Today is Dec. 1 — the start of meteorological winter.
While winter doesn’t start on the calendar until the winter solstice just days before Christmas, meteorologists measure winter as the months of December, January and February.
Of course, a flip of the calendar from November to December doesn’t mean that cold air and snow are going to magically appear — especially here in the South! Still, we had already received our first accumulating snow by this time last year, and most of us can remember Decembers that featured appreciable snowfall. (Remember December 2010?!?)
This may very well turn out to be a December that features cold and snowy weather…but it’s not going to start that way. Temperatures are in the upper 50s this afternoon, and they’ll likely be in the 60s the next couple of days, perhaps even approaching 70° on Friday. That’s well above normal; our average high for this time of year is in the low 50s.
And while we aren’t going to be in the 60s every day, it looks like the warmer-than-average start to December will persist through at least the middle of the month, if not longer.
Here’s what’s driving our warm weather:
There are a number of teleconnections that help dictate the weather over the northern hemisphere during the winter months. Three of them that are the easiest to understand, and forecast, are the North Atlantic Oscillation, the Arctic Oscillation and the Pacific North American ridge index — or the NAO, AO and PNA for short.
If you’ve read this blog in the past, you’ve seen these teleconnections mentioned. It’s hardly an exhaustive list of the various teleconnections that help determine our weather, but they’re the first three that many medium- and long-range forecasters often look at. In a nutshell, the NAO is a measure of storminess in the northern Atlantic Ocean, the AO is a measure of storminess in the Arctics, and the PNA is a measure of atmospheric ridging over the eastern Pacific Ocean. And although there are certainly exceptions, the general rule is that a -NAO and a -AO tend to mean colder weather for much of the eastern U.S., while a +NAO and a +AO tend to mean warmer weather for much of the eastern U.S. Meanwhile, a +PNA lends itself to colder air in the eastern U.S., while a -PNA lends itself to colder air in the western U.S.
Continuing with the “nutshell” explanation, you can think of it like this: A -AO allows colder air to be unleashed from the Arctics and spill into the continental U.S. Ridging in the eastern Pacific (+PNA) helps to steer that cold air east of the Rockies, while a lack of ridging in the eastern Pacific (-PNA) helps the cold air filter in west of the Rockies, and often co-exists with ridging in the eastern U.S. that helps pump warmer air into this part of the country. A -NAO, meanwhile, can create a roadblock to help the colder air penetrate deeper into the eastern U.S. and have more staying power, while the lack of storminess in the northern Atlantic removes that barrier.
So all of that is to say that a +AO/+NAO/-PNA combination is not conducive to cold weather in the eastern U.S. — particularly here in the South — and that’s exactly the setup that we’re going to be looking at for the next couple of weeks.
The AO has been more negative than positive for much of the fall season, but is currently trending positive and is forecast to stay that way for at least the next couple of weeks. The PNA has been more positive than negative for much of the fall season, but it looks like there will be a dearth of ridging in the PNA region for at least the next couple of weeks.
What does that mean? It means we’re probably going to see a lot of mild air here in the South.
That doesn’t mean every day will be warm. We’ll see a cold front on Monday that could knock temperatures down into the 40s for a day or so, with sub-freezing temperatures Monday night. And we’ll see more cold fronts further down the pike. But the cold air won’t last long; it’ll generally be in-and-out, and there are no potential winter storms showing up for the first half of December.
Will the pattern change in time to deliver weather that feels more like Christmas before Christmas actually arrives? That remains to be seen, but for at least the next couple of weeks, it appears that we’ll see far more days with high temps in the 60s than we will with high temps in the 40s or colder.
The NWS’s Climate Prediction Center is forecasting above-average temperatures for most of the continental U.S. for the next couple of weeks, and for the month of December as a whole.
Indeed, it looks increasingly as though this winter will be a mild one, if not a warm one, with below-average snowfall, as we enter a classic La Nina winter pattern. That doesn’t mean we’re going to see wall-to-wall warmth with no snow threats at all this winter, but it does mean that we’re not going to see the South transformed into a frozen tundra with day after day of subfreezing temps and snow on the ground.