Wednesday, September 21, 2011

Basin Cold Pools of the Rush Valley

As mentioned yesterday, we are in a pattern that lends itself to the development of strong cold pool, especially in topographic basins.  The Peter Sinks are the king of all Utah cold spots, but the Rush Valley is a close second and, more importantly, is well instrumented.

The Rush Valley is poorly named as it is really a closed hydrologic basin located south of Tooele and East of Provo.

Map Background Source: ACME Mapper
It is surrounded by a jumbled mess mountain ranges including the Stansbury and Onaqui Mountains to the west, the Sheeprock and Tintic Mountains to the south, the Oquirrh Mountains and Thorpe Hills to the east, and South mountain to the north.  Much of the valley floor is below 5100 feet, but the low point is at Rush Lake, just south of the Stockton Gap, at about 4960 feet.  The topography surrounding the Rush Valley has two major weaknesses.  One at what I call Stockton Gap (~5120 ft), the other at Five Mile Pass (~5300 ft).  Between the West and East Tintic Mountains, Boulter Summit, sits at about 6000 feet.

On a clear night like last night, the Rush Valley gets cold.  Really cold.  This morning, the minimum temperature at stations near the floor of the valley ranged from 29–34F, with the coldest temperature at the Pony Express Marker.

Minimum 24-h temperatures reported to MesoWest.  Winds
plotted are the most recent observation within the hour
ending at 1427 UTC 21 Sep 2011.
The time series from the Pony Express Marker shows a diurnal temperature range of about 50F.  As is often the case in basins, the temperature fell most rapidly during the first part of the night, then fell more slowly, reaching a minimum near sunrise.

Contrast this with Ophir Station (5561 ft) on the east bench or Vernon Hill (5761 ft), which sticks up in the southern part of the Rush Valley, where the minimum temperatures were only 53 F and 58F, respectively.   At Vernon Hill, temperatures remained between 58 and 63 F for much of the night.

One of the "cool" (pun intended) things about cold pools is that they tend to be most intense below the lowest gap, and this is supported by the evidence above.  In the case of the Rush Valley, that's Stockton Gap, where all sorts of wild things happen.  At the Stockton Bar MesoWest site, the maximum temperature yesterday was about 75 F.  There was a modest drop in temperature around sunset, followed by relatively steady temperatures until about 0430 UTC.  At this time, the temperature dropped from 65 to 58 F.

This temperature drop coincided with a wind shift from northerly to southerly, which likely occurred when the cold pool in the Rush Valley reached sufficient depth to pour through Stockton Gap.

Following this wind shift, the wind remained from the south for the remainder of the night, although you can see some warming and cooling cycles that are somewhat regular.  These could be due to sloshing of the cold pool (sometimes referred to as a seiche).

Curiously, the temperature at Five Mile Pass, which is about 180 feet higher than Stockton Gap, only dropped to 44F.  The shallow, intense cold pool near the valley floor does not extend as high as Five Mile Pass.  Whether or not this is due simply to nocturnal radiative and turbulent processes, which perhaps are not sufficient to deepen the cold pool that much, or due to a loss of mass through Stockton Gap, remains for further investigation.


  1. Jim, recently started reading your blog and really enjoy it. I have a question tangentially related to this post. Looking at daily temperatures in PC for the last month I see daily highs very close to average yet nightly lows significantly above average. What could contribute to such an anomaly? It seems to the untrained mind that the two would be more closely coupled, whether we are having a warm or cool spell.

  2. ... that is, absent unusual cloud cover or high humidity.

  3. Evryn,

    Thanks for the positive feedback. Spread the word!

    My first thought is that you could be comparing apples and oranges. What site is the average from and what site are the current observations from. Minimum temperatures are extremely sensitive to site location or changes. If the sites are different, that's a possible culprit. Even if it is the same site, it could be that the sky-view factor has changes (e.g., buildings or trees added to the surrounding area), the land surface has changed, or the site was moved just a bit. That would be the first thing I would check.

  4. These are not personal measurements but what I assume to be official temperatures reported by accuweather
    Surely such issues have been addressed by the NWS? If not, I've got to call up Al Gore and let him know(!)
    Even under the conditions this last week that should be conducive to night-time cooling, the lows are higher. Are they within an expected range? I don't see how there could be the 7 degree differential on the 20th, for example.

  5. I don't know where these temperatures and averages come from. They could be from the Park City volunteer National Weather Service cooperative observer. If so, there's a very good chance that the current observer is in a different location than the old. The only long-term climo I am aware of at Park City is They could be using that for the Park City averages (I'm just guessing here) and then have a local observer that provides them with data.