You probably ought to mention altitudes. Enchantments is at 1500m, Rainier's main visitor station is at 1900m. Mosquitoes, and flies, are highly altitude-dependent, as well as water-dependent. I remember bailing out early from a weeklong hike in the Pyrenees because any dips below 1000m brought deerflies. This was a crest-valley-crest-valley-... hike, I got plenty of experience of that altitude dependency.
Past that, slow-moving water, as opposed to swift rivers in steep downhill areas, is what mosquitoes prefer. Lake breeding might (I guess) be kept in check by fish.
I also think prevailing wind speed affects mosquitoes. They just don't fly very quickly so they'd get carried away by steady winds. That or wind mess with their prey-homing mechanisms.
Mosquitoes, at least in some areas, also depend on the time of day. Caribbean mosquitoes, from living 8 yrs there, are most active at dusk/twilight. So timing your trip through some known-infested areas can be useful. The few times I've had real problems in BC were in late afternoon, but I've had so few problems here that I hesitate to think much of it, especially when forest cover gets factored in.
interesting reading: https://underc.nd.edu/assets/216551/fullsize/mcreynolds2003.pdf note the remarks about temperature and the types of nearby water bodies.
At a guess:
- regional aspects - are mosquitoes in the general area or not? I don't mean to be flippant here - Canada is known for mosquitoes in summer. Coastal British Columbia, not so much. It happens, in spots, but it's not the general expectation.
- temperature - seasonal and altitude-dependent
- water availability, but not a lake or river. Think puddles/swamps/stagnant pools. For areas with dry/rainy seasons - look at during/after rainy times. For temperate climates, seasonal temperature matters more.
- time of day
- windspeed, which you can probably gauge by regional factors and topology.
- Tree cover/shade also seems to affect many species. I've rarely seen them in full sun, open terrain. Then again, up in the far North, an area I have no experience with, they seem to do pretty well, in areas that I assume are without extensive cover.