Friday, November 21, 2014

Peering into Forecasts of the Coming Storm

Time-height sections are remarkably powerful tools for weather forecasting (see Forecast Tools: The Time-Height Section for a basic introduction).  The forecast NAM time-height section for Salt Lake City for the next couple of days is an absolute classic, showing beautifully the likely storm environment changes over northern Utah.

With time-height sections, we typically reverse the x-axis so that time increases to the left, as is the case above.  So, scan from right to left to see the passage of various weather systems between now and Monday afternoon.  Instead of temperature, we also use a thermodynamic variable known as equivalent potential temperature, also known as theta-e or θe, which considers temperature and moisture and can be useful for examining atmospheric stability.

As can be seen above, we begin today with a mainly dry (and polluted environment).  The leading edge of the precipitation system is marked by a pronounced warm front that descends overnight and passes at mountaintop level prior to noon (Sat 18Z) tomorrow.  As the warm front approaches, southwesterly low-level flow will increase and potentially mix out our the inversion overnight.  

Moisture moves in with the approach of the warm front at low levels, and precipitation will develop over the mountains tomorrow morning.  Snow levels will max out around the time of warm-frontal passage, reaching 7500 feet (note that the blue line is the freezing level and that the snow level is typically 1000 feet below this).  Later in the day, a surge of low-θe aloft will precede the surface cold front and destabilize the lower atmosphere.  Thus, I would expect the precipitation to become more convective tomorrow afternoon, with even the potential for a bolt or two of lightning and rumble of thunder.  During this period, there might be graupel at times, and snow levels will lower, but remain above the valley floor.

Cold frontal passage occurs late tomorrow or early tomorrow evening, at which point snow levels will crash to the valley floor.  This will be the first full-bore period of winter weather at all northern Utah elevations this year (hooray!).  Get your old bald tires replaced today!

Following frontal passage, we see a prolonged period of unstable northwesterly flow.  This is what I call the post-frontal crap shoot since much of what happens during this period depends on factors that are difficult to forecast, including lake-effect.  

Of course the hard part is figuring out how much snow is going to fall.  The graphs below show what the NAM is producing for Alta.  Precipitation develops during the day tomorrow, and becomes heavy during the frontal passage, and then tapers off overnight.    

Based on this, I'd probably go for a total accumulation of 12-20 inches tomorrow and tomorrow night (i.e., through  5 am Sunday) in upper Little Cottonwood (typically the NAM underestimates total snowfall, hence the increase, although there are unfortunately exceptions). After that, based on a survey of the various models, I think it's likely we will see periods of snow through Monday afternoon and that we'll get more snow than indicated by the NAM above.  Nevertheless, I consider the post-frontal environment to be more of a crap shoot, so I'm not sticking my neck out for specific accumulations in a 48-84 hour forecast, especially given the fickle nature of the lake effect, which could play a role beginning on Sunday morning.  I think you should be optimistic, but keep your fingers crossed.  

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