LP, classic, HP, and the regional variants

One storm type, three amounts of rain

LPLow 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., classic and HPHigh 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. are all supercells by the lesson 6 definition. What differs is how much precipitation the storm makes and where it falls relative to the updraftThe 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., and that changes almost everything about what you can see and what it can do.

What decides which one you get

Rasmussen and Straka found that storm-relative flow at 9 to 10 km above ground discriminates the three morphologies. Strong upper-level storm-relative flow ventilates the anvilThe 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. far downwind and leaves the updraft bare, which is an LP storm. Weak flow lets the precipitation fall back around the updraft, which is HP.

Moller and colleagues made the wider point in 1994: this is a continuum, not three boxes. A storm can move along it during its life, and an HP storm that started out classic is one of the commonest and most dangerous evolutions in the Southeast, because the view you calibrated on is gone by the time it matters.

Small supercells

Low-topped and miniature supercells have every dynamical feature from lesson 6 at a fraction of the scale: shallower mesocyclones, lower echo tops, smaller hooks, and a much smaller visual signature from the ground. They occur in shallow-instability environments, which in the Southeast means cool-season and tropical setups, and they are consistently harder to detect on radar because everything about them is smaller than the sampling.

The trap is calibration. A spotter who has learned supercellA 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. structure from Plains photography is looking for something two or three times the size of what is in front of them, and it looks wrong, so they discount it.

Tornadoes in tropical cyclones

Landfalling tropical cyclones produce tornadoes from mini-supercellsA supercell with a shallower, smaller and weaker rotating updraft than a classic one, typical of tropical cyclone rainbands. Produces tornadoes in an environment with a fraction of the instability a Plains supercell needs. in their outer rainbands. Compared with a Plains supercell they have lower echo tops, smaller horizontal scale and shallower mesocyclones. They are most frequent 100 to 500 km from the centre, in the right-front quadrant relative to storm motion, which for a northward-moving landfalling system is the northeast side. Most are rated below EF2.

They are also unusually hard to spot, for reasons that stack: rain-wrapped by definition, embedded in a background wind field that is already damaging, at night as often as not, with low cloud bases and no contrast. A tornado inside a hurricane does not sound different from the hurricane.

The anchor case is on our own coastline. Hurricane Ivan came ashore as a Category 3 near Gulf Shores and Orange Beach, Alabama, around 0650 UTC on 16 September 2004, and produced 118 tornadoes between 15 and 18 September, a record at the time, surpassing Hurricane Beulah in 1967. Edwards' tropical-cyclone tornado accounting puts 18 of them at F2 and one at F3. Locally, a tornado family in Baldwin County injured four; NWS Tallahassee documented 23 tornadoes in its adjacent area on 15 and 16 September. Notably, 57 of the 118 came in a roughly ten-hour window on the last two days, in the Mid-Atlantic, long after landfall.

Now the part that is more useful than the record. Here is what recent landfalling systems actually produced in the NWS Mobile county warning area.

High shear, low CAPEConvective Available Potential EnergyHow much energy is available to an updraft, in joules per kilogram. Bigger numbers mean a stronger updraft is possible, and a number on its own says nothing about what kind of storm you get.

The environment that produces most of our cool-season severe weather has a name and a definition. Sherburn and Parker set HSLCHigh Shear, Low CAPEAn environment with strong wind shear and very little instability, common in the Southeast in the cool season and at night. It produces shallow storms that radar struggles to see and warnings often miss. as surface-based CAPE of 500 J/kg or less, most-unstable CAPE of 1,000 J/kg or less, and 0 to 6 km shear of at least 18 m/s, about 35 kt. All three conditions at once, which is why lesson 1 insisted on knowing which CAPE you are looking at.

• These setups occur at all hours and in all seasons, and they are frequent in the Southeast. They are not a winter curiosity.
• Warning performance degrades as CAPE decreases. The storms are shallow, the signatures are small and low, and the beam is above them at any range.
• HSLC setups account for a disproportionate share of tornado-watch false-alarm hours, which is its own hazard: the days that cry wolf and the days that produce are drawn from the same population.

Lesson 6 explained why a supercell can exist here at all with so little buoyancy: with strong shear the dynamic pressure contribution does the work that CAPE does elsewhere.

Elevated and embedded

An elevated supercell is rooted above a stable surface layer rather than in the boundary layer, which is what most-unstable CAPE finds. It can produce very large hail and damaging wind while being much less likely to be tornadic, because the low-level circulation has no connection to the ground.

An embedded supercell is a rotating storm inside a line or a larger precipitation shield, sharing the airspace with everything around it. It carries the full supercell threat with none of the visual isolation, and it is one of the two hardest things on this coast to spot. Lesson 8 covers the other one.

Why our spotters need more, not less

Take the four preceding sections together and the local reality is this: rain-wrapped storms, at night, in tree cover, with low contrast and small features. You will rarely get the Plains view. Every one of those conditions removes a visual cue that standard spotter training assumes you have.

That is the argument for this module. When you cannot see the structure, what is left is knowing where the structure would be: which side of the storm you are on, what the radar mode implies about where the rotation lives, what the environment supports tonight. A spotter who has only learned to recognise photographs is unarmed in a Southeast cool-season outbreak.