Sunday, August 11, 2019

The Meteorology of the Salt Lake City Tornado (1999)

Today is the 20th anniversary of the Salt Lake City tornado, which I would rate as the most significant meteorological event to affect my neighborhood (The Avenues) since we moved there in 1996.

A detailed account of the tornado event is described by Dunn and Vasiloff (2001).  The tornado formed just west of downtown Salt Lake City and tracked roughly northeastward through portions of downtown, Memory Grove, and the Avenues.  It produced a path of moderate to considerable damage consistent with a rating of F2 on the old Fujita Scale (since replaced by the Enhanced Fujita Scale).  The damage path looked as if someone had sliced across the area with a surgical knife producing a narrow swath of damage with no damage in the surrounding area.

Source: Dunn and Vasiloff (1999)
The tornado touched down at 1840 UTC (1240 MDT).  I recall that we had gone with a group out to lunch and were returning to the office, commenting about the darkness of the sky and joking as Utah meteorologists do about it potentially being a "severe" day (this isn't tornado alley).  Shortly after returning to the office, someone yelled "tornado to the west", a comment that was initially met with derision, but eventually we all ran outside to watch.  The photo below was taken by our department web cam, looking west from the Browning Building on the University of Utah campus.

Source: University of Utah, Dunn and Vasiloff (1999)
It was the first and thankfully only tornado I've seen personally.  In reality, the smart thing to have done would have been to call my family and the National Weather Service immediately.

The tornado tracked within 2 or 3 blocks of our home.  My parents were in town and as the story goes, my father was eating lunch, looked out the window, and said "holy s---, a tornado."  I do not know if that is true, but it's a good story if it isn't.  He took my year-old son and my mom into the basement where they spent a tense several minutes.  We found some debris on our roof, but that wasn't anything to complain about.

In these pre-twitter days, we had no idea how bad things were until we began looking around the neighborhood that evening.  It was heartbreaking.  The Deseret News published a good review of the event two years ago that is worth a read and included the photo below, which shows the catastrophe suffered by our neighbors.

Source: Gary McKellar, Deseret News
The tornado resulted in one fatality, 80 injuries, and 300 damaged or destroyed buildings, 34 of which were uninhabitable.  Total damage was estimated at $170 million.

Dunn and Vasiloff (2001) show that the tornado formed along a convergence zone that was may have been a lake breeze front.  A strong updraft, associated with convection that formed initially over the Oquirrh Mountains, moved over this boundary and led to tornadogenesis.  The tornado was what is sometimes referred to as a non-descending or nonsupercell tornado.  Such tornadoes are typically short lived and more challenging to forecast, and indeed, the National Weather Service issued no tornado warning.

One reason for the lack of warning is poor observation of the lower atmosphere over the Salt Lake Valley by the National Weather Service Doppler Radar on Promontory Point, KMTX.  That year, however, the FAA installed a Terminal Doppler Weather Radar (TDWR) near Layton.  The TDWR was designed for detecting wind shear and other hazardous weather conditions at the Salt Lake City airport and provides higher resolution and lower altitude surveys over the Salt Lake Valley.  The data it collected was used extensively by Dunn and Vasiloff (2001) to understand the event. 

Integration of the TDWR data into National Weather Service operations, combined with improvements in KMTX scan strategies and algorithms, offer some potential that a warning would be issued for a similar tornado if it occurred today in the Salt Lake City area, although the lead time would likely be short (minutes).  Issuing warnings for this type of tornado is still very difficult and poses many challenges for contemporary forecasters.

One thing I take away from this event is that it could happen again.  Perhaps not in the same way, but the ingredients that came together on August 11, 1999 are not all that uncommon to find.  We often see convection initiating over the Oquirrhs and a convergence zone somewhere over the Salt Lake Valley as the lake-breeze front penetrates southward.  Most of the time, things don't align properly, or the convection isn't strong enough, or the the land-breeze front is sharp enough.  We also see weak tornadoes from time to time in other parts of Utah, something that has become more apparent with the proliferation of smart phones.  We should be cautious about discounting the likelihood of these events and attentive during periods of severe weather.  Know to take action by moving away from windows and exterior walls and into the basement or small interior rooms of your house (e.g., bathroom).  See for more information.

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