Polar Vortex to Unleash Historic Cold Across Plains, Midwest

There are increasing signals that we will see the polar vortex unleash potentially historic levels of cold air upon the nation, mainly targeting the Plains and Midwest.

Kyle MacRitchie

The Madden-Julian Oscillation (MJO) is projected to move into Phase 6 as we progress into January. For those who don't know, the Madden Julian Oscillation involves the placement of enhanced tropical convection over the Equatorial Pacific, extending from the Indian Ocean to roughly the middle of the Pacific Ocean nearly due east of northwest South America. The MJO entering Phase 6 means that the enhanced tropical convection will be northeast of Australia. After that Phase 6 movement, it looks like we will head on in to Phase 7 of the MJO, and that's the one we need to keep an eye on. This forecast, made by four separate CFS ensemble members, shows us entering Phase 7 around January 15th.

MJO Phase 7 500mb Height Anomaly Composite

If we take a look over RaleighWx's Phase 7 500mb anomaly chart, we see that Phase 7 of the MJO in January sees major ridging forming over the Western US and well into Canada, even over the Bering Sea. This is a classic example of a positive Pacific North American (PNA) index, which alters the jet stream so that the jet stream spikes north in the West due to that ridging, and then plummets southward in the Central and East US. This leads to a pattern highly conducive for cold weather in those areas. Adding to that conducive pattern is the highly negative North Atlantic Oscillation (NAO), as is exemplified by the heavy ridging over Greenland. All of this ridging across the upper latitudes then allows a negative Arctic Oscillation (AO) to form, which only adds to the intensity of the cold air in the US. All in all, the Phase 7 really creates the best environment that we can look forward to in the medium/long range for some cold weather.

This is all good and fun, but the really interesting stuff comes up when you take a look at the Climate Prediction Center's analog-produced upper air outlook for the medium range.

Does this chart look familiar? It should, because the anomaly placements are almost identical to the Phase 7 composite above when viewing the North American continent. We see extreme ridging over the Gulf of Alaska, which then allows for the formation of not only a deeply negative EPO (a key player in this impending brutal cold spell), but also a classic McFarland Signature look.

This image from the National Weather Service in 2010 shows what the McFarland Signature looks like. You have the massive ridge set up across the Gulf of Alaska and into the West Coast, and this leads to deep negative height anomalies developing just to the east, usually in the Central and East US. These deep negative height anomalies allow frigid air to be transported from the upper latitudes down into the United States, and I anticipate a very similar situation to evolve here again in the medium range time frame.

Taking a look back at that analog image above the McFarland picture, we see not only the well-established McFarland Signature/-EPO, but those deep negative anomalies being projected as very pronounced across the Central United States. Based on how pronounced those anomalies are, as well as analyzation of the jet stream and temperature patterns during this timeframe, it does appear that the polar vortex would be entering the United States. This wouldn't be any low pressure system; if this were to happen, we're talking record cold. Remember that the polar vortex is the low pressure system that sustains the cold air in the Arctic, so bringing a portion of that vortex down south into the US could be disastrous for farmers growing crops not only across portions of the Central and Eastern US, but also down in the South.

Model guidance is in good agreement about the polar vortex first affecting the United States around January 6-8. Beyond then, we may have to wait a little while until the MJO gets into Phase 7 before we see a resurgence of cold.

ECMWF Temperature valid Tuesday morning.

GFS Temperature valid Tuesday morning.

Andrew