Monsoon Meteorology

It's been a while since we took a look at the large-scale atmospheric circulation, but it seems appropriate today.

Last night, moisture moved into northern Utah, giving us some clouds and thunderstorms that were greatly appreciated after a 104ºF maximum on Wednesday.  Clouds linger this morning and we will see more showers and thunderstorms across the state today.  The National Weather Service has issued a flash flood watch from noon today through this evening for a good portion of central and southern Utah.

Source: NWS

The pattern producing this weather is commonly seen over the southwest United States during the summer monsoon.  As shown in yesterday morning's 1200 UTC analysis, we are under the influence of a strong upper-level ridge that is centered over the southwest US with easterlies across northern Mexico and southern Arizona turning anticyclonically and streaming northward into Utah.  This flow is confluent with a drier airstream moving into northern California and northwest Nevada from the west and southwest.

This is a very common occurrence during the monsoon, and the confluent flow pattern often results in dramatic contrasts in weather across the western United States.  It enables moist air originating over the Gulf of Mexico and environs to penetrate into the southwest US and the surge northward into Utah and the interior west (the Gulf of California can also be a moisture source) while it remains drier further west.  Note in the loop below the strong contrast in integrated precipitable water (a measure of the total water vapor content of the atmosphere) across Nevada (color contours with warmer colors indicating higher values).

Fluctuations in the strength and position of the western US ridge and the Pacific trough, as well as smaller-scale disturbances moving through these features, modulate the monsoon system.  As a result, the intensity of the monsoon varies from year to year and, during individual seasons, there are waxes and wanes in monsoon moisture and convection known as monsoon surges and breaks, respectively.  We are currently under the influence of a surge, which was strongly influenced by a shortwave trough moving into the western US from the eastern Pacific and additional shortwave troughs moving anticyclonically around the upper-level ridge as seen in the tropopause and 500-mb height analyses below (top two panels).  

Being at the northern periphery of the monsoon influence, such surges frequently play an important role in the development of precipitation over northern Utah during the summer.    

Addendum @ 12:30 PM 12 July:

Bob Maddox of the Madweather blog sent me a note illustrating that moisture sources from west of the Continental Divide, including the Gulf of California and the remnants of Hurricane Erick likely contributed to the moisture surge.  Below are some trajectories going back about 4.5 days illustrating this.  Thanks Bob!