Sounds Like A Broken Record

Warm, breezy winds out of the south, and high status clouds on deck... The PNW continues to get hammered with atmospheric rivers (AR's), with another decaying AR on tap for northern Utah today through the first half of this weekend (12/19-12/20). This feels like a broken record. Decaying AR's seem to be the new normal this season, and they unfortunately don't elicit the same weather excitement as full scale AR's. 

Sometimes, these "partial" AR's end up being more of a nuisance rather than anything satisfying. You get almost none of the benefits of an AR - large precipitation totals - and instead get all the disbenefits, including warm temperatures and high winds. In addition, because there is never a strong push of cold air, snow levels remain high, snow water ratios remain low, and snow pack in some areas has the opportunity to melt out. It's all a bit frustrating. 

ECMWF 00Z 12/19/2025 four-panel forecast 12/19-12/22/2025: (Top left to bottom right) MSLP/precip and 1000-500 mb thickness; 300 mb gph and winds; ; 500 mb gph, vorticity, and winds; 700 mb gph, RH, temp, and winds

The best opportunity for precipitation across the Wasatch Front looks to arrive Friday night through Saturday morning as the best dynamics and moisture arrives. Some scattered shower activity, mainly focused over the mountains, is possible through Saturday afternoon as conditions become more favorable for terrain driven upslope. Cottonwoods could get a little extra during that time. 

Total Snow (10:1 in) 12/19-12/20/2025 Left: ECMWF 12Z 12/19/2025; Right: GFS 12Z 12/19/2025

Snow and liquid accumulations don't look particularly impressive, with quite a bit of uncertainty and a relatively wide range in lower and higher end totals. For the Cottonwoods, there is the potential for anywhere between 4-10" of snow (~0.5 - 1" liquid) through the storm event (Friday-Saturday afternoon). Note, that most all of the snow accumulations will be above 7500-8000' and the snow that falls in the higher elevations will be wetter and dense. 

GFS 12Z 12/19/2025 Derived Alta, UT Snow Accumulation (in) 12/19-12/26/2025 (credit: UoU Atmo Sciences)

ECMWF ENS 00Z 12/19/2025 Total Snow (10:1 in)

The bombardment of AR's along the PNW and California coast looks to continue through the late weekend into the holiday week. This will provide an opportunity for a few additional shots of moisture to graze Utah Sunday through Tuesday of next week. However, once again, these will be 'warm' storms, with high snow levels and strong gusty winds at higher elevations. There are hints that this pattern will change somewhat going into Christmas Eve and Christmas and offer Utah a more robust storm, but too far out in forecasting fairy tale land to say for certain. In the meantime, we will have to make do...

  

Some Records

A bit calmer and cooler today than 24 hours ago. The storm that rolled through yesterday morning through the afternoon was punchy and quick, but still managed to throw out some significant and noteworthy weather numbers. Strong wind gusts 70+ mph were recorded at many sites across northern Utah as the main energy/front passed over early afternoon. Top of Snowbasin reported a gust of 94 mph and a gust of 77 mph was observed at the Alta Guard House. 

The high winds aided in vigorous mixing and possibly some down slope warming off the mountains, which produced record high temperatures at many locations across the Wasatch Front for the date. 

If we look at soundings from KSLC from the morning and afternoon of 12/17, we can identify the valley was inverted in the morning, with cooler air pooled near the valley surface and warmer air overlying. The late afternoon sounding reveals the temperature inversion and cooler air in the valley had been eroded away in the afternoon by the strong winds. Note the afternoon KSLC sounding was observed well after the record high temps (67 F ~12:50 PM), but the sounding still exhibits how the temperature profile evolved with the storm.

So how did the winds create abnormally warm temps? The winds mixed warmer air from above the valley surface down to the ground. As the air was forced towards the surface, it compressed and warmed at the adiabatic lapse rate (9.8C/km). For example, when the 10C air at 800 mb (~1 km AGL) was mixed down to the valley floor it had the potential to warm to 20C (68F). This is how we observed record temperatures during the early afternoon hours from the storm yesterday. 

