|Mean monthly Intermountain cyclone|
frequency and genesis (Jeglum et al. 2010).
The intensity (or amplitude) of Intermountain cyclones also tends to be greater in the spring.
|Two-dimensional histogram of peak 850-mb Intermountain|
cyclone amplitude vs. month (Jeglum et al. 2010).
Not coincidentally, Shafer and Steenburgh (2008) found that the frequency of strong Intermountain cold frontal passages is also highest in the spring, although the peak is sharper and in June.
|Monthly frequency of strong cold frontal passages over|
the Intermountain West (solid) and western United States
(dashed, Shafer and Steenburgh 2008)
If the models are on track, we have a great Intermountain cyclone event on tap for Friday and Saturday. The 1800 UTC 23 Feb initialized NAM produces Intermountain cyclogenesis in the direct lee of the southern "High Sierra" for 0000 UTC 26 Feb (5 PM MST 25 Feb, Friday afternoon).
The low center then tracks into northeast Nevada, with Salt Lake City in the so-called "warm sector" and strong southerly flow ahead of the low center at 0600 UTC 26 Feb (11 PM 25 Feb).
As the low center moves into southwest Wyoming, the cold front rotates across Salt Lake City early Saturday morning.
The NAM cyclone is as close to a "classical" frontal cyclone as you can get in the Intermountain West. Although cyclogenesis occurs in the lee of the Sierra Nevada and orographic forcing is important, the low center forms along a pre-existing frontal boundary. The amplitude of the frontal wave then increases, resulting in a "open wave" cyclone as the low center moves across northern Utah. The Bergen School meteorologists responsible for the Norwegian Cyclone Model would be quite proud!
This is all fine and dandy, but a look at the GFS illustrates that there is great uncertainty in this case with regards to cyclone track. In particular, the GFS forecast for 0600 UTC 26 Feb puts the low center in southern Nevada, with a surface trough and frontal zone draped across central Utah, well to the south of the NAM trough and frontal zone.
In contrast to the southerly warm-sector flow predicted by the NAM, the GFS forecast puts Salt Lake in cold, post-frontal northerly flow for Friday night bar hopping! Further, the GFS puts a band of what would be heavy snow across Utah Country, whereas the NAM frontal band is near the Utah-Idaho border.
This case provides a great example of how you can have confidence in a synoptic event, but differences in positioning make weather forecasts for specific locations very difficult.