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.
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 weather.utah.edu and also at https://ensemble.ucar.edu/). 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.
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.