A little secret for you all – winter forecasting involves many variables that meteorologists are trying to track down to accurately communicate the risks involved with a big snowstorm.
And for this month’s edition of the newsletter, I’m going to try to break down some of that nuance with the help of Virginia Tech meteorology professor Dave Carroll – an especially noteworthy topic since we have likely already received more snow this winter than the last two, where there definitely wasn’t a lot to go around. And yes, this is about to get very nerdy. If you have any questions, feel free to respond to this email and I’ll do my best to further explain!
We should start where most modern forecasting starts – the tried-and-true weather balloon.
Twice a day, meteorologists at National Weather Service offices across the country launch those balloons at the same time.
“And what they are attempting to do is get a snapshot of the atmosphere in three dimensions,” says Carroll. “We want to get a vertical profile of the temperature, the moisture and the wind – and these balloons will go up to 100,000 feet or so before they pop. By doing that, we can assess the atmosphere, not only at the surface, but also through its depth. And when we’re talking about precipitation type, that is absolutely critical.”
That data from the balloons comes back in a format known as a Skew-T plot, or a sounding. For reference going forward, think of the bottom of the Skew-T plot as the surface where meteorologists launch those weather balloons – with the rest of the plot showing an increase in altitude along the vertical axis.
With those important guidelines out of the way, let’s take a look at various different soundings for winter precipitation types >>>
SNOW:
Snow is really a beautiful miracle because it represents every element necessary for winter weather coming together in the perfect way. To get just snow during a winter storm, you want your temperature line and your humidity line in the sounding to be right beside each other.
Sparing you some complicated physics equations, this essentially represents an atmosphere that is saturated. Any dry air throughout the atmosphere will play a big role in the precipitation type you see at the surface. And too much dry air may mean you don’t see anything at the surface at all.
Secondly, you want that merged precipitation and humidity line to be below the freezing mark of 32 degrees Fahrenheit all the way up through the atmosphere. These are vital to receiving snow at the surface.
Another key element of snowfall is what’s known as the dendritic growth zone.
“That is simply the ice crystal growth zone that we see,” Carroll explains. “So, we know through research that ice crystals are much more efficiently grown in an environment where the temperatures range in that kind of -10 to -20 degrees C (10.4 to 0.4 degrees Fahrenheit) is kind of the sweet spot where you are cold enough for efficient growth and you have enough moisture. Once you get below -20 degrees C, there’s just not a lot of moisture to work with. The air tends to be really, really dry at that point.”
Long story short, you want your saturated column to be within that zone for elevated snow formation to happen.
As we’ll see, other winter precipitation types begin to occur when these so-called “perfect” conditions aren’t present…
SLEET:
Think of sleet as the kind of middle ground between snow and freezing rain. As you can see in the sounding below, there’s a section of our saturated column that is above the freezing mark of 32 degrees. This is often called a “warm nose.”
“Above the surface, we may actually edge above freezing for a bit before we go back down below freezing,” Carroll says.
Sleet occurs when a snowflake falls through that warmer layer and partially melts. However, as you can see, the air towards the surface is below freezing. This means that partially melted snowflake will refreeze before landing at the surface.
There is a key distinction with this “warm nose” that differentiates it from our next winter precipitation type, too.
“If that warm nose only extends a degree or two above freezing and then backs down below freezing to the surface – then that is more of a sleet profile,” explains Carroll.
Otherwise, you’ll get…
FREEZING RAIN:
“If that warm nose sticking out edges more like a five degree increase, then we’re more looking at freezing rain. So, again, oftentimes, you’ll see this vary and you’ll go from one precip type to another,” he says.
For freezing rain, the warmer section completely melts a snowflake – resulting in just a regular, albeit chilly, rain drop. There isn’t enough time for it to refreeze before reaching the surface. However, in the sounding below, you’ll see that the temperature at ground level is again below zero.
That means our chilly raindrop will freeze on contact with the cold surface – creating a dangerous layer of ice.
You can also get freezing rain or drizzle when there’s not enough moisture or saturation in that special snow growth zone we talked about earlier.
These are just some of the factors that meteorologists are looking at when it comes to precipitation type during the winter months. We’ve already talked about the influence of dry air (which can be really hard to pin down before a storm rolls through, but will absolutely lower or even eliminate projected snowfall totals if there’s enough of it), but elevation, snowfall rates or intensity, how much cold air funnels into our region and the overall track of a storm – which may deem how much moisture is available – are incredibly important factors to consider as well. The exact details of these things can often still be hazy even a day ahead of when snow is expected to begin falling – further compounding the challenge of communicating risks to the public. The influx of dry air, too, may not be evident until the day of a forecasted snow event.
All of that to say, winter forecasting with its many, many nuances is tough! A degree or two difference all the way up through the atmosphere can really change things here at the surface. Hopefully this little guide will help you understand some of the many factors that go into winter forecasting.
And, if you are someone who loves to see snow in the winter months, Carroll breaks down the ideal signals to look for in this part of the world.
“Look for a big area of high pressure over eastern Canada [bringing in a sufficient amount of cold air] and then our favorable locations for the big snows where we really swing a lot of moisture into the area is that coastal low. Places west of the I-95 corridor typically will get the wintertime precip – and then it’s typically all snow for the Blue Ridge and points west.”
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