The Convective Outlook (AC) is the primary short-term severe weather forecast product issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. This product serves as a crucial communication tool for meteorologists and the public regarding the potential for organized severe thunderstorms across the contiguous United States. The outlook communicates the geographical location and the likelihood of different types of severe weather events developing. It provides a detailed, color-coded map and corresponding text analysis, highlighting areas likely to support storms capable of producing tornadoes, large hail, or damaging winds.
Defining the Forecast Window and Scope
The Convective Outlook specifically addresses the weather conditions expected during the Day 1 period, which covers the following 24 hours. The forecast is valid from 1200Z (Coordinated Universal Time) on the day of issuance until 1200Z the next day, establishing a defined window for potential hazards. This product is updated multiple times throughout the day to account for evolving atmospheric conditions. The SPC typically issues updates around 0600Z, 1300Z, 1630Z, 2000Z, and 0100Z.
Scope of Convective Weather
The scope of the outlook focuses strictly on convective weather, which refers to thunderstorms capable of producing severe weather. The definition of severe weather includes tornadoes, hail one inch in diameter or larger, and thunderstorm wind gusts of 58 miles per hour or greater. The forecast domain covers the entire contiguous United States, allowing for a broad, national-level assessment of the day’s severe weather potential. The Day 1 outlook provides a foundational perspective for more localized forecasts and warnings.
Interpreting the Categorical Risk Levels
The SPC uses a six-level categorical scale to communicate the overall potential for organized severe weather, regardless of the specific hazard type. The lowest level, designated as General Thunderstorms (TSTM), indicates a 10% or greater chance of non-severe thunderstorms.
The five levels of organized severe weather risk are:
- Marginal (MRGL, Level 1): Isolated severe storms are possible, but they are expected to be limited in coverage, intensity, or duration.
- Slight (SLGT, Level 2): Scattered severe storms are possible, but they are generally expected to be short-lived or not widespread.
- Enhanced (ENH, Level 3): Represents a greater threat, suggesting numerous severe storms are possible, which may be more persistent or widespread than those expected in a Slight risk area.
- Moderate (MDT, Level 4): Reserved for areas where widespread severe storms are likely, often including long-lived, intense storm cells, supporting a significant severe weather event.
- High (HIGH, Level 5): Reserved for the most dangerous and rare outbreaks when widespread, exceptionally intense, and long-lived severe storms are expected, often signaling a major tornado outbreak or a widespread, destructive wind event.
Identifying Specific Severe Weather Threats
While the categorical risk provides an overall sense of the threat, the Convective Outlook also details the specific hazards expected within those risk areas. The three primary severe hazards forecast are tornadoes, damaging straight-line winds, and large hail. For each of these hazards, the SPC uses probabilistic percentages to communicate the specific chance of an event occurring.
These percentages define the probability that an event, such as a tornado or a severe wind gust, will occur within 25 miles of any given point in the shaded area. For instance, a 15% probability for hail means there is a 15% chance of one-inch or larger hail occurring within a 25-mile radius of a location in that zone. The wind probability specifically addresses gusts of 58 mph or greater, while the hail category focuses on hail one inch or greater in diameter.
The outlook also draws attention to particularly intense events through the use of hatched areas. A hatched region on a probabilistic map indicates a 10% or greater probability of “significant severe” weather. This designation applies to tornadoes of Enhanced Fujita Scale 2 (EF2) strength or greater, wind gusts of 75 mph or faster, or hail two inches in diameter or larger. The presence of a hatched area signals that if severe weather occurs, it has a higher chance of being extreme and causing substantial damage.