SPC Outlooks: What does Slight, Moderate, High Risk mean?

Many times when severe weather is expected, meteorologists will refer to an area having a certain "risk" of severe weather.  When severe weather outbreaks are possible, the risk may be Slight, Moderate, or rarely High. So what do these terms mean?  

As an overview, the Storm Prediction Center (SPC), which is the NOAA/NWS organization responsible for severe weather forecasting in the U.S., issues convective outlooks covering the following 8 days.  On days 1-3, maps are drawn that may contain one of 4 colored areas: General Thunderstorm, Slight, Moderate, and High Risk.  General thunderstorm areas indicate the indicated zone will have a likelihood of thunderstorms which, for the most part, will be sub-severe.  (Remember a thunderstorm must produce 1" hail, 58 mph wind, or tornadoes to be considered severe.)  Slight, Moderate, and High risk areas represent progressively larger threat for organized severe storm episodes.

SLIGHT RISK:  Issued frequently during the peak severe weather season and implies that well-organized severe thunderstorms are expected but in relatively small numbers/coverage, or there is a small chance of a more significant severe event. Typically, Slight Risk areas produce scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes.  Significant severe events (75 mph wind, EF-2 tornadoes, or 2" hail) typically do not occur on Slight Risk days.

MODERATE RISK:  Issued much less frequently than Slight Risk areas (a couple of times a month during a typical severe weather season) and is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak.  A Moderate Risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a Slight Risk. Typical Moderate Risk days include multiple tornadic supercells with very large hail, intense squall lines with widespread damaging winds, or in some cases, land-falling tropical storm systems.

HIGH RISK: A High Risk area implies that a major severe weather outbreak is expected, with large coverage of severe weather and the likelihood of extreme severe storms (i.e., violent tornadoes or very damaging convective wind events).  On most High Risk days, a major tornado outbreak is expected.  The High Risk category is reserved for the most extreme events with the least forecast uncertainty, and is generally only used a few times each year.  In fact, 51 High Risk days have occurred in the past 12 years (an average of about 4 a year).

The forecast of Slight, Moderate, and High Risk is based on percentages indicating the probability of severe weather in the forecast area.  The tables below show the conversion from outlook probabilities to risk areas for Day 1 (today) and Days 2-3 (tomorrow, next day).  As an example, if there is a 15% probability of a tornado within 25 miles of a point on Day 1, a Moderate Risk area will be issued.

Graphic courtesy Wikipedia. 60% probability High Risk not used on Day 3.

Hopefully this helps explain the risk terms used frequently by MWN and many other meteorologists!  Learn more on the SPC website.

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