Monday, March 9, 2015

Freaky Februaries: 2015 and 1934

We have an official scientific name to describe last month: Ridiculous! Cold and snowy in the east.  Warm and not so snowy in the west.  The National Weather Service Forecast Office in Boston posted up the figure below showing the statewide average temperature ranks for the continguous United States.  Record or near record warmth across the west, and near record cold across the east.

Source: NWS
Salt Lake City International Airport had its warmest February, breaking the record set in 1934 (records go back to 1928).

Source: NOAA Regional Climate Centers
I didn't think much about 1934 until I saw in the paper that Buffalo had its coldest February, breaking the record set guessed it...1934.

As freaky as February 2015 was, February 1934 was nearly as freaky at these two sites.  In Salt Lake City, the mean temperature in February 1934 was 42.2ºF compared to 43.9ºF in February 2015.  In Buffalo, 11.4ºF and 10.9ºF respectively.

I thought it would be interesting to go back and take a look at the large-scale patterns during these two Februaries.  When meteorologists do this, we often look at the average 500-mb pattern (top figure below) and anomalies (bottom figure below).  The anomalies tell us where there is anomalous troughing and ridging.

As can be seen below, February 2015 was characterized by anomalous ridging over all of western North America and most of northern Asia, with anomalous troughing over eastern North America.  The resulting pattern favored frequent cold-air intrusions into eastern Canada and the United States, leading to a remarkably cold month.

Source: ESRL
Source: ESRL
February 1934 was a long time ago.  There were no routine upper-air observations and certainly no numerical modeling systems.  (The first attempt at numerical weather prediction was made by Lewis Richardson in 1922, but didn't know the tricks and techniques that were necessary to produce a stable forecast.  Those weren't developed until the middle 20th century).  However, we can now examine upper-air analyses for the month thanks to a remarkable project known as the 20th Century Reanalysis (more info here).  The pattern in February 1934 was similar, but not as amplified, as what was observed in February 2015.  Note the anomalous ridging over western North America and northern Asia, and troughing over eastern North America.  Over the southwest US, however, there was an anomalous trough, although I see this as a likely consequence of the overall upper-level pattern, which often favors a high-over-low blocking pattern in the western U.S. (we've seen that frequently this winter, but it hasn't dominated enough to be seen in the average fields).  There are a few other differences, but in general the pattern is fairly similar, and I suppose that is not hugely surprising since you need such a pattern to get extreme warmth in the west and extreme cold in the east.  There is some uncertainty in this analysis, but it's enough to provide a reasonable comparison.

Source: ESRL
Source: ESRL
So, it appears that February 2015 isn't the only black swan in the U.S. historical weather records (although I suspect that the scale of the warmth in the west and the cold in the east in 1934 might not have been as great and the snow in New England and the south bears further investigation as well).  This is one of the reasons why I'm always hesitant to blame unusual weather and short-term climate events on global warming.  There's a large natural component to such extreme events.  Climate is the statistics of weather and it is better to discuss how the odds or likelihood of weather events are changing.  We can see those odds shifting in recent decades, but as the easterners have learned this winter, Mother Nature can still bring the cold to a particular region when she goes into outlier mode.  Nevertheless, when one looks at the planet as a whole, I'm sure February 2015 will be one of the warmest Februaries on record when the climate centers release their global analyses in a few days.

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