The Rocky Mountains, a few hundred miles to our east, offer profound protection for our region from the bitter winter cold of the interior of North America.
During the next week, this protective shield will be put to a severe test and will not be found wanting. Let me show you.
The terrain map below show the double terrain protection our region enjoys. To the east are the Rockies, with plenty of terrain rising to 8000 to 10,000 ft. To the east of the Rockies are the Great Plains of the interior of our continent, which provides a flat, low-elevation conduit from the frigid, snow covered Arctic directly into the middle of the continent.
The Arctic is a particularly good place to generate cold air. Covered with snow, which is a very effective emitter of infrared radiation to space. Little solar radiation in winter. Generally light winds and high pressure dominating.
Think of the Canadian Arctic and the nearby ice-covered ocean as the refrigerator for North America.
But to drive the air southward effectively you also need the right large scale wind flow, one that produces strong northerly (from the north) flow over the interior of the continent.
And we will have that in spades!
Take a look at the forecast upper-level (500-hPa pressure level, about 18,000 ft) weather map for 4 PM PDT on Friday.
Wow. HUGE ridge (high pressure) over the West Coast, with a deep trough to the east of the Rockies. The result of this highly disturbed upper level flow pattern is strong northerly flow pushing southward over the Great Plains
Now, let me show you a sequence of surface air temperature forecasts this week, which will allow you to view the invasion of cold into the hinterlands of North America.
Start with today at 1 PM. The coldest air is found in the northern Canadian Arctic.
By Wednesday afternoon at 4 PM, the cold air is moving southward, with a notable extension into Alberta. Note how the Rockies keep the cold air to the east!
By 4 AM Sunday, frigid air has plummeted through the Canadian plains into the northern plains of the U.S., with the Rockies providing protection for the western U.S. Huge north-south extend of the cold air, with the source region in northern Canada.
And by 4 AM Wednesday of next week the cold air moves eastward, into the upper mid-West and the northeast U.S. But we in the Northwest remain untouched!
Finally, to impress you with the temperature contrasts, here are the forecast temperatures for 10 PM Saturday. 36F in Seattle…which is cold for us, but -25 to -30 over Albert and as low as -11 in Montana. A four hours drive from Spokane (28F) to Great Falls, Montana (-10F) would be quite a shocker.
So if you don’t get the cold air you grave during winter, you know one source of this warm affliction…the Rocky Mountains.