What causes upslope snow

25 Mar 2009
Authored by: Ron

We are about to get a facefull of upslope snow tomorrow along the Front Range of Colorado including Fort Collins. What does this have to do with web development? Nothing, except maybe I’ll get to ski to work.

Weather is one of my favorite topics, which is good for my health since I’m a pilot.

Upslope snow in Fort Collins happens this way: A low pressure area passes to our south. The flow around a low pressure area is counterclockwise, which causes the surface wind to come from the east, a little unusual for our area. That air originated well southeast of here near the Gulf of Mexico (visualize that large-scale counterclockwise flow), so it’s wet and moist and thinks it might be fun to take a little vacation in Colorado.

As the moist air flows from the east to the west aiming at the Front Range, it is moving uphill. The elevation in Fort Collins is just over 5000 feet MSL. The elevation at the eastern edge of Colorado is about 3500 feet MSL. The air coming at us is indeed moving “upslope”. As the air is forced westward it’s also forced upward and the gain in altitude causes the air to cool down, which (if it cools down enough) causes the moisture in the air to condense, or if it’s cold enough, to sublimate and precipitate out as snow.

As long as that low pressure sits to our south pumping the air westward and upward against the Rockies, it will continue to snow. If it sits there long enough like it did in 2003 we could get three feet of snow. This time the storm should keep mozying along and we’ll get 6 inches or so by Friday morning.

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  1. Comment by Dr. Baerchen

    Nice description of the upslope storm. This is my first winter in CO and I heard a lot of people talk about the upslope storms and how they just dump snow along the front range. I experienced my first one on 3/26/09, which left 14-18″ in Boulder. Then a few days later on 4/1 we got 4″ of snow. Now they’re forecasting another foot overnight tonight (4/3)! I love the snow, but I’m kind of bummed that this is happening now instead of 2-3 months ago.
    BTW – Not to be a douche, but sublimation refers to a solid transitioning to a gas, not the other way around. You mean to say precipitate, which refers to a solid forming out of a liquid/gas phase (i.e. snow). Trust me, I’m a doctor (but not the kind that helps people! ;).

  2. Comment by Phil Earnhadt

    It was a big surprise when I first learned that the moisture from front range upslopes was coming from the Gulf. The other thing that I wouldn’t have guessed was how localized the precipitation can be in an upslope. If the low pressure system gets parked, a small strip of the front range can just get nailed with snow and areas just a few miles away will get nothing.

    A few years back, one of the faster-moving hurricanes plowed through South Carolina and wound up creating an upslope in the Appalachians in North Carolina.

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