RIVERSIDE – A red flag warning signifying a high risk of wildfire because of high winds and low humidity went into effect this morning in much of Riverside County west of the mountains as late-season Santa Ana winds are expected to buffet Southern California over the next four days and maybe longer.
The red flag warning and a high wind warning, which indicates sustained wind speeds of 40 miles per hour and 58-mph gusts, were both expected to expire at midnight Thursday, according to the National Weather Service. But the Santa Ana winds could persist into Friday or Saturday, which would likely mean an extension of both warnings.
The red flag and high wind warnings both covered the Riverside metropolitan area, the Riverside County valleys and mountains and the San Gorgonio Pass near Banning. The high wind warning also was in effect in Corona, Idyllwild, Pine Cove, Banning and Desert Hot Springs.
Neither a red flag warning nor a high wind warning were issued for the Coachella Valley, which will experience typical winds unrelated to the Santa Ana event, with gusts of up to 40 mph, NWS forecasters said. High temperatures there today will be 70 to 75 degrees.
High temperatures will be 66 to 71 degrees in Riverside and the Inland Empire, 53 to 59 in the mountains and 65 to 70 at the San Gorgonio Pass near Banning and Desert Hot Springs.
The fire threat this week doesn’t come from high temperatures. Instead, it’s the strong winds and the low humidity levels, with humidity expected to be in the 10 to 15 percent range today in Riverside. The humidity is expected to be in the 7 to 15 percent range between Tuesday and Saturday.
The U.S. Forest Service’s Santa Ana Wildfire Threat Index, which categorizes Santa Ana winds based on anticipated fire potential, forecasts that each day this week will present wildfire danger.
According to the index, which ranks Santa Ana-driven wildfire threats as extreme, high, moderate, marginal or no rating, the threat in Riverside County will be moderate today and high tomorrow. The threat index predicts marginal fire danger Wednesday jumping back up to moderate Thursday and Friday and then marginal again Saturday.
A moderate threat like the one today means that “upon ignition, fires will grow rapidly and will be difficult to control,” according to the index. The high threat forecast for tomorrow means “upon ignition, fires will grow very rapidly, will burn intensely, and will be very difficult to control.””
The strongest and most widespread winds are expected for late tonight into Tuesday and again for late Wednesday night into Thursday. In most areas, winds are expected to be 20 to 30 mph with gusts to 55 mph, though stronger
sustained winds are possible in higher elevations and wind-prone areas. Isolated gusts up to 70 to 80 mph are possible on Tuesday and Thursday.
The NWS warned residents to “avoid activities that could spark a fire” and warned of the dangers of high winds that include power outages and damaged or toppled trees or power lines.