Dry microbursts – why do they happen?

TheWxMeister Wonders

Many times in Colorado, we’ll have enough instability and mid-level moisture to produce convection, the process that causes thunderstorms. Many times in Colorado, it also looks like it’s going to rain, but it doesn’t.

The main reason for this “more bark than bite” type of thunderstorm is due to the atmospheric conditions between the base of the thunderstorm and the ground. As dry, desert air moves in from Arizona and New Mexico, it descends down the eastern slope of the Rocky Mountains. This surface air, which is dry anyway, dries out even more as it descends down the hills.

Microbursts occur when a pocket of air beneath a thunderstorm is cooler than the surrounding air. This air is more dense too, and, because of gravity, will rapidly descend to the surface, where it will spread out in all directions.
GIF courtesy of the National Weather Service – weather.gov

These dry microbursts are a pretty typical part of our severe weather climatology. With the dry surface air, our storms are often high based, meaning the flat, bottom of the storm where the convective process begins, is often at or above 14,000 feet. As rain produced in the updraft of the storm falls from the cloud into the dry sub-cloud air, it will evaporate.

Evaporation is a cooling process (It’s why you’re chilly when you get out of the shower or pool–water is evaporating off of your skin!) that ends up producing air beneath the cloud that is cooler than the surrounding air. This cool or cold pocket will rapidly accelerate down to the surface, where it spreads out in all directions.

Dry microbursts occur frequently in Colorado, while wet microbursts are more likely in the central and eastern United States. Clouds that produce dry microbursts often have cloud bases that are above 14,000′.
Schematic courtesy of the National Weather Service – weather.gov

Forecasting these types of storms requires monitoring the characteristics of moisture from the surface up through the tops of the updrafts. We use several tools to do this, including radiosondes (weather balloons) and surface dewpoint temperatures.

A surface dewpoint temperature below 40 degrees indicate very dry air at the surface. While not the only part of the picture in forecasting microbursts or downbursts, surface dewpoint often indicates that some potential exists.
This SKEW-T sounding from Denver’s weather balloon launched on Thursday morning, June 27, shows very dry air in the lowest layers of the atmosphere. Note that the red (temperature) and green (dewpoints) lines at the bottom of the plot are very far apart, indicating a low dewpoint compared to the air temperature. The deeper this layer of dry air, the more likely microbursts and downbursts are to occur underneath convective cells.

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