Saturday, April 30, 2016

"Wake low" causes wind damage across the metro Saturday morning

For the second day in a row, a line of thunderstorms dissipated over the metro region. Both times a 'wake low' formed over the area. Saturday morning's was much stronger and produced the most impact. It made its appearance known in the form of high wind that occurred between about 10am and noon, starting in far eastern AR and making its way slowly east across the city and into the eastern suburbs.

So what is a wake low? According to the American Meteorological Society, a wake low is defined as:
"A surface low pressure area or mesolow (or the envelope of several low pressure areas) to the rear of a squall line; most commonly found in squall lines with trailing stratiform precipitation regions, in which case the axis of the low is positioned near the back edge of the stratiform rain area."
In other words, sometimes trailing a squall line a mesoscale (small scale) low pressure system will form. Typically it will trail a mesoscale high pressure system than forms immediately behind the squall line. In many instances, the meso, or wake, low will be aligned along the back edge of stratiform (steady rain) precipitation trailing the line of storms as air sinks, thus warming and also drying the back edge of precipitation. The pressure gradient between the meso high and meso low creates a scenario in which a localized area of high wind can develop. (Weather geeks will recall that wind increases when the gradient between two pressure systems increases. A strong high pressure system in close proximity to a strong low pressure system produces very strong wind.)

A wake low can form on the back edge of an area of precipitation due to sinking air that warms as it sinks. The pressure gradient between the meso high (shown as the 'cold pool' above) and the trailing low can contribute to increased wind speeds between the high and low.
In today's case, there was about a 3 millibar pressure difference between Memphis International Airport and West Memphis airport, a fairly large difference over such a short distance. The result was peak wind in the 50-55 mph range in many areas around the metro, including Tom Lee Park, where Beale Street Music Festival infrastructure was setup (though patrons were not in the park). Damage at Tom Lee Park included tents, trash cans, and many other loose objects overturned and blown apart and debris scattered throughout the park, which caused a 2 1/2 hour delay in opening the festival and 4 cancelled musical acts. There were also many reports of trees and power lines down throughout the Bluff City and into DeSoto County with over 22,000 customers without power on the MLG&W grid at one point.

Below are some pictures submitted to us on Twitter, as well as a 6 hour radar loop from 5-11am showing the decay of the squall line. The wake low formed in the last hour of the loop on the back side of the light precipitation.

Erik Proseus
MWN Meteorologist

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