Trough Brings Mountain Snow Wednesday

Posted Tuesday, January 22nd, 2019 by marcelcaron
TLDR: Wednesday’s storm will bring light to moderate snowfall to Wasatch resorts. Scroll down for the timing and specific accumulations.

For those of you that missed the MLK day powder rush, Tuesday’s bluebird ski offered plenty of fresh turns. I had a chance to squeeze in a few runs at Solitude and needless to say it was well worth the trip up the canyon.

trough
Black Bess Peak, above Honeycomb Canyon, Solitude Mountain Resort, 1 PM Tuesday, January 22 2019. PC: Marcel Caron (Twitter @marcelsiuswx)

With MLK Day storm totals under the belt, let’s talk about what we can look for this week.


Wednesday’s Shortwave Trough

First, Wednesday morning is the next chance to see accumulating snow in the mountains. As Taylor alluded to yesterday, this disturbance is a weak nudge and relates to a shallow shortwave trough. Furthermore, I can summarize the Synoptics of this system pretty easily.


Vorticity

In the mid-latitude cool season, troughs are often just a way of looking at a dip in the jet stream. These dips contain by nature a certain amount of spin, specifically “vorticity.” In essence, both the concentration of vorticity in an area, along with this area’s direction of travel, can suggest the strength and location of a disturbance.

Take tomorrow’s storm for example. To start, we can observe Wednesday’s passing shortwave trough as a moving area of moderately strong vorticity (shaded below) in the middle troposphere (about 19,000 feet ASL). Next, this area of vorticity is headed towards the southeast as indicated by wind barbs.

shortwave trough
01/22/2019 18Z GFS 500mb absolute vorticity (shaded) and wind (barbs), valid at 5 PM MT Wednesday the 23rd. Outlined is an area of maximum vorticity associated with Wednesday shortwave trough. Annotated arrows emphasize the wind direction, which suggests the path the outlined area will travel. A red dot marks the location of Salt Lake City, UT. (courtesy tropicaltidbits.com)

Finally, meteorology would place the disturbance where vorticity increases maximally. That’s just downstream of the area of vorticity (outlined in the figure) for the time stamped in the image, or just over northern Utah.

With this information alone we can assume that something could happen over northern Utah, and it’ll probably be weak.

Identifying the Front

From another perspective, we can see this weak front in the time heights below. Specifically, Wednesday morning’s front is accompanied by increasing temperatures, moisture, and instability. Furthermore, it passes aloft (around 7 AM Wednesday) before it crosses the surface. Altogether, this is a pretty typical warm front signature. Also, note that the front will be most potent in the 750-700mb range, or between about 8,000—10,000 feet, as indicated by a tight temperature gradient.

wednesday trough time heights
12km NAM Time Heights. The horizontal axis represents time and increases toward the left. The vertical axis represents height, in pressure units (meteorologists prefer to use pressure). Shaded is relative humidity, where areas of higher moisture are colored green. Red contours mark areas of decreased atmospheric stability. Annotated in red are time stamps of note. Annotated in black are elevations of note. See if you can spot the warm front. (courtesy weather.utah.edu)

Timing and Total Snowfall

Timing

In general, snowfall begins light with the frontal passage, hitting the northern Wasatch first. Then, snow rates increase throughout the day Wednesday.

Begin: Sunrise, Wednesday (light snowfall)
Peak: before midnight, Wednesday night (moderate snowfall)
End: before sunrise Thursday

Midmountain Totals

Cottonwoods (e.g., Alta) – 7” (Bust: 5”, Boom: 10”)
Northern Wasatch (e.g., PowMow) – 5” (Bust: 3”, Boom: 8”)
Wasatch Back (e.g., Deer Valley) and Sundance – 4” (Bust: 3”, Boom: 6”)


After Wednesday

At least one more storm is slated to hit the region this week. For a better perspective, we can zoom out from the first 500mb plot I showed.

trough #1, 2, 3
01/22/2019 18Z GFS 500mb absolute vorticity (shaded) and wind (barbs), valid at 5 PM MT Wednesday the 23rd. Annotated lines mark trough axes. Wednesday’s shortwave trough is marked “#1”. Two other troughs of note are marked “#2” and “#3”. (courtesy tropicaltidbits.com)

Note that Wednesday’s storm is marked by #1 and a blue dashed line over its trough axis. Also, we can observe two other troughs spinning around a stationary gyre over the northern Pacific. The GFS model stretches #2 as the system passes east of us and poses no threat. Next, the North Pacific gyre kicks #3 downstream sometime Friday, and the storm passes just south of UT. However, even if models push this last storm’s track northward, #3 may be too dry to deliver much or any precipitation.

Looking ahead, I think this weekend we’ll have a weak inversion, with moderately unhealthy air quality in urban areas. Then, the next best chance for sizeable resort precipitation is around Tuesday next week. Nevertheless, check future posts for more comments on Friday’s chances for snow, inversion possibilities and strength, and the next best chance for a storm.

-marcel

Leave a Reply

Your email address will not be published. Required fields are marked *