Wednesday, November 22, 2023

Thanksgiving Weekend Crap Shoot

Instead of arguing with your crazy uncle about politics this Thanksgiving, consider arguing about the weather instead.  There's certainly plenty of room for debate and disagreement if you are trying to forecast snowfall in northern Utah.  

The culprit is an upper-level trough that will develop and "dig" (move equatorward) over the Great Basin on Thanksgiving.  at 1200 UTC 23 November (5 AM MST Thanksgiving morning), the trough is forecast to be an open wave with no closed circulation and an axis that extends from Oregon and California. 

However, the trough is also diffluent, with an intense jet on it's upstream side and flow that weakens and fans out on its downstream side.  Such troughs are prone to amplification and digging, and that is exactly what is forecast to happen during the day on Thanksgiving.  By 0000 UTC 22 November (5 PM MST Thanksgiving), the trough has closed off over southern Idaho, and concomitant precipitation is moving into northern Utah.   

This is where things get interesting.  This is not a clean frontal system that just moves through the area with a well defined frontal band.  Instead, the system comes trough as a deep closed low.  At 0600 UTC 22 November (11 PM MST Thanksgiving), the circulation center at 700 mb is forecast by the GFS to be right over Salt Lake City. 

Systems like this tend to have an array of precipitation features that we can't reliably predict the intensity and location of at such lead times.  They also can produce a lot of changes in wind direction, which affects local orographic precipitation enhancement processes.  A look at the time-height section for Salt Lake City shows the arrival of cold, moist air associated with the trough at about 00Z Friday (5 PM MST Thanksgiving) with NW low level flow, then a shift to NE by 12Z Friday (5 AM MST Friday), NNE by 18Z Friday (11 AM MST Friday), NNW 00Z Saturday (5 PM MST Friday), etc.  I think you get the point.  Higher up, there is a progression from southwest to northwest over a two day period, but even that will depend some on the track of the low.  

The net impact of this is a lot of spread in what the models are putting out for snowfall.  For Alta–Collins, the latest GFS is putting out a storm total through about noon Saturday of 0.67" of water and 12" of snow.  Most of this falls from Thanksgiving afternoon through Friday evening. 

The euro only has about 0.38", but you can find heavier amounts elsewhere, such as in the Oquirrh Mountains and Uintas compared to the central Wasatch.  That's unusual, but it reflects the complexity of this system.  Who gets what and when depends a lot of the low-center track and embedded precipitation features.  

The SREF exhibits considerable spread, with a range from about 0.2-1.2" of water and 5–22" of snow.  

As they say in the Hunger Games, may the odds be ever in your favor.  


  1. Thanks for another great post. Nice to learn what digging means. Seems like this is the second time you've featured the SREF this fall. My failing memory remembers you saying it was getting old and outdated, not reliable. Is that an accurate memory or do you feel SREF is useful. Maybe you feel its useful but would be better if more reliable.


  2. Old and outdated yes, but sometimes useful for illustrating uncertainty

  3. You should forecast the bear Rivers as well