At its peak, there were more than a quarter of a million power outages across Massachusetts yesterday caused by a strong line of thunderstorms that moved from Ontario, Canada, all the way across southern New England. As the cleanup continues, you might wonder what caused all the tree damage and resulting power outages.
First of all, what occurred yesterday was not a tornado. Tornadoes are a specific wind phenomenon in which the wind is rotating; we did not have any rotation yesterday. What we did see were microbursts. A microburst is just a small downburst that is usually less than 4 kilometers across. You can contrast this with a derecho, another severe wind phenomenon, which is a line of straight-line winds that lasts a longer time and moves across a wider area. One might even argue that yesterday’s line of microbursts was in a sense a derecho, but I will leave that to the folks at the National Weather Service to decide.
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No matter what we call it, the winds yesterday were caused by strong wind coming down from higher levels of the atmosphere, where part of the thunderstorm was occurring, and being brought to the surface.
Thunderstorms are created when we have strong updrafts carrying moist air higher in the atmosphere, which is subsequently cooled, and condenses. This process releases latent heat and allows that storm to continue to develop as that warm air rises; sometimes, like yesterday, these storms go on to reach severe criteria. Severe thunderstorms need to have at least one of the following: wind gusts of at least 58 miles per hour, 1 inch of hail, or a tornado.
In these storms hail and lots of rain are created in the upper parts of the thunderstorm. When the updraft is no longer powerful enough to keep the hail and rain from falling, rain and hail will start to rapidly come down to the surface. As this happens, the strong winds that are in the upper part of a thunderstorm get pulled down to the surface with them. Here’s an analogy to help visualize this. Think about how you can feel air rushing by you when a large truck passes as you stand on a sidewalk. The truck is dragging the air in the same way the air is being dragged by rain and hail. In the truck example air moves horizontally by you, but in the thunderstorm that air hits the ground and then rushes outward, creating damage.
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Sometimes the winds can be brought to the surface without any rain and the updraft simply collapses on itself. These are called dry microbursts. When we experience heavy rain with strong winds we have a wet microburst. Either way, the damage that occurs tends to fall in one direction and indicate that there was no rotation. This is one of the ways in which scientists determine the difference between a tornado and a microburst.
Yesterday the winds were very strong from about 5,000 feet and higher. It was pretty clear that if we saw thunderstorms, some of those winds would be carried to the ground and damage would result. Unfortunately for many, the forecast was correct and the cleanup continues.
Follow Dave Epstein @growingwisdom.