What does TSTM mean in weather?

TSTM is the SPC abbreviation for General Thunderstorm — the lowest category on the convective outlook, used where ordinary (non-severe) thunderstorms are expected. Severe-weather probability is below 5%. A TSTM area does not mean storms will be quiet; it means SPC does not expect them to reach severe criteria (1" hail, 58 mph wind, or a tornado).

What does MRGL mean in weather?

MRGL stands for Marginal Risk — SPC's level 1 of 5 severe categories. Isolated severe storms are possible, generally limited in coverage and short-lived. A few storms may produce 1" hail or 58 mph wind; a brief tornado is possible but not the primary concern.

What does SLGT mean in weather?

SLGT is Slight Risk — SPC's level 2 of 5. Scattered severe storms are possible. Short-lived organized supercells producing 1-1.5" hail or 60-70 mph wind are plausible, and a couple of brief tornadoes can occur. The most common outlook category on active days.

What does ENH mean in weather?

ENH stands for Enhanced Risk — SPC's level 3 of 5. Numerous severe storms expected, with several being intense and longer-lived. Strong tornadoes (EF2+), 2"+ hail, and 75+ mph wind gusts become realistic threats. Spotters are typically activated and EMAs pre-position.

What does MDT mean in weather?

MDT is Moderate Risk — SPC's level 4 of 5. Widespread severe storms, several intense, with significant tornadoes likely (EF2+), large hail (2"+), and damaging wind clusters. Issued ~20-30 times per year nationally; warrants direct spotter activation and shelter readiness.

What does HIGH risk mean in SPC outlooks?

HIGH is SPC's top level 5 of 5 — a major severe-weather outbreak is expected. Long-track violent (EF3+) tornadoes, derecho-magnitude wind, or 3"+ hail are likely. SPC issues HIGH only ~3-4 times per year on average; it triggers maximum operational posture from NWS, EMAs, and broadcast meteorology.

What is the difference between MRGL and SLGT risk?

MRGL (level 1) means isolated severe storms possible — short-lived and few in number. SLGT (level 2) means scattered severe storms possible — multiple cells, some better-organized, with a higher tornado/hail/wind probability. SLGT is roughly twice as common as MRGL on active days and is the threshold at which most EMAs start coordinating.

How often does SPC issue convective outlooks?

Daily, on a fixed schedule. Day 1 outlooks are issued at 0600 UTC, then updated at 1300, 1630, 2000, and 0100 UTC. Day 2 and Day 3 outlooks update twice daily. Days 4-8 are issued once per day. Each outlook covers a 24-hour period and contains the categorical risk area plus probabilistic tornado/hail/wind areas.

What is the Probabilistic Tornado Outlook?

Separate from the categorical outlook (TSTM through HIGH), SPC issues 2%/5%/10%/15%/30%/45%/60% probability contours for tornadoes within 25 miles of any point. Higher probabilities map to higher categories, but since March 2026 the exact category also depends on the Conditional Intensity Group (CIG1–CIG3) — which replaced the old black "significant severe" hatching and conveys how intense a tornado could be (EF2/EF3/EF4) if one occurs.

What do supercell, QLCS, and MCS mean?

These are the three main storm modes SPC discusses. A supercell is a rotating long-lived thunderstorm — the most prolific tornado producer. A QLCS (Quasi-Linear Convective System) is a squall line that can produce embedded brief tornadoes and damaging wind. An MCS (Mesoscale Convective System) is a large complex of storms, often persisting overnight, that produces widespread wind damage and flash flooding.

SPC Convective Outlook Risk Levels — TSTM/MRGL/SLGT/ENH/MDT/HIGH

SPC Convective Outlook Risk Levels — TSTM/MRGL/SLGT/ENH/MDT/HIGHWhat the six SPC severe-weather risk categories mean, plus a glossary of common terms in SPC discussions: supercell, MCS, QLCS, CAPE, shear, helicity, and more.The Storm Prediction Center (SPC) issues a Convective Outlook for Day 1 (today), Day 2 (tomorrow), and Day 3 (day after tomorrow), grading severe-weather potential into one of six categories from "general thunderstorms" to "high risk."The six risk categories

TSTM (General Thunderstorms): Non-severe storms expected, but no organized severe risk.