Unfortunately, no records or even significant numbers related to snowfall were observed. Only a few inches of wet snow/graupel were reported at Alta. All said and done, Alta observed a total snowfall of 3" with a total liquid of 1.06" over the course of the storm. This probably did not all fall as frozen precipitation as temps were warm enough for rain early on 12/17. Either way, I would say the snow that fell is dense or mostly graupel/rimed snow..

Something

Breezy and relatively warm in the Salt Lake Valley this morning. Temperatures actually bottomed out near midnight last night before clouds moved over and winds kicked up out of the south. A low of 36 F was recorded at KSLC just after 12AM 12/17/2025 before winds increased and temperatures rose by about 12 degrees by 6AM. 

Observations from KSLC 12/16-12/17/2025 Top: Temperature, RH, and dew pt; Bottom: wind speed, gust, and direction

Typically, nights are coldest when skies are clear and winds are calm. The lack of winds and clouds allows the most efficient conditions for nighttime radiative cooling, where the Earth's surface loses heat by emitting  thermal infrared energy (longwave radiation) upward into space. This allows the surface to cool faster than the air directly above it, creating a nocturnal temperature inversion. 

Nighttime temps with calm winds and clear skies (Credit: NWS)

On nights when there is sufficient wind, the warmer air from above the surface and cooler air near the surface are mixed, which has the net affect of warming the air near the surface. This process is similar to a fan in a room mixing warmer air near the ceiling down to the ground. Clouds act as another layer to inhibit nighttime cooling due to the fact that clouds radiate infrared energy towards the surface.

Nighttime surfaces temps with breezy winds and clear skies (Credit: NWS)

Today 12/17/2025

A decaying atmospheric river currently impacting northern Utah, bringing strong winds and precipitation through the afternoon hours. The main upper-level low associated with this storm is centered across the northern and Canadian Rockies as evidenced in the mid-level water vapor imagery below. 

GOES-18 West 12/17/2025

A strong jet maximum and coincident tight surface pressure gradient has already brought strong southerly to westerly winds this afternoon, with gusts 60-70+ mph observed, particularly across the West Desert. Winds were strong enough to create a brief window of blowing dust that swept across the SL Valley prior to the arrival of a band of moderate to heavy rainfall. 

Blowing dust moving across downtown SLC 12/17/2025. Camera image is looking west across downtown. 

Mosaic radar loop 1830-2150Z

The heaviest and most consistent showers have already come to an end in the SL Valley, but will continue over course of the afternoon in the mountains. A few sloppy inches of snow are possible above 8000', with less possible at the tail end of the storm between 7000-8000' as cooler air moves in and snow levels drop. Not much...

Another possible system this weekend. But once again, it looks to be a 'warm' storm and any substantial precipitation amounts may be hit or miss across northern Utah.

Ask Santa for Snow

This fall/winter has yet to get truly cold, so the past few abnormally warm December days have felt par for the course. Nevertheless, it's still very odd to be having March weather in December. Salt Lake City set/tied a daily high record of 62 F yesterday afternoon (12/11). The normal high for the date is 40 F, so 22 degrees above normal is rather significant. 

The relatively mild and dry conditions will continue the next 3-4 days as a ridge continues to build over Utah. A weak and dry disturbance will pass over the state the second half of this weekend, which may knock temps down a few degrees from Friday or Saturday, but otherwise conditions will be quiet. 

The next opportunity for some actual weather across Utah looks to be the middle of next week. However, there's quite a bit of uncertainty and spread in the potential scenarios playing out. Ensembles are all over the place, with a large range in precipitation amounts next week. 

ECMWF ENS 12Z 12/12/2025 KSLC Total Precipitation

GFS ENS 12Z 12/12/2025 KSLC Total Precipitation

In the meantime, areas in the far NW part of the U.S. will continue to receive heavy rainfall and high mountain snow through the weekend and early next week.

The PNW

While Utah sits high and dry the PNW - specifically, Washington state -, will continue to get deluged with rain and snow. Many locations are already under flood risk as streams and rivers crest above flood stage. In the past week, areas along the Washington Cascades have received 10-20"+ of liquid. For reference, SLC receives about 16" total liquid in a year, so parts of Washington have gotten more precipitation in a week than SLC does in a year. 

