Dave Epstein

Tornado on the Cape? A look at how it happened

Tornado observed on radar in Yarmouth, weather service says

For the second day in a row, a tornado warning was issued across parts of Cape Cod. While yesterday’s thunderstorms brought strong, damaging winds, today’s brought an actual tornado.

The weather service in Boston confirmed a tornado based on radar around noon Tuesday. (NOAA Twitter)

First off, what causes a tornado? In order for this vortex of swirling winds to form, the atmospheric conditions have to be just right. The two main components to create a tornado are:

1. Powerful lift, and

2. Atmospheric spin

When meteorologists talk about “lift” they’re talking about the ability for the air to rise. We’ve all seen big, billowy thunderstorm clouds -- those are created from the lift of the air. Flatter clouds, by contrast, don’t have as much lift.


Spin is simply rotation of the air itself. When thunderstorms have rotation they actually spin and, if the conditions are right, the spin of the storm can lead to the spin of a smaller entity called a tornado.

This is a very simplified explanation, but without these components -- the lift moving the air higher and the spin causing the rotation -- you won’t have a tornado.

There was atmospheric spin present in the lead up to today’s tornado.
There was atmospheric spin present in the lead up to today’s tornado.(NOAA)

Tornadoes are not common in New England, and are certainly even less common on Cape Cod, which is surrounded by relatively cool ocean water. Cool ocean water tends to cool the air around it. This cooler air is not as buoyant and will not be lifted as easily, and without that lift you’re not likely to get a tornado.

The severe weather of the past couple of days comes at the conclusion of a heat wave. The frontal system that brought in the cooler air stalled across the south coast of New England. This boundary has acted as the focal point for severe weather the past couple of days.


This morning a small area of low pressure, called a mesoscale low, moved along this boundary. This low provided the right atmospheric conditions of both lift and spin to create a tornado.


Now that the storms have all passed offshore, the weather service will evaluate the damage and determine if there was one or more tornadoes and how strong they were. They will look at the way the damage occurred and look for tornado damage signatures, such as whether the debris is strewn in one direction or rotated. The degree of destruction will then be used to rank the tornado and determine the maximum wind speed. It’s likely this was an EF1 or very weak EF2 tornado on a scale that goes up to EF5.

Tornadoes on the Cape are very rare. There was actually a small tornado around the Woods Hole area of Falmouth last fall, but prior to that the last confirmed tornado to hit the Cape was in 1977.