weather eye
n.
An ability to recognize quickly signs of changes in the weather.
Idiom:

keep a/
one's weather eye open
To keep watch; stay alert.

[TahlilGaran] American Dictionary ▲

weather eye
n.
1. Eyes that can tell what the weather will be.
Grandfather's weather eye always tells him when it will rain.
2. Eyes ready or quick to see; careful watch.
■ Usually used in phrases like "keep a weather eye on", "open", or "out for".
Mrs. Brown kept a weather eye on the children so they wouldn't hurt each other.
Keep a weather eye out for Uncle George at the store.
Keep a weather eye open for deer.
The police have a weather eye out for the robbers.
Compare: LOOK OUT.

[TahlilGaran] English Idioms Dictionary ▲