Bow-echo tornadoes

John Farley

I am wondering what experiences others have had on chases involving bow-echo tornadoes. True bow-echo tornadoes (not gustnadoes) usually occur when surging bow echoes in an environment with strong shear encounter a pre-existing boundary or outflow from another nearby storm. Usually they occur at or north of the apex of the bow echo.

Research has found that up to 40% of tornadoes in Illinois and Indiana occur in this type of situation, though nationally the figure is about half that. Still a non-trivial number, though.

I have encountered these during chases three times, all in Illinois, and each time by accident after observing earlier supercell activity in the same general area area. One time, the most recent (June 13 of this year near Bunker Hill, IL), I missed the tornadoes as I was more interested in avoiding getting overrun by the bow ceho. [I did see the tail end of a brief tornado under the flanking line of the supercell that occurred a little earlier near Brighton, but the bow echo produced FOUR confirmed tornadoes in and north and east of Brighton, all of which I missed.]

On July 5 of last year, I did see a brief tornado just east of my home town of Edwardsville, though I got no picture because it was ver brief and I at first mistook it for a scud finger. And on July 27, 1997, I encountered and photographed such a tornado in the Quad Cities area (chase report at ). This looks a little like a gustnado, but there was strong rotation in the clouds as well as on the ground, and the tornado persisted for about 5 minutes.

Each time, I was caught by surprise, not expecting a tornado in an outflow-dominant storm. Yet, in reading about this recently, each case fit the pattern described above which is typical of bow-echo tornadoes. This makes me wonder whether, if one consciously placed oneself ahead of and to the north of the apex of a bow echo, one would have a decent chance of seeing tornadoes. This would, of course, be difficult due to the typical very fast movement of the bow echo.

In my experience, each time I have encountered this phenomenon, the following conditions have been present:

1. There were supercells in the area, which either evolved into bow echo storms or were overtaken by bow echo storms that developed nearby.

2. The tornado occurred in the typical portion of the bow echo, i.e. a little north of the apex (but not in the comma head).

3. They occurred as the bow echo surged forward very quickly and seems to have also encountered a boundary, as described above.

Anyone else have experience with this? What do you think about the possibility of intentionally positioning oneself to see tornadoes in this situation?
LEWP. bow echoes and the like..

I think I have the acronym right. Line Echo Wave Patterns..

I live in Ohio and I believe the figure would be something like 40% of tornadoes come from those systems here. There is a study posted online form a NWS office in North Carolina I believe about detecting the "hot spots" in a line on radar.

As for specifically targetting them based on previous boundaries... I do not get that detailed. Although based on your advice I may. My basic Great Lakes Line chasing strategy breaks down like this.

  • * Keep close to good data. This is usually easy to do in heavilly populated states.Look for hot spots, bows and developing breaks in the line.
    * Keep safe. Unfortunately this sometimes takes precedence. Dodging the rough spots of a powerful line is more dificult than dodging a well defined Super Cell.
    * Hope for good luck. Targetting a large area is rough. Good luck plays a part.
    * Tail end Charly. Getting on the tail end of the line with access to better inflow. I know that contradicts the first bullet but depending on position and storm motion I will take the tail end or look for breaks.
    * Breaks Breaks Breaks. I look for developing breaks in the line. I have read some research on Tornados from squall lines and that data pointed towards area near breaks in the line.

    • I will look for the data I found regarding LEWPs lines bow echos and breaks in lines and tornados. If I find it I will post a link.

      Tom Hanlon
Having done the majority of my chasing in Indiana and Illinois, I've been through my share of bow echoes. As John mentioned, they generally move quite rapidly. The strategy in those cases is either to keep doing short hops or to batten down the hatches. The hop strategy is more flexible. You stop to observe, then when the line starts to get too close, you get out ahead of it again. The disadvantage is that if something unexpected happens, you may not have a safe place to take shelter. The BDtH strategy involves looking for a sturdy structure, and parking next to it. Then you just let the line come over you. The disadvantage here is that its hard to adjust once the line has gotten over you due to heavy rain, high winds, and potential roadblocks.

As for the tornadogenesis aspect, I think outflow boundaries are the way to go. Last year when I was volunteering at LMK, we had several occasions of tornadoes from OFBs intersecting a line. I haven't read as much of the scientific literature on this as I should.

I wonder, did the BAMEX project do any research on these? I remeber a really intense storm back on the evening May 18, 1997, or was it 1998. A powerful bow echo moved through the Chicago metro area and dropped a tornado in Lynwood, IL The sirens went off all along the line of storms for Doppler-indicated rotation. I think the storms started out as some supercells in Iowa, does anyone remember chasing that day?