Turn Off Your Sprinklers

The storm is here.  The slow moving trough advertised in the prior post is now moving into the southwest US and it has tapped into tropical moisture, with atmospheric river conditions moving into northern Utah today. 

The image below summarizes the situation as we approach the noon hour.  The 500-mb trough axis lies along the Pacific coast (black contours).  Ahead of this feature, moisture laden air, indicated by a tongue of high precipitable water extends from the tropics into southern Utah (highlighted with red arrow).  

For midlatitude and mountain precipitation, integrated water vapor transport is a better indicator of atmospheric river conditions and you can see high values below pushing all the way into northern Utah.  

Source: CW3E

A time height section from the GFS shows deep moisture and instability predominating through Saturday morning.  The upper-level trough in this forecast passes over Salt Lake City at about 1200 UTC 9 October (6 AM MDT Saturday). After that, moist northwesterly crest-level flow continues through Sunday morning.  

Expect a wet period for valley and mountain locations, with some thunderstorms and snow levels lowering to 9000 feet this afternoon and 7500 feet by tomorrow morning.  We've been working on a new Little Cottonwood Canyon forecast product.  It's not ready for prime-time yet, but I share the GFS-derived forecast below to illustrate the development of high-density snow at Alta-Collins level this afternoon.  The GFS puts out 1.35 inches of water at Alta through 9 AM tomorrow (Saturday), with our snow-to-liquid ratio algorithms suggest will be about 10" of snow. 

The temperature at Alta-Collins at 11 AM was 37˚F, so the snow level is close.  Snowbird's Mineral Basin cam at 11,000 ft has a coating of melting snow with some white evident in places.

Source: Snowbird.  Downloaded 11:40 AM MDT 8 Oct 2021.

Our snow accumulation algorithms do not account for snow melting on warm ground and that will be an issue when the snow levels first lower.  I'd probably go for 5-10" of snow at Alta-Collins (9600 ft) through 9 AM tomorrow (Saturday) based on what I'm seeing now.  Accumulations will decrease rapidly with decreasing elevation from 10000 to 8000 feet and vary depending on the surface.   

Enjoy the cool and moisture.  We've earned it with a painfully hot summer.