In short: SCP and STP are recipes. Each takes ingredients covered in CAPE & Instability and Wind Shear & Helicity, divides each one by a typical value, and multiplies the results. A value of 1 means “the ingredients together look like a typical case.” SCP asks whether the air favors supercells. STP asks, if a supercell forms here, whether the air favors a strong tornado. Neither forecasts whether storms will form.

Why multiply?

Severe storms need ingredients at the same time and in the same place. Big CAPE with no shear makes short-lived storms. Big shear with no CAPE makes nothing. Multiplying captures that: one weak ingredient drags the whole product down, and a missing one sets it to zero.

1 2 3 4 5 6
  1. 1
    Instability, 1.4. Surface-based CAPE of 2,100 J/kg, divided by 1,500.
  2. 2
    Cloud base, 1.0. An LCL of 800 m. Full credit below 1,000 m.
  3. 3
    Helicity, 2.0. 0 to 1 km SRH of 300 m²/s², divided by 150.
  4. 4
    Shear, 1.25. 0 to 6 km shear of 25 m/s (about 49 kt), divided by 20.
  5. 5
    Inhibition, 1.0. CIN of 20 J/kg. No penalty until about 50.
  6. 6
    The 1.0 line. Each term's typical tornadic-supercell value.
One example sounding through the fixed-layer STP; the helicity term (amber) does the most work here. Multiply the bars: 1.4 × 1.0 × 2.0 × 1.25 × 1.0 = 3.5. Shrink any one bar to zero and the whole index is zero.

Supercell Composite Parameter (SCP)

SCP asks one question: does this air favor supercells, storms with a long-lived rotating updraft? SPC’s current formula multiplies four terms:

TermDivided byNotes
Most-unstable CAPE1,000 J/kgUses the most buoyant parcel, so elevated storms count
Effective SRH50 m²/s²Helicity in the layer the storm can actually ingest
Effective bulk shear20 m/s (about 40 kt)Zero below 10 m/s, capped at 1 above 20 m/s
Most-unstable CIN-40 J/kgNo penalty when CIN is weaker than 40 J/kg

How to read it:

  • Above 1: the ingredients for supercells overlap. Thompson and colleagues found supercells far more common in this range than ordinary storms.
  • Negative values are left off the map. A negative SCP marks air that favors left-moving supercells, the ones that rotate clockwise. SPC’s map shows only the positive part, the right movers.
  • It is not a tornado index. Plenty of high-SCP days produce big hail and no tornadoes. SCP is the first question, STP the second.

In MesoRadar: Supercell Composite, in the National tab's HRRR Composite Indices group (Pro). On the map it is a fixed-layer stand-in, built from the fixed 0 to 3 km SRH and 0 to 6 km shear in the HRRR files. The Skew-T tool's SCP row is the full effective-layer version, tagged eff.

Supercell Composite Parameter, fixed-layer (SCP)

Quick read

Where the air favors supercells, storms with a long-lived rotating updraft, from instability, low-level spin, deep shear and the cap together. This map is a fixed-layer stand-in for SPC's version, so it can run high where storms would feed on air above the ground. It is not a tornado index: plenty of high values bring large hail and no tornadoes.

  1. 1
  2. 2
  3. 4
  4. 6
  5. 8
  6. 12
  7. 20

No units. Clear below 0.5. Thompson and colleagues found supercells far more common than ordinary storms where SCP is above 1.

For forecasters

Formula
SCP = (MUCAPE / 1000 J/kg) × (SRH0-3 km / 50 m²/s²) × shear term × CIN term
Term limits
As in the table above, with the 0 to 6 km bulk wind difference in the shear term; negative results are set to 0.
Units
None
Parcel and layers
CAPE and CIN from the HRRR's most-unstable parcel (the highest theta-e level in the lowest 300 mb, GRIB level "255-0 mb above ground"), close to SPC's MUCAPE and MUCIN; fixed 0 to 3 km SRH and 0 to 6 km bulk wind difference.
How it is made
Computed by MesoRadar from native HRRR fields. SPC's SCP uses effective SRH and effective bulk wind difference. The HRRR files carry an effective SRH field but no effective bulk wind difference, and this map uses the fixed 0 to 3 km SRH and 0 to 6 km shear for both, so it is a fixed-layer stand-in.
Caveats
Fixed layers can read differently from SPC's effective-layer SCP in either direction. They tend to run high for elevated storms and where the inflow layer is shallow, because the fixed layers include air the storm does not take in. Like STP, it describes the air around storms, not whether storms form.
Sources
SPC mesoanalysis help, Supercell Composite Parameter; Thompson et al. 2003; Gropp and Davenport 2018 (the CIN term).

