Snowfall, who dis? Why you’ve been thinking about the snow all wrong

I am going to let you in on a little secret: Forecasting snow totals is really hard.

Why do I say that? So, here is the usual scenario: I put in a snow forecast calling for 10 centimetres of snow and we end up getting 12 centimetres.

The viewers’ response? “All you weather people are idiots and don’t know what you are talking about.”

But here is the thing — I am also going to tell you snow totals don’t matter as much as you think they do.

Here are three things that are more important than the amount of snow you are going to end up with:

Impact

How a snowfall is going to impact people’s lives should be question number one when it comes to looking at a snowfall.

Are we talking dangerous driving conditions, air travel delays, shoveling snow a few times a day or just once? This is not totally captured with a range of snow totals in a forecast.

Five centimetres of snow can look a lot different in terms of impact at different times of the day, or even what day it falls on.

Timing

When the snow falls is far more important in my books than how much.

A five-centimetre snowfall just as the morning commute is getting underway is going to be a much bigger deal than a five-centimetre snowfall Saturday at 11 p.m.

Or say we are expecting 12 hours of snowfall that produces 12 centimetres. If that starts at 5 a.m. and ends at 5 p.m., it is now impacting two commutes and snow removal crews are trying to play catch up all day. But if it starts at noon and ends at midnight — well one drive is impacted and the roads will likely be clear by the next morning.

So, same amount of snow, but very different impacts.

Intensity

Intensity is far too often overlooked.

You may have heard the term “snowfall rate” before. It is in reference to the amount of snow that is expected to fall per hour.

Typical snowfall rates range from under five centimetres per hour to one to two centimetres per hour, and in some instances of lake-effect snow it can reach four to five centimetres an hour.

The intensity impacts how quickly snow removal crews can respond and keep up on the roads and at the airport ground operations, which directly impacts quality of life to those not in the snow removal business.

But it also can impact the visibility on the roads. Heavy snowfall and some wind can lead to whiteout conditions and major multi-car pileups. We usually see one or two of those a year around here.

Snow-to-liquid ratio

Remember when I said forecasting snow is hard to do? Let me dig into that a bit.

Not all 10-centimetre snowfalls are created equal. Is it going to be heavier wet snow or super light and fluffy? Why does that matter? It has to do with the amount of water content inside each snowfall and a thing called the snow-to-liquid ratio.

If you have 10 millimetres worth of moisture in a storm system, if it comes down as rain, we would see 10 millimetres of rain or have a ratio of 1:1.

If it were freezing rain, it would likely be close to 10 millimetres of ice accretion.

But with snow it could be anywhere from five centimetres to 25 centimetres — all depending on the snow-to-liquid ratio.

Typically when we have a snowfall that occurs when temperatures are warmer, I am going to forecast the storm with a lower snow-to-liquid ratio. Something in the order between 5:1 or 10:1. With some colder storms here across southern Ontario, it is possible to see snow-to-liquid ratios hit 15:1 or even 20:1.

So the example of 10 millimetres of moisture could produce: five, 10, 15 or 20 centimetres (and all the numbers in between) of snow as related to the atmospheric temperature profile that a snowflake has to pass through.

 

Let’s bring this back to how this affects you, and not just me sitting here sobbing “woe is me.”

When I call for 15 centimetres of snow that you will have to shovel off your driveway, you might only be shoveling 10 millimetres of water content. That’s when the snow is light and fluffy. It is that water content weight that makes the difference between making the chore back-breaking labour or a walk in the park.

The next time we have snow coming our way, perhaps you will look a little beyond just the numbers. I know you probably won’t, but I can dream!