Monday, March 12, 2012

Oddities of the Intermountain Precipitation Climatology

The Intermountain West is truly a strange place, especially when it comes to the seasonality of precipitation.  For example, what is the wettest month of the year climatologically in northern Utah?  It turns out that the answer varies geographically.

At Alta, there is a broad winter maximum from Dec–Mar with the peak snow-water equivalent in January.  In contrast, at the Salt Lake City International Airport (KSLC), the peak is in April.

I've heard several meteorologists say that April and May are the wet season in northern Utah, but clearly the answer varies geographically and with altitude, as illustrated above.

The contrast between mountain and lowland precipitation, which can be expressed as an orographic ratio (that is, the ratio of the monthly mean precipitation at Alta to the monthly mean precipitation at KSLC), also varies seasonally.  In January, the mean monthly precipitation at Alta is more than 5 times what it is at KSLC, but by June it is only about 2 times as great.

This seasonality is orographic ratio is an important characteristic of the Intermountain precipitation climatology that is underappreciated, yet important for weather and climate prediction.  In addition, it is poorly understood.  I don't know of a single peer-reviewed paper addressing the spatial variations in precipitation seasonality and the seasonality of the orographic ratio over the Intermountain West.  We could hypothesize that fluctuations in the intensity of the mean westerlies (and cross-barrier moisture transport) contribute, but the problem is likely more complex than that.  This is fertile ground for research that could have important implications for regional climate prediction.


  1. I would almost be willing to bet that Utah is the most varied state in the U.S. in terms of precipitation seasonality. There is a December - January maximum primarily in the northern/central mountains, an April maximum along parts of the I-15 corridor, a July - August maximum in limited areas of southern Utah (which is a secondary maximum over a much wider area) and a September - October maximum in much of the eastern part of the state. So all four seasons are represented here.

  2. You are probably right. More to come in a couple of weeks...

  3. Really interesting findings-- would enjoy reading more about this. -Grant W