NRCS Site 7-day Precipitation Total (in)

PRISM 10-day Estimated Precipitation Total (in)

Multiple additional rounds of atmospheric river moisture and energy will impact the PNW the next 5-7 days. Accompanying warmer temps will let most of the precip fall as rain, with snow levels staying high. This could allow for more runoff than would typically occur when temperatures are colder. Another 5"+ inches of liquid are forecast for the region. 

ECMWF 12Z 12/12/2025 four-panel forecast 12/12-12/18/2025: (Top left to bottom right) MSLP/precip and 1000-500 mb thickness; 300 mb gph and winds; ; 500 mb gph, vorticity, and winds; 700 mb gph, RH, temp, and winds

ECMWF 12Z 12/12/2025 Total Precipitation (in) 12/12-12/18/2025

Low Tide

High cirrostratus clouds with a few lenticular-ish and mountain wave clouds out there this morning. The mountain wave and lenticular clouds are a sign that moderate to strong upper-level winds are interacting with the terrain in a similar manner as water in a river flows over boulders. 

Lenticular-ish clouds over the Wasatch 

If we check out the sounding from KSLC this morning, we can find strong winds 30+kts above 650 mb coincident with a moist stable layer - temperature profile is isothermal or inverted and temp/dew pt are close. Mountain waves and lenticular clouds form when air is forced upward, cools, and condensates within these types of stratified stable layers. Mountain wave clouds can be identified in satellite imagery by the ripples in the cloud field. 

GOES-18 West 12/10/2025

Left: KSLC 12Z 12/9/2025 sounding; Right: Simple schematic of how mountain wave clouds form (credit: theweatherprediction Habyhints)

When stable air is forced upward - due to underlying terrain - it becomes less buoyant than its environment. Less buoyant air will naturally accelerate in the negative vertical direction in an attempt to reach it's original equilibrium point. However, due to its downward momentum, it will initially pass its equilibrium point before correcting itself and accelerating back in the upward direction. The air mass will continue to oscillate in this upward/downward manner until it attains equilibrium. Conceptually, this phenomena is similar to how a spring oscillates when a force is enacted on it.

Short Range

Besides the interesting clouds over the Wasatch, not much is going on weather-wise today besides the abnormally warm temperatures. Highs in the northern Utah valleys will top out in the mid to upper 50s on Wednesday which is more like March than December. We can thank the placement of an upper-level ridge centered just off the U.S. Pacific coast for the warmth and relatively quiescent weather pattern. 

12Z 12/10/2025 analyzed 500 mb heights and temp with overlays

The upper-level ridge is forecasted to be stubborn and somewhat stationary over the next 5-7 days, shunting the storm track with the most energy and moisture around the northern periphery of the ridge and leaving areas to the south dry and warm. Currently, Utah sits on the downstream edge of this ridge but will become more firmly under it's grasp later this week into the weekend. This means moisture will be directed north of the state for the next few days to a week at least. 

Who is the winner with this setup? The PNW and northern Rockies win out in this scenario. They will be in the line of fire through the middle of December, with rounds of storms expected. 

ECMWF ENS 12Z 12/10/2025 Left: 7-day 500 mb height anomalies (m); Right: 7-day precipitation anomalies (in)

Long Range 

The bottom line?… the tap will be shut off for Utah the next 5-7 days, with warm and dry conditions expected. It may even get warm enough that daytime snow making becomes an issue at some of the ski resorts. This forecast is unfortunate, considering people just started feeling like the season was finally starting after this past weekend’s storm. 

ECMWF ENS 12Z 12/10/2025 forecasted Pacific North American Oscillation

The models are all over the place in terms of timing and intensity of the next potential storm that will hit the state. However, there are hints that the ridge will flatten or transition into a trough across western CONUS late next week or at least the later half of this month. Looking at the forecasted PNA (above), most models are suggesting that troughing across the western U.S. is likely after mid-December (negative PNA values). This would mean the Pacific would open up and a more active and wet weather pattern would be in store for Utah. I would take this information with a heavy spoonful of salt as models have been having a very difficult time beyond 3-5 days...

