A Fly in the Ointment

A high-amplitude upper-level ridge remains parked over western North America, but as shown in yesterday afternoon's 0000 UTC 23 Sep 500-mb height analysis, there is a potential fly in the ointment for Utah weather.

Source: NOAA/ESRL

The fly is the weak closed low that is sitting over the coast of Washington, which results from a weather feature that meteorologists call a coherent tropopause disturbance, or CTD.  A CTD is a localized cyclonic vortex, which is associated with counterclockwise rotation in the Northern Hemisphere and is strongest at the tropopause.  The tropopause separates the stratosphere from the troposphere.

Source: NASA

The CTD and closed low move into Nevada by tomorrow morning [1200 UTC (0600 MDT) 24 September].

A different perspective at this time is provided below by a 3-D colored representation of the tropopause, with cool colors indicating a lower tropopause and warm colors indicating a higher tropopause.  Note how the tropopause is locally depressed where the CTD and closed low are found over Nevada.

Next, we have a vertical slice taken through the CTD the line that bisects the image above.  In this slice, the tropopause is indicated by a red line.  Note how the tropopause is locally low just left of center where the vertical slice cuts through the CTD.  

The other lines are temperature contours.  Note how they dip down near and beneath the CTD, which is an indication of colder air aloft.  

Thus, as the CTD approaches Utah, temperatures in the upper-troposphere will fall.  This, combined with large-scale rising motion ahead of the CTD, will act to destabilize the atmosphere and initiate some showers and thunderstorms.  As a result, although we are still under the influence of a large-scale upper-level ridge, we have a chance of some isolated showers and thunderstorms today and scattered showers and thunderstorms tomorrow.