The Storm Prediction Center (SPC) is a division of NOAA and the National Weather Service, which has a team of forecasters who specialize in forecasting severe weather (and fire weather). The SPC offers several unique products related to severe weather, which can be used to alert people to the potential for severe thunderstorms before there's even a cloud in the sky. These products cover the Contiguous United States (CONUS), which is also known as "The Lower 48" (meaning these products do not cover Alaska, Hawaii, or United States territories).


i. Severe Thunderstorm Definition

To better understand the Storm Prediction Center's products, one must be familiar with the definition of a "severe thunderstorm". The United States National Weather Service defines a severe thunderstorm as a thunderstorm that produces one, two, or all of the following:

         - Tornado
         - Straight-line winds of at least 50 knots (58 mph)
         - Hail at least 1.00 inch in diameter (2.50 cm)

Heavy rainfall and frequent lightning are NOT official criteria for severe thunderstorms in the United States, though many severe thunderstorms produce heavy rain and frequent cloud-to-ground lightning.


ii. Convective Outlooks

The main product that gets the most attention is the convective outlook, which highlights areas where severe thunderstorms are possible and how intense the thunderstorms might be. Five main categories of risk are currently employed, and these risks are listed below (from least urgent to most urgent):

Marginal Risk (MRGL): Also known as a "Level 1 Risk" or "Category 1 Risk". A few thunderstorms may briefly become severe. Overall intensity should remain low. Marginal risks are mostly associated with very low (but nonzero) tornado potential, gusty to perhaps damaging straight-line wind gusts (40 to 60 mph), and small to perhaps large hail (up to one inch diameter).

Slight Risk (SLGT): Also known as a "Level 2 Risk" or "Category 2 Risk". Severe thunderstorms are likely, but overall coverage and intensity should remain low. Slight risks are mostly associated with low tornado potential, isolated straight-line wind damage, and large to perhaps very large hail (up to two inches diameter). A slight risk may also be issued when a variety of different severe weather scenarios are possible.

Enhanced Risk (ENH): Also known as a "Level 3 Risk" or "Category 3 Risk". Severe thunderstorms are expected, and overall coverage and intensity will be somewhat higher than a Slight Risk. Enhanced Risks are mostly associated with medium to perhaps significant tornado potential, scattered to perhaps widespread straight-line wind damage, and very large hail.

Moderate Risk (MDT): Also known as a "Level 4 Risk" or "Category 4 Risk". Widespread severe thunderstorms are likely, and overall intensity will be high. Moderate Risks are mostly associated with significant tornado potential, widespread straight-line wind damage, and destructive hail. The issuance of a moderate risk is somewhat rare and signals the potential for a volatile atmosphere capable of producing very dangerous weather conditions.

High Risk (HIGH): Also known as a "Level 5 Risk" or "Category 5 Risk". Widespread severe thunderstorms are expected, and overall intensity will be especially high. High Risks are mostly associated with tornado outbreaks and derecho events (long-lived intense straight-line wind storms). The issuance of a high risk is rare and signals the potential for a major threat to life and property.

The Storm Prediction Center also outlines areas where ordinary (non-severe) thunderstorms are expected. This is indicated by an unlabeled bright green line. If no thunderstorms are anticipated across the Contiguous United States (CONUS), then the outlook will be blank and have no risk contours at all.

Also part of the convective outlook is a probabilistic forecast for tornadoes, damaging wind, and large hail. A given probability is the likelihood of a particular hazard occurring within 25 miles of any point (roughly the size of a metropolitan area).

If a 10% tornado probability is issued, it means the probability of a tornado occurring within 25 miles of any point is (at least) 10%. Tornado probabilities above 10% are generally considered to be a "significant tornado risk", and a probability below 10% is usually classified as a "low tornado risk". The tornado outlook can also include a forecast area where strong tornadoes (EF2-EF5) are possible, which is plotted on WarnUS as a black line. 2%, 5%, 10%, 15%, 30%, 45%, and 60% are all probabilities that can appear on a tornado outlook.

If a 30% wind probability is issued, it means the probability of a severe wind gust (at least 58 mph) occurring within 25 miles of any point is (at least) 30%. Wind probabilities above 30% are generally considered to be a "significant wind risk", and a probability below 30% is usually classified as a "low wind risk". The wind outlook can also include a forecast area where some hurricane force wind gusts (74 mph or higher) are possible, which is plotted on WarnUS as a black line. 5%, 15%, 30%, 45%, and 60% are all probabilities that can appear on a wind outlook.

