Tuesday, December 23, 2014

Rainy Winter Days

Mean monthly temperatures at Minneapolis, 1981-2010. From MRCC/cli-MATE  
December 2014 has been a strange one here in Minnesota and the Upper Midwest. After one of the coldest Novembers on record, including a top-10 showing in Minneapolis, December is going to end up warmer than November (or nearly so) in many places. And that is no easy feat, considering that November is typically 13-16 degrees warmer than December. The only way to do it is to have an unusually cold November and then an unusually warm December. Check and check.

A lot of people have complained, or maybe just noticed, that it seems to be raining a lot this December. That is, raining rather than snowing. It's especially noticeable because November had been so cold, and so snowy (in some areas). Moreover, we have had a formidable run of cloudiness, such that by December 26, many parts of the Minnesota will have had 19 out of 20 days with cloudy or mostly cloudy conditions.

But how unusual are rainy days in this region, in the winter? Unfortunately, you can't just query some database and come up with a clean answer, so I took the "threaded" record of Minneapolis back to the winter of 1900-01, and extracted days with precipitation but either no snow or a trace of snow, and also with high temperatures exceeding 32 degrees (in order to exclude ice storms). There are other ways to do it for sure, but this one allows me to bypass fussing over snow-to-liquid ratios.

The "average" winter, defined here as December through February, has about 2.8 wet days at Minneapolis. As the graph below shows, the history of wet days during winter has a double-peak pattern, with a long period during the 1910s through 30s that tended to be above-average, followed by an at-or-below-average period into the 1980s, which ultimately gave way to the current era, in which we are more above-average than any other part of the record. Since 1990, only one winter in Minneapolis has failed to have two or more wet days.

As of this post, Minneapolis has had 4 wet days this December (and thus, this winter). That puts it above the long-term average but not into uncharted territory. Of the three winter months, December is most likely to have wet days, and accounts for about 49% of them (155 of the 319 identified). There were ten other Decembers in the record with four or more wet days (including one with 8 and another with 7). So, again, above average, but not at all standing alone.
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Wet days at Minneapolis, during winter from the 1900-01 winter into the beginning of the 2014-15 winter (plot ends December 22, 2014). The gray trace is a 5-year average centered on the reference year; the yellow trace is 10-year average ending on the reference year.  

Two notes: 1) the "up-down-up" pattern you see on graph, especially in the traces, is all over the Upper Midwest climate record. You'll see it with rainfall, with snow, and with temperatures; lots of things took a dip in the middle of the last century. I'll post more on that later. 2) The "threaded" climate record of Minneapolis is assembled from several different locations over time, to allow a longer total record. More info here.

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