Monday, March 28, 2016

Stuff We Know About This Storm and Stuff We Don't

It's always great to be greeted by post-frontal snowfall in the morning.

I've been taking a look at the various model forecasts and I have to say that this is one of the most complicated storms I can remember.  There are a variety of frontal positions, low-center tracks, and precipitation distributions and intensities being forecast by the various models and their ensembles that make forecasting details very difficult.

For simplicity, I'm going to use only the 0600 UTC NAM to illustrate some of the issues at play.  The red dot and yellow plus sign are located at the approximate location of Alta.  One can see the frontal band that is over us this morning in the early part of the loop,  Then a narrow band sets up very close to Alta, followed by more scattered precipitation.

Those precipitation features are very narrow and small in scale and predicting their intensity and position with any accuracy is very difficult.  One can imagine how a slight change in location is the difference between heavy snowfall and no snowfall at any given time, and that's assuming the model forecast has a reasonable handle on what Mother Nature will do.

If we look at the time series of 3-hour accumulated precipitation, we see quite a bit of "pulsiness" with some periods of moderate precipitation and periods of less precipitation.

One can look at the GFS, HRRR, or NCAR ensemble and find all sorts of variations in the timing and intensity of precipitation features over the next 48 hours or so.  Below is the NCAR ensemble.  The box-and-whisker plots in the bottom frame show the wider range of possible precipitation rates being forecast by the ensemble members each hour at Alta.  Total accumulations range from a bit under an inch to a bit over two inches.

So, let's start with what we know.  Winter has returned and we are going to see periods of snow over about the next 48 hour period.

What we don't know is the details of where and when the snow will come (other than some snow continuing this morning).  For Alta-Collins, I think the most likely range of totals by 6 AM Wednesday is 0.7 to 1.4 inches of water and 8-16 inches of snow.  We could do better or worse depending on how things develop.  I expect to see quite a bit of spatial variability in snowfall from this event (Powder Mountain is already reporting 5 inches).  Thus, be patient and go where and when it snows.  I expect there will be some good skiing to be had and a modest event would actually be better than a big one, at least for my skiing interests.    


  1. Hard storm to forecast for the Cottonwoods since the 700mb flow stays southerly until Wednesday morning before it finally turns to the north, areas that do well in a southerly flow should do well.

  2. ~4" at top of Gad 2 by 2pm (when I left). It really started coming down at 12:30.

  3. Park City Canyons side on 9990 had what I estimate to be around 8 inches by 3:00 pm yesterday. It was great skiing. Lower on the mountain was very wet snow - the kind that melts into your jacket when it lands on you. Today should be even better with the cooler temps.