Thursday, January 10, 2013

"Wake low" wind causes power outages in three states

Though it didn't necessarily form in the classic sense, a "wake low" developed this afternoon and moved across north MS. The northern edge of the feature brushed the Memphis metro.  So what is a wake low and what did it do?

Wake lows occur infrequently and bring strong and gusty wind to the area affected.  In some cases, high wind of 60 mph or more can result from the sinking air associated with these mesoscale (small-scale) features.  They typically trail precipitation areas, most of the time forming on the back side of a "meso-high," or small area of high pressure that forms behind a squall line or decaying thunderstorm area.

In today's case, though we had precipitation that was moving out about the time the low formed, there was no squall line to trigger the formation of a meso-high.  The small low pressure area is formed when air rises into a precipitation area, leaving a "hole" of lower pressure in it's wake. The air that is forced upwards then descends back down into the trailing low pressure area, trying to achieve equilibrium.  The air can descend fast enough to hit the ground with some force and spread out, creating a high wind event.

Diagram of a wake low. Credit: UCAR.
In general, wind speeds in the metro were in the 30-40 mph range. According to MLGW, as many as 7,400 customers were left without power following the strong wind that occurred between 3:30-4:00pm this afternoon. Trees and power lines were down and some poles were toppled or shifted due to saturated ground from the 1.5-2.0" of rain we have received in the past 36 hours, further complicating restoration efforts.

The wake low has continued to move across north MS into central AL.  At 5:15, Tupelo law enforcement reported trees and power lines down and as of 8:30pm reports of wind gusts up to 60 mph have been received from Tuscaloosa, AL with numerous power outages and blown transformers (credit: NWS).

A previous MWN Blog post on the topic from July 2010 includes radar imagery of the wake low, which was much more pronounced due to the presence of precipitation with it.  Damage was also reported with that event.

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