March: A Surprisingly Snowy Month For The Mountains

Not many people know that March, on average, is nearly as snowy as February for most of the Cascades and Olympics. Snoqualmie Pass, for example, averages 72 inches for February vs 71.7 inches for March. Yes, March does have an unfair advantage in terms of length, but with days rapidly becoming longer, storms becoming weaker, and the famous cold east-pass flow becoming much less of a factor as the Columbia Basin warms, how is it that March and February are so close in snowfall?

There are several reasons for March’s surprisingly significant snowfall, but the largest contribution comes from the fact that the mid and upper levels of the atmosphere over the Pacific Northwest are, on average, colder in early March than they are at any other time of the year. While the average temperatures at the surface in Seattle rebound from a minimum of 40 degrees in early January to an average of 45 by the 1st of March, temperatures at the 850 millibar (mb) pressure level of the atmosphere (about 5,000 feet in elevation) slightly decrease from around -1 degrees C to -2 degrees C during the same time frame. The reason for this discrepancy in timing is because the ground and ocean are more effective at absorbing heat than the atmosphere, allowing the higher sun angles in the spring to warm the lower atmosphere far more dramatically and quickly than the upper atmosphere.

But a cooler upper atmosphere does more than just lower snow levels. With warmer temperatures in the lower atmosphere and colder temperatures in the mid and upper-atmosphere, the atmosphere has a steeper decrease in temperature with height and thus becomes more unstable than usual, leading to a process known as convection. Convection can lead to the intense showers that lower the snow level via dynamic cooling, a process I touched on in yesterday’s blog. But to explain how these showers form in the first place, we first need to explain the concept of adiabatic cooling.

Adiabatic Cooling:

We all know what happens when you release a helium balloon into the atmosphere. Because helium is less dense than our atmosphere (mainly nitrogen and oxygen, with a little argon for comic relief), it rises and expands, finally popping when the strength of the balloon can no longer contain the volume of air inside it. The reason it expands is simply because air pressure decreases as you rise through the atmosphere, exerting less and less of a force on the outside of the balloon in the process. As this force decreases, the air inside the balloon pushes against the balloon, expanding it and eventually popping it in the process. The air inside the balloon does not interact in any way with the air outside the balloon, until it pops, of course.

The rising and expansion of this balloon is what is known as an “adiabatic process.” In an adiabatic process, no heat energy is exchanged with the surrounding environment. It is important to remember that heat energy is NOT the same as temperature. Since the volume of this balloon increases, the temperature must decrease to ensure that the amount of heat energy contained inside the balloon doesn’t change. This phenomenon is known as adiabatic cooling, and can be illustrated via two basic laws of physics that are ubiquitous throughout atmospheric science – the hydrostatic equation and the ideal gas law. And yes, this balloon would shrink and undergo “adiabatic warming” if it were to decrease in elevation.

We can apply the concept of adiabatic cooling to a “parcel” of air that rises. As air rises, it cools and expands. If it cools at a rate less than that of the surrounding atmosphere, it remains less dense and thus continues to rise since warmer air is less dense than colder air. The process by which air parcels rise due to them being warmer than the surrounding atmosphere is known as convection. Though we don’t get the type of extreme convection that gives rise to the colossal supercell thunderstorms experienced over the Great Plains, we can get enough to cause heavy showers and even a rumble of thunder or two – this past Monday being a prime example.

But it’s not only the increased instability in the springtime and cooler air aloft that contributes to March being a surprisingly snowy month for the mountains. The large-scale circulation of the atmosphere also changes between these two months as the surface warms and the temperature difference between the poles and midlatitude decreases. I promise this discussion will be a little less technical than our discussion about adiabatic processes!

Differences in Circulation:

In the winter, our prevailing winds are from the southwest, giving us relatively mild temperatures and some of the dreariest days known to mankind. This is due to a strong jet stream that often steams straight across the Pacific into the Pacific Northwest. But in the spring, this jet stream weakens, and instead of seeing strong southwesterly flow, we often see weaker, WNW flow as a large trough of low pressure directs cool and unstable air into our area. This is a great pattern for snow because (a) unstable air rises easily when it encounters terrain, (b) WNW flow is close to perpendicular to the Cascades, allowing for effective orographic uplift, and (c) cold air gives low snow levels (duh!).

The diagrams show the average 500 millibar heights over North America during February (top) and March (bottom). Notice how the height contours come into our area from the WSW in February but come from a more westerly direction in March. The less of a southerly component, the cooler your air mass will be!

Lowland snow threat Saturday night-Monday:

Though this post has been more of a science/research post focused on mountain snowfall rather than a lowland snow forecast, it is worth mentioning that models have shown periods of spotty lowland snow for both Seattle and Portland between Saturday night and Monday afternoon. The large-scale pattern is actually relatively similar to what we saw last week – a deep upper-level trough will direct moisture into our area, with temperatures right on the fringe for snow. It’s too early to pin down specific timing and accumulation amounts, but at least an inch of snow is likely in many (but not all) areas above 500 feet, and accumulating snow could once again fall to sea level. I’m pretty busy Friday and Saturday with a weather conference (the Pacific Northwest Weather Workshop, to be exact), but I’ll do my very best to keep you updated on the latest developments on this blog, on the WeatherTogether Forums, and on our Facebook and Twitter pages.

Thanks for reading! I hope you found this post enjoyable and informative.
Charlie