September storms near the Great Salt Lake

A number storms formed in the early afternoon of September 22nd in response to forcing associated with the North American Monsoon over the Intermountain West region of the U.S. The image below shows the accumulations of NWS severe weather warnings, storm reports from SPC, and the centroids of ProbSevere objects attaining 50%+, at each time. Each accumulation is over the timeframe of 12Z on 9/22 to 12Z on 9/23. You can get a quick-look at how the model performed using these accumulations (the previous day accumulations are here). On this day, the NOAA/CIMSS ProbSevere model performed reasonably well, with high probabilities corresponding to numerous wind, hail and tornado reports. There were a couple of false alarms to the south east of the Great Salt Lake and a couple wind reports missed to its west and south.

Figure 1: Accumulations of ProbSevere objects, reports, and NWS warnings for 9/22/2016.

The storm that produced hail, wind, and the one the tornado reported initiated well to the southwest of Salt Lake City. The time series below of its probability and constituent predictors in ProbSevere demonstrates its evolution.

The probability of severe is the thick red line, with the scale on the left. The six predictors in ProbSevere have varying scales on the right. The NWP predictors of effective bulk shear and MUCAPE are the dashed black and brown lines, respectively. The MESH is solid orange, and the total lightning flash rate is solid green. The lifetime max normalized satellite growth rate and glaciate rate are depicted by the solid blue and dashed cyan lines, respectively. Both satellite growth rates use the blue scale on the right with nominal 'Weak', 'Mod.' (moderate), and 'Strong' designations.

Figure 2: Time series of ProbSevere predictors and severe probability value for a long-lived storm affecting the Salt Lake City metro area.

We see that the normalized satellite growth rate from GOES-West was strong at 18:50Z, while the probability of severe jumped to 15%. The jump in MESH in a high shear environment also helped to jump the probability up to 50% at 19:12Z. Increasing MESH and flash rate helped the probability climb to over 90% by 19:50Z. The first severe thunderstorm warning was issued at 20:09Z. Golfball-sized hail was observed at the Antelope Island Marina at 21:37Z. The tornado in the city of Ogden was reported at 21:45Z, and left thousands without power.