MRGL (Marginal, 1 of 5): Isolated severe storms possible.

SLGT (Slight, 2 of 5): Scattered severe storms expected.

ENH (Enhanced, 3 of 5): Numerous severe storms; widespread damage possible.

MDT (Moderate, 4 of 5): Significant severe weather likely. This is the day you cancel plans.

HIGH (5 of 5): Major severe outbreak; catastrophic impacts possible.

A "Marginal" day does not mean "no risk." A weak SLGT day with poor timing has produced fatal tornadoes. The category indicates coverage and confidence, not the severity of any single storm. SPC also publishes specific probability maps for tornadoes, hail, and wind separately.Common terms in SPC discussionsThe following glossary is drawn from word-frequency analysis of 100 recent SPC Mesoscale Discussions and Convective Outlooks.Storm types

Supercell: Long-lived rotating thunderstorm — produces most strong tornadoes and the largest hail.

Discrete: Storms developing as separate, isolated cells with clear air between them. Discrete supercells are the highest tornado threat.

Cluster: A group of storms moving together, often interacting. Damaging wind and hail dominate.

MCS (Mesoscale Convective System): Large organized complex of storms covering hundreds of miles. Typical late-night summer convection.

QLCS (Quasi-Linear Convective System): Long line of storms (squall line). Wind damage and brief tornadoes ("spin-ups") common; sustained tornadoes rare.

Bow echo: Curved, bowed-out radar signature within a QLCS — sign of intense damaging winds.

Splitting storm: A supercell that divides into a right-mover (typically tornadic) and left-mover (typically hail-dominant).

Embedded: Smaller cells contained within a larger area of rain or convection.

The ingredients

CAPE: Convective Available Potential Energy — the fuel. How much energy is available for an updraft. Higher = stronger storms. Units J/kg.

CIN: Convective Inhibition — the lid. Energy a parcel must overcome to reach free convection.

MLCAPE / SBCAPE / MUCAPE: CAPE for Mixed-Layer (average), Surface-Based (afternoon storms), and Most-Unstable (elevated convection at night) parcels.

Lapse rate: How fast temperature drops with altitude. Steep lapse rates = unstable atmosphere = vigorous updrafts. ~8 °C/km is a typical severe value.

Cap / Lid: Warm layer aloft that suppresses convection. "Cap breaks" = convection initiates.

Boundary layer: Lowest ~1 km of the atmosphere where surface heating, moisture, and friction dominate.

Wind structure

Shear: How wind changes with altitude — the basic ingredient for organizing storms.

Bulk shear: Magnitude of wind difference between two altitudes. "0–6 km bulk shear" is the standard severe-weather metric.

SRH (Storm-Relative Helicity): Spin available to storms. 0–1 km SRH drives tornado potential; 0–3 km SRH drives general supercell organization.

Hodograph: Graph of wind speed and direction across heights — used for storm-mode prediction.

Low-level jet (LLJ): Narrow band of fast winds 1–2 km above the surface, important for nocturnal storms.

Veering: Wind direction turning clockwise with altitude — favors supercells.

Boundaries and synoptic features

Trough: Elongated area of lower pressure aloft. Provides lift and forcing for storms.

Ridge: Elongated area of higher pressure aloft. Generally suppresses storms.

Shortwave: Fast-moving disturbance in the upper-level flow — often the immediate trigger for an outbreak.

Warm front / Cold front: Boundaries where one air mass replaces another. Common storm initiation zones.

Dryline: Boundary between dry desert air and moist Gulf air, common over the southern Plains. One of the most prolific severe-weather producers in the US.

Outflow boundary: Surface boundary left behind by a previous storm's rain-cooled air.

Storm dynamics

Updraft: Rising column of air inside a thunderstorm. Drives hail growth.

Downdraft: Descending column of air. Drives damaging wind and microbursts.

Mesocyclone: Rotating updraft inside a supercell. Tornado prerequisite (though not all mesos produce tornadoes).

Mesovortex: Small-scale rotation embedded in a QLCS. Source of brief spin-up tornadoes.

