What is upslope and downslope?
You may frequently hear the meteorologists here at News 10 talk about upslope and downslope, especially when a storm is on its way. Why is that significant? These phenomenon can greatly impact the amounts of snow that you may see, with upslope the potential for more snow is there and with downslope you would get less, you may have heard us refer to this as a “snow shadow”. So how does this all work?
This process all has to do with the direction of the winds, and because we have changing terrain across the Capital Region it becomes a concern. If we were all flat in the Capital Region and did not have the mountains then this would not be something we would see here.
With upslope, or orographic lift, winds moving west or east, in our case here, will encounter the side of a mountain. Once this interaction begins the air, and moisture is forced upwards into the atmosphere. Air cools as it rises higher in the atmosphere and as it does the moisture within it will condense into clouds and eventually into precipitation, in this case, snow.
It all depends on the direction of the wind, in our area if we are seeing a westerly wind then the higher snow totals would be found on the west side of the mountains. Look at the mountains as a barrier for the air, so it would get forced up, it cools and condenses and the heavier snow would fall on the west side of the mountain. (This idea would be reversed if we were talking about an easterly wind).
Below is a hypothetical map showing “upslope” with a westerly wind. Especially when moisture is carried from Lake Ontario the wind can get funneled down the Mohawk Valley, however, once it hits the west side of the Berkshires, Taconics and Greens there will likely be a little “bonus” snow as the air and moisture are forced up and condenses. These are typical areas of enhancement thanks to this process.
As the air on the east side sinks the opposite process happens, this is called downslope.
You’ll often here us refer to a “snow shadow” when pointing out our snowfall forecast maps. This is the process of downsloping, Almost totally reverse of what upslope is. Instead of air rising, cooling and condensing the air on the east sides of the mountain is now sinking, drying and sometimes warming. This would rapidly cut down on snow totals in these locations because you are drying the air before the moisture ever had a chance to actually reach the ground.
Below is a hypothetical map showing this process with an easterly wind, very similar to the storm we had a few weeks ago in early February. The east slopes of the Greens and Berkshires got “bonus” snow while the west side, into the Hudson valley saw less, the snow shadow. This is why many times you will see us post a snowfall map with the “peaks and valleys” because we are trying to pick out and show you the upslope and downslope areas, or areas that will see more or less depending on the winds and the storm itself.
Do you have a weather question you would like answered? Send an email to Rob Lindenmuth (firstname.lastname@example.org) or Matt Mackie (email@example.com) to get it answered in Weather 101!