Monday, September 1, 2014

August Precipitation in Context

August was a great month in Salt Lake City.  I haven't run my sprinklers in quite a while.  If only every summer could by like this.  How unusual was it?  Let's have a look.

Although we often present historical weather data in chronological order (i.e., ordered by year), it can be helpful to instead present the data in ascending order, as I have done below for the August precipitation in Salt Lake City from 1928–2014.

As highlighted above, the median and average August precipitation in Salt Lake City for the period of record are 0.54" and 0.81", respectively.  The median represents the value at which 50% of the years are drier and 50% of the years are wetter and it is lower than the average, but the average is skewed higher by a small number of wet Augusts.   Thus, a curious aspect of the August precipitation climatology in Salt Lake City is that there is a 64% chance of precipitation at or below the average and only a 36% chance of precipitation above average (it is not uncommon for precipitation distributions to be skewed in this way, especially in arid regions).  So, if you want to make some money off your friends, bet them each year that the August precipitation will be below average.  You'll win about 64% of the time and slowly but surely build up a war chest of cash that you can blow in Vegas where the odds are certainly not in your favor.

So now let's look at 2014.  It seemed really wet and it was with 1.77" falling at the Salt Lake City airport.  That's more than double the average and more than three times the median.  It was also the 11th wettest August on record.

However, although it was a relatively wet August, the historical record shows that it can be far wetter.  In 1968, 3.66" of rain fell at the Salt Lake City international airport, more than double what fell this year (compare blue August 1968 accumulation with green August 2014 graph below).  In fact, 1968 was a very wet month across much of the region.

Given the spotty nature of monsoon rainfall, ideally one needs to look at a larger number of stations to gain a better context for the entire Wasatch Front.  Perhaps someone out there can dig through the data and add comments about accumulations at other sites.  Concentrate on sites with good data back to 1968 since that seems to be a critical year.


  1. I found a few other stations with data for August of both years (1968 listed first): Tooele 3.93 vs 2.34; Logan 3.44 vs 2.31; Spanish Fork 2.21 vs 2.73; Alpine 2.28 vs 2.91. So at least a couple of sites were apparently wetter this year assuming they have complete records. The majority of the CoCoRahs stations along the Wasatch Front (Ogden to Provo) were between 2" and 3" for the August 2014 total. My station in Taylorsville totaled 2.68" which was typical of many others around the Salt Lake Valley. The SLC airport total of 1.77" was one of the lowest that I could find in the area.

  2. Nice point about the chances of above or below normal precipitation. Your wording suggests that this might be somewhat unique to August in Salt Lake City, but I would not be surprised if it was generally true elsewhere, and I'm curious as to how the 64/36 ratio might compare with other locales. Might just have to pull out the CPC Precipitation Analysis and have a look, since I don't want to go searching for all the city data.