Challenges of Snow Level and Precipitation-Type Forecasting

Forecasts remain very much on track with the expectations from a couple of days ago (see An Exciting Forecast).  For the central Wasatch and Alta-Collins specifically, the models are calling for snowfall to being today in southwesterly flow ahead of the approaching trough and continuing for the most part through Saturday afternoon when we are in the colder, post-frontal storm phase.  

I wouldn't be surprised to see a break or two in the snowfall at times in there, but totals look to be impressive.  Through 11 PM MST Saturday, the GFS is coming in with 1.90" of water and 24" of snow.  The HRRR doesn't go out that far, but just through 11 PM MST Friday it's at 2.4" of water and 24" of snow, so it's quite excited about the warmer part of the storm.   Most members of the 82-member Utah Snow Ensemble are in the 1.5–3" of water and 20–40" of snow range through 0600 UTC 16 Feb (11 PM MST Saturday).  

This is also a statewide storm, not some localized miracle for Alta (at least until we're in the post frontal stage), so everyone should get some.  Good news for all. 

But the fly in the ointment for the forecast is what is going to happen in the Salt Lake Valley later today and tonight, with the forecasts providing a "teaching" opportunity for me with regards to how we forecast precipitation type using model soudings. 

We will begin with this morning's observed sounding from the Salt Lake City International Airport as it sets the stage for the changes that are coming over the next 24 hours or so.  Thanks to the cold surge earlier this week, the temperatures in this sounding (red line) are below 0ºC everywhere. It is also dry in the lower to mid levels.  Between the surface and 700-mb, the dewpoint depression (the difference between the temperature and the dewpoint) is more than 10ºC except right at the surface.  

As a result, the wet-bulb temperature, indicated by the thin blue line, is generally 2-3°C colder than the actual temperature.  Meteorologists use wet-bulb temperature to forecast precipitation type because it the temperature the air will cool to if you evaporate water into the atmosphere, as happens for example when precipitation begins to fall.  

So, this mornings sounding is cold and could get colder with precipitation.

However, that assumes that the sounding doesn't change and in reality it is going to change and change a lot today and tonight due to the transport of warm air and moisture in advance of the approaching system.  So the next step for the meteorologist is to use the computer models to try to get a handle on what those changes will be.  Typically this is done by looking initially at maps like the ones below which are from the GFS and provide the large-scale context for tonight's forecast.  Valid at 0600 UTC 14 February (11 PM MST Thursday), they snow northern Utah in warm, moist southerly flow with widespread precipitation over northern Utah except in the lower elevations of western Utah.  

After that, a meteorologist might look at forecast soundings from the model.  This is typically done using what are known as "BUFR" soundings.  BUFR is short for Binary Universal Form for the Representation of meteorological data.  That's a mouthful, but the long and short of it is that BUFR is a binary data format maintained by the World Meteorological Organization and widely used to store high-resolution forecast profiles from computer models, including those for the Salt Lake City International Airport.  

The GFS BUFR sounding for the Salt Lake City International Airport valid at 0600 UTC 14 February (11 PM MST Thursday) shows considerable warming at all levels as we would expect based on the maps above, but if you look at the temperature trace (red line), it is very different from the surface to about 800 mb (7000 ft) than aloft.  In that low-level layer, temperatures stay very close to 0°C (I've indicated the 0°C line with a dark grey line; it is skewed because of the design of this plot which is called a "Skew-T"...that's a story for another day).  

We sometimes call such a layer "isothermal" because the temperature is nearly constant with height.  An isothermal layer near 0°C is not unusual to find in winter storms because in heavy snow, the melting of the snow tends to lower the temperature to 0°C, but no lower, similar to adding ice to a cold drink.  

In that sounding, there is only a very shallow layer near 800 mb that is above 0°C, not enough to fully melt falling snow, so we would expect the precipitation to fall as snow all the way down to the valley floor if it were to verify.  

However, other models have different ideas.  The HRRR for example also warms things up aloft, but it has a very different profile below about 700-mb.  There is a larger dewpoint depression (meaning the relative humidity is lower) and it is much warmer near and below 800 mb.  In fact the surface temperature in this sounding is about 5°C (41°F) and even the dewpoint is about +1°C.  

In a sounding like that, we would expect rain or maybe mixed rain and snow.  At the airport, the snow would turn to rain much earlier in the HRRR (and precipitation would probably be lighter too). 

These differences reflect differences in the resolution and parameterization of physical processes in the two models.  The GFS is lower resolution and doesn't resolve the terrain as well as the HRRR.  It also deals with mixing due to the friction experienced by the atmosphere near the Earth's surface different than the HRRR.  As a result, during this period, the GFS produces more precipitation at the airport and the cooling effects of that precipitation combined with the difference in mixing leads to a stable layer over the valley and snow persisiting longer.  In contrast, the HRRR produces less precipitation at the airport and mixes out the atmosphere more readily resulting in warmer low-level temperatures and an earlier transition to rain. 

Neither the GFS or the HRRR is perfect, so the devil is in the details concerning what unfolds tonight on the valley floor and the amount of snow that falls before things change to rain could vary a lot across the valley simply because precipitation rates tend to vary a lot across the valley in patterns like this (as do elevations).    

For the airport, the National Weather Service Forecast calls for snow today, but "rain, possibly mixed with snow becoming all rain after 2 am" tonight.  

Screenshot taken at 8:03 AM MST Thursday 13 February

So, their expection is that we will see the snow turn into rain overnight.  

This is a time worth monitoring the forecasts, especially if you have to get around the valley tonight or in the early morning tomorrow as I do.  I have to get my wife to the airport at o'dark 30 and am keeping a close eye on this and planning an earlier awakening to see what happens and be prepared for winter driving conditions if the snow hangs overnight, especially on the east bench.