Finally, a decent storm! Our storm chasing activities this month were somewhat curtailed by Mother Nature. Storms were limited, but we made the most of the three major opportunities we had, including our efforts yesterday and today.
Yesterday, we set up camp in the South Jordan Train Station parking lot for the frontal passage and post-frontal precipitation.
We operated here in 2011 when it was in the boondocks. Although development is spreading westward (note the building behind the DOW), we were able to use it effectively yesterday, although we may need to find an alternative for future deployments.
There was a great deal going on and I suspect we have a great dataset for both an MS thesis and at least one paper. Here are a few snippets.
The photo below from U undergraduate Spencer Fielding is taken facing ESE shows the situation shortly after the surface cold front has passed and was located just south of Point of the Mountain. A pronounced cloud rope was evident above the frontal interface with the cloud structure suggesting ascent in the strong southwesterly flow over and downstream of the front. It was black as coal over the Cottonwoods, but we saw very little on radar during this period. Instead, the precipitation was falling further downstream (relative to the flow aloft) in the area east of Mill Creek Canyon. This appeared to be a classic situation of precipitation growth, transport, and fallout with the growth happening in the ascent region, but the particles needing sufficient time to grow big enough to fall out, which happened further downstream. It was unclear if the mountains really mattered at all in this period. Making all of this easy to see was the fact we seemed to be in a rain shadow east of the Oquirrh Mountains.
That Oquirrh rainshadow was a prominent feature for a couple of hours after frontal passage. The image below is a vertical slice taken facing northwest toward the Oquirrh Mountains (Grey region marked with an "OM." The color fill is Doppler velocity, which measures the speed of the flow toward (cool colors) or away (warm colors) from the radar. The northwesterly flow is clearly evident. Note how it descends into the Salt Lake Valley southeast of the Oquirrh Mountains.
That descending flow is quite consistent with surface observations at the time which showed WNW flow at most sites along and east of the Mountain View Corridor (highway 85) in the western Valley. This was about the time of the frontal passage at our location.
An hour later, the front had pushed through Point of the Mountain. There was a clear boundary over the southwest Salt Lake Valley between the downslope westerlies and the along-valley northerlies. Curiously, the downslope flow was colder
than the northerly flow to the east. Downsloping air warms compressionally, so for that to occur, precipitation must have cooled the airmass.
The photo below was taken shortly after the MesoWest analysis above and was taken facing NNW. Here you can see quite well the dark, low-level clouds that accompanied the front into the Salt Lake Valley. To the left, however, one can see snow (fibrous clouds) that is spilling over into the lee of the Oquirrhs. The sublimation of that snow presumably contributed to the cooling of the airmass. Note also how the subsidence has eroded the low-level cloud away on the west side of the photo.
Looking west, you can see this spillover really well in the radar reflectivity. Note in particluar the shallowing of the echoes toward the radar due to the downslope flow. One might think of this as a "snow foot."
Eventually, the cold air deepened and the flow aloft veered (turned clockwise) to westerly and eventually westnorthwesterly and we got quite a treat as the area around Little Cottonwood Canyon finally lit up. One example is below, which is a vertical slice taken facing east directly up Little Cottonwood Canyon. The bright yellow/orange colors are ground clutter produced by the sloping canyon floor. Above that clutter, there's not much happening in the lower canyon, but from mid canyon up, there are strong returns, indicating heavy snowfall.
The photo below was taken about 2 hours earlier when it was still light, but shows perhaps what was happening with little or no precipitation falling at the base of the mountains and in the lower canyon (v-shaped notch in background to left of radar dish), with heavy snow in the middle and upper canyon.
At times, away from the mountains, we also saw some beautiful fall streaks.
Google it and look at the photos and you'll know what I mean.
We eventually deployed to a site we could scan further north for the orographic and lake-effect precipitation that fell overnight. I went home and tried to sleep, but it was largely a night spent looking at the radar and hoping we were getting great data, which we were. You'll probably hear a lot of talk about the snow being lake effect, but in the mountains, most of it wasn't. It was just good old mountain lifting doing the job until early this morning when we got a bit of lake effect.
We surveyed the lake-effect showers for most of the morning. At one point, we decided to forget about doing anything except two rapid fire vertical slices taken through the lake-effect convection as it moved inland. A still is below, but these slices were taken every 16 seconds. I can't wait to process the data and see a video.
All of this brings to an end the OREO field phase. Although storms were limited, we actually have some great data from three major storm cycles, and the students enjoyed a smorgasbord of precipitation and wind phenomenon.
Special thanks to the National Science Foundation for sponsoring this visit and the Center for Severe Weather Research (CSWR) for making it happen. The CSWR deserves extra special thanks for letting us keep the DOW a few extra days to catch this latest storm.