RFD (Rear-Flank Downdraft): Downdraft on the back side of a supercell. RFD surge is often the immediate tornado trigger.

Threats

Large hail: Hail ≥ 1" diameter (severe threshold). Significant = ≥ 2" (golfball+).

Damaging wind: Wind ≥ 58 mph (severe threshold). Significant = ≥ 75 mph.

Tornado Watch: SPC-issued box (typically 4–8 hours, multi-state) where tornadoes are possible.

Severe Thunderstorm Watch: Same shape but for hail/wind threat without strong tornado risk.

STP (Significant Tornado Parameter): Composite combining CAPE, shear, helicity, and moisture; calibrated to highlight EF2+ environments.

Frequently asked questions

What does TSTM mean in weather?: TSTM is the SPC abbreviation for General Thunderstorm — the lowest category on the convective outlook, used where ordinary (non-severe) thunderstorms are expected. Severe-weather probability is below 5%. A TSTM area does not mean storms will be quiet; it means SPC does not expect them to reach severe criteria (1" hail, 58 mph wind, or a tornado).
What does MRGL mean in weather?: MRGL stands for Marginal Risk — SPC's level 1 of 5 severe categories. Isolated severe storms are possible, generally limited in coverage and short-lived. A few storms may produce 1" hail or 58 mph wind; a brief tornado is possible but not the primary concern.
What does SLGT mean in weather?: SLGT is Slight Risk — SPC's level 2 of 5. Scattered severe storms are possible. Short-lived organized supercells producing 1-1.5" hail or 60-70 mph wind are plausible, and a couple of brief tornadoes can occur. The most common outlook category on active days.
What does ENH mean in weather?: ENH stands for Enhanced Risk — SPC's level 3 of 5. Numerous severe storms expected, with several being intense and longer-lived. Strong tornadoes (EF2+), 2"+ hail, and 75+ mph wind gusts become realistic threats. Spotters are typically activated and EMAs pre-position.
What does MDT mean in weather?: MDT is Moderate Risk — SPC's level 4 of 5. Widespread severe storms, several intense, with significant tornadoes likely (EF2+), large hail (2"+), and damaging wind clusters. Issued ~20-30 times per year nationally; warrants direct spotter activation and shelter readiness.
What does HIGH risk mean in SPC outlooks?: HIGH is SPC's top level 5 of 5 — a major severe-weather outbreak is expected. Long-track violent (EF3+) tornadoes, derecho-magnitude wind, or 3"+ hail are likely. SPC issues HIGH only ~3-4 times per year on average; it triggers maximum operational posture from NWS, EMAs, and broadcast meteorology.
What is the difference between MRGL and SLGT risk?: MRGL (level 1) means isolated severe storms possible — short-lived and few in number. SLGT (level 2) means scattered severe storms possible — multiple cells, some better-organized, with a higher tornado/hail/wind probability. SLGT is roughly twice as common as MRGL on active days and is the threshold at which most EMAs start coordinating.
How often does SPC issue convective outlooks?: Daily, on a fixed schedule. Day 1 outlooks are issued at 0600 UTC, then updated at 1300, 1630, 2000, and 0100 UTC. Day 2 and Day 3 outlooks update twice daily. Days 4-8 are issued once per day. Each outlook covers a 24-hour period and contains the categorical risk area plus probabilistic tornado/hail/wind areas.
What is the Probabilistic Tornado Outlook?: Separate from the categorical outlook (TSTM through HIGH), SPC issues 2%/5%/10%/15%/30%/45%/60% probability contours for tornadoes within 25 miles of any point. Higher probabilities map to higher categories, but since March 2026 the exact category also depends on the Conditional Intensity Group (CIG1–CIG3) — which replaced the old black "significant severe" hatching and conveys how intense a tornado could be (EF2/EF3/EF4) if one occurs.
What do supercell, QLCS, and MCS mean?: These are the three main storm modes SPC discusses. A supercell is a rotating long-lived thunderstorm — the most prolific tornado producer. A QLCS (Quasi-Linear Convective System) is a squall line that can produce embedded brief tornadoes and damaging wind. An MCS (Mesoscale Convective System) is a large complex of storms, often persisting overnight, that produces widespread wind damage and flash flooding.

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