What is the cause of this?

[Dennis Sherrod]

The two radar grabs were from earlier this evening in Birmingham. As this cell moved by, the outer half ring of "precipitation" moved in front of and ahead of the storm. It expanded away from the main cell with each update and then slowly dissipated over the course of approximately 30 minutes. At the begining of the cell's life, this half ring was not visible, but became more apparent as the cell developed.
What would cause this or would this maybe be some strange thing being caused by being fairly close to the radar site?
Thanks

 

[Chris Allington]

Looks like an outflow boundary generated by those storms collapsing to me. Judging by the fact that they look more intense in the original scan and that it continues moving away in an arc shape.

 

[Joshua Nall]

Yep... outflow boundary. But usually there's not a lot of precipitation with that thin arc, even though it shows up well on radar. It's caused by the rush of cool wind from collapsing thunderstorm(s) as Chris said, slicing under the warm air out in front of it.

 

[Dennis Sherrod]

Thanks for the info. It was just pretty pronounced for around here. It was neat to watch on radar the entire cycle of it pushing outward.

 

[JIM SELLARS]

high humidity and a cold pool boundary

 

[Jason Foster]

Thanks for the info. It was just pretty pronounced for around here. It was neat to watch on radar the entire cycle of it pushing outward.

One thing to keep in mind too, with many of these, is how to also identify these boundaries when out in the field. Dennis, I don't know how close you were to the actual signature you were asking about on the radar, but sometimes, just going out and feeling the air, and seeing the clouds will help solve questions....but sometimes it also creates more, but end the end, it is good to actually get out there and see it.

 

[Mike Johnston]

One thing to keep in mind too, with many of these, is how to also identify these boundaries when out in the field. Dennis, I don't know how close you were to the actual signature you were asking about on the radar, but sometimes, just going out and feeling the air, and seeing the clouds will help solve questions....but sometimes it also creates more, but end the end, it is good to actually get out there and see it.

Yes, that's a good point. I remember back in my younger days living in Kansas (long before I ever heard of the term "outflow boundary"), seeing and wondering about these very long, thin, low-topped arcs of clouds that stretched across the horizon in clear air. They seemed to be roughly associated with severe thunderstorms, but it confused me that their appearance both followed and preceded the storms - often by several hours. Also, their direction of movement was often independent of the general flow. Of course, it's nice to know now what I was looking at those many years ago.