Discussion: The negative phase of the arctic oscillation (-AO) emerged in early December and so far, has displayed most of the attendant high-latitude high-pressure blocking across Greenland (negative North Atlantic oscillation or -NAO). The -AO/-NAO combination has produced early season chill in Europe, especially France and across the Northwest/North-central U.S. (enhanced by deepening snow cover). Interestingly, despite the usually cold-producing mid-latitude climate by the -AO/-NAO regime, south of snow cover in the U.S. so far in December is unusually warm. The warmth is linked in-part to unusually warm SSTA in both the mid-latitude North Pacific (marine heat wave NEP22A) and North Atlantic.
However, the character of the -AO regime is about to change. The high-latitude high-pressure ridge shifts from Greenland to Alaska and gradually an Alaska “ridge bridge” toward Siberia develops. The result is increasing risk of a cross-polar arctic air surge from Siberia into North America in 7-10 days. The pattern described is often referred to as the “Siberian Express” and the Alaska “ridge bridge” phrase became popular after the North America “polar vortex” winter of 2013-14. The Alaska “ridge bridge” pattern is present when sharp negative East Pacific oscillation (-EPO) is in-place.
In this pattern, zero (F) reaches as far south as Nebraska to Wisconsin and 20’s (F) will almost reach the Gulf Coast. The projected U.S. snow cover for Christmas Day by GFS ENS indicates all but the far southern U.S. will have snow on the ground!
Fig: 1-2: ECM ENS 11-15-day upper air forecast for the northern hemisphere identifies the Alaska “ridge bridge” and attendant very cold U.S. pattern.
Fig: 3-4: ECM ENS <24F forecast and the GFS ENS U.S. snow depth forecast for Christmas Day.