Significant Tornado Parameter (STP)

STP asks a narrower question: if a supercell forms here, do the ingredients match the ones seen with significant tornadoes, EF2 and stronger? SPC publishes two versions.

The fixed-layer version uses layers that are easy to compute anywhere:

TermFormulaLimits
Instabilitysurface-based CAPE ÷ 1,500 J/kg
Cloud base(2,000 m minus LCL) ÷ 1,000 m1 below 1,000 m, 0 above 2,000 m
Low-level helicity0 to 1 km SRH ÷ 150 m²/s²
Deep shear0 to 6 km bulk shear ÷ 20 m/s0 below 12.5 m/s, capped at 1.5 from 30 m/s
Inhibition(200 + CIN) ÷ 150 J/kg1 when CIN is weaker than 50, 0 at 200 or more

SPC’s summary, from a large sample of supercells: a majority of significant tornadoes came with STP above 1, while most supercells without tornadoes came with values below 1.

The effective-layer version

The effective-layer STP swaps in mixed-layer CAPE, cloud base and CIN, effective SRH and effective bulk shear. It fits the storm’s real inflow better, and SPC sets it to zero whenever the effective inflow layer starts above the ground. That rule exists because storms feeding on air above a cool surface layer rarely produce strong tornadoes.

In MesoRadar: Significant Tornado Parameter, in HRRR Composite Indices (Pro). The map uses the fixed-layer formula above. The Skew-T tool's STP row shows the effective-layer version when the sounding supports it, tagged eff, and the fixed-layer one otherwise, tagged fixed.

Significant Tornado Parameter, fixed-layer (STP)

Quick read

How closely the air matches the setups seen with strong (EF2 and up) tornadoes in supercells: instability, a low cloud base, low-level spin, deep shear and a weak cap, all at once. Any one of them missing sets it to zero. A bright area is a place to watch if a supercell forms there, not a sign that one will.

  1. 1
  2. 2
  3. 3
  4. 4
  5. 6
  6. 8
  7. 12

No units. Clear below 0.5. SPC: a majority of significant tornadoes in its sample came with STP above 1; most supercells without tornadoes came with values below 1.

For forecasters

Formula
STP = (SBCAPE / 1500 J/kg) × ((2000 − LCL) / 1000 m) × (SRH0-1 km / 150 m²/s²) × (BWD0-6 km / 20 m/s) × ((200 + SBCIN) / 150 J/kg)
Term limits
As in the fixed-layer table above; negative results are set to 0.
Units
None
Parcel and layers
Surface-based parcel (CAPE, CIN and LCL height above ground); fixed 0 to 1 km SRH and 0 to 6 km bulk wind difference.
How it is made
Computed by MesoRadar from six native HRRR fields: surface-based CAPE and CIN, 0 to 1 km helicity, the 0 to 6 km shear vector and the height of the lifting condensation level. It is SPC's fixed-layer version, not the effective-layer one.
Caveats
Surface-based parcel throughout, so it can differ from SPC's effective-layer STP (mixed-layer CAPE, LCL and CIN, effective SRH and shear, zero when the inflow layer starts above the ground). The limits shared by every composite are in What the composites do not tell you. The Skew-T tool shows the effective-layer STP for a point when the sounding supports it.
Sources
SPC mesoanalysis help, fixed-layer STP; Thompson et al. 2003; Thompson et al. 2012.

What the composites do not tell you

The single most important fact about STP and SCP: both were built from soundings near storms that already existed. They describe how favorable the air was around supercells. That leads to four easy mistakes:

  • No storm, no tornado. A bright STP area under a strong cap, or with no lift to start storms, produces nothing. Check CIN and where storms are actually forming.
  • Storm mode matters. STP assumes a supercell. If storms quickly merge into a line or a messy cluster, the tornado risk drops even with high values. A squall line’s tornado risk is better read from its own signatures in Squall Lines, Bow Echoes & Derechos.
  • It is not a count or a probability. STP of 6 does not mean six times the tornadoes, or a 60% chance. It means the ingredients overlap strongly.
  • The model can be wrong. A map of STP is only as good as the model’s moisture, cap and low-level wind, which are the hardest things to forecast. Compare runs, compare with a balloon sounding, and read the SPC’s own outlook and mesoscale discussions.

