In the previous post (Windy Storm on Tap) we discussed an approaching storm system that would come in today (Friday), but really pick up Friday night as strong flow and an inland penetrating atmospheric river move into northern Utah.
That forecast is still on track. Overnight last night and today we are seeing the dribs and drabs at the leading edge of the storm system, as expected. Overnight through 8 AM this morning Alta-Collins picked up about .16" of water and 2" of snow and Snowbasin-Boardwalk came in with 0.24" of water and 3" of snow.
But the big story is not today but tonight when the precipitation efficiency looks to increase as an upper-level trough moves through and high integrated vapor transport (IVT) noses into northern Utah. The GFS forecast valid 0600 UTC 6 Dec (11 PM MDT Friday) shows heavy precipitation over the mountains of northern Utah (or better put what the GFS thinks is the mountains of northern Utah) as the trough (dashed line below) as the trough moves through.
Our HRRR-derived forecast for upper Little Cottonwood (click to enlarge) shows relatively light periods of precipitation through 5 PM today (top left), afterwhich hourly liquid-precipitation equivalent rates really pick up as the upper-level trough approaches. Precipitation rates maximize at around 1 AM Saturday with periods of precipitation continuing through 7 AM tomorrow (Saturday) after which precipitation becomes widely scattered.
This is a warm and windy storm. Our forecast winds for Mt. Baldy peak at around 700 mph at 4 AM Saturday. Temperatures overnight are forecast to be around 27 F at Collins and 30 F at the base. As a result, snow-to-liquid ratios are low and generally less than 10 for the period. This keeps the HRRR-derived snowfall relatively low and around 13" including the dribs and drabs today. That said, at this point with the need for base, Cascade Concrete is better than Cold Smoke.
The overnight six-member RRFS ensemble shows a range of liquid precipitation equivalent of between about 1.1 and 2.3" and snowfall between about 13" and 26". Thus, the HRRR is on the low end of the RRFS.
My take is that 1.2-2.2" of water and 12-22" of snow from 7 AM this morning through 7 AM Saturday represent the most likely range of outcomes for Alta-Collins, with much of that coming this evening and tonight. The water amounts are most important right now to build base. Let's hope we can eclipse 2".
Looking to the north, the story at Snowbasin is much the same with the overnight period being the wettest and then things settling down towards morning. Snow-to-liquid ratios are quite low at Boardwalk, which is lower than Alta-Collins and thus warmer in this storm. I'm expecting this to be a very wet snow event up there.
The HRRR Skew-T for Ogden shows a freezing level near 775 mb or about 7000 ft. Conditions are saturated, so that also represents the wet-bulb zero level.
If we zoom into the plot of wet-bulb zero height with time we see that it just about reaches the altitude of boardwal (~8000 ft) and after midnight the base is below that level. This raises the possibility of a bit of rain or mixed rain and snow at the base depending on how things shake out.
The extended forecast after this storm is not great for us, but maybe we can get something. The GFS, for example, has a monster atmospheric river roaring across the northwest next week. Batten down the hatches for the Cascades, and mountains of the Idaho Panhandle, Montana, and northwest Wyoming. Goodness gracious that is a tempest of a forecast for them.
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