Thursday, December 15, 2016

Perspectives on the Storm from the NCAR Ensemble and Climatology

The NCAR ensemble is a 10-member, high-resolution (3-km grid spacing) ensemble forecast system run once a day (0000 UTC initial conditions) by the National Center for Atmospheric Research.  We have been working to evaluate the capabilities and limitations of the forecast system for forecasting mountain precipitation over the west.  It's not perfect (no modeling system is), but it does provide helpful guidance for winter storms in our area.

However, before delving into the NCAR ensemble forecasts, let me summarize the large-scale evolution of the event.   Forecasts for the large-scale conditions for the storm have changed little over the past couple of days.  Tonight we have warm, atmospheric river conditions in southwesterly flow.  Tomorrow (Friday), an upper-level trough and cold front move through the state, lowering snow levels and eventually shifting the flow to northwesterly by late tomorrow.

The time series below is from the 1200 UTC NAM and based on that model's forecasts for the Alta grid point and our algorithms for variables like snow ratio and accumulated snow.  Snow levels (typically about 1000 feet below the wet-bulb zero level at upper right) are forecast to be between 7000 and 8000 feet through tomorrow morning before dropping with the frontal passage.  Total water equivalent through 0000 UTC (5 PM MST) Friday afternoon is 2.17 inches.  Through tomorrow morning, most of this snow is high density, with snow-to-liquid ratios of less than 10, after which the snow gets drier (middle right) so that the later stages of the storm should be right-side up.  This ultimately yields 17" of snow through 0000 UTC (5 PM MST) Friday afternoon.

Of course, that is a single model solution and there is a good reason why meteorologists don't issue a forecast as specific as 17" of snow falling.  Ranges are used because of the inherent uncertainty in winter storms.  Ensembles represent an effort to quantify that uncertainty and the range of possibilities.  Typically, they are run at low resolution, but the NCAR ensemble is quite unique in that it is run at 3-km grid spacing so it better (but not completely) resolves the topography of Utah.  And, with 10 members, it provides better (but still imperfect) guidance on the range of possibilities.

The plots below provide a summary of the forecasts produced by the NCAR ensemble through 0000 UTC (5 PM MST) Friday Afternoon (Note: These forecasts are available daily at and also at  The mean (upper left), maximum (upper middle), and minimum (upper right) precipitation maximize at high elevations around Mt. Timpanogos and the northern Wasatch, as one might expect given the juicy, southwesterly flow.  All ensemble members generate at least an inch of precipitation along most of the Wasatch Range (bottom middle) and over 80% generate more than 2 inches in the high terrain of Mt. Timpanogos, the Northern Wasatch, and Little Cottonwood.
At Alta, there is actually a fairly tight clustering of 9 members in terms of total precipitation through 0000 UTC (5 PM MST) Friday afternoon with a range from around 1.6 to 2.6 inches.  Just to give me heartburn, there is an outlier member that produces just under an inch.  Ick.

One thing that surprised me in these forecasts is that the totals at Snowbasin are somewhat lower than at Alta and also exhibit quite a bit more spread.  That's not what I would expect, especially during the juicy, atmospheric river portion of the storm.  I wonder if the crest of the Wasatch Range there is well resolved.  That's something for us to dig into.  Even 3-km models have biases.

Putting all this together, the story pretty much remains the same as it has been the past few days.  We will experience a warm, sloppy storm with an eventual frontal passage and falling snow levels.  My best guess for Alta is 1.5 to 2.5 inches of water and 12 to 24 inches of snow through 5 PM MST Friday afternoon.  The 24 inches might be optimistic given the high snow densities.  A bit more is possible with snow showers Friday night.

A quick word on water equivalents at Alta.  There are only a handful of events that have produced more than 2.5 inches of water in 24 hours (see graph below, which is based on about 25 years of data).  3 inches in 24 hours is exceptionally rare.  This is one of the reasons why I'm cutting my total off at 2.5 inches through 5 PM Friday.  While I can't rule 3" out, it is a low probability possibility.

1 comment:

  1. From the GFS and NAM, it looks to me like we might have as many as 3 separate cold frontal passages or cold surges. The first is around midday Friday, with colder air of Pacific origin behind it in NW flow (-8 to -12 C at 700-mb level). The second is associated with artic air spilling over the Rockies in British Columbia currently, which races southward through eastern WA/OR and arrives here late Friday evening (down to -16 C at 700 mb behind it). The third possible one is artic air that spills over the Continental Divide in MT/WY into eastern ID and then into northern Utah by Saturday morning (down to as cold as -20 C at 700mb). So looks very interesting as this system is mixing together air masses of very tropical and very arctic origin.