After a few mild, spring-like days this week in the East, a cold shot from Canada will move in for the weekend and deliver a brief dose of February for a change. It does seem a little odd that an upcoming 2-3 day bout with chilly air — carrying temperatures perhaps as much as 8-16°F below average — deserves headlines. But given that winter 2011-12 has so far given us a temperature map like the one below, a cold snap of any kind is worth discussing.

Surface temperature anomalies (°C) averaged since December 1. (NOAA/ESRL)

A forecast of the jet stream (outlined by the black line) from the UKMET model for Thursday. Black text denotes upper-level low (L) and high (H) pressure systems. Image courtesy Penn State.

Some of this chaotic behavior is manifested in the variety of forecasted shapes and sizes of the waves embeded within the jet stream over North America (the L-H-L group in the image above).

Some recent forecasts of the jet stream and surface temperatures for the upcoming weekend — see below — have looked like the picture on the left (a recent forecast from ECMWF model), others like that on the right (a GFS model forecast from yesterday), and still others some combination of the two.

Recent forecasts of the high-altitude flow (outlined by the black lines) and surface temperatures (text) for Sunday from ECMWF (left) and GFS (right). Images courtesy of Centre Meteo UQAM-Montreal.

On the left, a cold, dense surface high pressure system over the Central Plains and an arctic high north of Lake Superior heading south would combine to deal parts of the eastern half of the country the coldest air of the season. On the right, a relatively mild southerly wind developing across the Plains would quickly send the cold air packing. This is truly a forecaster’s nightmare, especially when it’s not immediately clear where or when the sensitivity is coming from.

What is clear is that extremely cold air is right now beginning to slide southward from the Pole into Eastern Canada, toward Hudson Bay. This is associated with the inflation of the West Coast ridge. In the darkest shades of purple across far northern Canada (see below), surface air temperatures last night were less than -50°F.

Surface temperatures (°F) this morning. Image courtesy of Unisys.

Fortunately, the arctic air mass will gradually lose its punch as it moves southeastward. A modified piece of its southern edge will slide across our northern border Friday and Saturday and bring much-below average temperatures there. The National Weather Service in International Falls, MN predicts highs near 0°F on Friday. As it stands right now, a good estimate of the temperature anomalies on Sunday looks something like what you see below.

Temperature anomalies expected Sunday from the GFS ensemble mean. Image courtesy The Pennsylvania State University.

This transient move away from our recent bizarre warmth to a northwesterly flow that actually delivers February-like conditions is partially tied to the evolution of the Madden-Julian Oscillation (MJO), and its interaction with the midlatitude jet stream. The MJO is moving across the tropical Pacific right now and thumping the atmosphere in a way that encourages ridging at upper-levels near western North America.

As we move forward into the second half of February, there is good reason to believe that the overall mild pattern (for nearly everyone east of the Rockies) will resume, owing in part to the migration of the MJO to the other side of the world. In fact, places in blue in the image above will likely be yellow or red (above average) again by the middle of next week.