Reading the updraft base
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Striations, inflow bands and the beaver tail: what an updraft looks like when it is turning, and how precipitation placement decides how much of it you get to see.
In this lesson you will
- Identify a striated or corkscrew updraft, and state what striations indicate.
- Identify inflow bands and the beaver tail, and state what each one indicates.
- Distinguish the LP, classic and HP supercell visual signatures, and explain how each hides or reveals rotation.
Striations: the updraft showing its own rotation
All supercells contain storm-scale rotation, which may give a striated or corkscrew appearance to the storm's updraft.
Striations are horizontal banding wrapped around the updraft tower, and they are one of the few cues that says "rotating" before anything has lowered. The extreme version has its own entry in the glossary:
A thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole.
Striations are a cue, not a report. They tell you which storm to watch and where to stand. Nothing in this section is reportable on its own.
Inflow bands, and the one warning that matters
Inflow bands are ragged bands of cloud feeding into a storm from low levels, marking where the air being ingested comes from. Most of the time they say only that the storm is being fed. The NWS singles out one case:
Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone.
That sentence is doing something precise. It says curvature may indicate a mesocyclone, which is the only sanctioned route from something you can see to the word lesson 1 told you not to use. Even here it is an implication, not an observation.
Beaver tail is not tail cloud
A particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. [It is] attached to a supercell's general updraft and [is] oriented roughly parallel to the pseudo-warm front.
These two get swapped constantly, and the difference is where the feature attaches and how high its base is.
| Beaver tail | Tail cloud | |
|---|---|---|
| Attaches to | the general updraft | the wall cloud |
| Base height | about the updraft base | lower, down at wall-cloud level |
| Shape | broad and flat | narrow and tapering |
| Points at | roughly along the pseudo-warm front | from the precipitation toward the wall cloud |
Tail cloud is lesson 3, where it belongs, because you cannot have one without a wall cloud to attach it to.
Where the rain sits decides what you can see
Module 3 covered the three supercell archetypes. This is what each one costs you when the thing you are looking for is rotation.
- LP supercell
- Little precipitation, so the updraft is naked. Striations, the base and any lowering are all visible from a long way off. The easiest storm to read and the least common.
- Classic supercell
- The textbook. Precipitation on the forward flank, updraft base clear on the rear. This is the storm every training photograph is of, and the reason Plains-trained spotters struggle elsewhere.
- HP supercell
- Heavy precipitation wrapping around the circulation itself. The rotation can be entirely inside the rain. Common anywhere humid, and the reason lesson 8 exists.
None of the cues in this lesson is available in an HP storm at night in the rain, which describes a large share of the tornadoes in this platform's home region. Learn them, and do not build your whole method on them.
Check yourself
A broad, flat band of cloud runs into the storm at roughly the height of the updraft base, oriented away from the precipitation. What is it, and is it reportable?
- A beaver tail, and no: it is an inflow feature, not rotation
- A tail cloud, and yes: tail clouds indicate a rotating wall cloud
- A shelf cloud, and no: it is outflow
- A wall cloud, and yes if it persists
Hint
Two of these attach in different places at different heights. The height given is the clue.
Answer
A beaver tail, and no: it is an inflow feature, not rotation
A beaver tail: broad, flat, attached to the general updraft at about the updraft base. A tail cloud is narrower, lower, and attaches to a wall cloud, which means there has to be a wall cloud first. Neither is reportable by itself; both are cues about where to look.
Sources
- NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
- Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145
- NWS online glossary