Most areas of the US are in the heart of severe weather. The Storm Prediction Center is responsible for forecasting the risk of severe weather caused by severe thunderstorms, specifically, those producing tornadoes, hail of one inch (2.5 cm) in diameter or larger, and/or winds of 60 miles per hour The agency also forecasts hazardous winter and fire weather conditions. It does so primarily by issuing convective outlooks, severe thunderstorm watches, tornado watches, and mesoscale discussions.
There is a three-stage process in which the area, time period, and details of a severe weather forecast are refined from a broad-scale forecast of potential hazards to a more specific and detailed forecast of what hazards are expected, and where and in what time frame they are expected to occur. If warranted, forecasts will also increase in severity through this three-stage process.
Graphical categorical risk: Using numbers, words, and colors, the categorical graphic depicts general thunderstorm areas (TSTM-light green) and up to five risk types (1-MRGL-dark green, 2-SLGT-yellow, 3-ENH-orange, 4-MDT-red, and 5-HIGH-magenta) based on the coverage and intensity of organized severe weather such as supercells, squall lines, and multicell thunderstorm complexes.
The TSTM area encloses where a 10% or higher probability of thunderstorms is forecast during the valid period. A 1-MRGL-dark green risk area includes severe storms of either limited organization and longevity, or very low coverage and marginal intensity. A 2-SLGT-yellow risk area implies organized severe thunderstorms are expected, but usually in low coverage with varying levels of intensity. A 3-ENH-orange risk area depicts a greater concentration of organized severe thunderstorms with varying levels of intensity. A 4-MDT-red risk indicates the potential for widespread severe weather with several tornadoes and/or numerous severe thunderstorms, some of which may be intense. A 5-HIGH-magenta risk area suggests a severe weather outbreak is expected from either numerous intense and long-track tornadoes, or a long-lived derecho system with hurricane-force wind gusts producing widespread damage. Don't let the terms fool you.
Graphical probabilistic risk: The probability of severe weather drives the outlook categories, with the tables below showing the conversion for each threshold. On days 1 and 2, the outlook contains individual severe probabilities for tornadoes, wind, and hail. With greater uncertainty about severe-storm type into the future, the outlook on day 3 only forecast the combined probability of all three types of severe weather. For all outlooks, the probability values represent the chance of severe weather occurring within 25 miles of any point, which is about the size of a major metropolitan area.
As an example, if you have a 15% probability of tornadoes, this means you have a 15% chance of a tornado occurring within 25 miles of your location. This may seem like a low number, but a tornado is very uncommon at any one location. Normally, your chances of getting hit by a tornado or other severe weather are small, purely based on a statistical average. Let's say you have a 1% statistical (climatology) history of tornadoes within 25 miles on this day, which still is large. Having a 15% probability means 15 times the normal odds of a tornado nearby, meaning it should be taken seriously. The probabilities for severe thunderstorm wind and hail also have the same meaning as they do for tornadoes, but typically will be higher numbers than for tornadoes, since they are much more common.
Sometimes, a black hatched area will be overlaid with the severe probabilities. Black hatching means a 10% or higher probability for significant severe events within 25 miles of any point. "Significant" is defined as: tornadoes rated EF2 or greater, thunderstorm wind gusts of hurricane force (74 mph) or higher, or hail 2 inches or larger in diameter.
The outlook on day 3 combines all forms (tornado, wind, and hail) into a single black hatched area for a 10% or higher significant-severe risk. In addition, for tropical cyclones (hurricanes, tropical storms, and depressions), the outlook on day 3 allows a 5% total severe probability to be an SLGT risk because they are specifically tornado-driven.
Remember, these are "outlooks," not watches.
Forecasting rare events such as tornadoes and the occurrence of large hail and damaging wind gusts is a difficult process and one that contains a large amount of uncertainty. Once again, do not rigidly associate the type of risk area (e.g., 2-SLGT-yellow) with the severe potential for any given thunderstorm in the risk area. Just because a 2-SLGT-yellow risk is a forecast does not necessarily mean that the thunderstorms within the risk area will be slightly severe. There have been times, violent tornadoes occur in 2-SLGT-yellow, 3-ENH-orange, or 4-MDT-red risk areas as opposed to 5-HIGH-magenta. The reason for this is the synoptic situation producing the violent tornadoes may be confined to a relatively small area or a conditional, uncertain situation. Another 2-SLGT-yellow risk area may cover several states in which only one or two tornadoes are expected to develop. Some 2-SLGT-yellow situations won't involve a threat of tornadoes or supercells but sustained multicell storms with a threat of severe hail and wind damage.
SPC severe weather outlooks forecast events from organized convection (e.g., supercells, squall lines, and multicell thunderstorm complexes), most capable of damage and injury from tornadoes, damaging winds, or large hail. Since almost any thunderstorm can produce a brief severe weather event, it doesn't necessarily mean there is a conflict when a severe thunderstorm warning is issued by a local NWS office outside of an SPC severe weather risk area.
No two situations are alike, even within the same risk category. This is why a probabilistic forecast and text discussion accompanies the categorical outlook. The probabilities used in the SPC Convective Outlooks are known as subjective probabilities. The forecaster makes their best estimate of the probability of an event occurring. The probabilities that you see on the graphics represent the probability of one or more events occurring within 25 miles of a point during the outlook period. This definition is used as the probability of severe weather at a given point is quite small. How many times have you experienced a tornado in your neighborhood? For most people, the answer is never. Now think of how many times severe weather has occurred within 25 miles of your location. It's probably safe to say that you can think of some close-by severe weather events. You should be able to imagine that the probability of having severe weather occur within such an area is much larger than the probability of having it occur specifically within any one neighborhood.
