I don't have an official game lore answer, but I've lived in the Puget Sound area for over thirty years and might have some useful insight for you.
In terms of natural areas, we have four major types of terrain: Forested Foothills, Mountains, Deserts, and Rainforests.
Forested Foothills are the most common type of terrain west of the mountains. These are mostly 150-year-old conifers, with a mix of Pine and Douglas Fir. The topography usually involves a lot of steep hills leading to creekbeds, and elevation is going to be a major tactical consideration - flat valleys are few and far between. Ridgelines will give optimal sight but suboptimal concealment. Underbrush is a mix of low plants like Salal and ferns, occasionally broken up by large patches of impassible foliage like wild blackberries. The amount of passable versus impassible terrain is going to depend largely on how the SSC feels about periodic forest fires clearing brush.
Depending on the season, Mountains are either similar to the foothills or a snowy deathtrap. The steep elevation is even more pronounced, with lots of bare rock walls and deep chasms - plenty of roads border 100 meter drops onto rock or pines. The cold and thin air make the underbrush thin out at higher altitudes, with low plants like wildflowers common. At the higher elevations, the mountains are snowed in for most of the year. It's not uncommon to find two-meter snowdrifts near the passes in late spring - one imagines that thermal vision would be incredibly effective. It's also worth considering how the cold would affect characters, or vehicles, since the mild Seattle weather means most are unprepared for snow or ice. Pre-Ghost Dance, Mount Rainier was actually considered daunting enough to be used as training for ascending Everest, with a plethora of glaciers and ice caves, and lethal weather patterns.
East of the mountains, a sizable rain shadow creates an arid Desert. Farmers take advantage of the lack of natural irrigation to tightly control how much water their crops get, and apples and wines are both produced here with much success. Nonetheless, the weather is brutally hot and cold compared to the Seattle side of the mountains - it's not uncommon for Spokane to be below freezing for four solid months, while Seattle is lucky to get two weeks of snow a year. With the changes brought in the Shadowrun timeline, it would make a lot of sense to demand characters stuck East of the mountains adapt with desert or snow gear, which could lead to some new challenges. Aside from agriculture, the flora east of the mountains is mostly scrub brush, with sparse trees near the rivers. The rivers East of the mountains tend to be mighty compared to the network of small creeks to the West - it's not uncommon for the rivers to be more than 50 meters across and deep, with lots of hydropower facilities.
The most interesting terrain is the Temperate Rainforest of the Olympic Peninsula. This forest is incredibly lush and verdant, with terrain features similar to the Foothills. The constant drizzle here has caused an explosion of plant life, and it honestly looks magical even pre-Awakening. It's status as a national park means that it is far more overgrown than the forests across the Sound, with the ground choked from rotting timbers thick with moss and mushrooms. I would expect games here to be magic-heavy, as the lush forest is an astral beacon. Furthermore, visibility is going to be extraordinarily limited due to the density, and movement at speed is going to be extremely difficult for anyone without special training and/or magic powers.