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.

In this lesson you will

  • Define arcus and identify its two types: the attached shelf cloud and the detached roll cloud.
  • Identify the whale's mouth, the turbulent underside behind the gust front.
  • Identify scud and state that it does not rotate and is not reportable by itself.
  • Associate the shelf cloud with the leading gust front and with damaging straight-line winds.

The arcus 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 gust front... 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 fractus, 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 shelf cloud spanning the horizon ahead of a line is the thing most people photograph and post as a tornado warning. 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.

The test that settles it
Scud does not rotate. It drifts, it rises slowly, it changes shape, and it forms and dissipates in place. Watch it for a minute or two, which lesson 11 will tell you to do anyway, and the question answers itself. A funnel rotates continuously about a vertical axis and keeps its shape while it does.

Check yourself

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?

  1. A shelf cloud on the gust front. Report it as a wind threat, never as rotation
  2. A wall cloud. Report it and watch for rotation
  3. A roll cloud, because it is wedge-shaped and horizontal
  4. Nothing reportable. Outflow features are never reportable
Hint

Wind before rain, coming from the storm, is the whole answer to the first half.

Answer

A shelf cloud on the gust front. Report it as a wind threat, never as rotation

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.

Sources

  1. NWS online glossary (arcus, shelf cloud, roll cloud)
  2. NWS Louisville, "Shelf Cloud versus a Wall Cloud"
  3. AMS Glossary of Meteorology, roll cloud
  4. Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145 (2nd ed. 1996)