The Weekend Update

A soggy cold morning with some light to moderate snow showers. Temps aren't all that cold in the Salt Lake Valley, ranging from 32-36F across locations. Looks like temps remained a bit too warm and snowfall rates at bit too light for any meaningful accumulations on the roadways this morning. I think some minor slush or a skiff occurred up along the benches, but otherwise no significant valley impacts. 

View from Alta, UT base 12/5/2025

The mountains are fairing a bit better in terms of snow, with 2-3" already reported at the ski resorts. Moist northwesterly flow continues today through tomorrow, which will supply the fuel for ample orographic lift along the Wasatch. These are the conditions when the Wasatch can live up to their reputation. 

Will the Wasatch Whale provide?

The forecast for the storm today - Friday 12/5/2025 - through the first half of this weekend remains on track from the previous discussion I posted. 

1. An atmospheric river (AR), arching over an upper-level ridge located across the eastern Pacific, will continue to feed moisture and energy from the NW into Utah Friday through Saturday afternoon. 

   

   Analyzed 700mb heights, temps, winds, and RH 12/5/2025 @ 1600Z

2. A relatively strong jet streak, associated with the AR, with setup across northern Utah providing energy and lift for more enhanced precipitation. 

   

   Analyzed 300mb heights, temps, and winds12/5/2025 @ 1600Z

3. Warm air advection mid Friday through Saturday morning will bring warmer temperatures into the state, lifting snow levels from the valley floors to around 7000-7500' by Friday night/early Saturday morning. 

   

   Alta, UT - Top: Wet bulb zero level (proxy for snow level); Bottom: Snow ratio (density)  (source: UoU Atmo Sciences)
   

4. Valleys will see snow change over to rain Friday afternoon and continue into Saturday.
5. The heaviest snowfall rates in the mountains will occur late Friday through Saturday morning as the main surge of moisture and energy make across northern Utah. 

   

   ECMWF 00Z 12/5/2025 6-hr snow accumulation

   
   
6. Mountain locations could see 1-2.5" of SWE and 12-30" of snow. Potentially localized areas of 30"+ in favored locations such as the Cottonwoods. 

   

   Total Snow 10:1 (in) 12/5-12/8/2025 - Left: ECMWF 00Z 12/5/2025; Right: GFS 12Z 12/5/2025
   

   
   
7. Cooler air will fill back into the region at the tail end of the storm Saturday morning through the afternoon, but by this point precipitation coverage and rates will already begin to trend downwards. 
8. Snow showers in the mountains look to end by Saturday evening/night

That's pretty much this weekends storm in a nutshell. Should provide part of an answer to Utahn's prayers for snow. Nevertheless, even with the relatively significant snow and liquid totals in the mountains over the next 24hrs, this storm is still not this season's savior. Even if the Wasatch manages to squeeze out the top end storm totals - 2-2.5" SWE or 20-30" of snow -, this will not make up for the unprecedented lack of snow thus far. SNOTEL sites were still reporting the lowest snowpack to date on record for many sites in the Wasatch as of yesterday. There is still a long way to dig us out of - or rather fill in - the current state of the snowpack in northern Utah. 

Snowbird SNOTEL Dec. 4th Snow Water Equivalent (SWE) 1989-2025. Red star represents the SWE for 2025

I'm sure the powder fever is going to hit people hard this weekend, with many already shaking and foaming with excitement. I'll bet on the canyons becoming a mess and drivers shrugging off patience and decency in an attempt to get up to the ski resorts as fast as possible. I hope I am wrong and this weekend goes without a hitch. Some may even be of the impression that this is finally the light at the end of the tunnel, but unfortunately I consider this storm to only be a band aid on the open wound that is the Wasatch snowpack. A number of storms of this caliber are needed to get us to "normal", and unfortunately the long-term doesn't look so optimistic but that is always open to change. In the meantime, keep on praying hard or ask Santa for snow this Christmas... 

UDOT sign Big Cottonwood Canyon 2025

An Actual Storm??

