We're still struggling to unthaw from Friday night's very light snowfall and the associated temperature plunge that created icy conditions on untreated roadways, with dozens of churches across the region canceling services this morning and tonight. But another weather-maker is on the horizon, and will threaten the return to classes for students after the MLK holiday.
Even though I said early on that Friday's little storm system had greater snowfall potential than any we had seen here in the Cumberlands this winter, it turned out to be largely a nothing-burger (hey, it's a millennial term...sorry). It's true that we saw the most snowfall from that system that we had seen this winter, but that's little more than semantics, as most places wound up only with a trace of snow. At best, some isolated areas eked out half an inch of snow, but we are still without a snowfall of measurable proportions this winter.
This next system, which will roll through on Tuesday, has more potential for accumulating snow than the late Friday system. But it also carries what is perhaps a slightly higher bust potential. Here's why:
Tuesday's system will be what meteorologists refer to as a "clipper system." It isn't a bonafide low pressure system, like Friday's storm, that is tapping the Gulf of Mexico for moisture. Instead, it is a piece of energy that rolls in from the northwest, taking advantage of moisture that is already in place. For that reason, clippers are notoriously moisture-starved...but they're also able to maximize their potential because temperature profiles aren't a jeopardizing factor as they are with most low pressure systems.
So here's what's happening: temperatures today will warm to near freezing with plenty of sunshine, which will help the thaw process immensely on those icy roadways. There's a very slim chance of snow flurries or isolated snow showers tonight, which could put down a quick dusting of snow on the very cold ground. But then a southerly flow kicks in tomorrow, pushing temps to near 40 degrees. Cold air will be quick to return after the sun sets, however, and our potential snow-maker will ride a cold front into the region.
As I mentioned, clippers are notoriously moisture-starved, because they aren't tapping the Gulf of Mexico for moisture, and that usually means they're very minor snow events in our area. Typically they result in light accumulations that warrant only a winter weather advisory from weather authorities, rather than a winter storm warning. They're usually good for only an inch or two of snow at best. (I recall a clipper system about 13 years ago that resulted in a heavy snow warning for the northern Cumberland Plateau, but even then we wound up only with about four inches of snow.)
They're able to maximize their potential because they're not usually associated with warm air advection, which we see when low pressure systems tap Gulf moisture, and temperature profiles are so cold that snow ratios are often higher. The standard snow ratio is 10:1, meaning a tenth of an inch of liquid precipitation results in an inch of snow. But the ratio can even be lower than that when temperatures throughout the atmosphere are borderline. This results in snow with high moisture content — the heavy flakes that we refer to as a "wet" snow that make for good snowballs and snowmen and cling to everything in sight, even bringing down power lines and tree limbs when there's enough of it. As ratios climb above 10:1, the snow has less moisture content. It usually doesn't cling to trees or power lines, doesn't make good snowballs or snowmen, and will initially blow off the road as cars are driving over it. As a general rule, the colder the temperatures in the "snow-growth" region of the atmosphere, the drier the snow. However, if all you care about is snow deep enough to cover your grass, you like this — because a small amount of moisture can result in more snow. Ratios can be as high as 20:1 or even 30:1, which can turn a tiny bit of moisture into an appreciable snowfall.
Clippers tend to be fairly reliable snowmakers for our region. In cold weather regimes, like the one we're in now, we tend to see more snowfall potential from clippers than from low pressure storm systems. However, they also tend to hold relatively high "bust" potential because the energy that creates the snowfall is far less organized and more unpredictable. And with such low moisture content in the atmosphere to start with, the unpredictability factor is increased.
So, as Tuesday arrives, temperatures aren't going to be an issue like they were on Friday. Even though we'll climb to near 40 on Monday, temps will quickly plunge back below freezing Monday night and stay there. This isn't a rain-to-snow scenario where precious moisture is wasted as the atmosphere struggles to cool itself. Every drop of precipitation that falls Monday night and Tuesday will be in the form of snow. And ground temperatures won't be marginal as they were on Friday night; accumulation will start fairly rapidly.
That's why Tuesday has more snowfall potential than Friday did, even though there's far less moisture to work with. But exact amounts remain very uncertain. As is always the case with clippers, there won't be much moisture available.
If you are more than just a casual observer of the weather, you might say, "But won't Monday's southerly flow bring in warmer, more moist air, making more moisture available for the clipper system?" It's true that more moist air will flow in on Monday. You don't get those warmer temps from the Gulf of Mexico on the southerly winds without also getting that moisture from the Gulf. But by the time the lift and energy arrives Monday night into Tuesday, the colder air from the northwest will already be scouring out that moist air. Models show precipitable water values of almost one-half inch over the northern plateau early Monday evening, dipping to less than half that by midday on Tuesday.
The National Weather Service at Morristown is currently forecasting general snow accumulations of a half inch or less for most of East Tennessee, though it does say that up to three inches will be possible in isolated areas along the plateau and up into southwest Virginia. That's not an unreasonable forecast, as several trusty weather models have generally shown less than an inch of snow with this system.
But there's also potential for more, and that's being picked up on the models this morning. (Stand by to be geeked out a bit.) It has to do with the tilt of the trough that will be moving across the eastern U.S. Basically, models are picking up on the idea that the trough might be negatively tilted. Think of it as the vertical alignment of the trough (which is a broad area of low atmospheric pressure that is associated with cold fronts). If you were looking at it on a weather map and saw the U-shaped pressure lines, the base of the trough — the bottom of the U — would be pointed more towards Louisiana if it were positively tilted; more towards Florida if it were negatively tilted. A negatively-tilted trough tends to mean more explosive weather in general. In the spring, a negatively-tilted trough makes storm chasers salivate. In the winter, it can mean more snowfall potential.
Right on cue, this morning's models are starting to indicate more potential for accumulation. The GFS model is indicating 2-3 inches for the northern plateau, as is the high-resolution 3K NAM model. The original NAM, which is lower resolution, shows a much juicier solution, with 4-5 inches across Scott County and even a whopping 8 inches just to our west in the Cookeville area (that seems very, very unlikely given a system like this and I wouldn't consider it plausible). The European model is showing a bit more precipitation as well, though the Canadian model is still indicating an inch of snow or less for us.
Behind the system, much colder temperatures will crash in. Single digits are possible Tuesday night, and we may struggle to get out of the teens on Wednesday if we have snow cover. The timing of the system is in question. NWS-Morristown says it will primarily be a Tuesday afternoon event, but it could be earlier. The GFS model has accumulations knocking on our door at midnight tomorrow night; the NAM is several hours slower but has accumulations about to begin by daybreak Tuesday morning.
The NWS office in Nashville, which is predicting about an inch of snow across Middle Tennessee, said it perhaps better than anyone could in this morning's forecast discussion: "The physical difference in a cold atmosphere between a dusting of snow and 2 inches of snow is small, making it difficult to accurately forecast each location`s snowfall with decimal point precision."
A pessimist might say, "So, in other words, it might snow or it might not snow." And isn't that always the case with winter weather in the South? But, actually, the chances of at least a dusting of snow look quite high. And, if you're just looking for another day out of school on Tuesday, sometimes a dusting is all you need.
Eye to the Sky is a weather blog by Independent Herald editor Ben Garrett. Garrett is a weather enthusiast who has long blogged about interesting weather on his personal website. He is not a professional forecaster or a meteorologist and information on this blog should not be considered a substitute for forecasts, advisories or other products from the National Weather Service.