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Lesson 09 of 1130 minBeyond SKYWARN

Classic, HP and LP from the ground

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.

By the end of this lesson
M3.9.aDescribe the visual signature of classic, HP and LP supercells.
M3.9.bState which hazards each archetype most commonly produces.
M3.9.cExplain why HP supercells are dangerous to spot, because the mesocyclone is rain-wrapped.
M3.9.dIdentify an LP supercell by its striated, low-precipitation, sculpted updraft.

The three, from the ground

TypeWhat you seeWhat it does
ClassicThe smooth, flat, dark cloud base on the rear flank of a storm, where warm air is flowing in and rising. It is named for the air going up, not for being dry: hail and large drops may still fall from it., a visible and often rotating A localized, persistent lowering beneath the updraft base, on the rear inflow side of a storm, marking where air is being drawn up. Not all of them rotate, and a rotating one is what you report., precipitation core clearly separated from the 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.The full hazard spectrum, and the dominant share of significant tornadoes
High Precipitation supercellA supercell whose precipitation wraps around and can completely hide the updraft. Rain-wrapped tornadoes and flash flooding, and the common mode on the Gulf Coast.Precipitation wrapped around the updraft and The rotating updraft inside a supercell, typically a few miles across. Radar can see it aloft; whether anything is rotating at the ground is a separate question.. Often nothing identifiable at all from the wrong sideThe greatest flash-flood threat, plus wind, hail, and frequently Hidden inside the precipitation around it. A rain-wrapped tornado breaks none of the identification rules; it just denies you the observation those rules rely on. tornadoes
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.Little or no precipitation near a striated, flared, sculpted corkscrew updraft. Usually no A hook-shaped appendage on the reflectivity image, wrapped around the back of a supercell. One of the oldest radar clues that a storm is rotating.Large 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 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. 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 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. 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.

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.

Knowledge checkNot graded · the exam draws a fresh variant of this item

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?

What does an absent observation actually tell you about the storm?
Sources for this lessonMoller, Doswell, Foster & Woodall 1994, Wea. Forecasting 9, 327–347: the operational recognition of supercell thunderstorm environments and storm structuresDoswell & Burgess 1993, Tornadoes and tornadic storms: a review of conceptual models. Geophys. Monogr. 79, Amer. Geophys. Union, 161–172Rasmussen & Straka 1998, Mon. Wea. Rev. 126, 2406–2421: variations in supercell morphology, part IBluestein & Parks 1983, Mon. Wea. Rev. 111, 2034: low-precipitation severe stormsNWS Weather Spotter's Field Guide (YPA-201154, June 2011), glossary, supercell variations
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