Friday, October 18, 2013

Great Storm Anniversary

Editor's Note: Special thanks to Jeff Massey for contributions to this post.  

Today marks the 29th anniversary of one of the largest snowstorms to hit the Wasatch Front.  It desperately needs a name, but to my knowledge one hasn't been coined, so we'll just call it that Great Storm of October 1984.

A brief analysis of the Great Storm was produced by David Carpenter and published in the journal Weather and Forecasting in 1993 (Carpenter 1993).  David is now the meteorologist-in-charge at the Rapid City National Weather Service Forecast Office and recently made headlines for his office's dedication to public service when a huge blizzard hit western South Dakota during the federal government shutdown.

In the Salt Lake Valley, the Great Storm was a beast, producing 18 inches at the Salt Lake City airport, 27 inches inOlympus Cove, and 25 inches in Cottonwood Heights.  The heaviest accumulation was along the east bench, whereas there were locations in the western and southern valley that received no snow.
Storm total snowfall for 17–18 October 1984.
Source: Carpenter (1993).
Carpenter (1993) suggests that most of the snow fell from 0030–1830 UTC 18 October (1830 MDT 17 October–1230 MDT 18 October) and primarily the result of lake effect.  It is a shame that there was not meteorological radar operating in the Salt Lake Valley at the time to take a closer look at the event as this would surely make it the largest valley lake-effect event in the past 30 years if not in the entire historical record.  In any event, the Great Storm still stands as the largest 24-hour snowfall ever observed at the Salt Lake City airport.  All that snow fell while most of the trees in the valley still had leaves, creating quite a mess of downed power lines.

Carpenter estimated lake temperatures to be near 10ºC during the event.  Temperatures of the Great Salt Lake tend to track very close to the 7-day running mean temperature at the airport.  While I don't have access to lake-temperature data during this period (it might exist, but I'm being lazy), the week preceding the storm features temperatures near 68ºF (20ºC) on the 13th followed cooler weather.  In the three days preceding the event, temperatures fluctuated from near 32ºF (0ºC) to about 50ºF (10ºC).
KSLC Temperatures October 1984.  Courtesy Jeff Massey.
Thus, I'd go with a temperature of 10ºC or perhaps a bit lower.  Surprisingly, this is not unusually high. The climatological lake temperature for mid October is about 14ºC.  Thus, the extreme nature of this storm cannot be blamed on an unusually warm lake (note: the lake size, however, was quite large as during this period the lake level was near its historical high).

However, the event did feature the passage of an especially cold upper-level trough.  As shown in the looop below, 500-mb temperatures dropped to near -30ºC after 0000 UTC 18 October when the lake-effect raged.  At 700-mb, temperatures dropped to about -11ºC, which is not too shabby either.

500-mb and 700-mb analyses for the Great Storm.  Courtesy Jeff Massey.
There are, however, many cold troughs that come through Utah, generate large lake-atmosphere temperature differentials, and don't generate any lake effect.  The triggering and organization of lake effect depends on smaller-scale processes that we are just beginning to understand.  It appears that the key ingredients were all in place for the Great Storm, but a lack of radar data and surface observations precludes a careful analysis of the small scale processes that made it such a huge event.  We're just going to have to hope for a repeat.


  1. Browsing through Carpenter (1984), I didn't see lake level mentioned, but the lake level was pretty high in October of 1984, wasn't it? I don't know where to access the data, but you can see it is likely above 4205 feet in Figure 2 in this paper:, which is significantly higher than it has been in recent years.

    1. A graph showing the historical elevations of the Great Salt Lake is available at 1984 was clearly a well above average year and within about 3 feet of the historical record. WIth some effort, one can probably get a precise elevation at or near the time of the storm.

    2. Thanks for the link. The map shown on that website shows that the west-east fetch over the southern end of the lake is much larger during high lake levels than during average or below average lake levels. I wonder what role that plays, especially for events with a decent westerly component wind, like the October 1984 event. It would seem that a larger lake increases the probabilities of lake effect for most wind directions with all else equal other than maybe NNW (or at least makes more wind directions possible when they aren't with a low lake). In doing so, you might maximize your probability of a big event.

  2. A deeper lake would also hold more heat. Its temperature would not fluctuate quite as much, so that also improves early season lake effect potential.