Cold air floodgates opened

Snow-lovers waited impatiently throughout the winter for a strong negative Atlantic Oscillation (AO) to develop, and it never really happened.

Now, on the week when the calendar switches from winter to spring, the AO has gone strongly negative.

That's why the Carolinas -- and, for that matter, most of the eastern United States -- are in for a week of chilly weather. Someone to our north could be facing a big snowstorm later in the week, before the pattern changes.

The Atlantic Oscillation is one of several atmospheric steering patterns, and when the AO goes negative, it means steering currents are carrying cold air from Canada or even the Arctic into the eastern United States. In the winter, a negative AO puts the Carolinas in position for snow, sleet or freezing rain, because temperatures are cold enough for frozen precipitation if a storm system comes through.

The AO went negative a few times during the recent winter, but its impact was blunted by another pattern -- the Pacific Oscillation. Persistent low pressure off the West Coast often blocked cold air from pushing into the United States.

Now the Pacific pattern is not blocking the cold air, so temperatures that are well below average are likely during the next week in the Carolinas and elsewhere in the East.

And by the weekend, a couple of storm systems are expected to move across the United States.

If this were January or February, we'd be on "snow watch."  But it's March.  Average temperatures are almost 15 degrees warmer now in Charlotte than in early January, so an outbreak of cold air and a storm system probably means chilly rain, rather than snow.

Indications are that the cold pattern will persist for about 7 to 10 days before relenting. If this pans out, we'll have milder weather again by Easter.

But it will be chilly, especially Friday through Sunday, thanks to the long-overdue negative AO.