Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
Red Flag Criteria in the National Weather Service Glossary
| Factor | Criterion |
|---|---|
| Fuels | A dry spell of a week or two, with fire danger rated high to extreme |
| Wind | Sustained winds averaging 15 mph or greater |
| Relative humidity | 25% or less |
| Temperature | Above 75°F |
Local offices set their own thresholds. NWS Baltimore/Washington, for example, uses sustained winds of 20 mph or greater with relative humidity below 30% in Virginia and Maryland. Some states also count dry lightning and unstable air.
Key Points
- A Fire Weather Watch may be issued ahead of a Red Flag Warning.
- The warning is about the weather, not about a fire. It means a fire that starts could spread quickly.
Related Terms
Relative Humidity
The ratio of the current absolute humidity to the maximum possible humidity at the current temperature, expressed as a percentage. 100% relative humidity means the air is fully saturated. Relative humidity changes with temperature even if moisture content stays the same.
Humidity
The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.
AQI
The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's color-coded scale for reporting air pollution. It has six categories, from Good (0 to 50) to Hazardous (301 and higher). Values of 101 and above are unhealthy, first for sensitive groups and then, as the number rises, for everyone.
PM2.5
PM2.5 is fine particulate matter: airborne particles 2.5 micrometers or smaller in diameter. An average human hair is about 30 times wider than the largest of them. The particles are small enough to reach deep into the lungs, and some enter the bloodstream, which makes PM2.5 the particle pollution with the greatest health risk.
Watch vs. Warning
A watch means the risk of hazardous weather has risen but its timing, location, or occurrence is still uncertain: be prepared. A warning means hazardous weather is occurring, imminent, or likely and threatens life or property: take action now. An advisory covers events that are occurring or likely but less serious than a warning.
Dew Point
The dew point is the temperature to which air must be cooled to become saturated, so that water vapor begins to condense into dew or fog. The higher the dew point, the more moisture is in the air. In summer, a dew point of 55°F or lower feels dry and comfortable, and 65°F or higher feels muggy.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Feature
Wildfire Tracking
Follow active wildfires, smoke forecasts, and fire weather conditions.
Tool
Wind Speed Converter
Convert between mph, km/h, knots, and m/s for wind reports and forecasts.
Feature
Custom Alerts
Set weather notifications that match the places and conditions you actually care about.
Guide
NWS Outlook vs Watch vs Warning
Know how alert wording changes from early awareness to immediate action.
Sources
Explore Other Terms
Blizzard
A blizzard is a storm with sustained winds or frequent gusts of 35 mph or more and falling or blowing snow that frequently reduces visibility to below a quarter mile, lasting 3 hours or longer. The National Weather Service definition has no snowfall or temperature requirement: wind and visibility make a blizzard.
King Tide
King tide is a popular, non-scientific name for an exceptionally high tide. King tides happen when the higher tides around a new or full moon coincide with the Moon's closest approach to Earth, called perigee. They can flood low-lying coastal areas on a clear day, and they raise the water level any storm surge is added to.
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
Kp Index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
Liquefaction
A process in which water-saturated, loosely packed soil temporarily behaves like a liquid during strong shaking and loses its ability to bear weight. It can tilt or sink buildings, float buried tanks and pipes upward, and cause the ground to crack and spread sideways. Reclaimed land, river deltas, and sandy fill are especially vulnerable.
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
Browse by Category
In the WeatherAI App
WeatherAI displays official NWS alerts, SPC fire weather outlooks, and U.S. wildfire incident maps, and its free NWS hub includes the Fire Weather Forecast.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions. WeatherAI supplements official sources and is not an emergency notification service. For life-safety decisions, rely on your national weather service, local emergency management, and official warning channels.