I had never heard of this type of storm system until this weekend. (These storms are also called a derecho storm.) The mammoth tempest that struck the Maryland and Virginia areas on Saturday was a bow-echo storm system. It produced amazing damage in a very short period of time and for many people it was with little warning. The governor of Virginia stated that his state had suffered the largest “non-hurricane power outage” in its history. Wow! So what is this storm system all about?!?!
On radar, a bow-echo looks like a comma, with a round head on one end and a tail on the other. The leading edge has a sharp reflectivity gradient, and there are notches (of dry air) dug into the weak reflectivity gradient on the trailing edge. These rapid moving storms can produce hail, lightning, large amounts of rain, high winds, and even tornadoes across a great distance. Bow-echoes seem to occur most often along the corridors of the upper Mississippi Valley to the Mid-Atlantic States, and across the southern plains around Oklahoma.
The basic conditions that favor thunderstorm development (including multi-cell systems like bow-echoes) consists of four qualities:
- Atmosphere that is thermally unstable – the atmosphere must have substantial warmth at its lowest levels, and cool off rapidly with increasing height.
- Moisture laden – the atmosphere must contain a large amount of moisture. An atmosphere that is nearly saturated and contains great warmth is very buoyant, and will tend to rise easily if provided with some sort of lift.
- Contains some form of lifting mechanism – some form of lifting mechanism is needed to get the warm, moist air to start rising. There are several that will work including the orographic lifting of air up over mountains or a front of some kind is a provider of lift, forcing the less dense warmer air mass over the top of the colder dense one.
- Experiencing wind shear – there must be wind shear present. Wind shear is the change of the winds speed and/or direction with height.
If the stars and planets have come into alignment and the four areas above are all present, how does the squall-line mutate into a bow-echo? The line of storms can be bent outward in a couple of ways. One possibility is by very strong winds slamming into the back of the line of storms at the mid levels of the atmosphere, accelerating that section of the line forward faster than the rest of the line. Another possibility is by the development of strong surface winds, possibly even by just one very intense thunderstorm cell, that distorts the shape of this cell enough to rip it apart and form a bent (bowed) line segment of several cells.
The main damaging effect from a bow-echo is not large hail, but severe, destructive straight-line winds. The winds from a bow-echos downburst can exceed 110 miles per hour! Cloud to ground lightning also accompanies these storms, as well as torrential rains. And as if the wind, rain, hail, and lightning is not bad enough,bow-echoes can spin-up tornadoes from three different locations along their extent. Tornadoes can form from the rotation in the comma head, in the supercells (if present) at the (right) tail end of the bow-echo, and just to the left of the bows apex and the strongest winds (due to extreme horizontal wind shear). Wow!
All and all, a very impressive storm system! And yet another reason to be prepared…because, as mother nature keeps showing us…anything is possible!