Globally, surface temperature is near a record high. Yet those of us in central and eastern North America have had an abnormally cold winter followed by an unusually cool summer. Why?

The reason for this is a series of persistent ridge-and-trough patterns, where upper-level winds have steered warm air far north into Alaska, then cool Arctic air deep into the Midwest and eastern region. Typically, polar winds blow from west to east in a fairly well constrained zonal pattern. Occasionally, the low-amplitude zonal pattern will stretch into a meridional (ridge-and-trough) pattern that lasts several days before the zonal pattern resumes. What has been happening more often is that the meridional flow pattern just won't go away. A ridge channels warm air up the west coast into Alaska and western Canada, setting records for heat and dryness. The trough part of the formation sets up over the central part of the continent (the locations are influenced by mountain ranges and the like), opening the door to cool air from the north.

Here is an excellent primer that everyone interested in weather should read. It's all you ever wanted to know about the polar vortex -- but were afraid to ask. A weak polar vortex is what allows the persistent, exaggerated pattern...so the question becomes, What makes a weak polar vortex?

The growing realization is that it is due to polar amplification. The temperature gradient between polar and sub-polar zones is what powers the jet stream, which serves to separate the two air masses. As the planet warms, the Arctic warms faster, reducing the gradient and weakening the jet stream. As the temperature gradient becomes "flatter" the upper-level winds are more free to meander, like a river does crossing a flat plain. With a weaker polar jet, the pattern is also slower to move off, meaning extended warm or cool periods.

So far as a weather forecast, I can't answer that. But the way things have gone so far this year, I would plan on a cool fall. I would also guess that an early frost is more likely this year than for an average year. Weather is very hard to predict, though, so it is certainly "possible" that an extended warm pattern will set in just when we need it to finish the crop. I wouldn't be counting on that to happen, though.