We finally have a weather system to discuss, which is expected to reach Utah on Friday. Yippee! Before we get to the details, let's take a look at the current stagnant pattern.
We have a strong ridge centered just offshore, providing sinking air over much of the interior West. Notice the ridge is positioned a little further westward than several days ago, something is changing!
This stagnant air mass and resulting valley inversions are creating poor air quality, mostly in the moderate category-- unhealthy for sensitive groups. This may continue into Thursday, but we should see a vast improvement Friday as a relatively weak storm arrives.
Now that the ridge has shifted from the interior West to just offshore, the door is open to a system arriving from the north. Systems that arrive from the northwest, off the Pacific Ocean, tend to be associated with good moisture content. In this case, the Friday system will come from the north, moving over more land mass and with less moisture. There is a plus here-- it should be cold enough for precipitation to fall as all snow in the Wasatch Front Valleys. Here's what the 500 mb trough looks like Friday morning:
...And moisture? Below is a time-height forecast at SLC of the moisture in the vertical, through time, reading from left to right. Deeper blues or purples show high relative humidity values, a measure of saturation in the atmosphere. Browns/lightest greens are drier air. Notice the deeper blue colors start off at 700 mb which is roughly 4-5k above ground level at SLC, and drier near the surface. There could be some flurries or very light snow prior to 5 AM (12z) Friday, but not yet significant. After 5 AM, the brighter colors reach more toward the surface indicating the low levels are moistening up. Expect snow to pick up across the Salt Lake Valley at that time! By late Friday morning, moisture decreases and so will the snow, becoming dry by the afternoon.
This is a quick moving storm and with limited moisture. But, I do expect snowfall along the entire I-15 metro corridor from Nephi to the Idaho border. Remember, the front is coming from the north, so Logan to Ogden will see accumulating snowfall 2-3 hours earlier than the Salt Lake Valley. From Utah county southward (Provo to Nephi), expect snowfall to start 1-2 hours later. So, we all want to know how much snow will accumulate right? Here is the NBM (model blend) forecast for expected snow and I think it's reasonable:
It shows 1-2" for the I-15 metro corridor, including the Salt Lake Valley, which is very reasonable. The benches could see 3" (light blues). Higher mountains and ski areas, up to 6". This is not a blockbuster storm, but it is a stagnation-busting storm. It will clear out valley inversions and pollution. Better yet, it will provide a little fresh powder for the ski resorts-- you're welcome skiers!
Expect snowfall during the Friday morning commute. It could get a bit messy but probably not high impact overall. Temperatures in the lower valleys should be in the low 30s, so roads will be more slushy than icy. Conditions will still warrant driving with caution. Roadways near the benches and in the canyons could be especially snow covered and slick Friday morning-- be prepared! Check the most recent/updated forecasts as we approach Friday weather.gov/SLC
So, can we expect more of the white stuff anytime soon? For at least the next week (except Friday), probably not much. However, the larger-scale pattern change I discussed in previous posts still looks very possible-- great news! I have closely tracked the longer range models for a couple weeks now. They have waffled a lot from day to day, but still generally show the long wave trough currently over the Eastern U.S. retrograding to the West toward the end of this month. Remember, a long wave trough is a region where there tends to be active wintry weather, not indicative of any specific storm. I'm sure most of you have seen pictures on TV of the snow and ice the past couple weeks affecting the Eastern states. Blame the longe wave trough!
So, what does the long wave pattern look like for the next several days? Here are the 500 mb height anomalies showing the long wave trough will still reside over the Eastern states. It does not mean we will not see a storm in the West. In fact, we expect a weak one to reach Utah on Friday. The main storm track is simply, on average, over the East.
Blue colors above indicate lower height anomalies (trough), so expect more winter weather in the Eastern U.S. this next week. The ridge anomaly is just off the U.S and B.C. coasts, too close for large or successive storms. But this is expected to change. Take a look at the long wave pattern toward the end of the month:
Notice a difference? The trough anomaly back East is weaker, hence the light blues. More importantly, the high pressure anomalies have shifted further westward and extend north into Southwest Alaska. That's why I refer to this as a retrograding long wave pattern. The large weather influencers are shifting westward against the prevailing flow of individual weather systems that move with an eastward component. By the last few days of January, the 500 mb lines on the map are northwesterly oriented and the ridge is further west which is conducive for a storm track into the Great Basin, including Utah. It gets better! Here are the low height (trough) anomalies for the first week of February:
This is a great long wave set up for wintry weather in the West. The ridge is amplified and far enough offshore to drive colder air south from Western Canada. Storms moving into the Pacific Northwest would reach Utah, and with good moisture content. Significant mountain and even valley snow is possible, but those details will have to wait until we get much closer in time. Much of the Western U.S. would also experience unsettled wintry weather if this pattern develops. Again, we're not looking at any specific storm this far out, just the pattern. It is entirely possible to have a long wave trough over the West, under a poor flow regime, and not get strong storms here in Utah. So, stay tuned!
Here's one last graphic, showing the Euro model ensemble forecast (snow amount at SLC) that extends into the first couple days of February. The ensemble is 50 "tweaked" projections of the same model all at once to account for possible errors and outcomes. Pay attention to the green line (average of all runs) at the bottom, reading left to right. You can see the average green line shows less than 2" at SLC Friday morning. There's a hint of a very weak system around next Tuesday but even less significant than the Friday system. Check out the green line moving up the last couple days of January into the first few days of February. Don't pay attention to the actual snow amounts that far out, there is no skill! It does imply the average of the 50 runs show storm activity picking up over our area. Given that January will likely end with below average precipitation across Utah, let's hope for a more wintry start to February!
