grammar - In this sentence why is 'the' omitted?

1. We shall start tomorrow, if the weather permits.


2. We shall start tomorrow, weather permitting.

Both sentences are equal. Rather, I think We shall start tomorrow, the weather permitting.

In the second sentence, why has 'the' been omitted?

Answer

As you said, both sentences are equal; your

We shall start tomorrow, the weather permitting.

May be acceptable in certain locales, but at least it the western US it sounds odd. However, it is still easily understood.