Light flurries and low clouds out there this morning. Compliments of a shortwave trough moving south and east out of the state. Areas on the east side of the Salt Lake Valley have managed to get a skiff of snow earlier this morning, while other locations saw no accumulations. The Wasatch mountains picked up a few inches of snow over the course of this storm. Ski resorts like Alta, UT managed to squeeze out 3-4". Not nothing, but that amount would be judged as light relative to most storms. 

Left: Alta snow stake; Right: Snowbird snow stake 12/3/2025 

Residual moisture and light northerly to northwesterly flow continues to bring some light snow showers in the mountains of northern Utah and some areas in the central and southern parts of the state. However, conditions should improve and snow showers should gradually taper off across locations as the trough and attendant moisture exists the state over the course of the day. But can't rule out some pop up snow showers in the mountains with diurnal heating this afternoon.

GOES-18 West True Color Imagery 12/3/2025

A brief respite in the weather is expected Wednesday afternoon into Thursday as a transient shortwave ridge brings some subsidence and drier conditions. But, this break in the weather is short-lived. Things start to get interesting again late Thursday into Friday morning as Utah remains firmly within an upper-level northwesterly flow regime. Starting late Thursday, a slug of moisture and energy - associated with an atmospheric river (AR) making it's way inland from the PNW - is expected to descend NW to SE into Utah. Depending on which model you look at (GFS or ECMWF), snow showers arrive across the northern Wasatch Front sometime between late Thursday night into Friday morning and continue consistently or with a few breaks through Saturday. Heaviest snowfall rates in the mountains look to occur late Friday through Saturday afternoon.

ECMWF 00Z 12/3/2025 four-panel forecast 12/3-12/8/2025: (Top left to bottom right) MSLP/precip and 1000-500 mb thickness; 300 mb gph and winds; ; 500 mb gph, vorticity, and winds; 700 mb gph, RH, temp, and winds

Generally speaking, deterministic models have been struggling all week with how far south this storm will dig into Utah, but have now reached more of a consensus that this storm will bring a robust push of moisture with an accompanying jet streak into central to northern Utah the first half of this weekend. Additionally, jet streak dynamics look to be more optimal than previous model runs, with the left front exit region (LF) of the jet setting up across northern Utah Friday through Saturday. The LF region of a jet is typically associated with enhanced large scale ascent/lift due to the balance of forces and mass continuity in the atmosphere.   

Total Precipitation (in) 12/3-12/7/2025 - Left: ECMWF 00Z 12/3/2025; Right: GFS 00Z 12/3/2025

Total Snow 10:1 (in) 12/3-12/7/2025 - Left: ECMWF 00Z 12/3/2025; Right: GFS 00Z 12/3/2025

Timing and precipitation amounts with this weekends storms will still likely vary from current model runs, but I think there is more confidence this will be a substantial snow event for the northern Utah mountains this weekend. Looks like ensembles are calling for at least 1-2" of liquid or 10-20" of snow in favored areas of the Wasatch (i.e. Alta). 

Total Snow 10:1 (in) Alta, UT 12/3-12/18/2025 - Left: ECMWF ENS 12Z 12/3/2025; Right: GFS ENS 12Z 12/3/2025

This will be a warmer storm as it's origins are from the Pacific, so snow levels will initially start low at valley levels on Friday morning before gradually rising late Friday into Saturday to around 7000-7500' as warm air advection brings slightly "warmer" temps across the state. A resurgence of cooler air late Saturday closes out the storm, bringing snow levels back down to the valleys for any remaining showers. In terms of snow characteristics, snow density will see-saw similar to snow levels. 

Alta, UT GFS derived snow density (source: UoU Atmo Sciences)

Snow will likely start out as lighter/less dense early Friday then become heavier/more dense with the arrival of warmer temps late Friday into Saturday. A bit of lighter snow could end out the storm late Saturday as cooler temps return. This means the storm will likely produce a stratified snowpack with lighter-denser-lighter layering. Could loosely be described as having "upside down" (not related to Stranger Things) characteristics. 

Beyond this weekend models diverge quite a bit, with the GFS keeping things active through next week and weekend and the ECMWF keeping things a bit quieter. We'll have to see how things shape up after this weekends storm.