Clear Weather

Storms possible today, cooler tomorrow

The Triangle has a 30% chance for thunderstorms this afternoon, and a few may become severe. High winds and hail are our biggest threat today. How do we know? What parameters do meteorologists look at to decide if storms will happen and how bad they could be?

The most basic pieces for building a thunderstorm are moisture, heat, and a source of lift, meaning something to cause the air to rise from the surface and build the clouds. With a dew point of 70º as of the time that I'm writing this, we definitely have the moisture available. Our forecast high temperature this afternoon is 90º, so the heat will be available to help warm our surface air. Plus, a cold front will cross the state this afternoon providing the source of lift.

Remember that air that is warmer than its surroundings will rise and cooler air sinks. By definition a cold front is the boundary between cool, dry air and warm, moist air. As a cold front moves across the surface, the pool of cooler air pushes the warmer air up into the atmosphere. With enough heating at the surface ahead of the front, the warm air can rise so high that it shoots through the top of the troposphere - the layer of the atmosphere closest to the surface. A visual clue that this is happening is when you look at a storm from a distance and see the top has flattened out like the top of an anvil, but there is still a small puff of cloud above that flattened surface called "the over-shooting top." The process requires a lot of energy, and when that much energy is available, we know the storms have the chance to become severe.

Meteorologists call the energy needed to create storms Convective Available Potential Energy (CAPE), and it is measured in Joules per kilogram. The larger the number, the stronger the storms could be. Our CAPE values today are a very respectable 2000-2500 J/kg, so we know the atmosphere is primed for severe weather if all the pieces come together.

In order to build a strong storm, there must be an updraft, which is created by that warm air being lifted. Hail occurs when the updraft is so strong that it can carry water droplets up into the level of the atmosphere where the water freezes into ice crystals and hold it up there long enough for the ice crystals to grow big enough to remain frozen when they finally fall to the surface. The stronger the updraft, the larger the hail.

What goes up must come down, and that goes for air as well as water droplets. Dangerous winds occur when the rain cooled air sinks through the storm and crashes to the surface. The wind can't go into the ground, so it spreads out, usually ahead of the storm as straight-line winds, and sometimes in a circular pattern as a microburst. These winds can be as dangerous as a weak tornado and flip mobile homes and high profile vehicles, and down trees.

If the winds are turning strongly with height (wind shear), then meteorologists also look for rotation in the storms. Rotation means that tornadoes are also a possibility. Any time there is a chance for severe weather, it is safe to assume a tornado is possible, however they are not always probable. In today's case there is very little in the way of wind shear so tornadoes are improbable.

All of these pieces need to line up perfectly for a major severe weather outbreak, and today is not one of those days. While the risk for severe weather this afternoon is just marginal in the Triangle and points east, it only takes one storm to do a lot of damage. As long as there is any risk, it is a good idea to be prepared, have a way to receive storm alerts, and keep an eye on the weather.