"I'm dreaming of a white . . . Thanksgiving?"
It's that time of year when snow-lovers start to look longingly at the seven-day forecasts and students and teachers dream of those early-morning phone calls announcing that classes have been canceled because of snow.
Let's be clear on the front end: There are no signs of any sort of winter storm on the table for the remainder of this month. That should surprise no one; the northern Cumberland Plateau region of Tennessee rarely sees accumulating snow in November, and only occasionally sees it in December.
But if you're looking for snow, and any snow will do, there's a slim chance that you might get your chance later this month.
Technically, our region has already seen its first snowflakes of the season. Those came on October 28-29, after a strong cold front swept through the region. But you had to look close to see them.
Atmospheric conditions should begin aligning for a possible outbreak of cold weather somewhere right around the Thanksgiving holiday, and the later we get into November and then December, any cold snap has to be monitored for snow potential.
Here's what's happening: The North Atlantic Oscillation — a measure of storminess in the northern Atlantic Ocean — is projected to dip into negative territory over the next week, going to at least -1 and perhaps closer to -2 before beginning to relax as we get to the end of the month. Meanwhile, the Arctic Oscillation — a measure of the climate pattern in the arctics — is projected to go sharply negative in the days ahead, plummeting at least to -4 within the next week.
It isn't as simple as this, but a simplistic, nutshell description of what these two particular indices mean for our weather can be presented like this: when the AO is in a negative state, the counter-clockwise winds ringing the arctic become weaker and allow frigid cold air to spill over into the continental U.S. And when the NAO is in its negative phase, it generally means storms in the northern Atlantic are serving as a roadblock, which helps things "dam up" back in North America. That tends to drive storms further south here in the eastern U.S., which results in colder weather for our region and less of an influence from the tropics. It can also slow down the progress of our weather systems and air masses that circulate the Northern Hemisphere in a counter-clockwise direction, allowing cold patterns to become "locked in," so to speak.
These are hardly the only "teleconnections," as they're referred to in meteorological circles, to influence our weather here in Tennessee and in surrounding states. But they are two that matter. Another to keep an eye on is the Pacific North American ridge index, a measure of ridging in the eastern Pacific. When the PNA index is negative, there is no ridging in that region; when it's positive, ridging is building in the PNA region. Ridging in the eastern Pacific is important because as the colder air drops in from the northern latitudes, ridging out west helps funnel it into the eastern U.S. In the absence of ridging over the eastern Pacific or the Rockies, it becomes easier for heights to be pumped up over the eastern U.S., allowing for an influx of warm air from the tropics. Again, that's an overly simplistic description, but that's how the PNA works in a nutshell. For the past couple of months, the PNA has been primarily negative, but it is forecast to make a run at neutral territory as we get into the latter part of November.
So what does all of that mean? It means there are atmospheric signals that an outbreak of cold air could be lurking for our region in another week or so. Right now, there are pretty significant differences between the two major weather forecast models that meteorologists rely on for that time frame. The GFS, which is operated by the National Weather Service, is primarily cold. The ECMWF, which is operated by our friends across the pond in Europe, keeps most of the cold well to our northwest.
Right now, the GFS model is showing signals of a storm system right around Thanksgiving, which is what led to the teaser at the top that some snowflakes could potentially fly around the holiday. The model is hardly consistent (it rarely is that far out), but it has generally shown colder weather settling in right around Thanksgiving, which could lead to the potential for any residual moisture to change over to snow.
As I said at the top, we aren't talking a major winter storm, or even accumulating snow. Just something to make this Thanksgiving different from the last couple of years, which have featured shirt-sleeve weather on Turkey Day.
For now, if you want a verbatim take on what the weather model we're discussing is showing, it has temps topping out in the low-to-mid 30s on Thanksgiving Day, and struggling to get to 40 degrees on Black Friday. But it also shows moisture moving out by late Wednesday night, when temperatures are too warm for snow. This is one run of the model, which printed just about an hour ago, and it will likely change by the time the next run comes off the computers in about five hours.
Still, the general idea is on the table. If I were looking for snow, and I didn't have to have accumulating snow to appease me, I'd look for the period right around Thanksgiving or Black Friday. The models may not really be showing it at the moment, but the potential is there.
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.