Why is the North American west so much warmer than the east?

[unobtainium]

Originally Posted by Satellite_Anthem

Those arctic blasts are still heavily moderated though because as Thealpinist commented, they get compressed by downslope heating as they cross first the Rockies, then the Cascades. That's why record lows are so much higher for cities in Western Oregon and Washington than the eastern halves of those states, and even moreso when compared to, say, Montana. On the East Coast record lows are still higher closer to the water, but not as dramatically as the West Coast because the Appalachians have so much less relief.

But if that’s the case why are record lows still so much higher near the Pacific than in cities near the foothills of the mountains, where compressional heating should be strongest? For example, look at record lows for Chilliwack, BC (in the Fraser Valley) vs Vancouver. Chilliwack has much colder lows. The downslope warming could be less because a lot of this cold dense air is flowing out at low elevations through gaps, not rising up and over the mountains.

There’s also the fact that even when the predominant flow is offshore, there will always be local breezes near the water moderating temps near the Pacific. Same reason why east coast cities are a bit milder than inland locations, even though the atmospheric flow there is predominantly offshore. There’s still local sea and land breezes.

[Thealpinist]

Originally Posted by unobtainium

But the gaps do exist, not just the Fraser and Columbia but all up and down the coast. The Central Coast of BC is a series of fjords and gaps where cold air comes streaming out. All mountain chains have such gaps and so cold air has no trouble crossing the mountains. It may be a factor that reduces the amount of arctic air reaching the coast but I think it’s a minor factor. Whenever the flow reverses, the coast gets arctic air. I’ve never seen a case where offshore flow stops at the mountains. It’s just that offshore flow is not all that frequent.

You are ignoring the fact that the largest sea level gaps are the Fraser valley and Columbia gorge though. Pretty much every major arctic outbreak between the lower mainland in BC down to Seattle is due to Fraser outflow.

Places further up the BC coast (Squamish) for instance is generally colder than Vancouver. It’s much closer to the outflow source despite being at sea level.

You are ignoring the fact that cold air moves over the continent like a fluid. Obvious bumps (mountains) will hinder any significant arctic intrusions. No one is debating the power of the prevailing winds off the Pacific Ocean.

Simply pointing out that significantly high mountains have an additional moderating influence on arctic air masses. Without them, arctic outbreaks along the NW coast of North America would be more common.