In the latest round of our weekly series entitled “Hey- it snows a lot in the Wasatch” we will feature a long duration event. From Sunday until Tuesday of next week a large western trough will swing into the intermountain region and bring a prolonged period of mountain snowfall.
Saturday 11/20/10 Update
Things are looking still very good for a large and wet pacific storm to impact the Wasatch range over the next 96 hours. Starting today – later in the day- snow will begin to break out across the range along a sw flow. As the trough moves inland tonight, tomorrow snow will peak in intensity. High res gridded models show a very achievable 1.75 to 2.25 liquid equivalent precipitation by around midnight sunday. Then, as the trough moves over the great basin, a break in the snowfall should be ended by an unstable westerly flow. The latest time height diagram shows a good saturation of water vapor and pretty decent lapse rates from monday evening through at least Wednesday morning. With the orographic lifting the western slopes of the Cottonwoods should do very very well. Another 1 to 2 inches of liquid equivalent are very possible. Notably I use liquid equivalent here because with the steep vertical motion and sw flow gruapel is very likely in the first 1/2 of the storm. Accordingly total snow depth will be a little tougher to judge as graupel doesn’t really pile up.
As we move into the weekend a large pacific trough will deepen as it swings down out off the coast of the California. As it deepens and swings into california and the west it will bring with it upper level divergence and an unstable moist airmass. Overall the system will first bring precip on a south west flow, then as the trough moves through region it will bring precip enhanced by a western flow. Precip will begin on sunday and last through at the least tuesday.
The models have a very clear that the first precipitation should begin later in the day sunday:
Then the trough moves in full bore by Sunday.
As we can see clearly the beginning of the event will feature snow on a strong S/W flow. 24 hours later the strongest upper level divergence will spike as the trough remains it’s eastward progression through the intermountain west. Clearly the models have picked up on this and spiked the precip. rates in the central wasatch.
Once the trough flattens out, a moist airmass will persist. As the trough moves east, the flow will turn westerly. Given the moist airmass and the western flow, precip should remain over the Wasatch on Monday into Tuesday
And still persisting….tuesday into Wednesday.
Looking at the higher resolution time-height series for Alta-UT shows the long duration event with two spikes of upward motion that co-incide with the heightened model periods of precip- sunday night into monday and tuesday.
Adding all this up I’d suspect upper cottonwoods storm totals to reach 2-3 feet with more possible if the western flow remains steady and the vertical motion matches up.
Models are a little more progressive and show snow breaking out later in the day on saturday across the wasatch. With Omega values greater than -6 likely there will be adequate instability. Upward Vertical motion is also wvery high with spikes Sunday afternoon and night and then again tuesday early morning. All in all things look good.
Totals still look around 3 feet in the Cottonwoods. However this will have to be updated when the High Res. models come in range over the weekend.
Should be a great opening weekend at Alta!
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