Ragged cloud fragments, rain shafts and outflow features generate most bad rotation reports. Here is what they are, and the checklist that stops you filing one.

In this lesson you will

  • State the NWS definition of scud.
  • Apply the motion test that separates scud from a funnel.
  • Distinguish a shelf cloud, which is outflow and never reportable as rotation, from a wall cloud.
  • List the common look-alikes and what gives each one away.

Scud

Scud, SR-145, verbatim
Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.
Field Guide, verbatim
Scud clouds are low cloud fragments that may attach to a storm's base and can strongly mimic the appearance of a ragged funnel. Watch for persistent rotation of the suspicious feature to rule out scud.

Note the giveaway in the first definition: scud is cool moist air, associated with outflow. A funnel hangs from a rotating updraft, which is inflow. They are produced by opposite halves of the storm, and rising scud in front of a dark base is the single most photographed false tornado there is.

The five questions

The Field Guide gives a checklist for exactly this moment. It is worth memorising, because it works in the order the questions are written: a no anywhere near the top makes the rest moot.

  1. Can I see it clearly?
  2. Is the feature attached to a thunderstorm base?
  3. Is the feature in the section of the storm where tornadoes and funnels typically develop, near the updraft?
  4. Is there organized rotation present within the feature?
  5. If it appears to be a tornado, is there debris?

Question three is the one that eliminates most bad reports before motion is even considered. If the feature is out along the leading edge of the storm rather than back under the updraft, you are looking at the outflow side, and nothing there is a tornado cue.

Shelf cloud: outflow, and never a rotation report

Module 3 lesson 6 is the full treatment of shelf versus wall, and that discrimination is not repeated here. What belongs in this lesson is the one-line consequence: a shelf cloud is outflow. It is not reportable as rotation, whatever it looks like.

A wide, layered arc of cloud advancing across a lake at sunset
Shelf cloud: wide, layered, spanning the view, attached above to the storm behind it, on the leading edge. Dramatic, frequently reported as a wall cloud, and outflow every time. Public domain federal imagery
Why this one keeps happening
A shelf cloud is more visually impressive than most wall clouds, it arrives first, and it is what a storm looks like from the direction most people are facing. Being scared of it is reasonable. Reporting it as rotation is the most common single error in spotting, which is why module 3 gave it a lesson of its own.

The rest of the field

Rain shafts
A dense column of rain, seen edge-on against brighter sky, looks like a wide dark funnel. It has no rotation and its base is diffuse where it meets the ground.
Dust devils
Fair-weather, surface-heating driven, not attached to any cloud base. Question two of the checklist disposes of them.
Gustnadoes
A real whirl, on the gust front, unconnected to storm-scale rotation. Lesson 6.
Towers and structures
Grain elevators, chimneys and radio masts against a dark base at distance. Sounds ridiculous until it is dusk and raining.
Cold-air funnels
Small funnels under showers with no severe storm present. Rarely reach the ground and are usually harmless, though a local office may still want to hear.
The rule underneath all of it
Field Guide: report only what you see, not what you think you see. The whole of this lesson is that sentence applied to the six things most likely to make you think you saw something.

Check yourself

A dramatic wedge-shaped cloud spans the horizon on the leading edge of an approaching line, and the wind at your back has just picked up sharply. Reportable as rotation?

  1. No. It is on the leading edge and the wind shift says outflow, so it is a shelf cloud
  2. Yes, if it is persistent and localized
  3. Yes, because a wedge shape indicates a developing tornado
  4. Only if it is rotating about a horizontal axis
Hint

Question three of the checklist asks which part of the storm you are looking at.

Answer

No. It is on the leading edge and the wind shift says outflow, so it is a shelf cloud

Leading edge, plus a sharp wind increase from behind you as it arrives, is outflow: a shelf cloud. Rotation cues live back under the updraft on the inflow side, not out front. Nothing about the shape or the drama changes that, and this is the most common false report in spotting.

Sources

  1. Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145
  2. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
  3. NWS Northern Indiana, "Low hanging clouds mistaken for Funnel Clouds and Tornadoes"
  4. NWS online glossary