The National Weather Service has issued a Red Flag Warning for the Santa Clarita Valley through 6 p.m. on Tuesday, Oct. 24. A Red Flag warning signifies extreme fire danger. Should a fire break out during a Red Flag warning, says the National Weather Service’s website, it would have the potential to spread quickly and threaten structures and lives. The warning was issued on Saturday to large swaths of Los Angeles and Ventura Counties, which are also expected to reach triple-digit temperatures on Monday and Tuesday. The Signal spoke with the National Weather Service about what constitutes a Red Flag warning and why one was issued for the Santa Clarita Valley. Why a Red Flag warning? Red Flag warning are issued mainly to firefighting agencies, to ensure they are aware of heightened fire danger – but they are only issued if a certain set of criteria are met. Each region has its own criteria, said Meteorologist Kathy Hoxsie of the National Weather Service. The necessary conditions in the Antelope Valley, for example, are different from those in the Santa Clarita Valley. Santa Clarita’s are based on relative humidity and wind. Relative humidity, the amount of water vapor present in the air, must be less than or equal to 15%. Wind is a little more complicated. “We look at wind speed and duration over a six hour period,” Hoxsie said. Winds must be sustained at 25 mph with gusts of 35 mph. The seasonal Santa Ana winds, which are currently hitting the Santa Clarita Valley, usually satisfy this. “Anytime there are Santa Ana winds, 25 mph is easy,” said Hoxsie. But there is a caveat; if relative humidity is equal to or less than 10%, wind speed “does not need to be very strong.” As of 3 p.m. on Sunday, the Santa Clarita Valley meets these criteria. The Newhall Pass weather station reads 84 degrees, 11% humidity and 25 mph winds. Hoxsie explained that wind speed often picks up as it comes through the pass’ canyons. Relative humidity at the Saugus station is at the 10% mark. Saugus is also experiencing 86 degree heat and 28 mph wind gusts. What next? Once a Red Flag warning has been issued, the National Weather Service and local fire authorities employ another key measurement to assess the risk of fire – fuel moisture. The NWS defines ‘fuel moisture’ as the amount of water present in vegetation that could potentially catch fire. A high fuel moisture indicates low fire danger; low fuel moisture, the opposite. “It’s the key interpretation that we look at for fire weather,” said Hoxsie. “It indicates the temperature the brush has to be before the fire will ignite.” Fuel moisture, she explained, is measured by weight. Weather and fire department stations use a plank of wood, of which they know the dry weight and susceptibility to moisture, to gauge the measurement. The heavier the weight of the wood, the more ambient moisture it has absorbed – signifying higher humidity and lower fire danger. During a Red Flag warning, such as the one currently issued, the fuel moisture percentage is quite low. Weather stations in Santa Clarita have mirrored this trend on Sunday, indicating that the valley’s rolling foothills are dangerously dry.