I want to go into the high pressure ridge building over the Gulf of Alaska. This is a negative East Pacific Oscillation, or EPO. When a negative EPO sets up, cold air is then provoked to move into the central and eastern parts of the nation. It is thought of as one corner of a triangle of teleconnections that can bring cold air to the US. The second piece is the Arctic Oscillation, or AO. When the polar vortex is weak, cold air is not able to be held by the vortex and slips south from the Arctic. This is a negative Arctic Oscillation. Considering the polar vortex is going through heavy damage, the AO will be strongly negative and could bomb out to extreme negative values. The third item is the North Atlantic Oscillation (NAO). In the negative NAO, the jet stream buckles south to give parts of the nation cold weather. We see a battle ongoing between low pressure just south of Greenland (typical of a positive NAO) and high pressure to the north of Greenland (typical of a negative NAO). I think the NAO is definitely the wild card here, and could determine whether the cold stays in the Plains and Midwest (what I am thinking) or moves East into the Northeast and East Coast (which will happen, but may not be as cold).
An issue we are seeing on both the ECMWF and GFS model (image on the left from WeatherBell) for this event is the negative Pacific North American index, or PNA. In the negative PNA, low pressure builds over the West US and Rockies and provokes high pressure to build in the East. This whole cold outbreak will start with a negative PNA, which is why warm air will be maximized on the East Coast prior to this warm up, as the top ECMWF image is showing.
So we're seeing a tricky NAO and a bad PNA. Is this cold really going to happen??
YES. Both models have the polar vortex (yes, THE polar vortex) sliding into southern Canada and the north US. As it does so, the low pressure causing the negative PNA will also move east, bringing the unforgiving Arctic cold into the Plains and Midwest. The dreaded Southeast Ridge will then move out of the East Coast, allowing for some moderated (but still harsh) cold to bleed into that region.
Why am I so confident? The GFS has been giving a stellar performance with the stratosphere thus far, meaning it has a better handle than that of the ECMWF. If the GFS is to be believed, a negative NAO, negative AO and negative EPO will be in place when the cold breaks off and moves east in about 10-15 days. However, I think the ECMWF has a better handle on the North Atlantic Oscillation, hence my thinking that the strongest cold will be confined to the Plains and Midwest-Great Lakes.
Beyond January 20th, things become very murky, and I don't even want to go into what could happen after this initial wave of cold. We will have to wait a few more days to get a clearer picture of what happens post-January 20.
How could it mimic January 1985?