Tuesday, December 6, 2016

Snow Prospects in the Short, Medium, and Extended Forecast

Plenty to discuss today as there's all sorts of action in the short-, medium-, and extended-range forecast periods.


When it comes to snow, sometimes the most difficult forecasts are for light accumulations.  

The 1300 UTC initialized HRRR simulated reflectivity forecast below shows the challenges for today.  A relatively weak weather system over Oregon and Idaho is forecast to strengthen some as it moves into the Wasatch Front later today.  

For Salt Lake City, most ensemble members produce less than 0.1" of water equivalent through 0200 UTC (7 PM MST), although there are a couple that to for around 0.15".  
NCAR Ensemble forecast for the Salt Lake City International Airport

Thus, this could be everything from a few snow showers to an inch or two of snow.  On a meteorological scale of 0 to 10, that's about a 1, but it's coming in around rush hour.  If precipitation is light and intermittent, it probably won't be a big deal, but a brief burst of heavier precipitation, while a lower probability possibility, could cause some snafus.  Best to keep an eye on this as the day evolves.

A couple of members go for some significant snow overnight due to lake effect.  As usual, that's a tough forecast, so consider yourself fortunate if you get some.  

Best guess is perhaps 2-4" of cold smoke in the Cottonwoods later this afternoon and tonight.  Emphasis on guess as the models have been erratic and the lake is always a dicey proposition.  These amounts are a little lower than one might infer from the NCAR ensemble or overnight NCEP model runs (not shown), but reflect the fact that the system seems pretty weak right now.  Thus, hope we do better.  

Medium Range

We've already discussed the big cool down for tomorrow (Wednesday) and how it will be short lived.  The models bring in a pretty juicy pattern beginning late Thursday through Saturday night.  The NAEFS plume for Alta shows the weak system passing through later today (~0000 UTC 7 December), a break for tomorrow and early Thursday, and then the action picking up.  By 1200 UTC 11 December (5 am MST Sunday), our downscaled accumulation estimates fringe from about in inch of water equivalent to just over 4 inches of water equivalent.  You can further excite yourself by looking at the snowfall plume.  

NAEFS forecast for Alta
   Personally, I think it's too early to be talking about accumulations, but the pattern does suggest we will be adding to our base at upper elevations.  There will, however, be some yo-yoing of the snow levels, which could reach or even exceed 7000 feet Friday night.  

Extended Range

Most ensemble members suggest an active, stormy pattern through next week.  The Utah Avalanche Center and snow-safety teams at the resorts and along the highways will have their work cut out for him if the advertised pattern verifies.  I've already said more than I should as I put little faith in 5-10 day forecasts, but lets keep our fingers crossed. 


  1. At one point I think you said the rain/snow line was about 1000 feet below "wetbulbzero," which varies from a high of 8500 feet Fri 11am in the NAM12 to a low of 5000 feet Sat 11am in the GFS, at

    So that's rain between 4000 and 7500 feet for a period of time Friday into Saturday? Otherwise snow on the valley floor. Am I getting that right. Any subtleties in wetbulbzero you want to flesh out?

    This is fun, hopefully it verifies and the rain/snow line stays in the foothills, or we just get snow

    1. It's too far out to get into details on the snow level. Variations between the range you indicate are likely, but exact timing, etc., isn't worth pinpointing yet.

  2. So what is "wetbulbzero" and why is the rain/snow line 1000 feet lower?

    1. The west bulb zero is a thermodynamically derived quantity that basically represents the height of the freezing level if the atmosphere were cooled through evaporation to a relative humidity of 100% (that is what the wet bulb temperature is and the wet bulb zero is the altitude of the 0ÂșC wet bulb temperature).

      The snow level is typically lower than the freezing level since it takes time for snowflakes to melt as they fall. Typically, the begin to wet, become slushier, and then transition to rain.