When does the cold return?

The number one question asked of me in the past few days has been this. With the recent warm spell and the calendar turning to December the nature tendency is to wonder, but how soon we forget November. November in Charlotte was 1.5° below average and was the 12th coolest November since 1939.

Yet this is December and even though it’s been abnormally warm we didn’t break any records yet. Though many were set across the eastern half of the country over the past 5 days.

There are signs pointing to a major pattern shift, though it may take some time to fully develop. The first part of the equation is the cold air in Canada and Alaska. While the eastern half of the lower 48 is baking Alaska and Canada are in the deep freeze. These were the temperatures over Alaska this morning.

Almost all of Canada and Alaska is under a very early snowpack which is keeping the air very cold.

So what’s going to bring that south?

We need the Jetstream to buckle and force it south. The indices we look at for signs of this happening are the Arctic and North Atlantic Oscillations. They aren’t full proof but have a very high correlation to cold air moving into the eastern half of the U.S. when they are negative. Both of these are forecasted to go negative and stay there over the next 10 days.

The question is will the cold air come?

Signs are yes but not all at once. It make take several fronts and several storms to lay down snow cover to our north for progressively colder air masses to move south. Short range climate models point to warm air staying put across the U.S until about Wednesday of next week (bottom left). Then by the time we get to Christmas week the same climate model points to below average temperatures for most of the eastern U.S. (bottom right). This is a second half of the month pattern flip.

Any signs this is happening now?

The long-range weather models are pointing to the cold air coming, but likely in pieces. The ECMWF model and the GFS both have a significant cold front arriving next week. The air mass behind it is cold but modifies a bit on the way down. It’ may take severe of these fronts to bring a larger chunk of the arctic air down. It’s important to get the pattern changed first. That will lay the ground work and snowpack to the north for the colder air reaching the Carolinas. Below you see next weeks cold front left and then another strong front and east coast storm the following week on the 19th(right). These are just model guidance so specifics aren’t as important as the pattern they allude too.

Christmas Forecast?

Still too early to know for sure and as I’veblogged about before snow on Christmas is extremely rare in these parts. There is though all indications it will be cooler than normal by Christmas week. Can’t promise anything this far out but for fun here are 4 runs of the climate model for Christmas week snow cover. Enjoy!