Upcoming Snowy Weekend and an Arctic Freeze

Posted Thursday, February 7th, 2019 by marcelcaron
TLDR: Expect two back-to-back storms, the weaker storm hits mountains Saturday and the other dumps snow Sunday through Monday at all elevations.

We’re sitting pretty given the torrent of storms and arctic blasts to hit resorts over the past few days. To that regard, I’ve gathered some statistics about the snowfall since Sunday and the current state of the snowpack (just a handful of locations):

storm totals
Summary data at a selection of Wasatch ski resorts. Storm totals cover the period between Sunday evening February 3rd until early Thursday morning February 7th. [These use the NWS SLC’s preliminary precipitation reports, linked here. Also, for current snow depth, I used the MesoWest station data archive. For percent normal snowpack, I used the Colorado River Basin Forecast Center’s archived data. These are percentages of normal snow-water equivalent (SWE), not percentages of normal snow depth. Here, “normal” SWE means average Feb. 7 SWE]
Setup for this weekend

To summarize, an arctic vortex and a highly amplified jet stream interact to bring another strong storm to UT by Sunday.

Why are we getting this storm, and why are we in an “active pattern” for storms? These storm periods exist by nature of a weak, split, and highly-amplified jet stream.

Some people attribute this waviness of the jet stream to a few intraseasonal cycles. The Arctic Oscillation (AO), for example, results in a shift between a highly- and slightly-amplified jet stream. Perhaps the AO explains the currently frequent storms over the western US; in fact, the AO is now in its weak/highly-amplified negative phase. Just an idea.

Event Components

Given the setup, we can understand the Sunday storm’s two components. The first component is a shallow trough, traveling eastward across the North Pacific. The second part is a cold-core low whirling around a “daughter” of the arctic vortex, which is centered over northeast Canada.

vortex
12Z GFS Potential Vorticity analysis valid at 5 AM MT Thursday, February 7, 2019. Components #1 and #2 are annotated. Potential vorticity (PV) generally increases toward the northern side of the jet stream, so PV fields are useful for examining arctic air. Here, dark red colors generally represent arctic air. Courtesy tropicaltidbits.com

As the first component travels eastward, it interacts with the second, and both troughs dip southward along the west coast. Next, the second component becomes the main trough and begins to kick the first out into the western US.

vortex gif
12-18Z GFS PV valid from 5 AM MT Thursday, February 7 to 5 AM MT Saturday, February 9. Courtesy tropicaltidbits.com
Event Timing

You can expect, then a sort of one-two punch in the coming days. The first storm is delivered by the first component of this event and starts to impact Northern Utah Saturday daytime. The second component brings precipitation and an arctic blast Sunday daytime. With that, here’s a look at that timing at Alta-Collins station, just beware that precipitation totals will change before the storms hit:

QPF
12Z NAEFS QPF and Snowfall forecast for Alta-Collins station. Annotated are components #1 and #2 of the upcoming storm series. Courtesy weather.utah.edu

Lastly, you can expect the first storm to be weaker than the second. Both should impact the mountains, but only the second will impact the lowest elevations.

Tomorrow, we’ll cover storm details like temperatures, snowfall totals, and their uncertainties. Until then get hyped! More snow is on the way …

-Marcel

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