Friday, February 1, 2013

Valley vs. Mountain, January 2013

I planned a post today on all the weather oddities during January, but Tom Wharton has a pretty good summary in today's Salt Lake Tribune that does the job quite well.   It is, however, concentrated heavily on Salt Lake and snowfall, so I want to add something about what happened with temperature in the Wasatch Mountains.  To do this, I need to do some hand waving as I don't have the time to collect and process the data right.  Thus, concentrate on the big picture in this post and don't get too hung up on the gory details.

There was a dramatic difference in the temperatures experienced in the valleys and mountains of northern Utah this January.  The National Weather Service reports that January 2013 was the coldest month at the Salt Lake City airport since 1949, the 6th coldest January since 1874 (presumably the late 19th and early 20th century observations are from Salt Lake City proper), and the 9th coldest month of any month since 1874.

The NWS has some very nice graphs on their web site showing the departures from the daily averages, but I need to cook something up for comparison with Alta.  Thus, the image below shows the temperatures observed at the Salt Lake City International Airport during January (red line) compared to the average (1948–2012) maximum and minimum temperatures for the month (enclosed by green box) as provided by the Western Region Climate Center.  Note the prolonged periods of well-below average temperatures in early January and mid January, both of which were associated with inversions.

Salt Lake data source: MesoWest
In contrast, the picture at Alta is different.  The image below shows the temperatures observed at the Alta Guard weather station maintained by the Utah Department of Transportation (red line) compared to the average (1905–2012) maximum and minimum temperatures for the month (enclosed by the green box) as provided by the Western Region Climate Center.  These sites may not be collocated, and that does affect the comparison, but the big picture is clear.  During inversion periods when Salt Lake was well below average, Alta ran either near average (e.g., early January) or above average (e.g., mid January).

Alta Guard data source: MesoWest.  
Thus, while Salt Lake City experienced a remarkably cold January, Alta probably ended up either near or perhaps even above average.  It will be interesting to see how things average out for the Alta climate site when all the data is processed and available at the National Climatic Data Center in a couple of weeks.  

This illustrates one of the paradoxes of northern Utah weather and climate.  During winter, when under the influence of a large-scale upper-level ridge, it is possible to have below average temperatures in the valleys and above average temperatures in the mountains.  The contrast can become most exaggerated when there is a deep snowpack in the valleys.  Consideration of these effects is important for those who utilize long-term climate statistics in Utah and surrounding states that also see strong valley inversions and cold pools.  

1 comment:

  1. If you think about it, the stable layer above the cold pool is very consistent with the low orographic ratio that has been observed in some of the recent snow storms.