Rapidly Evolving – Great End to the New Year

Posted Wednesday, December 26th, 2018 by Mike Wessler

As a progressive pattern goes, we can expect rapidly evolving weather over the next few days. Little precipitation is expected overnight with a ridge axis moving through, though clouds will stick around as well as the chance of a flurry or two. The ridge overhead is wedged between two significant upper-level troughs, one now well to our east, and another beginning to dig in from the northwest. By early Thursday morning, the trough will already be progressing through and continues to amplify, pushing well south of Utah (not ideal for the Wasatch).

NAM 12 km 500 mb heights and vorticity highlighting the amplifying trough moving through the region on Thursday. Note the broad nature of the trough with the strongest height gradient well to our south - producing similar synoptic-scale forcing to an upper-level closed low.
NAM 12 km 500 mb heights and vorticity highlighting the amplifying trough moving through the region midday Thursday. Note the broad nature of the trough with the strongest height gradient well to our south – producing similar synoptic-scale forcing to an upper-level closed low.

While not quite a closed low, this upper-level setup leads to similar problems: weak, at times easterly flow, limited moisture transport, and very limited synoptic forcing. Working in our favor is the relatively deep moisture in the column, with relative humidity > 80% up to 500 mb or so. A weak upper-level front will produce the heaviest precipitation early Thursday morning, with lighter snowfall lingering into Friday morning. The snow level for this round should be right to the valley floor, and the snow should come in upright: close to 20:1 at first and finishing dusty cold at 30:1. Currently, short-term model forecasts suggest 3-6” of snow accumulation in the upper Cottonwoods by Friday morning. On top of the previous snowstorm, this will be a really nice top-off for those who can find it unskied.

SREF Downscaled Snowfall for Alta Collins highlighting Friday's passing snowstorm and the anticipated snowfall totals and snowfall rates. Width of the violin plot at each hour highlights likelihood of occurrence (widest - most likely).
SREF Downscaled Snowfall for Alta Collins highlighting Friday’s passing snowstorm and the anticipated snowfall totals and snowfall rates. Width of the violin plot at each hour highlights likelihood of occurrence (widest – most likely). Snowfall density highlighted in gray shading.

Beyond that? The deep, slow to progress trough over the desert southwest will keep unsettled weather around with mostly cloudy skies in the mountains. Friday provides the best chance for sunshine, though high elevation exposed areas will be subject to some seriously gusty winds. Thankful for the active pattern, this break in precipitation won’t last long. By Sunday, a weak but deep trough will swing through the region. Shallow moisture and a strong upper-level front will allow for a brief period of heavy precipitation during the day Sunday into the evening hours. By early Monday morning, dry air will fill the upper levels while a shallow layer of moisture lingers near the surface. This will keep clouds around snow showers going to finish off the year! As an upper-level ridge builds in overnight, we can expect a sunny powder day to start off 2019. Not too bad of a way to start if you ask me!

Happy holidays to you all.

-Mike

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