14 April 2009
It sounds like something right of an episode of Doctor Who. High winds arrive suddenly, and disappear 4 hours later in a flash. That’s just what happened over North Alabama Sunday night. Sonic screw drivers, and a Tardis might help explain it, but I have a pretty good idea, just the same. I can promise that the Daleks had nothing to do with it. (Although I would not put it past the Cybermen).
So what did happen to cause roaring winds of up to 70 mph that knocked down trees and power lines across 5 counties? Simply put, mother nature through a rock in the pond!
The heavy rain Sunday evening was the key, but you may have noticed that the winds followed, behind the rain. This is the big clue. It’s rare, but sometimes a large band of convection can act a barrier to the winds aloft.
As the wind flows over the thunderstorms, a wave develops in the atmosphere. Just like any wave, it has a high point and a low point. As the wind flows to the lowest point, it can reach the surface. This can bring the strong winds aloft, down to the ground.
John McLaughlin of KCCI TV in Des Moines Iowa captured a gravity wave event on his stations tower cams, a few years ago. Take a look. You can clearly see the waves.
There is growing interest in understanding these events, and in the operational forecasting of them.
Here is what we do know.
They tend to occur, when there is a stable layer of air near the ground, and unstable air aloft. Meteorologists call this elevated convection. IF, the depth of the stable layer is just right, then a wave can set up, that accelerates winds, in this stable layer, toward the surface.
Clues to an event include a jet stream pattern, with a trough to the west and a ridge to the east. A warm front, is usually located to the south of the area, affected by the gravity wave. A Wake low also develops behind the rain bands. The change in pressure, tends to track with the increase in winds.
So did we see that here Sunday night??
You better believe it.
Still, these conditions happen frequently, and they do not all bring 100 km/hr winds! Understanding, is the first step in forecasting them though.
You can find out more here.
For fellow nerds:
It’s been a crazy Spring so far. It may not be over yet!
UPDATE: Dr. Tim Coleman is the expert on these events and has written a guest post about this event here. Check it out!
Further update on Monday April 20:
The local Metr. community is still discussing this event, and an analysis by the Huntsville NWS office has some interesting information. The winds peaked before the lowest pressures were recorded. This seems to indicate that this was primarily a wake low event, rather than a gravity wave. As I wrote above, these things are still poorly understood, and believe me, this is the kind of thing that gets we weather nerds very excited.