Two More Days

Posted Wednesday, February 28th, 2018 by Alex Weech

In case you didn’t notice with all the excitement, it snowed last night. Parts of the Cottonwoods got an inch, and I woke up to a skiff on the east bench. It’s always nice to have freshly fallen snow on the ground, even if it’s just a trace.

After last night’s system passed, we had partly cloudy skies. Now that the next storm is approaching, they will start to cloud up. Multiband infrared satellite imagery shows a classic comma cloud draped across the far western United States with some high clouds preceding it near northern Nevada/southern Idaho. I could have used imagery from the old GOES-West satellite to get a better angle, but I’m so stoked for the GOES-S launch tomorrow that I’ll stick with GOES-16. Also the details in the loop are gorgeous. Notice all the small, low clouds over the ocean. This is indicative of how insanely cold this storm is.

Via NOAA
Via NOAA

Likely a surface cold front is underneath where the thickest clouds are in the loop–aka it just came ashore. Before a cold front makes it as far inland as Utah, we’ll have the typical pre-frontal weather that happens here. The wind will blow, and the dust will gather (unless it’s all weighed down by wet right now). This NAM forecast from weather.utah.edu for mountaintop winds shows rocking gusts from the south starting tomorrow afternoon and continuing into Friday. It won’t really be a good time to be on any exposed ridges. There will also be snow showers Thursday afternoon.

NAM12 00Z 2018-03-01 via weather.utah.edu
NAM12 00Z 2018-03-01 via weather.utah.edu

I still can’t get over that gif because it’s so stunning. Like wow. Anyways, the real fun starts Friday afternoon as a cold front finally reaches northern Utah. As per usual, it will come from the northwest and affect the northern Wasatch Front first before moving into Salt Lake and Utah Counties. The timing of the front is somewhat uncertain, but it’s looking more and more like late afternoon. From a meteorological perspective, late afternoon is the most exciting time to have a cold front because then the air mass ahead of the front can warm up during the day. This makes for a storm that packs a wallop, and it often makes for good photos. The timing could fall apart, or it could be cloudy here Friday during the day, so I’m not making any promises. But check the forecast Friday morning and maybe have a camera ready to point northwest.

With the front will come the snow. The way this storm is setting up, however, is that the precipitation band will stall out somewhere near northern Utah. Someone is going to have a good night. The problem is that there is still quite a bit of uncertainty as to who is going to have a bonanza and who is merely going to have a good storm. For example, the 00Z NAM 12 km tonight shows Salt Lake County and the Cottonwoods sitting under heavy precipitation for most of the night. Its forecast would bring a foot of snow to the valley and more to the mountains. However, that model’s 4 km child stalls out the snow band over Box Elder and Cache Counties. That would bring feet to the Wellsville and Bear River Mountains and maybe a foot and a half or so for the Cottonwoods. I’m bringing this up to illustrate the uncertainty that we’re still dealing with two days out.

Via weather.utah.edu
Via weather.utah.edu

Above is an ensemble plot of precipitation at Alta for the next few days. The first storm are the showers Thursday afternoon. Those will bring 3-6″ to the Cottonwoods. For the next part of the storm on Friday, the uncertainty increases. The orange lines and blue lines represent ensemble members that use two different “dynamical cores”, or the piece of the weather model that deals with moving air. What’s important is that the two cores will represent the front differently, and it shows in how the ensemble splits. The orange ARW lines show heavy precipitation in the Cottonwoods Friday evening. If you go to the website and click around you’ll see that the NMB members show a spike like that for sites in Weber County, indicating that it positions the frontal precipitation up there Friday night. Then Saturday during the day a precipitation band forms over the Cottonwoods in both models.

Looking across all the models, two different stories emerge. Story A is that a frontal band sets up across far northern Utah near Weber/Box Elder/Cache counties Friday night. Then the band reforms Saturday and pushes south across Salt Lake County and the Cottonwoods. Out of the 00Z models tonight, this story is supported by the European model, NAM 4km, and SREF NMB members. Story B is that a frontal band sets up across Salt Lake and Davis Counties Friday night. Then the band reforms Saturday and gives the Cottonwoods a second heap of snow. Out of the 00Z models tonight, this story is supported by the GFS, NAM 12km, and SREF ARW members. At this point there’s really no way to know what story is right. Hopefully tomorrow will bring some more clarity, and, either way, it’s going to be a good weekend.

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