with severe weather than during the rest of the summer months.  One reason is that cold air still remains above the surface as we transition out of winter into spring.  The cold air aloft helps create severe weather in a couple ways.  First, the cold air helps de-stabilize the atmosphere as warm air at the surface wants to rise to form thunderstorms.  The ground warms rapidly with the increasing sunshine during the spring months with the longest day of the year being the first day of summer or the summer solstice which occurs in June.  Second, the cold air allows hail to form in the updrafts at lower elevations in the cloud which makes the hail stones larger and more likely to reach the surface.  Many summer storms contain hail as well, but the hail travels through a larger warm layer and often melts before reaching the ground.  A second reason is wind shear or changes in wind speed, or direction, or both with height in the atmosphere.  In the spring the jet stream can still be rather active bringing disturbances to Colorado area before it retreats northward during summer.  The stronger winds aloft associated with the jet stream can be tapped into by strong thunderstorms.  Typically, but not always the strongest winds aloft are from the southwest ahead of an approaching trough of low pressure.  Winds at the surface often vary from the upper level winds by both speed and direction.  This in turn causes the wind shear with height in the atmosphere.  When a strong thunderstorm develops in this these conditions the updraft carrying moisture laden air from the surface upwards can rotate.  Storms with a rotating updraft are called super cell thunderstorms, like the storm in May that resulted in a strong tornado that devastated the Windsor area.  The rotation of the updraft can be concentrated by stretching the column of air like a figure skater spinning faster and faster as she brings her arms into her body.

There area many other factors that come into play when it comes to severe weather, but cold air aloft and wind shear are two of the major players when it comes to spring severe weather in the state of Colorado.  As we move into the summer months the jet stream weakens and travels north impacting the state less often and reducing wind shear.  As the overall atmosphere warms in the northern hemisphere during summer the air aloft is just not as cold near the surface allowing hail to fall as large rain drops instead of frozen precipitation.  Hail still quite often occurs here in Colorado during the summer months due to our elevation above sea level, but is typically smaller in size and more isolated.