Meteorology of a Monsoon Surge

Last week, we discussed how we were in a pattern characterized by persistent upper-level ridging over the western United States, but that "critters in the woods" would modulate the monsoon moisture and shower and thunderstorm activity (see Persistent Large-Scale Pattern with Critters in the Woods). 

One of those critters gave us the relatively pleasant weather on Saturday and another will be affecting northern Utah on Wednesday. 

The setup for Wednesday illustrates how both large-scale and small-scale features play an important role during the monsoon.  The situation at 0600 UTC 30 July (0000 MDT Monday) shows a high-amplitude upper-level (500-mb) ridge centered near the 4-corners area.  Anticyclonic (clockwise) flow around this larg-e scale feature results in westerly flow to our north and easterly flow over northern Mexico as depicted below. 

However, there is also an important smaller-scale circulation feature over northern Mexico.  I have identified this feature using contours of vorticity, a quantity used by meteorologists that essentially represents the circulation density.  Vorticity helps identify areas of strong shear and curvature in the flow.  Note that there is a local maximum, denoted by an X, over northern Mexico.  Meteorologists call such a feature a vorticity maximum, and this is our critter in the woods for Wednesday.

Typically, the large-scale flow "steers" vorticity maxima, but vorticity maxima also interact with the large-scale flow and can strongly affect the weather.  In the GFS forecast panels below, which cover the period through 0600 UTC 1 Aug (0000 MDT Thursday), the vorticity maximum is coaxed northward by the circulation associated with the large-scale ridge while at the same time it intensifies, resulting in enhanced southerly flow to its east. 

Together, this pattern leads to a very pronounced northward surge of monsoon moisture into Utah, as depicted below by the loop of precipitable water.

For the forecast, however, there are some differences in where the models place the moisture.  For instance, the GFS forecast for 0000 UTC 1 Aug (1800 MDT Wednesday) has the moisture plume covering central and western Utah, whereas the NAM has the moisture plume in central and eastern Utah. 

We'll see how this all plays out, but keeping an eye on forecasts and recognizing the potential for thunderstorms and related hazards is essential for Wednesday and Wednesday night.