The Storm Prediction Center issues severe weather forecasts for the continental United States

The Storm Prediction Center (SPC) is a division of the National Weather Service that forecasts severe thunderstorms, tornadoes, and fire weather across the lower 48 states. It does not forecast rain, snow, or routine weather — only events that pose when ready danger: hail larger than one inch, wind gusts above 58 miles per hour, tornadoes, and wildfires driven by dry conditions and wind.

SPC issues forecasts in layers of urgency and geography. A Convective Outlook covers a three-day window and divides the country into risk zones — from general thunderstorm risk to high risk for organized severe weather. A Mesoscale Discussion zooms into a smaller area and shorter timeframe when conditions are changing rapidly. Watches (issued by SPC) mean severe weather is possible within a specific region over the next few hours; Warnings (issued by your local National Weather Service office) mean it is happening or about to happen in your when ready area.

Key Takeaways

  • SPC forecasts only severe weather — tornadoes, large hail, damaging wind, and fire weather — not routine rain or thunderstorms.
  • A Convective Outlook covers three days and shows risk zones across the country; a Mesoscale Discussion focuses on a smaller area when conditions are changing fast.
  • SPC issues Watches (severe weather is possible); your local National Weather Service office issues Warnings (it is happening now).
  • You can check SPC forecasts on weather.gov/spc or through weather apps that pull from National Weather Service data.
  • Risk levels range from General Thunderstorm Risk to High Risk, and each level describes what type of severe weather is most likely.

How SPC divides the country into risk zones

SPC's Convective Outlook uses five risk categories, each with a specific color and meaning. General Thunderstorm Risk (green) means thunderstorms are possible but not expected to be severe. Marginal Risk (yellow) means isolated severe storms may occur. Slight Risk (orange) means scattered severe storms are possible. Moderate Risk (red) means numerous severe storms are expected. High Risk (magenta) means widespread severe weather is expected and conditions are particularly dangerous.

These zones change daily. SPC meteorologists update the Convective Outlook each morning, and again in the afternoon if conditions warrant. If you live in an area under Slight Risk or higher, severe weather is more likely than on a day with no risk marked. The size of the zone also matters: a High Risk covering a small area is more precise than one covering several states, because SPC is more confident about where storms will form.

The difference between Watches and Warnings

A Severe Thunderstorm Watch or Tornado Watch issued by SPC means conditions are favorable for severe weather to develop within a specific region over the next few hours. A watch does not mean a storm is imminent — it means you should monitor forecasts and be ready to take shelter if a warning is issued. Watches typically cover a county or multi-county area and last two to eight hours.

A Severe Thunderstorm Warning or Tornado Warning issued by your local National Weather Service office means severe weather is occurring or about to occur in your when ready area. A warning is the signal to take action: move to shelter, avoid windows, and stay alert. Warnings are much smaller in area than watches and last minutes to an hour. You will receive warnings through weather alerts on your phone, local news, or sirens if your area has them.

Reading a Mesoscale Discussion

When SPC meteorologists see conditions changing rapidly in a smaller area, they issue a Mesoscale Discussion to explain what is happening and what they are watching for. A Mesoscale Discussion is not a watch or warning — it is a detailed explanation of the weather setup, the timing of storms, and which type of severe weather is most likely (tornado, hail, or wind).

Mesoscale Discussions are written for people who want to understand the "why" behind a forecast, not just the "what." They describe atmospheric conditions like wind shear, instability, and moisture that meteorologists use to predict storm behavior. If you see a Mesoscale Discussion for your area, it means SPC is actively monitoring the situation and may issue a watch within the next hour or two.

How to find SPC forecasts and updates

The main SPC website is weather.gov/spc. The homepage displays the current Convective Outlook map, with risk zones color-coded by day. You can click on any zone to read a detailed discussion explaining why that risk level was issued and what type of severe weather is expected. SPC also publishes Mesoscale Discussions, fire weather outlooks, and a list of current watches.

Most weather apps pull SPC data automatically, so if your phone shows a Severe Thunderstorm Watch or Tornado Watch, that information comes from SPC. Local National Weather Service offices (weather.gov/[your city]) issue warnings specific to your county. You can also sign up for alerts through your phone's weather app, your local news station, or a weather radio that broadcasts National Weather Service alerts.

What SPC does not forecast

SPC focuses only on severe weather. It does not issue forecasts for routine thunderstorms, rain, snow, cold, heat, or wind that does not meet the severe threshold. Those forecasts come from your local National Weather Service office or from broader National Centers for Environmental Prediction models. If you want to know whether it will rain tomorrow or how cold it will be, check your local forecast instead of SPC.

SPC also does not issue warnings — only watches. The moment a severe storm is confirmed by radar or a tornado is sighted, your local National Weather Service office takes over and issues a warning for your specific area. This division of labor means SPC can focus on the big picture (where severe weather is possible across the country) while local offices focus on the when ready threat (what is happening in your neighborhood right now).

Understanding fire weather outlooks

SPC also issues Fire Weather Outlooks for areas where dry conditions, low humidity, and strong winds create a high risk of wildfire spread. These outlooks use the same three-day format as the Convective Outlook and divide the country into risk zones: Marginal, Moderate, and High. A High Fire Weather Risk means conditions are dangerous for fire growth and spread — not that a fire is burning, but that if one starts, it will be difficult to control.

Fire Weather Outlooks are most common in the western United States during late spring and summer, and in the Southeast during fall and winter. If you live in an area under a Moderate or High Fire Weather Risk, avoid outdoor burning and be extra cautious with anything that could spark a fire. Local fire management agencies use SPC fire weather outlooks to decide whether to restrict burning or issue fire bans.

Frequently Asked Questions

What should I do if I see a Severe Thunderstorm Watch?

Monitor weather forecasts and local news for updates. A watch means conditions are favorable for severe weather, but storms may not develop or may stay weak. Keep your phone charged, know where your safe shelter is (interior room on the lowest floor, away from windows), and be ready to move quickly if a warning is issued for your area.

Can SPC predict exactly where a tornado will form?

No. SPC can identify the region where tornadoes are possible and issue a watch, but the exact location, timing, and strength of a tornado cannot be predicted more than a few minutes in advance. That is why your local National Weather Service office issues warnings based on radar and storm spotters — they can see the storm forming and warn your specific area.

How far in advance does SPC forecast severe weather?

The Convective Outlook covers three days, but confidence decreases with each day. Day 1 (today) is most accurate. Day 2 (tomorrow) is less certain. Day 3 is a general outlook of where conditions may be favorable, not a detailed forecast. Mesoscale Discussions focus on the next 6 to 24 hours when conditions are changing rapidly.

Is a Marginal Risk the same as no risk?

No. Marginal Risk means isolated severe storms may occur — they are just not expected to be numerous or widespread. You should still monitor forecasts if you live in a Marginal Risk area, especially if you are planning outdoor activities or traveling. Severe weather can happen even on days with low risk.

Why does SPC sometimes change the forecast during the day?

Atmospheric conditions change as the day progresses. New data from weather balloons, satellites, and radar help SPC meteorologists refine their forecast. If a system moves faster or slower than expected, or if moisture or wind patterns shift, SPC updates the Convective Outlook to reflect the new information. Checking the forecast multiple times on a day with severe weather risk is always a good idea.