Forecasting early spring fire danger

While out of control brush fires and grass fires can happen anytime of year given the right conditions, fire season generally peaks from late February through mid April across the Great Plains and Midwest, including here in the Mid-Missouri River Valley. 

The presence of dormant vegetation coming out of the winter months combined with periods of dry windy weather make early spring an especially vulnerable time for wildfires. The risk is compounded by an increase in agricultural burning, a commonly used practice to clear out old vegetation, control invasive plant species, and improve soil quality.

Fire weather forecasting is crucial when it comes to alerting the public about elevated fire danger, including days where burning of any kind is prohibited. In the United States, the National Weather Service uses a system of alerts to get word out during high risk periods. 

A Fire Weather Watch is issued when fire weather conditions are possible, while a Red Flag Warning is issued when fire weather conditions are imminent or occurring. Local criteria for a Red Flag Warning to be issued includes sustained wind speeds >=20 MPH , and relative humidities <= 25%. 

The Fuel Moisture Index is also assessed before a Red Flag Warning is issued, derived from averaging weather conditions in a particular area over a specified period of time. FMI values are expressed in the form of a percent, and measured in different increments of time such as 10, 1000, or 1000 hours. These “lag times” represent the number of hours it would take for ⅔ of a particular dead vegetative fuel to respond to changes in atmospheric moisture. For example, 10 hour fuels include grass, leaves, and mulch. While 1000 hour fuels refer to sizable fallen trees, or brush piles. Each of these fuel increments can have an FMI value between 0 and 100%. A Red Flag Warning is issued when 10 hr FMIs drop below 9% while specified wind speed and relative humidity thresholds are met.

Just last week, a large brushfire broke out at Sugar Lake in southwest Buchanan county, triggering evacuations and scorching over 200 acres of land. Over a dozen local volunteer fire departments and the Missouri Department of Conservation worked together to get the fire under control. According to authorities, the original source of the fire is still unknown. 

Notably, a Red Flag Warning was not in effect at the time of the Sugar Lake brush fire. Relative humidity was low that day, however winds were too light to issue any fire danger related alerts. This highlights how quickly fires can spread on dry days in the presence of dry fuels, even if winds aren’t strong. The take home message here is to always burn with caution, and be mindful when disposing of fire causing agents like cigarette butts, with or without the presence of a Red Flag Warning.