Storm clouds: the formations and threats that come with them
With the threat of storms in the forecast almost daily this week, many may be keeping an eye on the horizon. While in the dark, turbulent skies of a Missouri spring can be intimidating, not every low-hanging cloud is a cause for alarm. There are several things to look for to distinguish between a "scary-looking" sky and a truly dangerous one.
The shelf cloud: perhaps the most photographed feature of any storm, the shelf cloud often looks like a massive wave or a dark wedge moving toward you. This is just the "outflow" of a storm. As cold, rain-cooled air drops out of the clouds, it spreads out along the ground, forcing the warm surface air upward. While it is visually terrifying, shelf clouds are rarely tornadic. Instead, they are more known for straight-line winds. If you see a shelf cloud, you should expect a sudden, sharp increase in wind speed (potentially 60+ mph) and heavy rainfall within minutes.
The scud cloud: if you've ever seen a "finger" of cloud reaching for the ground and began to worry, you likely saw this formation. These are the #1 source of false tornado reports to any given area. This formation, formally known as pannus, is wispy, fragmented pieces of moisture that condense in the humid air being sucked into the storm's updraft. The golden rule of storm spotting is rotation, and this is what helps to determine the threat level. Scud clouds move erratically; they may rise, fall, or drift, but they do not possess organized, sustained spinning.
The wall cloud: if you want to know where a tornado might form, this is the formation to look for. Unlike the shelf cloud, which sits at the front of the storm, the wall cloud is typically found at the rear, attached to a "rain-free" base. This is the "inflow," and it represents the area where the storm is breaking in warm, unstable air to fuel its updraft. A wall cloud is a localized lowering that is often very persistent. If you see a wall cloud that is visibly rotating like a slow-motion top, the storm is becoming organized. This is where the danger of tornado formation becomes more likely.
The final stages of storm development are the transition from a rotating wall cloud to a funnel cloud. This is a rotating column of air extending from the base of the cloud, but it has not yet reached the ground. Once that rotation makes contact with the earth, or if you see a debris cloud/dust whirl at the surface, it is officially a tornado.
In Missouri, "rain-wrapped" tornadoes are common, meaning the "classic" cone shape might be hidden behind a curtain of heavy rain and hail. If the sky turns a deep green and the winds go calm, don't wait for the visual confirmation of a funnel. Head to your basement or an interior room immediately and use resources like a local news station or a NOAA weather radio to receive the most up-to-date information on storms in your area.
