The Storm Prediction Center (SPC) is the national center that forecasts severe thunderstorms and tornadoes for the contiguous United States. Part of the National Weather Service's National Centers for Environmental Prediction (NCEP) and based in Norman, Oklahoma, it issues convective outlooks, mesoscale discussions, and tornado and severe thunderstorm watches.
From days out to hours out
SPC's convective outlooks cover Day 1, Day 2 and Day 3, plus a Day 4 to 8 outlook. In the Day 4 to 8 product, a highlighted area means a 15 percent, 30 percent or higher chance of severe thunderstorms within 25 miles of any point.
When models disagree, the ensemble spread is large or runs lack continuity, the Day 4 to 8 outlook can say predictability is too low. That label is a statement about uncertainty, not a forecast of quiet weather.
Discussions and watches
As storms near, SPC issues mesoscale discussions that explain what forecasters are watching in a smaller area. Those concerning a possible watch can include a stated probability that one will be issued.
SPC issues tornado and severe thunderstorm watches for the whole country. These are a notable exception to the usual pattern of local NWS offices issuing watches and warnings for their own areas.
Beyond severe storms
SPC also issues fire weather outlooks for Day 1, Day 2 and Days 3 to 8, using categories such as Elevated. The National Oceanic and Atmospheric Administration (NOAA) describes SPC as keeping a continuous watch on mesoscale atmospheric processes.
SPC shares a building with the NWS Norman forecast office, a reminder of how closely the national and local layers work together.
Reading SPC products in context
An outlook is a probability over a broad area and a long window. It does not say that any one town will see a severe storm, and a low-probability area can still produce a significant event.
Use the products as a sequence: outlooks frame the day, discussions show where attention is narrowing, watches mark where conditions favor severe storms, and local warnings address individual storms.