Weak Troughs Matter

Forecasting during the monsoon is often very challenging.  We talk a lot about monsoon surges, which sound quite dramatic, but sometimes the changes are more subtle.  

Currently there is a weak upper-level trough off the coast of California that will serve as an important driver of convective storms as it moves across the western United States.  

This afternoon the trough will be near the coast of California. It will help to coax monsoon moisture northward over the Sierra Nevada and western Nevada, with the HRRR producing clouds and showers (and presumably thunderstorms) that through that area and into Oregon.  

By tomorrow afternoon, the trough is near the Oregon-Nevada-California triple point and shower and thunderstorm activity has shifted eastward into eastern Nevada and western Utah.  

By Friday afternoon, the trough is moving into northern Utah and the GFS has moved the action into northern Utah (switching to GFS forecast below since the HRRR does not run past 48 hours).  

By most meteorological standards, the trough is quite weak.  Meteorologists often look at 500-mb geopotential height with contours every 60 meters to identify upper-level features and if you do that this morning, there's not much of anything to be seen off the coast of California.  

Source: TropicalTidbits.com

However, the trough is readily apparent if one looks at dynamic tropopause pressure and wind (upper left in the image below) or 500-mb vorticity (upper right).  These are variables that are more sensitive to flow curvature and shear and are often helpful for identifying weaker features (and they are still useful for strong features).  

Yesterday's high at KSLC was 100˚F, the 22nd time we've hit that mark this year (a new record).  As of 1:30, it is 96˚F at KSLC and we should be very near 100˚F for the maximum again today.  I personally am looking forward to a bit more cloud cover and the chance of showers and thunderstorms later this week.