Severe Weather Outlooks Explained: Marginal to High Risk

When a local forecast says "some storms could be severe," that wording usually traces back to the Storm Prediction Center (SPC) in Norman, Oklahoma, the national forecast center for thunderstorms and tornadoes. Days before a storm exists, the SPC publishes a convective outlook map showing where severe weather is possible and how likely it is. Learning to read it gives you the earliest heads-up you can get for severe weather, often three or more days in advance.

The outlook schedule

The SPC issues outlooks for several days ahead, and updates them as the event approaches:

• Day 1 (today and tonight): updated five times a day, most often around early morning, mid-morning, midday, mid-afternoon, and evening.
• Day 2 (tomorrow) and Day 3: each updated twice a day.
• Days 4–8: highlight areas with at least a 15 percent chance of severe weather when the forecast is confident enough to say anything at all.

Confidence in the details increases as the day approaches. A Day 3 outlook might show a broad Slight Risk; by Day 1 it might be split into an Enhanced Risk in one area and nothing in another.

The five risk levels

The outlook uses a categorical scale, with general thunderstorms shown separately below it:

The names can mislead. "Slight" sounds minor, but a Slight Risk day can include a few significant tornadoes or very large hail; it means the coverage is expected to be scattered, not that any storm that does form will be weak. Many significant tornadoes happen on Slight and Enhanced days, simply because those days are far more common. Moderate and High Risk days are uncommon. High Risk is issued only a few times a year at most, and some years not at all, but they account for a disproportionate share of major tornado outbreaks.

The probability maps behind the categories

The colored categories are built from separate probability forecasts for tornadoes, damaging wind, and large hail. Each probability means the chance of that hazard occurring within 25 miles of any point in the area. So a 10 percent tornado probability means a 10 percent chance that a tornado touches down within 25 miles of you, not that 10 percent of the area will see a tornado.

That's a much bigger number than it sounds. For a given spot on a typical day, the chance of a tornado within 25 miles is close to zero; 10 percent is a significant, elevated threat, and tornado probabilities of 15 to 30 percent or higher drive Moderate and High Risk days.

Hatched areas on the probability maps mark where significant severe weather is possible: strong tornadoes (EF2 or stronger), hail two inches or larger, or wind gusts of 75 mph or more. A hatched area inside even a Slight Risk is a signal that the storms that do form could be dangerous. Read the outlook's text discussion too, which explains the forecaster's reasoning and the expected timing.

From outlook to watch to warning

Severe weather information narrows in time and space as an event approaches:

1. Outlook (days ahead): the SPC highlights regions where ingredients may come together. Use it to plan: check your alert methods and keep an eye on the forecast.
2. Mesoscale Discussion (hours ahead): the SPC sends short technical updates on developing situations, often saying whether a watch is likely soon.
3. Watch (hours ahead): the SPC issues a Severe Thunderstorm or Tornado Watch for a specific area, typically covering dozens of counties for six to eight hours. Watches labeled "Particularly Dangerous Situation" (PDS) are reserved for rare, high-end events.
4. Warning (minutes ahead): your local National Weather Service office issues a warning for a specific storm, drawn as a polygon over its path. Take action now.

See NWS Watches vs. Warnings Explained for details on the last two steps.

How to use outlooks in daily life

• Marginal or Slight: go about your day, but know how you'd receive a warning and keep an eye on the radar in the afternoon and evening.
• Enhanced: plan around it. Check that weather radios and phone alerts work, know where you'll shelter, and consider moving outdoor events.
• Moderate or High: treat it like an approaching hurricane. Review your shelter plan with everyone in the household, charge devices, make sure you can be woken overnight, and stay informed through the day. See the Severe Weather Preparedness Checklist.

Keep in mind that a risk area is a regional forecast. Most places inside even a High Risk won't see a tornado, and severe storms can occur outside any risk area. Use the outlook to decide how alert to be, and the warnings to decide what to do.

Outlooks and RadarBug

RadarBug doesn't draw SPC outlooks on the map. They're available at spc.noaa.gov, and local TV and NWS offices share them widely. RadarBug covers the next steps: once watches and warnings are issued, they appear on the NWS alert layer, with watches drawn in blue and warnings in red, and the radar loop and lightning layer show the storms themselves. A good routine on an active day is to check the SPC outlook in the morning, then keep RadarBug open with alerts, radar, and lightning on through the afternoon and evening.