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.

Red flag criteria is generally when relative humidity is at 15% or less, combined with sustained winds and/or gusts of 25 mph or greater. Both of these conditions must occur simultaneously for at least 3 hours in a 12 hour period, according to the NWS.

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.

A short reprieve in the strong destructive winds has granted firefighters an opportunity to curb the spread of the deadly Eaton Fire near Pasadena.

The National Weather Service has issued a Red Flag waning for a wide swath of Southern California as Santa Ana winds are expected to bring dangerous fire weather to the region later this week.