SPC Risk Categories — Marginal to High Risk, Explained
What do the SPC risk categories mean?
The Storm Prediction Center (SPC) rates severe thunderstorm potential on a five-level scale: 1-Marginal, 2-Slight, 3-Enhanced, 4-Moderate, and 5-High, with a sixth unnumbered "general thunderstorm" area below them. The categories are shorthand for underlying probabilities of tornadoes, damaging wind, and large hail within 25 miles of a point. Marginal means isolated, mostly low-end severe storms; Slight means scattered severe storms; Enhanced means numerous, more persistent severe storms; Moderate means widespread severe weather with significant tornadoes or destructive winds likely; High — issued only a few times a year — means a severe weather outbreak is expected, historically including violent, long-track tornado days. The number to remember: Moderate and High days are rare, and they are rare on purpose.
What is the SPC and what are these outlooks?
The Storm Prediction Center in Norman, Oklahoma is the NWS national center responsible for forecasting severe thunderstorms and tornadoes. Its flagship public product is the Convective Outlook: a map of the continental US shaded by risk category, issued for Day 1 (today, updated five times daily), Day 2, Day 3, and in probabilistic form for Days 4-8.
Outlooks answer a different question than watches and warnings: not "is a storm threatening now?" but "how favorable is the environment, over a broad area, hours to days ahead?" The escalation ladder runs outlook → watch → warning, with each step narrowing in space and time.
What does each category actually mean?
- TSTM (light green) — general thunderstorms. Storms expected; severe weather not anticipated (though lightning is always a hazard).
- 1 — Marginal (dark green). Isolated severe storms possible, generally limited in intensity and duration: gusty winds, small severe hail, perhaps a brief tornado.
- 2 — Slight (yellow). Scattered severe storms. Short-lived and not widespread, but tornadoes, 1-2 inch hail, and damaging gusts are all on the table. Many tornado days in the Southeast are Slight-risk days.
- 3 — Enhanced (orange). Numerous severe storms, more persistent and/or widespread. A meaningful step up: organized severe weather with multiple tornadoes or a swath of wind damage is plausible.
- 4 — Moderate (red). Widespread severe weather likely, with the wording reserved for days when strong tornadoes (EF2+) or a major damaging-wind event are expected. A Moderate risk should change your plans for the day.
- 5 — High (magenta). A severe weather outbreak is expected: either a major tornado outbreak with violent, long-track tornadoes possible, or an extreme derecho-class wind event. Issued only a few times a year, and some years not at all for tornadoes.
What are the probabilities behind the colors?
Each category is defined by underlying probabilities — the chance of a severe event within 25 miles of any given point in the shaded area, with separate curves for tornado, wind, and hail on Day 1. A 2% tornado probability supports Marginal; 5% supports Slight; 10% Enhanced; 15% Moderate; 30%+ High. Hatched areas add the "significant severe" flag: a 10%+ chance of EF2+ tornadoes, 65+ kt winds, or 2"+ hail within 25 miles.
Two consequences worth internalizing. First, even a High risk is probabilistic: most points inside it see no tornado — 30% within 25 miles is enormous by forecasting standards and still not a coin flip. Second, the categories compress a lot of nuance: a "hatched Slight" for tornadoes can be a more dangerous tornado day for you than an unhatched Enhanced driven by hail probabilities. When it matters, read the probabilities and the SPC discussion text, not just the color.
How should I respond to each level?
A reasonable personal playbook: at Marginal/Slight, know storms are possible and make sure your alert channels are on. At Enhanced, check the timing (afternoon? overnight?), keep devices charged, and mentally walk your shelter plan. At Moderate/High, treat it like the forecast weather event it is: adjust plans that would strand you outdoors or on the road during the threat window, pre-position to sturdy shelter if you live in a mobile home, and expect watches during the day. For sheltering actions when a warning ultimately arrives, follow weather.gov/safety/tornado.
The categories' real gift is time: warnings give minutes; outlooks give hours to days.
How do outlooks relate to VORTEX's risk score?
They're complementary layers. SPC outlooks are broad-brush by design — one category can cover several states for a whole day. The VORTEX risk score works underneath that: it computes a local, hour-by-hour score from the actual atmospheric ingredients (CAPE, storm-relative helicity, shear, moisture) at your coordinates, so you can see when and where within an outlook area conditions genuinely spike. On a Moderate-risk day, your town's window might be 7-10 p.m. — that's the gap the score fills. It's situational awareness, not an official forecast: SPC outlooks and NWS warnings remain the authoritative products.