What a change! The atmosphere across the northern hemisphere is going to undergo a major transformation as winter finally appears ready to arrive. The current pattern (which has been remarkably persistent) features low heights over Alaska, the Gulf of Alaska, the Bering Strait, and Kamchatka. This +EPO pattern is essentially flooding the United States with mild Pacific air.
In addition the Arctic Oscillation has been strongly positive since October but is finally falling to near neutral numbers. You can see the changes over the North Pole over the last 7 days. Here’s a look at 500mb height anomalies from 1/1/12 across the northern hemisphere.
You can see the negative height anomalies over the North Pole and across the Arctic with huge positive height anomalies across the mid latitudes. Here is the composite for Arctic Oscillation phases and you can see how closely the pattern fits the +AO.
Fast forward to 1/7/12 and you can see the huge changes underway in the Arctic. The negative height anomalies over the North Pole are gone and heights are rising thanks to a ridge north of the Caspian Sea across Russia and into the Arctic Ocean.
The height rises over the Aleutians is very impressive with a large omega block developing. Winter lovers rejoice! This will effectively dislodge the polar vortex and the cold from the Arctic and send it south.
With little upstream blocking this will not be a prolific snow producing pattern. It will, however, be a chilly pattern and with an active northern stream of the jet stream I expect some snow threats to be around.
The question becomes how long does this pattern change stick around? I’m fairly confident a strong +AO will not return. One reason for this is a stratospheric warming event that’s ongoing. Temperatures are beginning to warm in the Arctic stratosphere and zonal wind anomalies are beginning to drop and in some cases reverse.
With the Arctic Oscillation on our side I doubt we’ll see a return to the December and early January torch. That said, I’m not sure the Pacific remains favorable for cold. For example the omega block over the Bering strait may retrograde into Kamchatka which will lower heights over Alaska and bring a return to the +EPO.
Though this may result in a warmer pattern overall by January 20th without the +AO I doubt it is anywhere near as warm as we’ve been. In addition renewed MJO convection over Indonesia (as forecast by the Euro weeklies) may help skew the pattern warmer over the U.S. by February 1 even with a neutral or negative AO.
Bottom line is that winter weather is coming with seasonable to below normal temperatures and some threats for snow. This won’t be an “epic” stretch of winter weather by any means but I do expect more winter than we’ve seen in since Halloween. What’s to be determined is how the pattern shakes out by the end of the month. Will stratospheric warming lead to a sustained -AO? Will a renewed push of MJO convection teleconnect to a warmer pattern overall? Will we see the +EPO vortex return? We shall see!