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Just like with anything strong, to support it you must have a solid foundation. Same thing with air. To sustain cold or warm air masses for any extended period of time, you must have an extensive supply of one or the other. The past 5 weeks, the cold that typically is seen in North America has been lacking. Look at the extensive positive temperature departures, especially in Canada which is the source region for cold in the Midwest.

That's led to average temperatures running 3-6 degrees above normal around the Midwest since December 5th.

To turn this around the pattern needs to be reversed so that cold can return and re-build over North America. In my last 2 posts I've shown lots of evidence why I think that is going to happen using teleconnections and analogs. Today, our mid-range models are starting to trend in that direction. Here's a good example. This is the 500mb jet stream flow currently in place that's bringing the mild conditions.

By January 18th (day 14) look at the dramatic difference. The west coast trough has been replaced by a ridge. A complete reversal.

That has big temperature implications. Look at the forecast temperature departures in 2 weeks.

The cold that was absent from Canada has returned and is strengthening as it pushes south into the Midwest. A driving factor is a massive 1057mb high pressure centered over Greenland. (The pink area near the top in the graphic below).

The ridge is bridging southward into into the northern Plains. Pressures are anomalously high over the Arctic and Canada indicating the extensive cold that's building.

This also is the set-up that drives a negative NAO (North Atlantic Oscillation). That and the negative AO (Arctic Osclilation) are drivers that lead to below normal temperatures over the central and eastern United States.

Here's the temperature anomalies for a negative NAO in January.

The temperature anomalies for a negative AO in January.

Another factor that will aid the advancement of the cold air if it indeed develops is the healthy snow pack that resides over the upper Midwest and northern Plains. That will keep the cold from modifying until it encounters bare ground in southern Minnesota. With time I expect the snow cover to advance southward into my area. However, it's too early to say precisely when...maybe the last 7-10 days of January.

While today will see temperatures in the 50s in many parts of the central Midwest, the foundation for the road to cold is being built over North America. Enjoy the warmth, the clock looks to be ticking. Roll weather...TS

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