The importance of rotating updrafts in severe weather

TheWxMeister Wonders

This timelapse of a rotating thunderstorm updraft over Castle Rock was taken on Friday by local resident Steve Davis. It’s an excellent example of what wind shear in the atmosphere will do to a thunderstorm updraft.

Wind shear is defined as wind moving at different speeds and/or directions in different levels of the atmosphere. On this day, wind in the lower levels was from the southeast. The wind in the mid and upper levels of the troposphere was faster and from the southwest.

The amount of wind shear present in a thunderstorm environment is of interest to forecasters and storm chasers because wind shear allows the storm updraft to tilt, separating the updraft and downdraft. This allows a thunderstorm to sustain. Without wind shear present, updrafts collapse back upon themselves.

A thunderstorm updraft that develops in a non sheared environment will collapse upon itself as a downdraft develops with precipitation production.

As the timelapse begins, you’ll note that the storm has a well-defined base. The base of the storm has a bell shape, as is often observed with storms that have a meso-cyclone, which is the meteorological term for an organized rotation on the storm scale.

Meteorologists and storm chasers use slang to describe the barber pole look that individual updraft pushes will take when a storm is rotating. Rotating storms are of interest because the rotation allows the updraft and downdraft wind currents to become separate and sustained.

Storms that exhibit meso-cyclones for extended periods of time are called supercell thunderstorms. While supercells only make up a small percentage of total thunderstorms that occur across the earth, the produce a remarkable amount of the severe weather that occurs (tornadoes, large hail, damaging straight-line wind).

This image shows identifiable features often observed with supercell thunderstorms. (Courtesy: National Severe Storms Laboratory)

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