Deja Vu All Over Again

We have another surge of monsoon moisture into Utah today.  Altostratus has overspread northern Utah and the PW has risen sharply to about 2.25 cm at KSLC.

Source: NOAA/ESRL

Similar to the event from last monday, the approach of a mid-lattitude trough in the mid-latitude westerlies may be playing a role in the surge.

Two day loop of PV (color fill), 925 mb wind, and precipitable
water (contours) ending 1600 UTC 25 July 2011.

At issue in these events is to what degree the cyclonic circulation associated with the upper-level trough and associated PV anomaly induces southerly flow at low levels and the advection of monsoon moisture into the Intermountain West.  Is this an important player, or are changes in the large-scale circulation associated with the ridge over the central United States and perhaps embedded disturbances in the easterly flow over Mexico and the Southwest US important?

This issue has been explored to some degree by Stensrud et al. (1997), Fuller and Stensrud (2000), Adams and Stensrud (2007), and Ladwig et al. (2009).  These papers show that easterly waves play an important role in modulating monsoon precipitation and in surges of moisture over the Gulf of California.  Stensrud et al. (1997) suggest that troughs in the midlatitude westerlies may also play a role in surges from the Gulf of California into Arizona.  For our part of the world, troughs in the midlatitude westerlies may be of fundamental importance and could be an important consideration for seasonal and subseasonal forecasts, as well as understanding interannual variability of monsoon precipitation in the Intermountain West.  There could be a good research project here for a motivated student.