Thursday, January 24, 2013

How Unusual Is This Ice Storm?

Trevor Alcott provided me with an analysis of hourly surface observations from the Salt Lake City International Airport that help illustrate the unusual nature of our ice storm:

  • Since 1940, there have been only 9 measurable freezing rain events (0.01 inches or greater) at the Salt Lake City International Airport
  • None of those events occurred with temperatures lower than 26ºF.  Today's began at 20ºF.  
  • The latest accumulation (0.08 inches as of 11 am) is the largest freezing rain accumulation since 31 Dec 1983. 
  • There are only three events that top this one in terms of freezing rain accumulation:
    • 0.13 inches that fell at 32ºF on 9 Feb 1976
    • 0.16 inches that fell at 29–32ºF from 26–27 Dec 1983
    • 0.21 inches that fell at 30–32ºF from 30–31 Dec 1983 (.11" of this fell as heavy sleet)
So, it appears this is the coldest measurable freezing rain event in the period of record and it is the fourth largest (so far) in terms of accumulation.  


  1. The inversion leading up to it was the strongest of any I can remember. Yesterday's 00Z sounding showed an incredible 20C temperature discontinuity at the top of the cold pool. I was amazed to observe first hand how distinctly separated the cold pool was was from the overlying air mass. I am curious about the climatology of ice storms in general... it seems like they are almost always a very low elevation phenomenon, but have never seen any data specific to this. If this is true, perhaps it is related to increasing storm wind fields with elevation, or thermodynamics related to air density, etc.

    1. While riming is common at upper elevations (freezing of supercooled cloud droplets on common), ice storms are largely a lower elevation phenomenon. This can be the result of topographic effects (e.g., topographic cold pools - esp. Columbia Basin, cold-air damming - east of Appalachians, gap flows - Columbia gorge) or synoptic processes (e.g., most commonly along warm and stationary fronts). Papers of interest include Robbins and Cortinas (2002,;2) and Cortinas et al. (2004,;2). These should be freely accessible. The latter includes a freezing rain climo for North America.

    2. Thanks for the references. Despite Utah's frequent cold air pooling during winter dry periods (similar to the Columbia Basin and low elevations in western MT), it seems like the higher elevation cold pools experience more mixing during storm periods than do those closer to sea level. Of course it is also much more difficult to achieve an overlying melt layer at higher elevations. A deep tropical moisture tap was responsible for that in this case.