It's a somewhat surprisingly complicated weather forecast for the weekend, but not that atypical for September as we transition from the so-called monsoonal flow pattern to the cool-season westerly storm track.
We'll begin with a look at the GFS forecast valid 0600 UTC 18 September (0000 MDT Saturday). At this time, a deep upper-level trough (black contours) and frontal system are pushing onto the northwest coast. Ahead of this system is a much weaker trough over California. It doesn't look like much, but it contributes to a surge of monsoon moisture (color contours are precipitable water – a measure of the total atmospheric water vapor) into Utah (red arrow).
As the deep upper level trough and cold front push into the Pacific Northwest, the weak trough slowly moves northeastward and the monsoon moisture streams into northern Utah. By 1800 UTC 18 September (1200 MDT Saturday) the GFS is generating some scattered precipitation in northern Utah, mainly in a narrow region coincident with the monsoon surge.
|Source: NWS, downloaded 8:35 AM MDT 17 Sep 2021|