Incredible that they verified, not because of the weather, but because of the low population density. Kudos to SPC.
John & Neil - Have you ever plotted the mean vertical motion by month over the Intermountain West? I'm wondering if that shows the same bimodal seasonality as the Intermountain cyclone stats.
Jim, I have not done so in some sort of regionally averaged sense, but I have done it for each grid point for each month. It does show some weak bimodality if I recall, but is strongly dominated by the spring maximum in ascent. I only calculate these stats through april, so I am not sure how May compares, which I seem to recall was the cyclone peak that you Matt document.
If you get motivated, it might be interesting to plot the monthly-mean VV for the Intermountain region that Matt and I investigated. I can send you a copy of the paper off line if you'd like.