Saturday, November 19, 2011

How Unusual is this Cold Snap?

An interesting, and increasingly asked question is: "how unusual is this cold snap?"

First some more numbers, just looking at Fairbanks (1904-2010):

Saturday (Nov 19) was the fifth day (in a row) with the daily minimum temperature 35 below or lower. Only four other Novembers have had five or more days with low temperatures of 35 below or lower.

Thus far, three days have had maximum temperatures of -20F or lower (Saturday will likely be the fourth). Eleven other Novembers have had three or more such days and six have had four or more.

The lowest temperature (thus far) has been -41F; this been exceeded in nine other Novembers.

However, this event is occurring not toward end of November, but around the middle of the month.

At the close of Saturday, the 7-day average temperature will be about -22F; there have been five other years that had a 7-day average temperature of -20F or lower ending November 20th or earlier. The coldest 7-day average temperature ending on or before November 20th is -26.5F, which occurred on the week ending November 13, 1989. In second place is Nov 11-17, 1956 at -23.9F.

Another way to look at this, though probably of dubious value, is to ask when the last time a cold snap set four daily record lows? Answer: September 1992, a "black swan" event if ever there was one in Interior Alaska. I say dubious because three of the four records broken were what are sometimes called "low hanging fruit", as can be seen in this plot of November daily record lows for Fairbanks:

Disclaimer: While I have doubts about the veracity of some of the pre-1930 data, it's what the NWS uses for daily records, so I include it here.

The three record lows of 33 below were the only records that high after November 4th. From this point onward only one day has a record lower warmer than -36F.

So, is this event unprecedented; no. Unusual? You bet; similar cold snaps at this time of year have occurred only a handful times in the past 106 years, and this may approach the coldest early season prolonged cold snap (before late November), though I'd guess at this point the 7-day record low mean of -26.5F set in 1989 will stand.

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