Thursday, February 8, 2018

How Unusually Warm Has This Winter Been?

Answering that question is not as straightforward as it sounds.  It is probably most easily answered at the Salt Lake City International Airport where data records go back to 1928.  There, Dec-Jan came in with an average temperature of 36ºF, making it the 4th warmest Dec-Jan on record.

Source: NOAA Regional Climate Centers
But let's suppose you want to know what has happened in the mountains.  Here's where things get tricky.  One option is to look at data collected by National Weather Service Cooperative Observers.  If you pull up a map of the stations, it looks like there are many options. 

Source: Western Region Climate Center
However, there really isn't.  Those are all stations that provided any data for any period going back for many decades.  The Snyderville Basin site started in 1991.  The site near the Park City Golf Course started in 1986, which is great, but ended in 1991.  That sounds like a long time, but there's data for temperature on only 26% of the days in the period.  Alta is a favorite of yours truly, but has lots of data gaps, even in the post WWII period.  The site near Brighton has records back to 1937, but with data gaps and, unfortunately for today's post, the observations from this January haven't come in yet.  If one does tempt fate and looks at the Alta coop site, one finds that this Dec-Jan is rated as the third warmest (27.2ºF, no missing days) behind Dec 1985 - Jan 1986 (27.2ºF, 11 missing days) and Dec 1980 - Jan 1981 (30.8ºF, 1 missing day).  

Source: NOAA Regional Climate Centers
There are other observing sites in the Wasatch, including those operated by ski areas, as well as the NRCS SNOTEL stations.  Data records for these are also often limited and don't go back a long ways in time.  

One option is to use estimates for average temperature in the "Northern Mountains" Climate Division produced by the National Centers for Environmental Information (NCEI).  These estimates integrate the available observations and attempt to account for data gaps and other data warts and biases (see Rose et al. 2014 for gory details).  If we pull that data, we see that this Dec-Jan had an average temperature in the region of 27.8ºF, which is the 2nd highest on record, behind only Dec 1980–Jan 1981.  Several other periods were close to as warm, including Dec 1917 – Jan 1918.  

Source: NCEI
Unfortunately, the Northern Mountains Climate Division covers a pretty varied region in northeast Utah with pretty dramatic changes in climate.  Trends could vary dramatically within that region.

Source: NCEI
If we pull all of this together, we find that we can be confident that this Dec-Jan was unusually warm relative to the climate in the historical record.  It does not appear to be the warmest such period at most sites, but probably sits in the 5-10% warmest Dec-Jan periods since the early 20th century.  

All of this highlights the desperate need for improved monitoring of weather, climate, snowfall, and snowpack in the Wasatch Range if we are to better anticipate, observe, and understand future climate change and it's impact on our snow climate and water resources.  For the most part, weather, climate, and snow scientists must act like vultures and piece together what we can with datasets that are incomplete and have lots of warts.  We have developed many techniques to do this, but nothing beats a well-maintained monitoring network, preferably one that observes more than just temperature and snowfall, but also snowpack characteristics and important drivers of snowmelt (e.g., solar radiation).  Establishing such a network would be a wise investment that could be taken advantage of by the generations to come as they attempt to anticipate changes in snow and water resources in a rapidly changing climate.  

1 comment:

  1. The SLC airport site has looked a little warm recently compared to nearby sites. I know that happened before, and I think it turned out to be a couple degrees too high that time, before it was adjusted.