Heading Toward Unstable Weather

Posted Saturday, February 23rd, 2019 by marcelcaron

To summarize, Sunday-Tuesday sees a gradual shift from stable to unstable weather. Then, a storm Wednesday and another around Friday drop substantial snow over the interior West.


Overview

Compared to the previous week, we’re under somewhat zonal flow during the weekend. Zonal flow means air in the jet stream moves along latitude lines or east-west. The difference between zonal and north-south flow is that zonal flow tends to propagate systems more quickly. However, these systems tend to have less “spin” to them. Nevertheless, zonal flow doesn’t necessarily mean fewer storms.

18Z GFS 250 mbar wind speed valid 11 AM MT Sunday. The synoptic pattern over this weekend is short-lived zonal upper-level flow. Courtesy tropicaltidbits.com
18Z GFS 250 millibar wind speed valid 11 AM MT Sunday. The synoptic pattern over this weekend is short-lived zonal upper-level flow. Courtesy tropicaltidbits.com

In fact, I think the next few days are looking like intermittent low-impact storm systems before a few harder-hitting late-week storms hit the area.


Unstable Weather

Sunday through Tuesday sees gradually decreasing low-level stability and increasing mid-level moisture.

18Z GFS time heights. Vertical axis is in Pressure (reads like elevation), the horizontal axis is time, and increases from right to left. Green shading indicates moist air, black contours mark theta-e values, blue line marks the freezing level. Note that low-level theta-e increases over time, which suggests that low-level stability might decrease over time. Courtesy weather.utah.edu
18Z GFS time heights. The vertical axis is in Pressure units (reads like elevation), the horizontal axis is in time units (UTC) and increases from right to left. Green shading indicates moist air, black contours mark theta-e values, blue line marks the freezing level. Note that low-level theta-e increases over time, which suggests that low-level stability might decrease over time. Courtesy weather.utah.edu

At the same time, an upper-level trough moves in slowly until it’s blocked outside of the jet stream just off-shore of Washington state. It seems like this setup works well for pivoting shortwaves around the upper-level trough and into the interior west. Our area’s mountains may get some weak precipitation from passing unstable air between now and Wednesday. The chance of experiencing these weak systems increases every day as moisture infiltrates the mid-levels. However, their timing is not easily predictable at this point.

Two Late-Week Systems

Otherwise, we’re looking at two stronger storms that arrive here next week. The first hits sometime Wednesday. It is a shortwave that loops around the blocked upper-level trough I mentioned earlier. Another system sometime Friday may be stronger. This system is a product of the strengthened upper-level trough itself, as it’s finally pulled into the jet stream and propagated across the US. Ultimately, the impact of each of these systems will depend on their available moisture and instability.

12Z (5 AM MT) NAEFS snowfall plumes for the next seven days. Vertical axis is snowfall in inches, horizontal axis is time in UTC. Each dark and line blue line is a deterministic precipitation forecast. Altogether these attempt to predict the range of possibilities in accumulated precipitation. Note that while precipitation amounts are still highly uncertain, the timing of each event is moderately consistent. Annotated are the general periods of time for each event. Courtesy weather.utah.edu
12Z (5 AM MT) NAEFS snowfall plumes for the next seven days. The vertical axis is snowfall in inches; the horizontal axis is the time in UTC. Each dark and line blue line is a deterministic precipitation forecast. Altogether these attempt to predict the range of possibilities in accumulated precipitation. Note that while precipitation amounts are still highly uncertain, the timing of each event is moderately consistent. Annotated are the general periods of time for each event. Courtesy weather.utah.edu

Precipitation Totals

As far as precipitation goes, I’m staying away from the late-week systems. Look out for that forecast later this week. Otherwise, NAEFS ensembles are confident that Sunday will be clear of new snow, but Monday and Tuesday each have a chance for 1-3 inches of snowfall per day. The confidence in the occurrence of this somewhat random precipitation increases closer to Wednesday. On the other hand, snowfall totals are still quite uncertain.

Wind and Temperature

Next, expect winds to increase over the next few days gradually. Short-term I don’t think this means anything more than a bit of a breeze for resort skiers, Monday and Tuesday for example, even at the summits. Wednesday-Friday, however, may be pretty windy. Just a heads up.

Finally, we can expect that temperatures this week will decrease somewhere between 7 and 9 degrees C (or 12-16 degrees F) for every kilometer above the surface because low-levels are conditionally unstable. You might think resort temperatures would then be frigid. However, early-week daytime temperatures in the valley will be increasingly mild (the 40s and 50s), so resort temperatures may be somewhat average in the 20s and 30s.


That’s all for now. Enjoy the unsettled weather if you can early this week!

-Marcel

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