One rule of thumb if you’re a weather-watcher in this part of the world: Never get carried away and say winter is over. Not in late January, not in February, and usually not even in early March. Because mother nature would love nothing better than to prove you wrong.
So we won’t say that winter is over, on Jan. 21. That would be premature; we’re only one month into winter on the calendar, and there are still more than five weeks of meteorological winter left to go. But it’s probably not premature to say winter is on life support, at least in terms of sustained cold air and enhanced chances for wintry precipitation.
At the very least, the much-ballyhooed pattern we anticipated as we entered January has faded. The almost universal support among the teleconnections for transient blasts of arctic air and accompanying snow chances has been replaced by teleconnections that are mixed — some supportive of cold and snow, some not.
The biggest hindrance to cold and snow in the eastern U.S. right now is a complete lack of ridging in the eastern Pacific region. The Pacific North American ridge index (PNA) has been positive since the middle of November, but it’s flipping to negative right in the heart of winter, and will likely stay there for the next couple of weeks. With no ridging to our west, we don’t see troughing here in the east — which means a general lack of cold air.
A -PNA can sometimes mean that atmospheric heights are pumped up in the eastern U.S. and we see very mild weather, with storm system trekking to our north and introducing thunderstorms and even severe weather chances to our weather here in Tennessee. That’s not happening this year, mostly because of other teleconnections that remain favorable for colder weather. Temperatures will generally be in the 40s and 50s for highs the next two weeks, with a few very brief shots of cold air filtering in.
Both the Arctic Oscillation and the North Atlantic Oscillation remain in negative territory, where they’ve been for a while. That’s a good look for cold and snow in this part of the country, but without the ridging in the Pacific and with the Madden-Julian Oscillation stuck in an unfavorable position, there’s not going to be much cold air filtering into the Southeast.
Can this change? Absolutely. The -PNA should begin to relax in a couple of weeks. If we can save the -NAO/-AO combination (both look to begin to relax as well, with some models showing the NAO flipping to positive but it’s too soon to say for sure that will happen) and if the MJO migrates into a more favorable position, it could be game-on for cold and snow in early-to-mid February. But climatology suggests this is unlikely; in fact, climatology would argue for a warmer-than-average month of February.
So why is winter on life support? The two most reliable medium-range models — the GFS and the ECMWF — are showing no real changes for the next 15 days. It wouldn’t be a smart bet to trust the models more than 5-7 days out at this point, but they’re pretty consistent right now in suggesting that there won’t be much cold air intrusion into the Southeast for the next couple of weeks.
If that holds true, then by the time we get to the end of that two-week period, we’ll be almost a week into February. By that time, the sun angle is rising higher in the sky, the average temperature is warming, and the day length is increasing. Big snows and arctic outbreaks can absolutely happen in February and even in March (five years ago, Scott County recorded some of its lowest temperatures ever in late February). So that’s why it’s too soon to declare winter over. But it’s probably not too soon to say that if you like snow and cold air, our best chances could very well be behind us.
In the meantime, a powerful storm system will impact our region early next week. Early indications are that this could be the biggest rain-maker we’ve seen in quite some time. A couple of inches of rain are not out of the question at this point.