High Shear, Low Cape on November 1

Some buzz out there about a tornado threat as a strong cold front comes through on Friday (11/1).

Massive thunderstorms are not a threat.  But, a large area of rain, squalls, and scattered thunderstorms is expected between 2am and 10am Friday.

Remember, the two primary factors in forecasting tornado development are wind shear and CAPE.

And there will be lots of wind shear in the atmosphere during this time.  Wind shear measures the change in wind speed and direction with increasing altitude.

Conversely, there will not be lots of energy to develop tall, deep thunderstorms (i.e. supercells), the ones that are responsible for large tornadoes.  We call that CAPE, for convective available potential energy.

Tornadoes are a very small, but non-zero threat during this time frame Friday morning. Admittedly, there is very limited skill in forecasting the size of tornadoes, but when they do occur in these these low-cape/high-shear environments, they tend to be very small and short-lived.

On the EF-scale, they are generally EF-0 or EF-1.  Usually, they last 20 minutes or less and are about the size of a typical elementary school.  These are not the type of tornadoes that scour homes off of their foundations; instead, they are more likely to do partial roof damage and knock down trees.

This is why we remind you to go to the lowest room of your home and stay away from windows when a Tornado Warning is issued.  This action is usually sufficient to remain safe in these type of events.

As a reminder, you are invited to sign up for the ABC13 WeatherCall service. If your area is included in a Tornado Warning, you will get a phone call to your landline or mobile phone. If you prefer, you can get a text message.  There is a small annual administrative fee, but it is an exceptional value.  I have it for my phone.

About seansublette

Meteorologist at Climate Central. Broadcast meteorologist in Virginia from 1995 to 2015. Born and raised in Richmond, VA. Penn State alumnus. Loves baseball and the rock band Rush. Views are independent of my employer. Past performance does not guarantee future results.
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