How to read the severe weather outlook: Marginal to High risk

The severe weather outlook is issued by the NOAA Storm Prediction Center in Norman, Oklahoma, and it is the map behind every "risk of severe storms" segment on local news. Each color is a category, and each category is a probability of severe weather within 25 miles of any point. About 67,000 people a month search for it directly, and many more see it without knowing what the colors are calibrated to. This is the key.

The five categories

• 1, Marginal (dark green): isolated severe storms possible, limited in coverage and intensity. Roughly a 5 percent chance of damaging wind or large hail, or 2 percent for a tornado, within 25 miles.
• 2, Slight (yellow): scattered severe storms. About 15 percent for wind or hail, 5 percent for a tornado. Slight is the most common category on any given spring day and is not a reason to cancel, but it is a reason to know where you would shelter.
• 3, Enhanced (orange): numerous severe storms, some intense. About 30 percent for wind or hail, 10 percent for a tornado.
• 4, Moderate (red): widespread severe storms, several intense. About 45 percent for wind or hail, 15 percent for a tornado with strong tornadoes likely. Moderate days produce most of the year's outbreaks.
• 5, High (magenta): a major outbreak expected. Reserved for roughly 30 percent tornado probability with violent tornadoes, or 60 percent for destructive wind. Issued only a handful of times a year and often not at all.

Below the numbered categories is a light green general thunderstorm area: storms, but not expected to be severe. "Severe" has a fixed definition: hail of one inch or larger, wind gusts of 58 mph or more, or a tornado.

Hatched areas and the significant threshold

Where the map is hatched, the SPC expects significant severe weather: tornadoes of EF2 or stronger, hail of 2 inches or larger, or gusts of 75 mph or more, with at least a 10 percent probability within 25 miles. A hatched Slight risk can be more dangerous than a plain Enhanced one, because the hatching is about intensity, not coverage. Under the map, the SPC publishes separate probability maps for tornado, wind and hail on Day 1; the category is derived from whichever of the three is highest.

When the map updates

The Day 1 outlook is issued at 6:00 AM Central and updated at 1:00 PM, 4:30 PM and 8:00 PM, with an overnight update at 1:00 AM. Day 2 and Day 3 are issued in the morning and updated once. Days 4 to 8 are simpler: a 15 percent or 30 percent area for any severe weather, with lower confidence. The afternoon updates matter most, because they incorporate the morning weather balloons and the first real look at how unstable the air actually became.

Outlook, watch, warning: three different clocks

The outlook covers a day and a region. A Tornado Watch or Severe Thunderstorm Watch, also issued by the SPC, covers the next several hours for a smaller area and means conditions are now favorable. A warning comes from your local NWS office for a storm that is happening, valid for under an hour over a polygon. The outlook tells you whether to pay attention today; the watch tells you when; the warning tells you where to be in the next fifteen minutes. The tornado watch vs warning guide goes into the last two.

What the models add

The outlook is the SPC's expert judgment; the raw material is model output on instability, wind shear and moisture. When the NWS blend, the American GFS and the European model disagree on the timing of a cold front by three or four hours, the risk can shift from an afternoon problem to an overnight one, which changes everything about how dangerous it is. On our city pages, Dallas or Nashville for example, the three models sit side by side hour by hour, and our accuracy scoreboard shows which one has earned trust in your city. Once storms are up, the radar replaces all of them.

Related: is a car safe in a lightning storm, and Red Flag Warnings for the dry-line days that produce both fire and storms.