If a 30% hail probability is issued, it means the probability of large hail (at least one inch in diameter) occurring within 25 miles of any point is (at least) 30%. Hail probabilities above 30% are generally considered to be a "significant hail risk", and a probability below 30% is usually classified as a "low hail risk". The hail outlook can also include a forecast area where very large hail (at least two inches in diameter) is possible, which is plotted on WarnUS as a black line. 5%, 15%, 30%, 45%, and 60% are all probabilities that can appear on a hail outlook.


iii. Mesoscale Discussions

The Storm Prediction Center also issues what are known as "Mesoscale Discussions (MDs)", which highlight areas of short-term interest for severe thunderstorm potential (or winter weather potential). If a mesoscale discussion is issued for severe weather, it means the potential for severe thunderstorms is expected over the next 1-6 hours, and it may be necessary to issue either a tornado watch or a severe thunderstorm watch (if a watch is not already in effect). Severe weather mesoscale discussions are colored red. Winter weather mesoscale discussions are colored blue. Mesoscale discussions may also be issued for potential risk upgrades (e.g. upgrading a Moderate Risk to a High Risk). Any such mesoscale discussions will be colored pink.

If a severe weather (red) mesoscale discussion is issued for your location, be on the lookout for rapidly changing weather conditions.


iv. Convective Watches

The Storm Prediction Center is responsible for issuing all tornado watches and severe thunderstorm watches that affect the Contiguous United States (CONUS). Watches are generally issued where conditions over a large area favor the development of severe weather. Severe thunderstorms that are isolated in nature rarely prompt the issuance of a watch. A watch is intended to heighten the awareness of people within a specific region and let them know that hazardous weather may affect them in the near future. A watch is not the same thing as a warning. A warning means that immediate action should be taken to protect life and property. A watch means to be alert.

Standard severe thunderstorm watches are plotted in blue and cover an area where conditions favor the development of large hail and damaging winds. The threat for tornadoes is expected to be low, but tornadoes can still occur within a severe thunderstorm watch.

Standard tornado watches are plotted in red and cover an area where conditions favor the development of tornadoes and other forms of severe weather (e.g. large hail and damaging winds). Even though a tornado watch may be in effect, the potential for damaging winds and large hail should not be neglected. In fact, storms that produce tornadoes can also produce destructive hail and/or destructive straight-line winds.

If there is high confidence in an unusually significant threat to life and property, a watch can designated as a Particularly Dangerous Situation (PDS) watch. If a PDS tornado watch is issued (plotted as a pink color), it means that there is potential for several destructive and long-tracked tornadoes (as opposed to weak and short-lived tornadoes). If a PDS severe thunderstorm watch is issued (plotted as purple), it means there is the potential for widespread significant damage caused by a long-lived straight-line wind storm. All watches and warnings should be taken seriously, but PDS watches and the warnings within PDS watches should prompt immediate and extensive action for those who are affected. PDS watches are normally reserved for Moderate Risk and High Risk events.

Watches are drawn as a four-point polygon, but the watch can cover areas outside of the polygon and can exclude areas within the polygon. It is best to look at the counties affected by the watch. If you're in a county that is specifically included in the watch or near the watch, you should pay close attention to the weather as long as the watch is active for your location.


v. Storm Reports

A live compilation of storm reports is also maintained by the Storm Prediction Center website. These are eyewitness reports of severe weather (tornadoes, damaging winds, and large hail). Details about individual reports can be obtained by clicking on a report plotted on the map. Tornado reports are plotted as red dots, wind reports are plotted as blue dots (significant wind is plotted as a dark blue square), and hail reports are plotted as green dots (significant hail is plotted as a dark green square).

These reports are mostly used for verification purposes (i.e. assessing how accurate the severe weather forecast is), but the confirmation of hazardous weather is often incorporated into the issuance of warnings. However, some reports may be deemed low priority in certain situations and thus won't appear in a warning (e.g. if a large tornado is about to hit a major city, hail of one inch in diameter is not the main concern).


More detailed information about SPC products can be found here.