AR Flavor

KSLC 12Z 10/22/2025 sounding skew-t: temp (red), dew pt. (green), and winds

Not a cloud in sight out there earlier this morning, but a very light haze does seem to have developed over the Salt Lake Valley. If one is thinking it will be repeat of yesterday's conditions for the northern Utah valleys - sunny with a high in the low 60s - then they would be mostly correct. The light haze can be blamed on the marginal cold air pooling and ridging over Utah. 

At this time of year, strong radiational cooling overnight really takes off on clear calm nights. This creates a near surface nocturnal temperature inversion that is difficult to erode with weaker solar heating due to the lower sun angle. When you add a subsidence inversion due to ridging/high pressure on top of that, it easily creates stagnate conditions where pollutants can build up. A quick look at the sounding obs from KSLC this morning (10/22/2025) reveals an inversion over the SLV from the surface to about 775 mb (~1000 m deep). The weather today will be close to yesterday's, with a slight variation due to a weak system progressing into the region. 

GOES East True Color 10/22/2025 with 500 mb Rap analysis overlaid

Satellite imagery this morning shows a closed upper-level low rotates just off the So Cal coast, with Utah lying downstream of the upper-level trough axis. Not much cold air or moisture is associated with this system, but the low will shift onshore through the day, spreading clouds northward into Utah. Clouds will increase over the course of the day for northern portions of the state. As the system translates inland across the Mojave Desert a few scattered elevated showers/thunderstorms are possible for the southern 2/3rd's of the state going into the afternoon. Weak upper-level forcing and limited moisture will largely hamper any organized shower or storm development, however, some instability due to surface heating this afternoon will aid in forming some disorganized precipitation. Shower activity will focus across the higher terrain. 

ECMWF 00Z 10/22/2025 forecasted precipitation for 00Z 10/23/2025 (1800 MDT)

The closed low continues to translate across the southern tier of the state tonight through Thursday, spreading more low to mid-level moisture into the state. A few isolated showers may make their way as far north as the Wasatch Front by Wednesday night into early Thursday morning. However, the focus of shower/storm activity will remain over the southern half of the state. The northern valley areas will likely just remain dry. Isolated showers/storms, focused over the higher terrain and SE Utah, continue Thursday before the low exits the state by Friday morning. 

The Weekend

Going into the weekend it looks like the weather gets a bit more interesting. After the passage of the closed low, a short-wave ridge quickly migrates over Utah, bringing southerly flow and warm air advection Friday through Saturday. In effect, this will create for some nice sunny and relatively warm fall days, with some breezy southerly winds come Saturday. 

Model forecasted surface temperature (F) for the SLC

The short-wave ridge is 'short' lived though, with a broad Pacific trough proceeding closely behind it. The trough looks to be associated with an Atmospheric River (AR), which is defined as a concentrated or focused region of atmospheric moisture transport from the lower to higher latitudes. AR events are difficult to forecast due to their complex nature and lack of observations over the oceans - where AR's originate. 

ECMWF 00Z 10/22/2025 four-panel forecast for 10/24-10/28/2025: (Top left to bottom right) MSLP/precip and 1000-500 mb thickness;700 mb gph, RH, temp, and winds; 300 mb gph and winds; 500 mb gph, vorticity, and winds

I think going into detail about how this AR is forecasted to impact Utah this weekend would be unwise at this point. But I will attempt to outline the potential timing and impacts from this system. Models have been trending on a solution that brings moisture and energy from the AR/trough across northern Utah sometime Saturday night into Sunday morning. This means a cold front along with precipitation will likely pass over the SLV the second half of this weekend. 

How much moisture makes it into the state is highly suspect and will change with each new forecast model run, so I would not get hung up on amounts at this point. The bulk of the moisture looks to miss Utah as of now. Like I mentioned, forecasting AR events are difficult. It's analogous to determining the movement of an unattended fire hose spraying water. Modeled integrated vapor transport shows the main region of moisture transport associated with the AR. It seems that due to terrain and trough characteristics the moisture field breaks up some upon landfall, with a more dispersed and weakened moisture field making it's way into Utah late Saturday into Sunday. 

ECMWF 00Z 10/22/2025 forecasted integrated vapor transport (IVT) through this weekend

Some rain likely for the valleys and snow for the mountains. Snow levels could potentially be pretty low (~5500-6000'), depending how how the trough tracks across the state. Temperatures will take a quick dip Sunday, with highs in the SLV 20-25 degrees cooler than Saturday only maxing out near 50F. A few additional bouts of energy and moisture associated with the AR are possible through early next week, keeping cooler air in place and bringing periods of showers into the state. However, there is a high probability of details changing during this period so there will likely be updates to this. Anyway, a period of unsettled and cooler weather is possible the second half of this weekend into early next week. Keep the liquid coming. Utah needs it...