What is fire weather?

Fire weather may be an unfamiliar term for some and is often overlooked or not taken seriously. While fire is commonly used for activities like yard work, camping, grilling, and farming, it can also rapidly spread out of control given the right set of circumstances. The warm, dry, and windy days of early spring and late fall can result in the highest risk for spreading fires. During the most susceptible periods, the National Weather Service will issue a Red Flag Warning.

Fire weather conditions must meet specific criteria before a Red Flag Warning (also known as a Fire Weather Warning) is issued. Locally, to meet this criteria, winds must be sustained at 20 mph or more for at least two hours, relative humidity must be 25% or lower, and there must be sufficient dry fuels. 

When we think of dry vegetation that can provide fuel for fires, drought conditions or rainfall deficits might come to mind.The most common reason for dry fuels through the year locally has less to do with drought, and more to do with dormant vegetation that hasn’t had the chance to green up during the early spring and late fall. Even if soils are somewhat moist, just a few days of dry and windy weather can leave dormant grass and brush bone dry.

In addition to Red Flag Warnings, there are also fire weather watches. The key difference is timing. A watch is issued up to 72 hours before the criteria mentioned above occurs, while a warning means those conditions are occurring or imminent within 24 hours.

Regardless of whether a watch or warning is in effect, burning is strongly discouraged when fire weather conditions are elevated. Even small actions, like discarding a cigarette butt or any kind of open burning, can spark erratic brush or grass fires when conditions are right.