Significant severe is the Storm Prediction Center (SPC) label for the high end of each severe hazard: a tornado rated EF2 or stronger, hail 2 inches or larger, or thunderstorm winds of 65 knots (about 75 mph) or more. It is the baseline for the Conditional Intensity Groups that SPC began issuing on 03/03/26 and now shows on its Day 1 to 3 outlooks.
What counts as significant severe
SPC splits the significant severe label by hazard. A tornado qualifies at EF2 or stronger, hail at 2 inches or larger, and thunderstorm wind at 65 knots or more.
Any one of the three is enough. A single 2-inch hailstone report counts as significant severe even if no tornado or high wind occurs that day.
How it connects to Conditional Intensity
Each hazard's lowest intensity group starts at the significant threshold. For tornadoes, CIG1 pairs with a reasonable maximum of EF2, for hail it points to greater potential for stones 2 inches or larger, and for wind it points to peak gusts of 65 knots or more.
An SPC presentation on the change lists EF2, 65 knots and 2 inches as the current significant definition. SPC research verified experimental conditional intensity forecasts by comparing significant and non-significant severe reports in each group.
The Conditional Intensity entry explains the groups themselves and how they combine with the outlook probabilities.
The older 10 percent area
Before 03/03/26, SPC outlined areas with a 10 percent or greater chance of significant severe events within 25 miles of a location, carried under a label called SIGN. NWS Little Rock described that area as an unconditional significant severe probability.
Conditional Intensity Groups replaced the SIGN label and are not tied to the 10 percent floor. SPC's change notice said its significant severe probability grids for tornado, wind, hail and total severe would continue, with even-numbered levels such as 2 and 4 percent added to the 10 percent threshold.
What it does not tell you
Neither the old 10 percent area nor a current intensity group promised high-end weather at any one place. Both describe potential across an area, and many points inside them see nothing severe.
Outlooks, case studies and archived discussions from before 03/03/26 still show the old area. Read those maps with the 10 percent meaning in mind, and read current outlooks through the intensity groups.