I noticed that in the PNW in USA and Canada, the rainy season is in the winter whereas the summers are relatively dry. However, move a bit inland to the interior of Washington/BC, and the rainy season becomes summer whereas the winters are fairly dry.
What causes the flip?
In: Earth Science
Your premise just isn’t true.
* [Yakima, WA](https://en.wikipedia.org/wiki/Yakima,_Washington#Climate): Driest month July, wettest December.
* [Spokane, WA](https://en.wikipedia.org/wiki/Spokane,_Washington#Climate): Driest month August, wettest November.
* [Bend, OR](https://en.wikipedia.org/wiki/Bend,_Oregon#Climate): Driest month September, wettest December
You have to go further north, to places like [Kelowna, BC](https://en.wikipedia.org/wiki/Kelowna#Climate) (driest month February, wettest June, but Kelowna’s preciptation is much more distributed through the year) or [Kamloops, BC](https://en.wikipedia.org/wiki/Kamloops#Climate) (same) to find wet summers.
The reason for the seasonality is the migration of the [subtropical ridge](https://en.wikipedia.org/wiki/Horse_latitudes), which moves north in summer and south in winter, but doesn’t get far enough north to dry out British Columbia (which lies in the wet band north of it during the summer).
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