The crest is the top of any mountain range, ie the Continental Divide in the Rockies.
In the Cascades the true crest would be along a line connecting the highest points. In Oregon and as far up as Rainier this is a line connecting the dots (or volcanoes). In the northern WA Cascades its probably more complicated.
The weather and conditions change across the crest. When storms come from the west they get lifted, compressing the airflow, on the west side. On the east they sink again and expand.
When the conditions on the two sides differ air can be forced through the passes and canyons, and other local effects such as a foehn may occur.
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