In each of the three sentences below, the internal punctuation employed does not support the sentence construction. Discussion below each example explains how to provide sufficient scaffolding.
1. “Enjoy the break in the weather, a major storm is set to hit Northern California tonight.”
This sentence contains a comma splice, it includes two independent clauses separated by a comma, which is insufficient to carry the load. (That sentence also suffers from a comma splice.) When a sentence is constructed like the example or the first sentence in this annotation — when it could be divided into two smaller sentences — do so, or at least replace the intervening comma with the sturdier semicolon: “Enjoy the break in the weather; a major storm is set to hit Northern California tonight.” (Alternatively, a colon or an em dash is sometimes appropriate.)
2. “But one thing is certain, belief in a fair press is gone.”
If the first clause in a sentence is a setup for what follows, follow the first clause with a colon; as with the comma splice, the comma is too weak to sustain the transition: “But one thing is certain: Belief in a fair press is gone.”
3. “He had two handguns, a Glock 10 mm and a Sig Sauer 9 mm, and a Bushmaster rifle.
The context may be clear in this specific sentence, but often, this construction is ambiguous. What appears to be two elements, “two handguns” and “a Bushmaster rifle,” interrupted by an interjection that precisely describes the first element, could also be interpreted as five objects described in three elements: “two handguns,” “a Glock 10 mm and a Sig Sauer 9 mm,” and “a Bushmaster rifle.” To eliminate the ambiguity, use parentheses or a pair of em dashes (whichever seems appropriate in the particular case) to set the annotation off from the rest of the sentence: “He had two handguns — a Glock 10 mm and a Sig Sauer 9 mm — and a Bushmaster rifle.”