What Causes "Flat" Light

The quality of light strongly affects one's confidence as a skier.  On a sunny day, most of the solar energy (about 85% of it when the sun is high in the sky) is direct, meaning that it is traveling along a straight line from the sun to the Earth's surface.  This results in large brightness contrasts between directly illuminated surfaces and those in shadows.  Even subtle textures in the surface of the snow are easily seen.  

Descending a groomed run at Ischgl, Austria on a sunny day dominated by "direct" solar energy.

All else being equal, it is much easier to ski on such days.  It is easier to see the steepness of the terrain, variations in the snow surface, and subtle changes in the snow conditions.  

However, on day with thick overcast, even high overcast, the light can be "flat." Flat light is a colloquialism for situation in which there are no shadows, as was was the case at times at Alta today (Sunday).  

Flat light at Alta on Sunday, March 30th

Flat light is produced when most of the light from the sun isn't direct but is instead diffuse.  When skies are covered by thick clouds, the sunlight is not direct because it has been scattered by the liquid water and ice particles in the clouds.  As a result, the Earth's surface receives sunlight from all parts of the sky, resulting in a lack of shadows.  This is particularly problematic for seeing contrasts in a white surface like snow.  

High clouds sometimes vary in depth, so on a day like today, there can be variations in the flatness of the light.  Below is a photo I took looking up upper Sleepy Hollow off the Supreme Chair at 11:07 AM.  At that time, the sun was only partially obscured and there was enough direct radiation for the trees to cast shadows and for one to see some texture in the snow, as evident in the foreground of the picture.  

On our next run, at 11:33 AM, the sun was nearly obscured and there was considerably less direct radiation.  Shadows were less apparent and the texture and variability of the snow were harder to see.  

Such conditions though are not anywhere near as bad as they can get.  Add fog and one can have a hard time telling up from down.  Balance becomes difficult and one can even suffer from vertigo.  

Goggles can help some.  Color tints with higher light transmission that enhance contrast are usually best on flat light days.  Skiing closer to trees and other darker objects can also help.  

One of Bruce Tremper's ten commandments of avalanche safety is "thou shalt never go first."  This commandment is especially important on powder days with flat light, as a set of tracks helps to provide some contrast in an otherwise featureless landscape.  Be generous to your friends on these days and let them have first tracks.

Dusty Spring Morning

With yesterday's strong south winds, dust moved into Salt Lake Valley and environs yesterday and lingers this morning with visibility somewhat reduced as one looks to the central Wasatch from the University of Utah.

Source: https://home.chpc.utah.edu/~u0790486/wxinfo/cgi-bin/uunet_camera_explorer.cgi

Observations from the University of Utah show strong south winds yesterday afternoon and evening with PM2.5 levels increasing after about 1600 MDT from 3 to 8 ug/m3.  After settling some through 0200 MDT, they then climbed again to 13 before settling some this morning.  

These numbers are not high and are at good to low-moderate air quality levels, but nevertheless, there's plenty of dust in the air.  

Give the overall flow yesterday, the dust in the Salt Lake Valley was likely from sources to the south and the southwest given the flow direction and not from the exposed Great Salt Lake bed.  The flow direction was such that if there was dust emitted from the Great Salt Lake bed, especially Farmington Bay, it would have been transported to the northern Wasatch Front.  I'm not sure if there could be a some dust from the lake bed mixed in now given the shift to northwest flow last night but I suspect most of this is still from origins to the south and southwest. 

Dry Spring -> More Typical Spring

The work week has proven spectacular with mostly sunny skies and mild temperatures.  "Anything season" is here, meaning that you can do pretty much any type of recreation you desire, sometimes on the same day.  The lower-elevation trails have dried out now in many areas and are quite passable for hikers and bikers. Meanwhile, we still have a deep snowpack at upper elevations.  

Today's sunrise from the Avenues foothills.

We will, however, be transitioning from the dry spring pattern we've experienced the last few days to a pattern that will bring the occasional mild spring storm system to northern Utah.  This transition won't necessarily be abrupt as a couple of weak systems move through Friday and Saturday.  After that, there's a series of troughs that move through.  It's not easy for me to summarize my expectations for the timing and strength of these storms as there's a good deal of variability in the Utah snow ensemble.  Just look at the spread at the end of the 10-day period.  

The best way to summarize this is perhaps simply to say changeable, with the occasional mountain snowstorm interspersed with breaks.  It's a time for adaptation to what Mother Nature brings rather than having strong expectations for what's to come.  Fortunately, spring in northern Utah means a plethora of options.  

Downtime for weather.utah.edu

Due to utility work at the University of Utah data center, weather.utah.edu is expected to be down for approximately two days.  Don't shoot the messenger. 

In the meantime, enjoy the sunny, warm March weather and stop worrying about the next storm.  The forecasts looked like spaghetti anyway, so who the hell knows what's going to happen.  

In the meantime, for your entertainment purposes, there were some beautiful lenticular clouds over the central Wasatch yesterday.  

