The same storm type in three costumes. One of them shows you everything, one shows you enough, and one shows you nothing while doing everything.

In this lesson you will

  • Describe the visual signature of classic, HP and LP supercells.
  • State which hazards each archetype most commonly produces.
  • Explain why HP supercells are dangerous to spot, because the mesocyclone is rain-wrapped.
  • Identify an LP supercell by its striated, low-precipitation, sculpted updraft.

The three, from the ground

TypeWhat you seeWhat it does
ClassicUpdraft base, a visible and often rotating wall cloud, precipitation core clearly separated from the updraftThe full hazard spectrum, and the dominant share of significant tornadoes
HPPrecipitation wrapped around the updraft and mesocyclone. Often nothing identifiable at all from the wrong sideThe greatest flash-flood threat, plus wind, hail, and frequently rain-wrapped tornadoes
LPLittle or no precipitation near a striated, flared, sculpted corkscrew updraft. Usually no hook echoLarge hail, and occasional weak tornadoes

Moller et al. 1994 and Doswell & Burgess 1993 for classic and HP, Bluestein & Parks 1983 for LP. The amber row is not the strongest archetype, it is the one that is hardest and most dangerous to spot.

Module 2 lesson 7 gave the environmental control: storm-relative flow at anvil level, strongest for LP and weakest for HP, with classic in between. The visual consequence is direct. Strong flow aloft carries the precipitation away and leaves the updraft bare; weak flow lets it fall back around the updraft and hide it.

A genuine disagreement about which is commonest

The Field Guide describes the classic supercell as the majority case. Moller and colleagues describe the HP supercell as "the most common supercell form". Those are different claims and this course is not going to pretend otherwise.

Teaching point
Some of the difference is regional and some is definitional, because the three types are a continuum rather than three boxes and where you cut it changes the counts. Which one you meet most is largely a question of where you stand: HP is the common mode across the Southeast and the Gulf Coast, classic supercells are the Plains archetype the photographs are all taken from, and LP storms belong mostly to the High Plains. A course built only around Plains classic-supercell photography would be training two thirds of its readers for somebody else's sky.

Why HP is the dangerous one to spot

Every visual cue this module has taught you depends on being able to see the updraft region. An HP supercell removes that. The precipitation wraps around the mesocyclone, so the wall cloud, the funnel and the tornado can all be inside a curtain of rain that you cannot see through and should not drive into.

  • "No visible tornado means safe" is false in an HP storm. It means you cannot see, which is a different fact about you rather than a fact about the storm.
  • "LP is harmless" is also false. LP storms are the best-looking and they drop very large hail out of what appears to be open sky.
  • Position matters more than optics here. With an HP storm the useful decision is usually where to stand, not what to look at, and that is module 8.

The honest summary wherever HP storms are common: in one, your eyes are a degraded instrument, and you should say so in the report. Lesson 11 covers how to word that.

Check yourself

You are watching a supercell whose updraft region is completely buried behind a wall of heavy rain. You can see no wall cloud, no funnel and no clear base. What is the correct conclusion?

  1. Probably an HP supercell. You cannot see the mesocyclone, so you cannot rule out a rain-wrapped tornado
  2. No visible rotation means no tornado threat from this storm
  3. It is a classic supercell viewed from the forward flank
  4. It is an LP supercell, because no wall cloud is visible
Hint

What does an absent observation actually tell you about the storm?

Answer

Probably an HP supercell. You cannot see the mesocyclone, so you cannot rule out a rain-wrapped tornado

Precipitation wrapped around the updraft is the HP signature, and it is the common mode across the Southeast. The critical move is not concluding anything from what you cannot see: an absent observation is a fact about your visibility, not about the storm. Rain-wrapped tornadoes are exactly what this archetype produces.

Sources

  1. Moller, Doswell, Foster & Woodall 1994, Wea. Forecasting 9, 327–347: the operational recognition of supercell thunderstorm environments and storm structures
  2. Doswell & Burgess 1993, Tornadoes and tornadic storms: a review of conceptual models. Geophys. Monogr. 79, Amer. Geophys. Union, 161–172
  3. Rasmussen & Straka 1998, Mon. Wea. Rev. 126, 2406–2421: variations in supercell morphology, part I
  4. Bluestein & Parks 1983, Mon. Wea. Rev. 111, 2034: low-precipitation severe storms
  5. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), glossary, supercell variations