Outflow features: gust front, shelf, roll cloud, scud
The cold air a storm makes has to go somewhere, and on its way out it builds the most dramatic and most misreported clouds in severe weather.
The The low horizontal cloud along the leading edge of a storm's outflow. It comes in two forms: the shelf cloud, attached to the storm, and the roll cloud, detached from it. family
Arcus is the NWS glossary term for a low horizontal cloud on the leading edge of thunderstorm outflow. It has two members, and the difference between them is simply whether it is joined to the storm.
- Shelf cloud
- "A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm The leading edge of cool air rushing out of a storm, ahead of the rain. It is what makes the wind pick up sharply just before a storm arrives.... attached to the base of the parent cloud above it. Rising cloud motion often can be seen in the leading (outer) part... while the underside often appears turbulent, boiling, and wind-torn."
- Roll cloud
- "A low, horizontal tube-shaped arcus cloud... completely detached from the thunderstorm base... Roll clouds usually appear to be rolling... but should not be confused with funnel clouds."
- Whale's mouth
- The turbulent, sagging, chaotic underside of the cloud deck behind a gust front, seen looking back up at it after the shelf has passed over you. Spectacular, and by then the wind has already arrived.
- Scud
- Ragged fragments of low cloud, formally Ragged low cloud that forms in humid air near a storm and drifts about. It rises slowly and does not rotate, which is what separates it from a funnel, and it is the single most common thing mistaken for one., forming in the humid air near a storm. NWS Louisville: they "can look ominous, but by themselves are not dangerous", and "They will not rotate and will rise slowly."
What outflow feels like
You do not have to identify the cloud to know you are in outflow, because outflow announces itself. The wind arrives before the rain, sharply, with a temperature drop, and it is blowing from the storm toward you. That sequence is the single most reliable outflow cue there is, and it does not depend on light, distance or viewing angle.
A A low, wedge-shaped arcus cloud on the leading edge of a storm, attached to the cloud base above it. It is an outflow feature, it means damaging wind rather than rotation, and it is the feature most often misreported as a wall cloud. spanning the horizon ahead of a line is the thing most people photograph and post as a Severe weather is happening or about to happen for the people inside a drawn polygon. Issued by the local forecast office. It means act now.. What it actually means is damaging straight-line wind, imminent, which is a real hazard, a module 5 report, and not rotation.
Scud
Scud is the other half of the misreporting problem, and it deserves its own paragraph because it is genuinely hard the first few times. It is ragged, it hangs below the base, it moves, it is dark, and from a distance a piece of it can look exactly like a funnel beginning to descend.
A wide, layered wedge of cloud is advancing toward you across open ground. The wind has just picked up sharply from the direction of the storm and the temperature has dropped. What is it, and what do you report?
Wind before rain, coming from the storm, is the whole answer to the first half.Wind first, from the storm, with a temperature drop, is outflow, and a wedge attached to the cloud above it is a shelf cloud. It is genuinely worth reporting, as a wind threat, because damaging straight-line winds are right behind it. What it is never worth reporting as is rotation. A roll cloud would be detached and tube-shaped rather than a wedge attached to the base.