A comma before that
Here is a perhaps more convincing authority recommending
Using Commas to Prevent Misunderstanding:
Sometimes you must use a comma to make the reader pause in the appropriate place in the sentence in order to prevent misreading.
Becoming a good writer means developing an awareness of how your sentences will sound to the reader. Reading your work aloud, to yourself or to a friend, is a good way to identify the places in the sentence where pauses--or commas--are needed.
Example 1:
Without comma: Before leaving the soldiers demolished the fort.
A comma is needed here to prevent [an initial] confusion in interpreting the sentence. Without the comma, the reader might [at first sight] think that the soldiers were being left, rather than doing the leaving. The sentence might have gone on to end this way: "Before leaving the soldiers, I kissed them all goodbye".
With comma: Before leaving, the soldiers demolished the fort.
- 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 . . .