Monday, September 28, 2015

Record Setting Late September Temperatures and Their Potential Hydrologic Implications

We've had a remarkable period of sustained warmth over the past week in Salt Lake City.  Not only have we set two maximum temperature records (93ºF on Sep 25 and 90ºF on Sep 26), the average temperature for the past week (Sep 21–27) of 74.1ºF is the highest on record, easily eclipsing the 72.9ºF observed in 2001.

A look at the time series above suggests that this past week is indeed quite an outlier and one of only 5 such September periods going back to 1874 with a mean temperature above 70ºF (note that the records above are a composite of observations collected in the Salt Lake Area as the airport didn't exist back then!).

An unfortunate aspect of this hot dry weather is a loss of moisture from the big rains we had from September 14–16.  As shown in the chart below for the Mill D North SNOTEL in Big Cottonwood Canyon, those big rains greatly increased the soil moisture content of the soils at both 2 and 8 inch depths.

Data Source: NRCS
 Similarly, soils elsewhere at all elevations, including in the Salt Lake Valley, received quite a boost.  Unfortunately, as can be seen in the plot above, we've now lost soil moisture to evaporation and transpiration, the process by which moisture is carried through plants for photosynthesis.  Sun exposed lawns that haven't been irrigated since mid September are starting to feel crunchy and look thirsty.

In the mountains, if that moisture is not replaced before the first snows, the soils will be the first place that the snowmelt goes in the spring (rather than to the rivers and streams).  That might be a good thing if we end up having a huge snow year, but if our run of meager winters continues, it would exacerbate the crappy runoff situation even further.  As we have discussed previously (see Wasatch Weather Weenies Survival Guide for El Nino), the current Super El Nino doesn't really load the dice one way or the other for us, so at this point, we have to have a wait-and-see approach to what happens between now and the first snows, as well as during the winter.


  1. From what I can find online, the Great Salt Lake is currently near 70 F which I assume is above average. If the forecast of a cool and moist trough late this week actually pans out, maybe that will contribute to some lake enhanced precipitation locally.

  2. Even an average winter would be a welcome change!

  3. It's also been extremely warm (and dry) all month here on Colorado's Front Range. I suspect it will be a near-record warm September for Boulder/Denver.

    Question: What is the URL for the tool you used to generate the time-series plot at the top of the post?