The Significant Severe Area, shown as a hatched overlay on SPC probabilistic maps, marks the region where forecasters expect at least a 10 percent chance of a high-end severe report within 25 miles of a point. It signals that the environment supports not just severe weather, but severe weather at the upper end of the intensity scale.
What counts as significant severe
SPC uses three fixed thresholds to define significant severe reports. For tornadoes, an EF2 or greater rating. For hail, a diameter of 2 inches or greater. For wind, gusts of 65 knots, about 74 mph, or greater. Any of the three qualifies its hazard as significant.
The 10 percent value is a probability threshold for one of these high-end reports occurring within 25 miles of a point, layered on top of whatever the ordinary severe probability happens to be. It is a conditional intensity signal. Given that severe weather is expected, the hatch says forecasters believe a meaningful share of it may be high-end.
How to read the hatched area
Hatching narrows attention within a broader risk area. An Enhanced Risk with hail hatching means significant hail is a real possibility somewhere in the outlined region. A Moderate Risk with a tornado hatch means the environment supports strong or violent tornadoes if storms fire and organize. Chasers and emergency managers weight these areas heavily in their planning.
The hatch also composes across hazards. A region hatched for both tornado and hail is one where multiple high-end modes are on the table, which is often more informative than the category label alone.
What the hatching does not do
The hatch does not localize anything. A hatched area might cover several counties, and the eventual significant reports, if they occur, will land somewhere in that footprint but not everywhere. Being inside the hatch is not a guarantee that any given point will see EF2-plus damage or 2-inch hail.
The hatch is also not a warning. It is a probabilistic signal about intensity potential over hours to a day. If the setup evolves faster or slower than expected, or if storms fail to organize, a hatched region can go entirely quiet.