Tuesday, July 27, 2010

"Wake low" impacts areas north of Memphis with damaging wind

A line of thunderstorms moved south to north through the Mid-South and Memphis area late last evening. The line, termed a mesoscale convective system (MCS), was trailed by light to moderate steady rain. It reached the TN/MS state line around 10pm and continued north, collapsing after 11pm into an area of rain.

While Severe Thunderstorm Warnings were issued for the main line of storms as it moved through the metro area, the more interesting weather phenomena that occurred last night was the formation of a wake low. A High Wind Warning was issued around midnight for counties north of greater Memphis as a result of damaging wind of 40-60 mph that occurred from this phenomena. Numerous trees were downed across Lauderdale County.

So what is a wake low? According to the AMS Glossary, it is "a surface low pressure area or meso-low (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." This setup is what happened last night - as the squall line collapsed after passing through the metro area, downward moving air warmed and formed a small-scale (mesoscale) low pressure system, which, in combination with a small-scale high pressure system directly behind the squall line, created a pressure gradient that supported the development of high wind.

Radar data clearly show the presence of this wake low with wind data (shown in the first loop below) depicting 40 mph+ wind over the affected area and precipitation data (second loop below) showing a lack of rainfall in the area. The descending air (subsidence) creates clearing conditions in the vicinity of the wake low, so a lack of rainfall is expected (as opposed to a severe thunderstorm which can create high wind due to descending air from the core of the thunderstorm).

More information on wake lows can be found on Wikipedia. In addition, a damage survey from north Alabama in April 2009 also describes the formation of a wake low that brought damaging wind to the area.

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