Storm "Risk" Areas Defined - 04/17/13
When severe weather is a threat in the United States, the meteorologists at the Storm Prediction Center (SPC), in Norman, Oklahoma, are typically way ahead of the game. Because certain factors in the atmosphere, when acting in concert, are known to bring about thunderstorms capable of producing severe weather (hail 1" diameter or greater, wind gusts 60 mph or greater, tornadoes), the folks at the SPC work on predicting days in advance which areas of the country are at risk for seeing such storms. Simplified, the more favorable factors present in the atmosphere, the greater the risk.
There are four categories of risk used by the SPC to alert areas of the country to the possibility of storms. The categories can be found in the "Convective Outlook" section of the SPC's website (www.spc.noaa.gov), and are typically divided into "Day 1, Day 2, Day 3, and Days 4-8" outlooks. The lowest level of alert in a convective outlook is "general thunderstorms," and it means exactly what it says. In such an area, thunderstorms are possible, but none is likely to reach severe levels.
One step up from "general thunderstorms" is the first severe weather risk category, "slight risk." In an area at slight risk for severe thunderstorms, some of the parameters critical for severe weather development are present in the forecast model data (for the period noted in the outlook), but perhaps they're not strong enough to give a high level of confidence for the likelihood of such storms; or, perhaps there are also mitigating factors present which would counterbalance the likelihood of such storms.
The next step up from "slight risk" is the "moderate risk" category. Typically, the SPC issues a moderate risk when there is a relatively high level of confidence that severe storms will occur (somewhere in the risk area). This category usually indicates the presence of many, convincing severe weather parameters occurring simultaneously, creating an environment conducive for powerful thunderstorms. Operational meteorologists (a.k.a. weather forecasters) with much experience perk up when seeing a "moderate risk" forecast from the SPC because they're not issued very often, as opposed to the "slight risk" category.
The highest SPC risk category, "high risk," is reserved for the most serious of storm outlooks. In a "high risk" environment, numerous severe weather parameters are present, many of them at impressive levels. Not only is there a high likelihood of severe weather in such an outlook area, the storms that occur will be quite dangerous. A "high risk" outlook is often an indicator of the worst thunderstorms can bring, including a tornado outbreak.