A comma before that - English Language & Usage

• Original) So dangerous did weather conditions become, that all mountain roads were closed.

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So, a normalized version could be:

• 1.) Weather conditions became so dangerous [that all mountain roads were closed].

And that seems fine to me, as it is, without the comma. Note that the content clause "that all mountain roads were closed" is licensed by the adverb "so".

With the insertion of the comma:

• 2.) Weather conditions became so dangerous, that all mountain roads were closed.

To many, a comma probably isn't acceptable here -- or at least, the preference would be for no comma.

When the matrix clause is inverted:

• 3.) So dangerous did weather conditions become that all mountain roads were closed.

and that seems fine too (without the comma).

If the comma is now inserted, then we get the OP's original example:

• 4.) So dangerous did weather conditions become, that all mountain roads were closed.

In this version, the comma doesn't seem to be so bad, imo -- that is, it seems to be okay or fine. The comma being acceptable in this version might be due to the presence of the inversion in the matrix clause, and/or due to poetic license by the writer.

Just some thoughts . . .