Onto December…and We Have a Storm!

Posted Friday, December 1st, 2017 by marcelcaron

TLDR: November was hard. However, December is kicking off with a storm sometime Sunday evening. Frontal Passage will bring small amounts of frozen precip to the valleys, increasing with elevation. Later, a ridge parked over the West keeps us warm and dry for a week.


The Quick Recap:

November was not easy on the resorts, and while we are in for some ski-lief, it will take at least another week before we can hope for a big pattern change. What I think has been the essence of November this year can be described in the figure below from earlier this evening. (Description in the caption)

Approximate 500 mbar Heights at 5PM today. Uninteresting zonal flow pattern. (Courtesy tropicaltidbits.com)
Approximate 500 mbar Heights are shown in colors for 5PM today. Zonal flow patterns don’t typically scream awesome weather. (Courtesy tropicaltidbits.com)

Nevertheless we are in for some snow. WOOHOO!


The Forecast:

Over the last few days we were watching a system materialize in some global forecast models, and finally we have good agreement for frontal passage sometime late Sunday–indicating good probability of a moderate event. Here’s the Euro (top) and the GFS (bottom) around forecasted frontal passage in SLC, for comparison:

ECMWF 500 mb Relative Vorticity forecast (shaded) with trough centered in N. NV. Surface FROPA generally east of the trough axis. (courtesy weather.us)
ECMWF 500 mb Relative Vorticity forecast (shaded) with trough centered in N. NV. Surface FROPA generally east of the trough axis. (courtesy weather.us)
GFS model (Courtesy weather.us)
Lower Res GFS 500 mb Relative Vorticity forecast (shaded) with trough centered N.E. NV at the same time. (courtesy weather.us)

Notes: The GFS features a signature of strong vorticity stretched from southern Nevada into southeastern Montana, and the trough axis less positively tilted than it is in the Euro. It is important, however, to consider the obvious differences in the resolution between the model output that can play a role in what we are seeing.

The result is slightly heavier precip in the GFS compared to the Euro. It’s likely that forecasted precip will change, and my gut tells me it will decrease until the event.

Lastly, I’m considering the temperature forecasts for Sunday evening. This will change, but currently the GFS Model Output Statistics (MOS) at SLC Airport are showing a slow decrease in temperature during the event.

GFS MOS at SLC Airport. Outlined are hourly temperatures. Hours are in zulu time (subtract 7 for local military time).
GFS MOS at SLC Airport. Outlined are hourly temperatures. Hours are in zulu time (subtract 7 for local military time). (courtesy National Weather Service)

I think the mountains will see snow, lower elevations might see a changeover from rain to snow sometime during the event. All in all snow levels will decrease with this storm.

Not quite ready to shoot numbers, but I don’t think lower elevations are pushing much more than an inch. High terrain of course would see higher amounts.

With the ski bus kicking off this Sunday, might be worth seeing this happen from up high.


The Horizon:

Looking ahead, things will ridge up for about a week with no event in sight.

GFS 500 mb Geopotential Height Anomalies December 5-10. Sunday's storm leaves behind a ridge that seems to last at least through next week.
GFS 500 mb Geopotential Height Anomalies December 5-10. Sunday’s storm leaves behind a ridge that could last through next week. (courtesy pivotalweather.com)

Hopefully we will see a pattern change on the horizon soon. We will be the first to let you know.

TGIF! Have a great weekend.

-Marcel

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