If you haven’t lived in the Rocky Mountain region or the Pacific mountain ranges, you likely haven’t experienced what is known as “upslope precipitation.” It is a unique phenomenon that will typically only be seen with rapid increased in elevation – such as mountain ranges or foothills.
We are expecting quite a bit of snowfall as a result of this method of snow creation in the Denver, Colorado, area on up north into Fort Collins and into Cheyenne, Wyoming. What happens is that as air flows into higher elevations it is forced up into the atmosphere. It has no other choice of where to go. As air rises, it cools. Warmer air can hold more moisture than cooler air, so when you get cooling air that has the same amount of water vapor in it – you’ll see a quick increase in relative humidity. Eventually the air cools to a point that it can’t support the water vapor in the air – and the water vapor condenses. Voila! You have clouds!
Eventually the condensation continues to a point that the clouds can’t hold onto any more moisture either – and that is when you get upslope precipitation. Depending on the temperature in the air you’ll get either rain or snow as a result. That is why as you look over the eventual storm-total snowfall for the Denver area for today through tomorrow you’ll see rapidly increasing snowfall totals from the east side of town to the west. As of this writing we are expecting nearly half a foot of snow on the east side – including some of the eastern suburbs of Denver, with closer to a foot of snow on the west side nearest the foothills of the Rockies.
If you live in these spots, or are visiting, you can actually watch this phenomena take place when you know winds are being forced up a nearby mountain range. Look at this webcam shot from Casper Mountain, in Casper, Wyoming:
In a few days, check back here and click here to see the latest image from this webcam, and you’ll be able to compare the two images!