I finally found some information along the lines of what I'm looking for in "A corpus-based regional dialect survey of grammatical variation in written Standard American English," Jack Grieve's PhD dissertation.
Grieve used spatial autocorrelation over text corpora from 200 American cities to identify dialectical features. For all findings below, other regions either showed no preference or a preference in the opposite direction.
Here is a summary of some of his findings relevant to the Northwest:
- Have not contraction is relatively common in the Northwest, especially, Oregon, Washington, and Idaho.
- Do not contraction is relatively common in the Northwest.
- Pronoun have contraction is relatively common in the Northwest, especially Oregon.
- No large clusters of pronoun modal contraction were found, however "relatively high values" were found in the Northwest (as well as Texas).
- Infinitival to contraction is relatively common (to a "lesser extent") in the Northwest.
- Where/in which alternation: where is more common in the Northwest.
- Temporal adverbs and instead are more likely to be found sentence-initial in the Northwest than embedded in the sentence.
The following features were found to cluster in the PNW, but were not globally autocorrelated across the country:
- Will/shall alternation: will is relatively more common in the PNW, especially Idaho.
- Until/till alternation: until is relatively common in the PNW, especially Oregon.
- Auxiliary and infinitive splitting is more common in the PNW.
Speakers of PNWE are significantly more likely to:
- use contractions in some situations (but not all);
- place temporal adverbs and instead in sentence initial position than within the sentence;
- split auxiliaries and infinitive with adverbs;
- and to prefer "where" to "in which," "will" to "shall," and "until" to "till."