Wednesday, July 26, 2017

The Early Morning Deluge

Thunderstorms with heavy precipitation moved into and intensified over the Salt Lake Valley last night, leading to some urban and small stream flooding.  The radar loop below runs from 0800–1300 UTC (0200–0700 MDT) and shows some especially strong storms in and around Sugarhouse and near and around Draper.

Radar estimated precipitation amounts key in on both of those areas reaching over 1.5 inches in the Sugarhouse area and nearly 2 inches near Draper.  

Gauge-measured rainfall amounts since midnight as reported to the National Weather Service include 2.38" at Sugarhouse near Parleys Way, 1.35" at our mountain meteorology lab on the University of Utah campus, and 1.20 inches in Draper.  

Contrasts between radar estimated and gauge-measured precipitation are not uncommon and careful analysis will be needed to identify the magnitude of peak precipitation amounts.  Nevertheless, it is clear from both Sugarhouse and Draper received an impressive amount of rain for such a short period of time.  Data from the National Weather Service Precipitation Frequency Data Server ( suggests that in the Sugarhouse area, precipitation amounts of about 2 inches in 6 hours have a return interval of 100 years.

Here's another perspective from the same web site showing the magnitude of 6-hour precipitation with a recurrence interval of 100 years for Utah (click to enlarge).  In the Salt Lake Valley, values are generally around 2 inches, but are slightly larger on the east bench, reaching about 2.2 inches along I-215.

Additionally, precipitation fell in somewhat less than six hours and probably was especially intense in short bursts.  Local news media reports some flooded basements, road closures, power outages, and transit disruptions.  

A quick word on recurrence intervals.  Perhaps most importantly is that they are estimates of the recurrence of precipitation at a point, not in an area.  Hence, given the chaotic and localized nature of thunderstorms, we might expect a storm like this to strike somewhere in the Salt Lake Valley more frequently than every 100 years.  

1 comment:

  1. There could be a decent chance for severe storms this afternoon/evening in the west desert. SPC has a marginal risk up.