The SPC convective outlook, explained

The SPC convective outlook is the Storm Prediction Center’s forecast of where thunderstorms and severe thunderstorms are expected across the contiguous United States over the next one to eight days, issued as a map of risk categories backed by probabilities and a written discussion. Everything below comes from the SPC’s own product documentation unless another source is named.

The outlooks and forecast models guide introduces the categories and why they are not intensity ratings. This page goes further: the issuance schedule, how the categories are actually derived, the conditional intensity levels the SPC added in March 2026, and what comes after the outlook.

What “severe” means here

The National Weather Service defines a severe thunderstorm as one that produces a tornado, wind of 58 mph (50 knots) or greater, or hail 1 inch in diameter or larger. The SPC separately defines significant severe: a tornado rated EF2 or stronger, wind of 75 mph (65 knots) or greater, or hail 2 inches or larger. The outlook forecasts both, and it targets organized convection — supercells, squall lines, multicell complexes. Brief pulse storms in weak shear may not be included in any risk area at all.

Day 1 to Day 8, and when each is issued

• Day 1: five issuances, at 0600, 1300, 1630, 2000 and 0100 UTC. Each runs until the next 1200 UTC.
• Day 2: 1:00 a.m. Central time (0600 UTC during daylight saving time, 0700 UTC otherwise) and 1730 UTC, covering 1200 UTC tomorrow to 1200 UTC the day after.
• Day 3: 2:30 a.m. Central (0730 or 0830 UTC) and 1930 UTC.
• Day 4–8: once, at 4:00 a.m. Central (0900 or 1000 UTC).

The SPC publishes the Day 2 through Day 4–8 times in Central time; the UTC equivalents above are a plain time-zone conversion.

Detail falls away with lead time. Days 1 and 2 carry separate probabilities for tornadoes, damaging wind and large hail. Day 3 uses a single total-severe probability. Day 4–8 has only two thresholds, 15% and 30%, equivalent to Slight and Enhanced; when no 15% area is drawn, the text says predictability too low or potential too low. The first means models disagree too much to place a risk; the second means 15% probabilities look highly unlikely.

The six categories

• TSTM (light green): general thunderstorms, a 10% or greater probability of thunder. Not a severe risk.
• 1 – MRGL (dark green): severe storms of limited organization and longevity, or very low coverage.
• 2 – SLGT (yellow): organized severe storms, not widespread.
• 3 – ENH (orange): greater coverage than Slight, varying intensity.
• 4 – MDT (red): widespread severe weather with several tornadoes and/or numerous severe storms likely, some intense.
• 5 – HIGH (magenta): a severe weather outbreak is expected, from numerous intense, long-tracked tornadoes or a long-lived derecho-producing complex.

How to read the probabilities

Every percentage on an SPC outlook is the forecaster’s estimate of one or more events occurring within 25 miles of a point during the valid period. The SPC notes that 25 miles is roughly the size of a metropolitan area. The radius is why the numbers look small: severe weather at any single spot is rare, so a 10% tornado probability is a large departure from normal.

The SPC suggests comparing against climatology. Its own example: a 30% tornado area over northwest Texas in mid-May, where the background rate is about 1.5%, is a forecast of roughly 20 times the usual risk.

Conditional intensity, since March 2026

For years, significant-severe potential was shown as a hatched area, labeled SIGN in the SPC’s text points product. Starting with the 1630 UTC Day 1 outlook on March 3, 2026, the SPC replaced it with three Conditional Intensity Groups, CIG1 to CIG3 (Service Change Notice 26-11). Tornado and wind use three levels; hail uses two.

“Conditional” is the key word. A CIG describes how strong events are expected to be if they occur, not how likely they are. The SPC derived the groups from 20 years of mesoanalysis data at observed tornadoes: EF2-or-stronger tornadoes make up 7% of the sample where no area is drawn, 20% at CIG1, 30% at CIG2 and about 40% at CIG3. The same notice added 75% and 90% thresholds to the Day 1 and Day 2 wind probabilities.

The category you see is computed from the probability and the intensity level together. The tornado table:

Two consequences are worth knowing. A 2% tornado probability can produce a Slight risk on its own if the intensity is CIG2. And hail alone tops out at Moderate. A HIGH requires tornado or wind probabilities of at least 30% tornado with CIG2, 60% tornado with CIG1, 45% wind with CIG3, or 60% wind with CIG2. The SPC says the tables were built to match historical practice, so the categories should occur about as often as before.

After the outlook: mesoscale discussions and watches

A mesoscale discussion covers the next six hours, with emphasis on the first one to three. A severe weather MD either discusses developing storms in relation to a possible or existing watch, or alerts users to a coming upgrade in the Day 1 outlook. When no watch is in effect, its concerning line gives the odds of one: watch unlikely (5 or 20%), possible (40 or 60%) or likely (80 or 95%).

A watch means conditions favor organized severe storms in a defined area, typically 20,000 to 40,000 square miles for 6 to 8 hours. A Tornado Watch is issued when multiple tornadoes or a single intense one are favored; a tornado can occur in either type. Warnings come from local NWS offices, not the SPC. Act on National Weather Service warnings.

Common misreadings

• “Marginal means nothing will happen.” Marginal is a severe risk category: severe storms are possible, just limited in organization or coverage. Categories are not ceilings either — the SPC states that violent tornadoes sometimes occur in Slight, Enhanced or Moderate areas when the threat is small in area or conditional.
• “High risk is a bad storm day.” It is rarer than that. An SPC study by Davis, Dean and Guyer found 29 High risk days from 2003 to 2009, a median of five a year, ranging from two in 2009 to seven in 2003.
• “The map says when.” A Day 1 probability applies to the whole valid period, up to 1200 UTC the next day. Timing lives in the written summary, and in the MDs and watches that follow.
• “No risk area, no severe storms.” The SPC notes that a warning outside a risk area is not a contradiction; nearly any thunderstorm can briefly produce severe weather.

WeatherStory Radar shows SPC outlooks as issued, alongside NWS warnings and full-resolution Level II radar.