SPC Severe Weather Outlooks

David Imy (ret.) and Roger Edwards, Storm Prediction Center

Days 1, 2 and 3

Overview: The Day 1, 2 and 3 convective outlooks consist of a text narrative and graphic depicting severe and general thunderstorm threats across the continental United States. The outlook text is written in scientific language for sophisticated weather users. A synopsis usually is included, except at 20 UTC when a short updates-only section appears above the previous outlook's text. The synopsis gives a general overview of the weather pattern, emphasizing features that will influence the thunderstorm and severe-weather threats.

Text outlooks:The text discussion is broken into geographic areas by headlines. Within the individual geographic threat areas, the text offers meteorological reasoning for each risk area as well as information on timing, specific severe weather threats and the expected intensity of the event.

Risk categories: Three risk categories (SLGT, MDT, and HIGH) stand for the coverage and intensity of organized severe weather such as supercells, squall lines, and multicell thunderstorm complexes. Pulse type thunderstorms, consisting primarily of solitary brief severe updrafts (often found in weakly sheared environments) are not considered organized. Isolated severe storms with marginal intensities or short durations of severe weather likely will not be included in one of the three severe risk areas, due to their random nature. The unlabeled (brown) thunderstorm line on the outlook graphic depicts, to the right of the line, a 10% or higher probability of thunderstorms during the valid period. A SLGT risk implies well-organized severe thunderstorms are expected, but in small numbers and/or low coverage. A MDT risk indicates a potential for a greater concentration of severe thunderstorms than the slight risk, and in most situations, the storms and associated severe weather are more intense. A HIGH risk area suggests a major severe weather outbreak is expected, with the expectation of either a concentration of strong tornadoes or an enhanced likelihood of a long-lived wind event (derecho) with the potential of higher end wind gusts (80+ mph) and structural damage.

On days when the forecast severe-storm threat is not organized enough or is too conditional for a SLGT risk, but marginal threat still exists, a SEE TEXT label will be placed on the graphic where a 5% severe probability exists.

Hazard probabilities: The probability of severe weather drives the outlook categories. The tables below show the conversion from probabilities to SLGT, MDT and HIGH categories for the day-1, day-2 and day-3 outlooks. The day-1 outlook contains individual severe probabilities for wind, hail and tornadoes. Because uncertainty about severe-storm type increases into the future, the day-2 and day-3 outlooks 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 are in a 15% probability for 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 statistical average. Let's say you have a 1% statistical (climatology) history of tornadoes within 25 miles on this day, which still is unusually large. Being in a 15% tornado area means 15 times the normal odds of a tornado nearby, meaning it should be taken seriously. The probabilities for hail and damaging thunderstorm wind also have the same meaning as they do for tornadoes, but typically will be higher numbers than for tornadoes, since severe hail and wind are much more common.

Sometimes, a blue hatched area will be overlaid with the severe probabilities. Blue hatching means a 10% or greater probability for significant severe events within 25 miles of a location in the area. "Significant" is defined as:

  • Tornadoes rated EF2 or greater,
  • Thunderstorm wind gusts of hurricane force or higher--65 kt or 74 mph;
  • Hail 2 inches or larger in diameter.

    Just as with the other severe event probabilities, the day-2 and day-3 outlooks combine all forms (hail, wind and tornado) into a single blue hatched area for a 10% or greater significant-severe risk. A SEE TEXT label means a 5% total probability of severe weather. The exception is for tropical cyclones (hurricanes, tropical storms and depressions). With tropical cyclones only, 5% lines on day-2 and day-3 outlooks become SLGT risks because they are specifically tornado-driven.

  • Day 4-8

    The Day 4-8 outlook is a single graphic and short narrative covering well-organized severe-weather potential 4-8 days out. There is no general thunder forecast, significant-severe area or SEE TEXT because of the long time frame and uncertainties involved. Each line represents a 30% probability area for a day, or the same as an "enhanced slight risk" on day-1. If the threat looks lower than 30%, "POTENTIAL TOO LOW" will appear. If the threat might exist, but is too unclear to draw a specific area, "PREDICTABILITY TOO LOW" is the label. The lack of a line does not mean that a severe threat is absent--just that the threat is too low, disorganized or uncertain for a 30% area that far out in time.

    On the day 4-8 outlook, each day's area (if any) is color-coded on the same graphic, using the table below.

    D4Day 4 D7Day 7
    D5Day 5 D8Day 8
    D6Day 6
    Each day is 12 UTC-12 UTC

    Page last modified 16 April 2013