LP, classic, HP, and the regional variants

The supercell you were shown in class is a photograph from Kansas. This lesson is about the ones that actually arrive here, most of which you cannot see.

One storm type, three amounts of rain

LP, classic and HP are all supercells by the lesson 6 definition. What differs is how much precipitation the storm makes and where it falls relative to the updraft, 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 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 supercell 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-supercells 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 center, 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 the central Gulf Coast. 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. Near the landfall point 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 CAPE

The environment that produces most of our cool-season severe weather has a name and a definition. Sherburn and Parker set HSLC 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 in the Southeast it is one of the two hardest things to spot. Lesson 8 covers the other one.

Why spotters in the Southeast need more, not less

Take the four preceding sections together and the reality across the Southeast 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 recognize photographs is unarmed in a Southeast cool-season outbreak.

Check yourself

A February evening sounding gives SBCAPE 350 J/kg, MUCAPE 800 J/kg and 0 to 6 km shear of 45 kt. Is this an HSLC environment, and what follows?

1. Yes, all three criteria are met. Expect shallow storms, small low signatures and degraded warning performance
2. No. MUCAPE of 800 J/kg is above the HSLC threshold
3. No. With that little CAPE, severe weather is not a realistic concern
4. Yes, but only if the storms are elevated, since HSLC is defined for elevated convection

Sources

1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), section 2, supercell variations
2. Rasmussen & Straka 1998, Mon. Wea. Rev. 126(9), 2406–2421: variations in supercell morphology, part I
3. Moller, Doswell, Foster & Woodall 1994, Wea. Forecasting 9, 327–347: the operational recognition of supercell thunderstorm environments and storm structures
4. Edwards 2012, Electron. J. Severe Storms Meteor. 7(6), 1–61: tropical cyclone tornadoes, a review of knowledge in research and prediction
5. Edwards 2010, tropical cyclone tornado records for the modernized NWS era. 25th Conf. on Severe Local Storms, P3.1. Source of the 118 / 18 F2 / 1 F3 figures for Ivan; other databases differ by a few counts
6. Nowotarski et al. 2021, Wea. Forecasting 36(5), 1589–1609: tornadoes in Hurricane Harvey
7. McCaul 1991, Mon. Wea. Rev. 119(8), 1954–1978: buoyancy and shear characteristics of hurricane-tornado environments
8. Sherburn & Parker 2014, Wea. Forecasting 29(4), 854–877: climatology and ingredients of significant severe convection in high-shear, low-CAPE environments
9. NWS Mobile hurricane event pages (Ivan, Katrina, Dennis, Michael, Sally, Zeta)
10. NCEI Storm Events Database, for per-county ratings and times