By 2:30 or a little after, I was on my way up to the pass. I was there somewhere around 3:15, or so. Patience would be the name of the game, as there were some light snow showers in the nearby mountains, but nothing strong enough for thunder yet. But the updrafts seemed to be growing, and soon a light shower of graupel (snow pellets) told me that I was high enough for snow, not rain. After a few minutes, a break in the clouds brightly illuminated Alberta Peak, providing a sharp contrast with the dark updraft base above it and just behind it:
This picture was taken looking south from the pass toward Alberta Peak, a couple miles away. As I continued to wait, the updraft towers slowly grew, and the snow to my southeast intensified, hiding the peaks at times. By shortly after 4:00, one snowshower was intensifying to my southeast, and another to my southwest. Before any precipitation from either of these cells reached my position, I heard a couple rumbles of thunder. Not sure which cell the first rumble was from, but the second one was definitely from the cell to my SSW, which was moving right toward me. Here is a picture from around this time:
The cell on the right of this picture is the one that would give me thundersnow. A few minutes after the second rumble of thunder, a few snow pellets began to come down. Within a minute of that, around 4:15 or 4:20, I had thundersnow!
Here is a short video clip:
The precipitation, mostly graupel (snow pellets) but with a little ordinarly snow also mixed in at times, continued for around 15 minutes or so. I kept the video camera rolling, but did not get any more thunder. By around 4:30 or a little later, it was evident that the storm had mostly passed over and there would be no more thundersnow with this cell, so I headed down. The heaviest part of the cell seems to have passed just to the west of where I watched it, and as I headed down from the pass I encountered a stretch of road that was slushy from perhaps 1/3 of an inch accumulation. P-type in the intermittent showers remained mainly wet snow/graupel down about to the scenic overlook, then predominantly rain below that. However, about halfway back to Pagosa Springs I encountered a heavy burst of precipitation from a new cell that was forming overhead - mostly heavy rain, but also something frozen mixed in - either very small hail or partly melted graupel, hard to tell.
By the time I got back into Pagosa Springs around 5:15, extensive convection was ongoing to the SE, south, and SW of town. I noticed that the sun was creating some interesting effects with precipitation that was occurring just to the south or southeast of town, so I stopped to get a couple pictures:
In these pictures, the white streaks are snow and/or graupel coming out of the base of the cloud, illuminated by the sun. This precipitation was melting to rain before reaching the valley floor, and a rainbow formed as the sun hit the rain. When I took the first picture, I was mainly interested in the white streaks of snow illuninated by the sun, and did not notice the rainbow. But if you look at the picture closely, you can see that there is actually a double rainbow, with the main rainbow barely visible through the trees on the left and the secondary rainbow on the right.
For anyone interested, here is a somewhat longer video, around two and a half minutes, that includes footage showing what was going on just before and after the thundersnow occurred:
Although the cell that produced the thundersnow resulted in only minor snow accumulations, it was the beginning of a multi-day storm that produced substantial accumulations around the pass and above it. The nearlby Wolf Creek ski area received nine inches of snow from Monday afternoon through Wednesday morning.
Total chase distance: Around 60 miles.