Thursday, July 27, 2017

Another Look at the Deluge

I've had an opportunity to dig in a little deeper to the deluge that hit the Sugarhouse area early Wednesday morning.

Storm-total precipitation was greatest at a citizen weather station between Foothill Drive and Parley's Way near the mouth of Parley's Canyon.  Maximum precipitation rates at the site were 1.64 inches in 30 minutes, 2.28 inches in 60 minutes, 2.39 inches in 2 hours, and 2.41 inches in 3 hours.  If accurate, those all have recurrence intervals near 200 years, as also reported by Salt Lake City Public Utilities.

Soundings collected at the Salt Lake City airport at about 6 AM either during or in the wake of the storm (depending on when the sounding was actually released) showed a precipitable water value of 1.41 inches.  Precipitable water is the depth of water that you would have if you condensed all of the water vapor out of an atmospheric column.  Such values are high, as one would expect in a flash-flood scenario, but not unprecedented, as illustrated by the chart below (red line represents the daily highest value).  The all time record is 1.66 inches.

You might wonder how you can get more precipitation than would be produced if you condensed all of the water vapor out of an atmospheric column.  The answer is horizontal moisture transport, which allows storms to process water vapor from the surrounding area.  Thus, storm intensity has to do with many factors besides how juicy the airmass is.

Radar imagery shows very nicely how a well-defined band of precipitation developed early in the event and extended from WSW to ENE across the Sugarhouse area.

Below is a radar estimate of the 3-hour accumulated precipitation (inches) for the period ending at 1109 UTC (0509 MDT), which captures most of the precipitation.  Note the banded area of high precipitation with a maximum located along Foothill Drive just south of campus where values exceed 1.5 inches.

The southern diamond is the site that observed 2.41 inches, the northern diamond covers our sites near the mouth of Red Butte Canyon where 1.33 and 1.21 inches were observed from midnight to 6 AM.  The former is much higher than the radar estimates, whereas the latter are closer.  Further sleuthing is needed to reconcile the differences.

Any observations from the area along I-15 just north of the Point of the Mountain that also appeared to get pounded (with even more precipitation)?  I'm curious what happened there.


  1. I was sacrificing some sleep to watch things develop during much of this, and noticed visually (city lights helped even though it was dark) that cloud bases were very low, some probably within a couple thousand feet of the surface. I think that some of the precipitation may have been forming under the radar beam elevation over SLC (I don't remember exactly what the beam elevation is here). Also, the bases of the clouds were essentially stationary even though the portions of the cells observed by radar were moving northeastward. So I think there may have been some stationary low-level features that were not seen very well by the radar.

  2. I agree with David that the likely reason for the mismatch between radar-retrieved rainfall and surface gauges is collision-coalescence growth of raindrops beneath the greatly elevated radar beam. If this is the dual-pol retrieval you are showing, then it would potentially give a very different answer near the surface, picking up on high concentrations of oblate raindrops, as opposed to 1.5+ km off of the surface, where the DSD could have been quite different. Additionally, there is likely horizontal advection of the rain as it falls beneath the radar beam in the cold pool outflow, which could cause 0.25 to 0.5 mile errors in this case.