This animation shows the retreat of glacial ice from North America between 18,000 and 8,000 years ago.

Based on what you have learned about mid-latitude oceanic and atmospheric circulation patterns, attempt to explain the rapid retreat in western North America relative to the eastern part of the continent.

Hint: Consider the possible effects of the ice sheet (a persistent thermal high-pressure cell) on the path of the jet stream, and how that might have affected temperatures. [Source]