From time to time, we communicate severe weather risk levels from the Storm Prediction Center. These are areas that have been marked by the SPC for the POTENTIAL of severe weather formation. Many who are put under risk levels do not end up getting a severe thunderstorm over them at all. The risk levels were not designed to communicate that all within a level will get severe storms.
REMEMBER: Any time you are in a risk area you should pay attention to weather reports and heed any and all watches and warnings. Some severe storms will occur without a watch being issued (especially in areas marked SLIGHT or lower). Being in a risk area is NOT the same thing as a guarantee of storms but, no matter what level of risk you are under, it could take just one storm to impact your life.
ALSO IMPORTANT: Be sure to have multiple means to receive weather information in severe situations. A combination of NOAA Weather Radio, phone app, local media, and online sources should do the trick.
WATCH ISSUANCE CRITERIA
“When conditions become favorable for organized severe thunderstorms or tornadoes to develop, the SPC issues a Severe Thunderstorm or Tornado Watch. A tornado can occur in either type of watch, but Tornado Watches are issued when conditions are favorable for either multiple tornadoes or a single intense tornado. A Severe Thunderstorm Watch is issued when there is a forecast of organized convection producing at least 6 total severe weather events.”
– Storm Prediction Center
When the Storm Prediction Center believes that the severe threat is extremely isolated and will likely result in less than 6 severe weather events (wind, hail) or less than 2 tornadoes, the SPC will only issue what is known as a Mesoscale Discussion. The Mesoscale Discussion is the SPC summary of atmospheric conditions in an area that is being monitored for watch issuance. The SPC also assigns a percentage likelihood of watch issuance. The lower the percentage, the less likely the SPC will issue a Watch. We post the Mesoscale Discussions as a heads up for those times when very few severe events are expected but require a heads up and in situations where severe weather may break out and the discussion is a good way to give an hour or two of additional lead time.
SPC RISK LEVELS
Color on Map: Light Green
Severe Risk: Less than 5% (within 25 miles of any point in the indicated region, same margin of error applies to all risk areas) (SPC officially says none, but we’ve seen rare instances where severe storms pop up in a “general” area)
What does this mean: Main threats are lightning and flooding due to heavy downpours
Color on Map: Dark Green
Severe Risk: Between 5 and 14.9 percent (within 25 miles)
What does this mean: Isolated severe storms possible that will likely die out quickly (like when we say one or two storms may reach severe qualifications)
Color on Map: Yellow
Severe Risk: 15 to 29.9 percent (within 25 miles)
What does this mean: Scattered severe storms possible and a few that could be intense
Color on Map: Orange
Severe Risk: 30 to 44.9 percent (within 25 miles)
What does this mean: Widespread severe storms possible with intense storms more likely than in a Slight Risk.
Color on Map: Red
Severe Risk: 45 to 59.9 percent (within 25 miles)
What does this mean: Long-lived and widespread storms, significant severe weather possible
Color on Map: Purple
**Used only in extremely high confidence situations**
Severe Risk: 60 percent or greater (within 25 miles)
What does this mean: Used in tornado outbreaks, or conditions which may favor derechos.