Such clouds are produced by mountain waves, up and down motions produced by the interaction of the atmospheric flow with the topography.  The clouds form where the flow is forced up and dissipate where the flow is forced down.  The layering is produced by vertical variations in relative humidity, which causes saturation in areas of ascent to be reached at different levels of vertical displacement.  

Lenticular clouds have been confused with flying saucers.  Let's not let this start a social media conspiracy theory.  

Avenues Avalanche!

Let's have some fun with a post based on what I'll call "sidewalk science."  

The Hall of Fame Baseball catcher Yogi Berra once said "you can observe a lot just by watching."  This applies for sure to both meteorology and snow science.

This afternoon, right near my house, I encountered an avalanche in the front yard of a house on 16th Avenue in the upper Avenues.

Oh, I didn't actually see the avalanche happen, but I did see the aftermath, which I've summarized in the below.  It contains many of the hallmark characteristics of an avalanche.  The first is the starting zone, which in this case I measured to be 35°.  Most avalanches start on slopes between 30 and 45 degrees, so the slope here was right in that range.

There is also the crown or crown line, which is the upper fracture line of the slab that broke away to create the avalanche, as well as the flank or fracture line along the side of the avalanche.  There is one on the other side, although I have not labeled it.   

Most avalanches that kill people are slab avalanches in which a cohesive layer of snow slides down the slope.  Indeed that was the case here with the slab sliding about 20 inches or 50 cm down the slope as a cohesive layer.  In this case, the length and speed of the avalanche was such that the slab remained intact.  However, eventually it encountered the lower angle sidewalk where the slab decelerated, debris built up, and the toe of the avalanche was found.  

The bed surface for this avalanche was the grass rather than a weak layer in the snow.  The trigger was likely solar heating, which led to a lubrication of the grass by liquid water as the snow melted, weakening the bond between the snow and the grass.  Such an avalanche is referred to as a glide avalanche, which is defined as a release of snowcover as a result of gliding over the ground.  

So much snow science in such a small space!  However, there are real-world applications.  Glide avalanches can be deadly and destructive.  In the Wasatch Range, they are probably most common in Broads Fork, a subdrainage of Big Cottonwood Canyon where glide avalanches commonly occur on steeply sloped rock slabs.  Here's a link to a report about one in Broads Fork: https://utahavalanchecenter.org/avalanche/69596. Note the fully exposed rocks in the starting zone of the avalanche.  

Glide avalanches are hard to predict, but can have deadly consequences.  They are also common in the Alps where the mountains are not only steep, but there are a lot of grassy, high altitude meadows that are grazed in the summer.  The resulting steep, grass slopes can make the perfect bed for an avalanche.  I saw the aftermath of many glide avalanches when I lived in Innsbruck, Austria.  Below is a photo looking down at one during a ski tour on the Gammerspitz, a 2500 meter peak in the Alps south of Innsbruck.  It was on a south facing slope and likely was triggered by solar heating like the avalanche I saw today in the Avenues.  We avoided slopes like this on our tour! 

I found the number of glide avalanches and the expansive coverage of glide cracks in the Alps to be very disconcerting and often made conservative terrain choices because of them.  Fortunately it was easy to avoid avalanche terrain today in the Avenues.

It Shall Get Warm

It's sort of weird after a winter with little valley snow to look out the window and see what might be the deepest snowpack of the season in my yard.  Admittedly, "deepest" in the valley this year is a low bar, but it will be good for the gardens eventually.  It was a wonderful storm for skiing too.

I can summarize the forecast for the next several days in two words: warming trend. Oh, today will be a cloudy "meh" day and Friday night and Saturday we'll have a trough passage that could bring a bit of snow, but after that a big ridge builds into the west.  Below is the GFS forecast valid 0000 UTC 27 March (6 PM Wednesday) showing the high-amplitude ridge parked just upstream of Utah.  

The NWS National Blend of Models maximum temperature forecast for Salt Lake City shows the warming trend nicely after Saturday, with temps climbing through Thursday.  

The median maximum temperature forecast for Thursday afternoon is 81°F, which would be the earliest 81 on record.  I thought this NBM forecast seemed a little jacked, but mid 70s seems doable.  

Bottom line: It shall get warm.

Another Spring Cold Front

The weather yo-yo of spring continues today and tonight with the approach and passage of a spring cold front.  

The 6Z GFS shows the cold front moving across northern Utah tonight at 0600 UTC 18 March (0000 MST Tuesday). 

Ahead of the front, today will be mild and windy with the possibility of blowing dust.  The prefrontal environment today looks to be mainly dry, although I wouldn't be surprised to see a spritz or two of valley rain or mountain snow.  Precipitation is not expected for the evening commute.  

The front drags through tonight.  Onset time, duration, and accumulations for the valley vary some from model to model.  For brevity, I'll focus here on the 1200 UTC initialized HRRR.  The time-series below is for the Salt Lake City International Airport.  Precipitation onset is just before 11 PM local time, but the turnover to snow doesn't occur until after 2 AM.  I'm not sure if I'd count on that timing given the uncertainties at play, but expect precipitation to move in tonight and at some point to turn over to snow at all elevations.  

