Mobs spawn in desert sunlight during storms

If you are in a desert during the day while it is raining in another biome, even though it is sunny in the desert, mobs will spawn and not burn.

Steps to reproduce:

1. Go to a desert

2. Use /Weather rain

3. Watch mobs spawn before sunset just barely when the sky starts getting darker.

Expected result:

Mobs don't spawn because it is still sunny.

Observed result:

Mobs spawn.