A Storm About Orographics

This morning dawned as expected with grey skies and a bit of light snowfall over the Wasatch Range, providing an illustration of the orographic processes that will play a major role in the storm for today and tonight.  

Orographic means related to mountains.  Meteorologists use words like orography, orographic, orographics pretty regularly in places like Utah because the mountains have such a dramatic influence on our weather.  

Orographic precipitation is precipitation that is generated or enhanced by the interaction of moist flow with orography.  If one looks at the 24-hour GFS accumulated precipitation forecast for northern Utah through 1200 UTC (0500 MST) tomorrow, one can see to some degree the influence of the mountains with precipitation greatest along a north-south corridor that is roughly aligned along the Wasatch, with the precipitation maximum in the central Cottonwoods.  

The corridor of precipitation is, however, much wider than the Wasatch Range.  This is because the orography in the GFS is very smooth.  The GFS uses grid cells that are approximately 13-km on a side, and there are no variations in topographic height within each of those cells.  Someday we will look back on models like this and laugh, but for now constraints like this are necessary as computer resources do not allow for smaller grid cells.  As a result, the mountains in the GFS are very smooth.  The GFS mountains start over the Great Salt Lake with a smooth slope up all the way to the western Uintas.  One of my students used to call the GFS (and NAM) mountains the "Uintasatch mountains" because they were all smoothed together.  

We can, however, use statistical techniques to adjust that precipitation into something that reflects what we might expect based on climatology.  This is one of the things that we do to produce the fine-scale snowfall forecasts on weatehr.utah.edu.  Below provides the "downscaled" GFS precipitation forecasts for the same time period.  The pattern is closer to what one might expect, with a maximum over the Wasatch Crest.  

Finally, we can go a step further and use some simple approaches to determine the precipitation type and snow-to-liquid ratio and you have a high-resolution snowfall forecast from the GFS.

This storm is expected to feature a strong increase in snowfall with elevation for two reasons.  First, the large-scale "dynamics" in this event is relatively weak.  There is no strong front or trough moving through (there is, however, a weak one and so there are some dynamics to help with large-scale precipitation development).  Second, there is strong and moist northwesterly flow, leading to flow over the topography.  If one looks at the GFS precipitation for one period of the storm, it looks a lot like the GFS topography.  

This is not to say that there won't be any valley precipitation.  There is a weak trough moving through this afternoon that will probably produce some valley snow, but for the most part, this is a storm that should feature a strong orographic precipitation gradient, with snowfall increasing with elevation. 

The downscaled GFS forecast above is the snowier of the various models I've consulted this morning.  The SREF has somewhat lower numbers with precipitation heaviest this afternoon, an average snow total at Alta by 1200 UTC (0500 MST) tomorrow morning of about 15 inches, and a range of 8-22 inches.  

That range reflects differences in the characteristics of the flow over northern Utah and the techniques used to simulate the fine-scale processes used in the various members of the SREF to generate snowfall.  

I lean toward something in the 9-18" range for that period at Alta.  This looks like a good day to go up for afternoon laps as the storm intensifies with time and probably reaches peak intensity in the afternoon.