Upper storm structure
Overshooting top, backsheared anvil, striations, flanking line, mammatus. Four of those tell you about the updraft. One of them tells you nothing, and is the most misreported cloud in the sky.
The features that mean a strong The column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is.
- Overshooting top
- A dome pushing up through the The flat, spreading top of a thunderstorm, where the updraft has run out of buoyancy and is pushed sideways by the winds aloft. An anvil left behind by a storm that has died is an orphan anvil., where the updraft has punched past the equilibrium level on momentum. The Field Guide is specific about the timing: "If the A dome punching up through the anvil, thrown there by an updraft strong enough to overshoot the level where it should have stopped. How long one lasts tracks how strong the updraft is. is persistent and lasts for 10 minutes or longer, it is generally a sign of a very strong thunderstorm updraft."
- Backsheared anvil
- An anvil spreading upwind, against the flow aloft, instead of only downwind. It takes a strong updraft to push ice out into the wind, so this is a good sign of one.
- Striations
- The NWS glossary calls them "Grooves or channels in cloud formations, arranged parallel to the flow of air". They "often reveal the presence of rotation, as in the An updraft whose cloud striations curve around it like the stripes on a barber pole. One of the clearest visual signs that an updraft is rotating, and still a cue rather than a report. or corkscrew effect often observed with the rotating updraft of a Low Precipitation (Low Precipitation supercellA supercell that makes very little rain, so its structure is fully visible and often beautiful. The hazard is that large hail can fall out of air that looks clear.) storm".
- Flanking line
- A stair-stepped line of A tall, hard-edged, cauliflower-textured convective tower with no anvil. Capable of heavy showers, and not yet a thunderstorm. feeding into the main updraft, normally on the south-west side, each tower shorter than the one ahead of it. Module 2 lesson 6 put this on the A thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds. plan view; here it is what you look for in the sky.
Read together, those four are a single sentence: overshooting top plus An anvil spreading upwind, against the flow aloft, instead of only downwind. It takes a strong updraft to push ice out into the wind, so it is a good sign of one. plus Grooves or channels in a cloud, arranged along the airflow, that often reveal rotation. The corkscrew or barber-pole look on a low-precipitation supercell updraft is the classic example. means a vigorous and probably rotating updraft. That is a storm worth staying with.
Note the word "barber pole" in the striations definition. It is spotter slang rather than a defined term, and so are "Spotter slang for a broad, flat inflow band attached to the updraft base, so it sits higher than a wall cloud. Frequently misreported as a funnel when seen end-on from a distance." and "tail-end Charlie" in the lessons that follow. Slang is fine on a net where everybody shares it. It is not fine in a report to a forecast office, which is a module 7 problem and flagged here so it does not surprise you there.
Rounded, sagging pouches on the underside of an anvil. They often accompany severe storms but do not produce severe weather, and despite what you will hear, they are not a sign a tornado is coming., and the myth about it
Mammatus are the rounded, sagging pouches on the underside of an anvil. The Field Guide glossary describes them as "Rounded, smooth, sack-like protrusions" and then says the part that matters: "Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well."
What mammatus is actually worth: it tells you there is a large anvil overhead, which usually means the storm itself is somewhere else. Photograph it. Do not report it.
A caller tells your net that they have dramatic mammatus overhead and asks whether they should take shelter from a tornado. What is the correct response?
Which part of the storm are they looking at, and is that the part that makes tornadoes?The NWS states that mammatus clouds "do not produce severe weather" and accompany non-severe storms too. They form on the underside of the anvil, which is the storm's exhaust, spread out downwind and often nowhere near the updraft. The 10 minute rule in one of the wrong answers belongs to the overshooting top, which is a genuine updraft-strength cue.