The model runs are pretty spicy and exciting this morning with a significant storm moving later today. The latest GFS suggest precipitation beginning in the central Wasatch this afternoon or evening and continuing into Friday. Let's have a look.
The GFS forecast valid 0600 UTC 6 March (11 PM Wednesday) shows that the early part of the storm is characterized by an deep upper-level trough (upper left panel below) with a lower-tropospheric cyclone (red L in images below) over Nevada. Associated with this system is an inland penetrating atmospheric river atmospheric river that approaches northern Utah via the lower Colorado River Basin. The crest-level 700-mb flow (lower left panel) is southerly or even south-southeasterly in the vicinity of the central Wasatch with 700-mb temperatures near -5°C.
Thus, the overnight storm period has all the hallmarks of a warm, high-density storm period. Through 9 AM tomorrow morning, the GFS generates 0.77" of water and 8.2" of 10% water content snow. The HRRR is even more excited with water with 1.65" of water and 13.8" of 12% water content snow. And that's just the start. For the Wasatch Back crowd, that little bit of easterly flow gets my attention and suggests this could be a period where you do pretty well. This could be a period where snowfall is greater at Deer Valley and less at Snowbird, but we'll see if that pans out.
The GFS drags the main cyclone very slowly across our area, resulting in a prolongued period of moist southerly to southwesterly flow that continues through the day tomorrow, By 0000 UTC 7 March (5 PM Thursday), the storm is so wrapped up that we actually have cooler air moving into northern Utah from the southwest, as illustrated by the 700-mb (crest-level forecast at lower left). By this time, the AR has moved downstream, but there's fairly deep instability and it's March, so things might get a bit convective tomorrow afternoon.
I'm not sure if we might get a break in there for a bit as sometimes there can be a dry slot that sets up in a pattern like this, but by and large I suspect we'll see periods of snow tomorrow that will continue to stack up.
Eventually the system moves through and we get into colder, unstable, northwest flow for Friday.
I've summarized the storm phases in the time-height section below. Time increases to the left. First there is the warm and juicy AR period Thursday night. Note the low-level southeast flow that might favor the eastern side of the central Wasatch. Then on Thursday, colder air wraps around the system and moves in aloft from the south and southwest. Call it an upper cold front if you like, but it will destabilize things. Temperatures drop further on Friday evening with a transition to colder, post-frontal flow.
For Alta-Collins, storm totals in the GFS through 7 PM Friday are 1.77" of water and 24.1" of snow. Expect that to be high density snow to start and lower density Thursday night and Friday. The HRRR only runs through 5 AM Friday but it has 2.22" of water and 20.2" of snow. It's less bullish on the post-frontal period whereas the GFS produces snow more continuously through the period. Such model inconsistencies are one of the joys of being a meteorologist!
I'm inclined to go for a storm total of 1.5-3" of water and 20-36" of snow for Alta-Collins through Friday evening. There are uncertainties in timing and details, but this looks like a pretty good storm. The early part could be a big producer on the Wasatch Back. It's a warm storm and perhaps snow levels will flirt with the lower Jordanelle/Mayflower area during the atmospheric river phase tonight and early tomorrow. At upper elevations, I suspect the snow will be fairly dense everywhere before trending drier late Thursday and Friday.
Buckle up and monitor forecasts. There's a lot of moving parts in this storm.
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