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Bipolar January redux: 67 Thursday, snow and ice on Saturday?

Forecast is surreal – and familiar.

By midmorning Thursday, the official temperature at Philadelphia International Airport already had reached 60 and jumped to 67 at 3:11 p.m.

And based on the official forecast, it could be snowing by daybreak Saturday with temperatures in the 20s — an inch or less is expected — then head back into the mid-50s during the workweek.

In other words, next week should bear an uncanny resemblance to this week.

As we observed, March has a reputation for volatility, but January has a well-cultivated bipolar side, quite evident in the period of record, long pre-dating the warming of recent decades.

One thing to keep in mind in this era of a warming planet, is that the temperature increases have been incremental; robust in the Arctic, less so elsewhere.

Extremes might be more extreme, but extremes by no means are anything new. For example, the record high for a Jan. 12 is 72 -- set back in 1890.

We remind our readers that despite the tidy look of weather maps, the atmosphere remains a three-dimensional fluid, 10 miles deep, attached to sphere spinning 1,000 mph, hurtling through space at 67,000 mph, with warm and cold air spilling and sloshing here, there, and everywhere.

Variability should be the norm; and it is. Thankfully, the atmosphere is still being the atmosphere.

In an earlier post, we mentioned the National Weather Service concept of "normal." It is a somewhat misleading term, and not the same as average.

"Normal" is based on the reality that climate is ever changing. At the start of each decade, the government tabulates temperatures, precipitation, and snowfall  for the 30-year period that just ended. Those computations become the basis of "normal."

The normal temperatures for this year are based on the 1981-2010 period. For Thursday, the "normal" high is 40. For the 1971-2000 period, the normal January high was 39. But the current normal seasonal snowfall, 22.4 inches, is higher than the 19.3 of the earlier period.

But on average, normal is abnormal.