Monday, September 1, 2014

Gravity waves from Missouri storms result in Labor Day showers in the metro

Pop-up showers around mid-day today were a bit of a surprise, as they were not indicated by computer models early this morning and, according to our forecasts over the weekend, Labor Day appeared to be a dry day. However, an event currently not easily "forecast-able" by the models was the cause for these showers - a set of gravity waves!

Sometimes these atmospheric waves can be detected in radar imagery, but if conditions are just right, they are more easily detected using visible satellite imagery, as was the case this morning. The image below, taken at 8:30am, shows an arc of clouds from OK through AR, into western TN and KY. This arc is the outflow boundary from the MO storms. However, the outflow was accompanied by a series of gravity waves as well, most clearly visible in OK in the circled region (the technical term for this phenomena is an undular bore). Click here for an animation of this satellite imagery clearly showing the waves propagating away from the storms between 7-10am CDT (animation download is large and recommended for high-speed internet users). Also, the waves can be faintly seen in this (large) radar loop from about 6am-10am this morning around Oklahoma City, Little Rock, and over east AR from the Memphis radar.

An outflow boundary from decaying thunderstorms in KS/MO moved across OK, AR, TN, and KY this morning as shown in the satellite image taken at 8:30am. Gravity waves were also present behind this outflow, best seen in the circled area in OK. Click here for a satellite loop clearly showing the waves propagating away from the storm complex (Warning: VERY large animation - best with high-speed connections).

So what conditions were responsible for the waves that occurred this morning and why did showers result in the Mid-South?

Just as when a pebble is thrown into a calm body of water, waves in the atmosphere spread out from a large disturbance such as a thunderstorm complex. In meteorology, these waves are called "gravity waves" and they form when the wave(s) are trapped in a stable layer of the atmosphere, rising like the crest of a wave on the water, then being pulled back towards earth by gravity. As long as the stable layer of air exists, the waves continue to propagate.

The waves are visible on satellite imagery when they move through an area of saturated air that exists in or just below the stable layer. Rising air from the wave causes clouds to form in the saturated air and sinking air promotes drying or clear skies. The alternating rising and sinking air result in cloud bands that form perpendicular to the direction the wind is blowing at that level (transverse bands). Satellite imagery is the best way to see these bands of clouds that are the result of the gravity waves!

The rising air from the waves can also be strong enough to promote precipitation from the resultant clouds, especially when they encounter less stable air, as was in place over the Mid-South at mid-day today. The showers were the result of the rising air in the gravity waves. Since we didn't know these waves would occur, nor that they would arrive as the atmosphere was destabilizing, we didn't have a good handle on the formation of showers either. As the waves dissipated, the showers ended by early afternoon.

Erik Proseus, MWN Meteorologist

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