The National Weather Service says it's due to low humidity drying out vegetation.
Sterling, Va. (NWS) Gusty winds are expected Saturday afternoon across much of the area. Low relative humidities are forecast during this time as well with values below 30%.
While some areas had rain in the last 24 hours, windy conditions and low humidity on Friday will have dried out fuels across the area. All of these factors combined will result in an elevated risk for the spread of fires Saturday afternoon. Outdoor burning is strongly discouraged during this time. Please refer to your local burn permitting authority on whether you can burn. If you do burn, use extreme caution and ensure fire suppression is readily available.
By The National Weather Service, Sterling, Va.