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Breakdown: Why the threat of damaging wind increases in June

Updated: Jun. 6, 2021 at 7:16 PM CDT
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MEMPHIS, Tenn. (WMC) --While the tornado threat goes down during the summer months, the threat of storms containing damaging wind can increase. The type of storms that we see more frequently in June and July are called an MCS or Mesoscale Convective System. A Mesoscale Convective System (MCS) is a cluster of thunderstorms that act as a single system. An MCS can spread across an entire state and last more than 12 hours. On radar they can appear as a solid line, a broken line, or a cluster of cells.

MCS’s are usually triggered by a weather element that can provide rising air in the atmosphere like an area of low pressure. It will develop in the afternoon when daytime heating is at the peak and when the atmosphere is more unstable. This intial stage is when the MCS has the most energy.

The cluster of storms can become more widespread or numerous as the cluster grows but as it grows, the severity tends to lessen. However, they can produce significant rainfall, gusty winds especially along the leading edge and frequent lightning. The storm cluster or MCS can remain intact through the evening and nighttime hours.

Cooler air near the ground at night can help intensify a jet stream much lower to the ground, only 1,500 to 3,000 feet, this low-level jet will ride atop cooler air near the ground, which then feeds storms humid air to a developing MCS.

As the low-level jet collides with the rain-cooled outflows from thunderstorms that formed earlier in the night, new thunderstorms will form and the process will continue over and over again.

Some MCS can be strong overnight thanks to the increased instability from the cooler air at the cloud-top level. As the clouds lose energy into space, there is no solar radiation absorbed, that along with heat giving off from condensation of water vapor into the clouds.

This type of system is common in many parts of the world, including the central and southern United States.

While MCS can produce hail and tornadoes they most often produce heavy rain, lightning and gusty wind. Damaging winds can occur when a MCS is pushed by a more strong upper level jet stream. When this happens, straight-line winds can down trees and power lines, in the heat of the summer.

Other type of convective storms are:

Mesoscale convective complex (MCC)—A particular type of MCS, an MCC is a large, circular, long-lived cluster of showers and thunderstorms identified by satellite. It often emerges out of other storm types during the late-night and early-morning hours. MCCs can cover an entire state.

Mesoscale convective vortex (MCV)—A low-pressure center within an MCS that pulls winds into a circling pattern, or vortex. With a core only 30 to 60 miles wide and 1 to 3 miles deep, an MCV is often overlooked in standard weather analyses. But an MCV can take on a life of its own, persisting for up to 12 hours after its parent MCS has dissipated. This orphaned MCV will sometimes then become the seed of the next thunderstorm outbreak. An MCV that moves into tropical waters, such as the Gulf of Mexico, can serve as the nucleus for a tropical storm or hurricane.

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