How should you interpret probabilistic values? The smallest values represent areas where the most uncertainty exists and correspondingly where the smallest expected coverage of storm reports exists. The higher the probabilities, the greater the perceived threat, and the greater the expected coverage of that hazard being forecast. The highest probabilities are generally reserved for more significant severe weather events and are used infrequently, if at all, during the year. Another way of thinking of the values is related to climatology. For example, if SPC forecaster drew a 30% area for tornadoes which included northwest Texas and southwestern Oklahoma in mid-May. The ratio of the forecast to climatology (30%/1.5%) yields a value of approximately 20. The SPC forecaster is stating they believe the risk of tornadoes in that region is 20 times larger than climatology. By comparing the forecast probability to climatology, you can better determine the magnitude of the risk on a given day. See the SPC Severe Weather Climatology page to find climatological values for where you live.
The most specific Convective Outlooks are those issued during the Day 1 period. Accordingly, SPC forecasters have the most information available to them to differentiate the threats of the individual severe weather hazards. During this period, the SPC produces probabilistic outlooks for each primary severe weather hazard (tornadoes, damaging wind, and large hail) separately. By producing separate forecasts for tornadoes, damaging wind, and large hail, the user is given substantially more information upon which to make decisions than in the categorical outlook. Users who are sensitive to one particular threat (e.g., car dealers and large hail) can make more informed decisions.
As with all weather forecasting, the specific details of severe weather forecasting can only be determined hours ahead of time, rather than several days, the severe weather probabilities for the Day 2 and Day 3 Convective Outlooks represent the probability of any severe weather hazard (large hail, damaging wind, or tornadoes) occurring. In addition, for tropical cyclones (hurricanes, tropical storms, or depressions), the outlooks on Day 2 and Day 3 allow a 5% total severe probability to be a SLGT risk because they are specifically tornado-driven.
Public Severe Weather Outlook (PWO) is issued on Day 1 for all category 5-HIGH-magenta risks and 4-MDT-red risks that are driven by tornadoes and/or damaging winds. This plain-language forecast is typically issued the morning of the event and is used to alert non-technical weather users concerned with public safety of a potentially dangerous situation.
SPC issues Mesoscale Discussions (MDs or MCDs) that focus on severe thunderstorm potential over the continental U.S. for the next 6 hours with an emphasis on the first 1-3 hours. SPC also issues MDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards, and freezing rain. All MDs contain areas affected line, concerning line, valid time, a paragraph for a summary, and a paragraph for a technical discussion, along with a graphical depiction of the highlighted area.
Watches (WWs) issued by the SPC are generally less than 20,000–50,000 square miles (52,000–129,000 km2) in the area and are normally preceded by a mesoscale discussion. Watches are intended to be issued preceding the arrival of severe weather by one to six hours. They indicate that conditions are favorable for thunderstorms capable of producing various modes of severe weather, including large hail, damaging straight-line winds and/or tornadoes. In the case of severe thunderstorm watches organized severe thunderstorms are expected but conditions are not thought to be especially favorable for tornadoes (although they can occur in such areas where one is in effect, and some severe thunderstorm watch statements issued by the SPC may note a threat of isolated tornadic activity if conditions are of modest favorability for storm rotation capable of inducing them), whereas for tornado watches conditions are thought to be favorable for severe thunderstorms to produce tornadoes.
In situations where a forecaster expects a significant threat of extremely severe and life-threatening weather, a watch with special enhanced wording, "Particularly Dangerous Situation" (PDS), is subjectively issued. It is occasionally issued with tornado watches, normally for the potential of major tornado outbreaks, especially those with a significant threat of multiple tornadoes capable of producing F4/EF4 and F5/EF5 damage and/or staying on the ground for long-duration – sometimes uninterrupted – paths. A PDS severe thunderstorm watch is very rare and is typically reserved for derecho events impacting densely populated areas.
Watches are not "warnings", where there is an immediate severe weather threat to life and property. Although severe thunderstorm and tornado warnings are ideally the next step after watches, watches cover a threat of organized severe thunderstorms over a larger area and may not always precede a warning; watch "busts" do sometimes occur should thunderstorm activity not occur at all or that which does develop never reaches the originally forecast level of severity. Warnings are issued by local National Weather Service offices, not the Storm Prediction Center, which is a national guidance center.
The process of issuing a convective watch begins with a conference call from SPC to local NWS offices. If after collaboration a watch is deemed necessary, the Storm Prediction Center will issue a watch approximation product which is followed by the local NWS office issuing a specific county-based watch product. The latter product is responsible for triggering public alert messages via television, radio stations and NOAA Weather Radio. The watch approximation product outlines specific regions covered by the watch (including the approximate outlined area in statute miles) and its time of expiration (based on the local time zone(s) of the areas under the watch), associated potential threats, a meteorological synopsis of atmospheric conditions favorable for severe thunderstorm development, forecasted aviation conditions, and a pre-determined message informing the public of the meaning behind the watch and to be vigilant of any warnings or weather statements that may be issued by their local National Weather Service office.
Watch outline products provide a visual map depiction of the issued watch; the SPC typically delineates watches within this product in the form of "boxes," which technically are represented as either squares, rectangles (horizontal or vertical) or parallelograms depending on the area it covers. Jurisdictions outlined by the county-based watch product as being included in the watch area may differ from the actual watch box; as such, certain counties, parishes or boroughs not covered by the fringes of the watch box may actually be included in the watch and vice versa. Watches can be expanded, contracted (by removing jurisdictions where SPC and NWS forecasters no longer consider there to be a viable threat of severe weather, in which case, the watch box may take on a trapezoidal representation in map-based watch products) or canceled before their set time of expiration by local NWS offices.