Slight Risk, Moderate Risk, High Risk. What does it all mean? - KMSP-TV

Slight Risk, Moderate Risk, High Risk. What does it all mean?

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THE SPC: What does Slight, Moderate and High Risk mean?

As an avid viewer, I'm sure that you've heard it a lot lately. "The SPC has us in a slight, moderate or high risk or severe weather" and "The main threats are Strong winds or large hail", etc. What does that all mean? As we enter the peak of severe weather season here in Minnesota, I thought it would be great to do a recap and explanation of the SPC and their role in forecasting severe weather outbreaks.

First of all, the SPC stands for the Storm Prediction Center, and they are located in Norman, OK. They may be far away from Minnesota but they are right in the middle of severe weather forecasting. They are responsible for forecasting severe weather outbreaks across the country.

What we as meteorologist most often look at are the Convective Outlooks.  Check out this link .

It's a forecast for the potential of severe weather for the next 3-8 days. Just like with any forecast, the short term forecast is much clearer. Therefore, you will see the outlook for Day 1, Day 2 and Day 3. On these forecasts, you will see 5 things….. General Thunderstorms, "See Text", Slight Risk, Moderate Risk and High Risk.


The green shaded area on the map below shows where there is a chance of convective activity, in other words, thunderstorms. These are your non severe, garden variety storms that pop up a lot in the warmer months.



The "See Text" label applies to when a slight risk of severe weather was considered, but not warranted, due to the scale and severity of any potential outbreak. On these days it's always a good idea to be on the lookout for the possibility of an isolated severe storm or two.


This Slight Risk is issued quite a bit during severe weather season. When we are included in a slight risk for severe weather, it mainly means that we have a chance of seeing some organized severe weather. Although this weather may be small in coverage and widely scattered, it still can reach severe limits. Keeping in mind that a severe thunderstorm is one that produces 58 mph winds or greater and 1' diameter hail. Also, when we are in a slight risk of severe weather, a widespread tornado outbreak is not expected; however, a few tornadoes are possible.



Now here is where we step it up a notch. When we are in a moderate risk, things can get a little more interesting. There is a good chance of you being affected by severe weather on a moderate risk day. There is usually some kind of large scale system in play, such as an advancing front, area of low pressure, or a large flow of warm, moist air, etc. There is also a higher confidence in the forecast and the threat of severe weather is more certain. On a moderate risk day, large supercell thunderstorms, strong tornadoes, strong wind, large hail are usually are usually a possibility. On average, a moderate risk is only issued a few times a month during the peak of severe weather season.


Rarely issued, a high risk day is when it all comes together for a major, life threatening outbreak of severe weather. A high risk is reserved for the time when there is little doubt in the forecast and is put out just a few times a year. The High Risk is mainly issued for wind events, such as the recent derecho, or a major tornado outbreak. When you hear of a high risk day, it is definitely the day to pay attention to the forecast as life threatening weather is a strong possibility. This is without a doubt, a day to stay "Sky Aware".



You may also hear that large hail or strong winds will be the main threats. There are many factors that go into this determination. A thunderstorm that comes through could be dealing with unusually cool air aloft, like in the early spring here in Minnesota, therefore resulting in a better chance of large hail. Also, a line of storms could form and then blow through the state with strong winds. A warm front could be lifting through with a cold front right behind it, resulting in an elevated risk of tornadoes.

The next time you are tuning in to your FOX 9 forecast and hear the phrases, Slight, Moderate, or High Risk, you will now have a better understand of what to expect.



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