Andy Kula, a friend and great meteorologist at the NWS office in Huntsville brought this to my attention, so I looked into it very closely. The area of thunderstorms over Mississippi that eventually moved into west Alabama last night was emitting gravity waves out the top, that were causing up and down motions in the high clouds around the tops of the thunderstorms. Notice the high freqeuency, short waves around the center of the storms in western Mississippi, with lower frequency, longer waves farther east. The waves show up nicely on this visible satellite image, basically a black and white photo from space. Very cool.
This shows a time-height section (from 6 pm to 10 pm, surface to about 50,000 feet) of the vertical motion in the atmosphere detected by the NOAA wind profiler at Okalona, MS (near Columbus Air Force Base). You can see the high clouds ahead of the storm moving in before the rain, and oscillations of upward and downward motion near the top of the clouds, associated with the gravity waves.
As the storms moved into Alabama and weakened some, two things happened. First, the earth’s rotation started to affect the wind around the storms, causing a circulation (known as an MCV, mesoscale convective vortex). You can see it on the radar loop below, west of BHM.
(Helena pressure)
As the cool air from the storms interacted with the stable air over central Alabama, a low-level gravity wave formed near the leading edge of the old rain area. It passes over the radar, and the weather observer at Helena’s pressure jumped quickly up and back down in 30 minutes. The pressure change was only about 0.04″, nothing like the 0.30″ to 0.40″ changes we saw with the wind-producing gravity wave in April.
For your meteorological consulting needs, Coleman and Peters, LLC can provide you with accurate information on past hailstorms, lightning, flooding, and wind damage. We work primarily for people involved in insurance cases and for attorneys. Please call us at (205) 568-4401.