Energy Helicity Index (EHI)

EHI is an older, simpler composite: CAPE times SRH, divided by 160,000. The idea is that rotating updrafts are most likely where CAPE and helicity are both large. SPC notes that 0 to 1 km EHI values above 1 to 2 have been associated with significant tornadoes in supercells. It has no cloud-base or CIN term, so it can light up where STP stays low.

In MesoRadar: Energy Helicity Index 0 to 1 km and 0 to 3 km, in HRRR Composite Indices (Pro), and EHI 0-1 / 0-3 under the Skew-T tool's More fold, from surface-based CAPE.

Energy Helicity Index, 0 to 1 km and 0 to 3 km (EHI)

Quick read

CAPE times helicity, scaled so that 1 is a meaningful value: it reads high only where both are large at once. Read it beside the tornado parameter on the same hour.

  1. 0.5
  2. 1
  3. 1.5
  4. 2
  5. 3
  6. 4
  7. 6
  8. 8

No units, on the same colors for both layers. Clear below 0.25. SPC: 0 to 1 km EHI above 1 to 2 has been associated with significant tornadoes in supercells.

For forecasters

Formula
EHI = (SBCAPE × SRH) / 160,000
Units
None
Parcel and layers
Surface-based CAPE, with SRH over 0 to 1 km (the 0 to 1 km map) or 0 to 3 km (the 0 to 3 km map).
How it is made
Computed by MesoRadar from the HRRR's surface-based CAPE and its 0 to 1 km or 0 to 3 km helicity. Negative values (negative helicity) are set to 0.
Caveats
Surface-based CAPE can overstate the energy on summer afternoons, and there is no check for a cap or a high cloud base.
Sources
Hart and Korotky 1991; SPC mesoanalysis help, Energy-Helicity Index.

SHIP: the significant hail parameter

SHIP is SPC’s composite for significant hail, stones 2 inches or larger. It multiplies most-unstable CAPE, the moisture of that parcel, the 700 to 500 mb lapse rate, the coldness at 500 mb and the deep shear, and divides by a constant. SPC then scales it down when MUCAPE is under 1,300 J/kg, when the 700 to 500 mb lapse rate is under 5.8 °C per km, or when the freezing level is under 2,400 m above ground. SPC notes values of 1.5 to 2 or more are typical where significant hail is reported.

  • Above 1: an environment favorable for significant hail.
  • Above 4: very high.
  • It is not a hail size. SPC says so directly. A SHIP of 2 does not mean 2-inch hail. More on reading hail in Hail on Radar.

In MesoRadar: the SHIP row, in the Hail group under the Skew-T tool's More fold (Pro).

Key points

  • SCP and STP multiply normalized ingredients; 1 means a typical case, and any missing ingredient gives zero
  • SCP asks about supercells; STP asks about significant tornadoes if a supercell forms
  • In SPC’s sample, most significant tornadoes came with STP above 1, most non-tornadic supercells below it
  • Both assume a storm exists and stays discrete; check the cap, the lift and storm mode
  • A value is not a count, not a probability, and only as good as the model behind it
  • EHI is a simpler cousin; SHIP is the hail composite and is not a hail size

Common questions

What does STP mean?

The Significant Tornado Parameter, an index from the Storm Prediction Center that multiplies five ingredients linked to strong tornadoes in supercells: instability, cloud base height, low-level helicity, deep shear and convective inhibition. Each is divided by a typical value for tornadic supercells, so 1 means the ingredients together match that typical value.

What is a high STP value?

In SPC's large sample of supercells, a majority of those producing significant (EF2 or stronger) tornadoes had STP above 1, and most supercells without tornadoes had less than 1. Values well above that, 4 to 10 on the biggest outbreak days, mean the ingredients overlap strongly. It is not a count of tornadoes and not a probability.

Why is STP zero where storms are happening?

Because it multiplies. If any one ingredient is missing, for example the cloud base is too high, the shear is too weak, or the cap is too strong, that term goes to zero and so does the whole index. Storms there may still produce hail or wind.

Does a high STP mean a tornado will happen?

No. STP was built from environments around storms that were already supercells. It says how favorable the air is if a supercell forms there and stays discrete. If no storm forms, or storms merge into a line, a high value means nothing happens.

Sources