Somebody’s Got a Case of the Mondays

Posted Friday, January 31st, 2020 by Luke Stone

TL;DR: Warm and dry the next few days, with snow to close out the weekend.

Current conditions:

It’s been a few weeks since I posted, and there have been a few good storms during that time. However, aside from a few snowy days here and there, it has been dry most of the time. There’s one good thing about that though; the fat bike trails get packed and into great shape, as evidenced by the photos below. First is Noodle, enjoying the snow as always.

The next picture shows a neat atmospheric feature known as Kelvin Helmholtz waves. These wave clouds occur when there is vertical wind shear, which is a difference in wind speeds at different heights, and strong stability.

Short-term forecast:

The weekend will feel more like April than the first week of February, with valley temps in the upper 40s and low 50s. A ridge will move in tomorrow, but fortunately it will be short lived. Even the cottonwoods will be quite warm this weekend, with temperatures reaching the upper 30s and low 40s. It certainly will feel like Spring.

That will change quickly though, as a strong and cold upper level through will move into the basin. This storm has all the ingredients for a big dump in the mountains. There’s plenty of moisture, good dynamics, cold temperatures, and orographic enhancement. I will go through each briefly.

First, we will look at the upper level setup. Below, you can see the storm move into our region at 500mb (~18k feet). One additional benefit with this storm is that it may close off at upper levels. This occurs when the storm, at higher levels in the atmosphere, becomes separated from the steering currents, and this slows down. This could lead to a longer duration event. At the end of the gif, you can see the upper level trough close off, as indicated by the closed circle forming over Utah.

(Image courtesy of Tropical Tidbits)

Next, we will look at the moisture available for this storm by examining the time height chart. These charts, which are read from right to left, show wind speed/direction, temperatures via the blue freezing level line, moisture via the coloring, instability in the areas circled in red, and fronts as well. Below is the 18z NAM time height for SLC.

There are several features to note here. Around 4-5z (10-11pm) the atmosphere becomes saturated, up to over 450mb. From this time, the relative humidity remains over 90% up to 450mb for most of the event, which is a good sign. This is the first ingredient.

The time height also shows the cold air in a few different ways. You can see the almost vertically oriented lines around Monday 6z, as well as the freezing line turn vertical around 4-5z, both of which indicate the cold front. The fact that the blue freezing line disappears shows that the freezing level (or height in the atmosphere where temperatures above the line are below freezing) will be below the valley floor for nearly the entire storm. This is a cold airmass that will result in low density snow and some valley snow as well.

Additionally, the time height shows a solid period of NW winds during the day on Monday. Although initially the winds are backing with height such that they’re not NW through the column, the NAM does show that this will occur sometime mid morning. During this time, areas favored by NW flow (LCC) will likely move ahead of the rest of the Utah mountains, as orographic precipitation will enhance the totals there.

The large scale dynamics are quite strong with this system as well. There is a prolonged period of strong vorticity, or lift, that is important for generating precipitation. See the gif below of the vorticity during the storm, with high levels for an extended period of time in Northern Utah. This will allow for heavy precipitation for much of the storm.

(Image courtesy of Tropical Tidbits)

All in all, there are many ingredients forecast to cook up something deep early next week. My initial snowfall forecast is 10-18″, with the highest amounts in Little Cottonwood. If the period of Northwest winds lasts longer, those totals could be even higher. Overall though, a healthy storm for all of Northern Utah.

Long-term forecast:

Beyond Tuesday, we will return to quiet weather for a few days. Our next chance for snow looks to br around Thursday, but right now it looks like a weak event. There is still time for that to change though.

-Luke

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