Benches (and Valleys) Beware

The south wind is roaring at my house right now in advance of a cold front that has been anticipated for days and is already moving quickly into northern Utah.   As of 1311 UUTC (711 MDT) this morning, radar echoes were covering portions of northwest Utah.  

It's going to be an exciting day and weekend!

This morning I woke up not wondering what will happen in the mountains, but what will happen in the valleys and on the benches.  Heavy snowfall looks to be a lead-pipe cinch in the northern Wasatch with the National Weather Service calling for widespread 8-18" and up to 24" of snow.  

However, the potential for snow also exists for the valleys and the benches, but the timing, locations, and amounts are unclear.

Below is the GFS time-height section for Salt Lake City.  After today's frontal passage, we enter a long period, roughly 36-48 hours, of cold, moist, unstable, post-frontal flow.  

This means mountain snow showers, but also the potential for valley and bench snow showers that develop due to post-frontal instabilities and/or lake-effect processes.  Such snow showers are notoriously fickle to forecast for a number of reasons.  First, the processes that generate them are small in scale and not directly simulated by current numerical forecast models.  Second, they are very sensitive to small change in the ambient flow characteristics, such as wind direction or humidity.  A good example of this is lake effect which can change a good deal with a small change in wind direction.  Finally, the features generated by these processes are more chaotic than large-scale weather systems such as fronts.  

What concerns me right now is the potential for heavy October snowfall at lower elevations.  Empahsis on potential because this is not a high-confidence forecast, but instead a statement that it is possible that some valley or bench areas receive significant October snow.  

This is an outlier event meteorologically.  It has been mild the last few weeks and we are now bringing in a deep, cold trough.  Great Salt Lake temperatures derived by satellite over the past week are around 17˚C, roughly 4˚C higher than average.  Although the lake is at an all-time historical low, it can still generate lake effect.  

Our lake-effect guidance derived from the GFS indicates elevated lake-effect probabilities from late Saturday afternoon through Monday morning.  On Saturday night, forecast temperatures suggest snow levels near the benches, but by Sunday night they are flirting with the valley floors.  

The probabilities (and the estimated affected areas) have been shifting from model run to model to run and if we were to apply this to an ensemble we would also see a lot of variability.  Thus, do not take the numbers or affected areas above as gospel, but instead as an indication that we could see lake effect or other post-frontal snow showers.  The situation is so complicated that the area forecast discussion issued early this morning by the National Weather Service was the longest I think I've ever seen.  I can't reproduce the whole thing as it would take up three pages, but the paragraph below contains the geeky summary of lake-effect and snow potential.  Note that they are also talking about lake-effect from Bear and Utah Lakes, which is consistent with this moist, early-season cold surge.  

This is a Devil Is in the Details forecast for sure.  I will be monitoring radar and forecasts and adjusting plans as needed.  

Buckle up.  Winter is here!