The rules
Every day at every track gets one flag, judged over the hours that kind of track actually races, in the track's time zone. The worst hour in them sets the flag. A shower at breakfast does not decide a Saturday-night dirt show.
- Ovals: 10 am to 10 pm
- Paved short tracks: noon to 11 pm
- Road courses: 8 am to 7 pm
- Street circuits: 9 am to 10 pm
- Dirt tracks: 4 pm to 11 pm
- Drag strips: 9 am to 10 pm
On the main day of a race weekend with a published start time, the day is judged from 3 hours before the green flag to 6 hours after it instead. A weekly track with a regular race night is judged the same way on those nights, around its published green flag.
- Green flag Race on. Under a 30% chance of rain every hour, no measurable rain, gusts under 25 mph, air between 40 °F and 95 °F, no lightning in the forecast.
- Yellow flag Caution. A 30 to 59% chance of showers in some hour, or a shower in the forecast with under a 30% chance of it (the amount is one run of the weather model; the chance is how many of its runs agree), or gusts of 25 mph or more, or heat of 95 °F and up, or cold of 40 °F and under.
- Red flag Weather hold. Rain likely (a 60% chance or more, or at least a millimetre in an hour with a 30% chance or more behind it), lightning possible (storm energy with rain in the forecast, or a thunderstorm code), or snow.
Dirt tracks also count the rain before the gates open, because it decides the surface: 0.2 in (5 mm) or more between 6 am and the start of the racing hours is a yellow flag, and 0.59 in (15 mm) or more a red one, whatever the evening looks like.
National Weather Service alerts
The forecast is a model. The National Weather Service is forecasters watching the radar. Every five minutes we read the NWS's active watches, warnings and advisories and match them to each track: by the warning's own outline when it has one, otherwise by the track's forecast zone and county. An alert shows at the top of the track's page with a link to the full text at weather.gov, and it raises the flag for the hours and the racing days it covers.
- Red flag Tornado Warning, Severe Thunderstorm Warning, Flash Flood Warning, Extreme Wind Warning, Hurricane Warning, Tropical Storm Warning, Blizzard Warning, Ice Storm Warning, Winter Storm Warning, Snow Squall Warning, Lake Effect Snow Warning, Dust Storm Warning.
- At least yellow Tornado Watch, Severe Thunderstorm Watch, Flash Flood Watch, Flood Watch, Hurricane Watch, Tropical Storm Watch, Winter Storm Watch, Winter Weather Advisory, Freezing Rain Advisory, High Wind Warning, High Wind Watch, Wind Advisory, Lake Wind Advisory, Blowing Dust Warning, Blowing Dust Advisory, Dust Advisory, Extreme Heat Warning, Extreme Heat Watch, Excessive Heat Warning, Excessive Heat Watch, Heat Advisory, Extreme Cold Warning, Extreme Cold Watch, Cold Weather Advisory, Wind Chill Warning, Wind Chill Watch, Wind Chill Advisory, Dense Fog Advisory, Freezing Fog Advisory, Dense Smoke Advisory.
- Shown, flag unchanged: Flood Warning, Flood Advisory, Special Weather Statement, Severe Weather Statement, Air Quality Alert, Red Flag Warning, Fire Weather Watch. The NWS "Red Flag Warning" is about fire danger; it is not our red flag.
We show an alert only while we can confirm it: if we cannot read the NWS feed for 45 minutes, alerts come off the page until we can again. For anything to do with your safety, go by the NWS itself and the track's officials.
The map's "roughness" list ranks tracks 0 to 100 by how much the forecast should bother a race day: rain chance and amount, gusts over 18 mph, lightning potential, heat past 95 °F, cold under 32 °F and snow, again over the racing hours, with a National Weather Service warning or watch on top. It is a ranking aid, not a prediction of a postponement.
Questions
What makes a day a red flag?
Rain likely in the racing hours (a 60% chance or more, or at least a millimetre in an hour with a 30% chance behind it), lightning possible, snow, or a National Weather Service warning for the track.
Why is my day yellow when there is no rain?
Yellow also covers gusts of 25 mph or more, heat of 95 °F and up, cold of 40 °F and under, a 30 to 59% chance of showers, or a National Weather Service watch or advisory. The reason is listed next to every flag.
Why did the flag change since this morning?
The forecast is refreshed through the day and the flag is worked out again each time. A shower that moves into or out of the racing hours, or an alert from the National Weather Service, changes it.
Does a yellow flag mean the race will be called off?
No. It means the weather is worth watching. Tracks and series make the call, often from the radar a few hours out.