Dozens of communities are now under mandatory evacuation orders in Southern California with evacuation warnings affecting dozens more as crews continued battling a pair of deadly wildfires in the region Friday.

Strong Santa Ana winds are hitting fire-ravaged Los Angeles on Friday morning, but the winds will relax later in the day and the red flag warning will end Friday night.

That’s leading to the creation of mountain waves, which arise when strong winds blow perpendicular to a mountain range. The waves shoot down the coastal side of the mountains, gaining speed along the way, and are more casually referred to as simple Santa Ana winds.