03.02.10

Thunderstorm Formation and Colliding Airmasses

Posted in Weather News at 4:13 pm by Rebekah

For the first time in 60 years, no tornadoes were reported in the United States in the month of February.

I’m not certain how many tornadoes the US sees on average each February; I tried to find this statistic in some weather archives, but was unsuccessful.  A BBC article claims there are, on average, 22 tornadoes in the US in February; however, considering the same article says that tornadoes form when cold air collides with warm air, I would not completely trust this source.

One of the most perpetuated myths about Tornado Alley states that tornadoes form because of collisions between airmasses.  Few myths grate on my nerves more than this one.

An airmass is a large body of air whose properties (e.g., temperature and humidity) are fairly uniform.  These airmasses may be thousands of kilometers across and up to several kilometers high.  The boundaries between these airmasses are what we call fronts, and that’s where a lot of our interesting weather takes place.  If a cold airmass dives south from Canada into the US, the leading edge of the airmass is a cold front.  If a warm, moist airmass moves north from the Gulf of Mexico into the US (i.e., cold air retreats northward ahead of the warm airmass), the leading edge of the airmass is a warm front.

If tornadoes occurred whenever airmasses collide, the entire world would be in a lot of trouble–as airmasses collide globally all the time, no matter what season it is.  Furthermore, with all of the cold fronts that we’ve had over the past few months, February should have been full of tornadoes in the US.

The primary conditions for strong thunderstorm (and possibly tornado) formation are lift, instability, low-level moisture, and wind shear.

Thunderstorms require rising air, and lift gets the air to start rising.  Lift may come in the form of a front, a dryline (similar to a front, it is the leading edge of a dry airmass, preceded by a moist airmass), a surface low pressure system, solar heating, an upper-level trough of low pressure, etc.

Once the air starts to rise, an unstable atmosphere will ensure that the air keeps rising.  Air cools as it rises, but as long as it doesn’t cool as fast as the surrounding environment (i.e., the rising air is warmer than the environmental temperature), we have an unstable atmosphere.  The best setup is one in which the environmental temperature is warm at the surface and rapidly decreases with height.

If the rising air is dry, we probably won’t get any clouds–so we want to have a warm, moist environment near the surface.  The higher the moisture content, the more likely it is that we will have low-based clouds (good for tornadoes).

Finally, in order to get long-lived, rotating thunderstorms, called supercells, we also need to have the wind increasing and changing direction with height.  This is called wind shear.

During the month of February, we had plenty of lift and shear, but not enough instability or moisture for many severe thunderstorms.  We continue to be in an active weather pattern in terms of upper-level troughs and a strong southern jet stream, but the Southern Plains needs to heat up and moisture needs to return before we can have any hope of the storm chasing season beginning.

When will this happen?  According to long-range weather and climate index models, it looks like it may be at least another couple weeks.

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