The HRRR produces a bit under 3 inches of snow at the Airport by 8 AM tomorrow morning.  Bench accumulations would likely be greater.  Below is the NWS infographic for this event showing the expectation for 1-3" for the Salt Lake Valley floor and 2-6" for the benches.  

NWS infographic downloaded 9:30 MDT 17 March

For the mountains, I suspect Tuesday will be a pretty good powder day, but you didn't hear that for me.  

Winter Begins (LOL)

We really haven't had much of a winter this year in the Salt Lake Valley.  Snow has been scant (only 12.7" through yesterday, the 3rd lowest on record through that date) and temperatures mild (7th warmest mean temperature on record through yesterday). 

It's been sunny and mild the past few days, so it definitely feels like spring.  Things will change late tomorrow with the passage of a cold front.  A couple of days ago I was going to call it the Start of Winter for the valley, but the storm is splitting and right now not looking as large as I thought a couple of days ago, although it could still have impacts. 

The GFS forecast valid 0000 UTC 14 March (6 PM Thursday) has the front and frontal band moving through northern Utah.  

Frontal passage at the airport in the GFS occurs at about 2200 UTC (4 PM Thursday) and the HRRR about 000 UTC (6 PM Thursday) so expect it to get here in the late afternoon to around dinner time.  Pre-frontal conditions will be windy and mild, with perhaps a few valley showers or maybe a thunderstorm.  Temperatures and snow levels will drop rapidly behind the front, with what looks to be a brief period of heavy snow to the valley floor.

Below is a time series for the Salt Lake City International Airport from the HRRR that includes our machine learned snow-to-liquid ratio and snowfall amount forecast.  Yes, it is possible to produce these for anywhere in the continental US on weather.utah.edu if you know where to look.  One can see the frontal passage at around 6 PM with the drop in the height of the 0.5C wet-bulb level.  Precipitation also picks up at that time.  Precipitaiton with the front persists through about 9 or 10 PM (although it's light( and then there is another brief period in the early morning hours associated with some HRRR lake effect.   

Snowfall with the front adds up to a bit more than an inch and then the lake band adds another inch or so, although that's a relatively low probability possibility.  

I think it's worth keeping an eye on official forecasts tomorrow for the evening commute.  A brief period of heavy snow with post-frontal wind can make things nasty quickly, although perhaps the warmth of the roads will help stave that off some.  A lot will depend on intensity and duration and perhaps elevation.  I took a look and the official NWS forecast is for 1-2" at the airport and a bit more on the bench, which makes sense to me.  Their forecast discussion expresses similar concerns to mine above: "A rapid changeover to snow could bring some impacts to the Thursday afternoon commute."

Good News for Skiers and Water Managers

A spectacular early March weekend is now in the books.  For skiers and water managers, March is an important month.  The spigot can stay open, extending the powder season while there is a deep snowpack and delaying the spring runoff (often leading to a more efficient runoff), or we can transition to spring conditions with the snowpack suffering a slow death.  

If you are a skier or a water manager, I think you will like the extended forecasts.  For the early part of this week we'll have our share of warmth, sunshine, and spring fever, but the models are calling for a major shift later this week to a more active pattern with a parade of troughs moving across the western United States thereafter.  

It's a long-enough range forecast that I don't want to talk about details, so I'll summarize with the Utah Snow Ensemble plume for Alta. It's flatlined for water equivalent and snow until late on the 13th when thigns really start to pick up.  There are some big numbers thereafter.  The driest member pumps out 1.63" of wter and 23" of snow through 0000 UTC 20 March (6 PM MDT 19 March), which is more than what we would expect over a 6 day period based on climatology (or average).  

Most of the members are between 2.25 and 4.5 inches of water and 30 and 60 inches of snow.  That would be an impressive storm cycle for mid-to-late March.  There will be breaks in the action over that stretch, so don't assume it will snow continuously, but right now it looks like a series of storms will bring the goods.  

Adios X

After more than a decade of engagement on Twitter/X, I departed from that platform yesterday.  

The post prompted people to ask if I was Ok or if I was leaving the U, but I was merely leaving X.  It is a wonderful platform for sharing weather and science information but for some time the meanness and cruelty there have worn on me.  The nonstop bombardment of click bait was affecting my mental health. Enough was enough.  You can find me on Bluesky for now (@professorpowder.bsky.social), although I'm not going to be on it as frequently.  

Thank you for keeping this blog a place where science, weather, and snow are discussed respectfully.

Problems I Want to Work On

Last night's storm period was one that I really enjoyed simply because it shows how remarkably variable snowfall can be in the central Wasatch.  As suggested in the prior post, the low-level ESE flow favored Deer Valley, which received more snow than Alta-Collins.  

Deer Valley/Ontario: 1.41"/14+" (snow depth sensor got flaky at 7:35 AM; 14 was the total at that time)
Alta-Collins: 1.02"/10"

It seems that a lot of data is not flowing into MesoWest from there resorts right now so I couldn't dig much deeper than that, but the Utah Avalanche Center report included storm totals as of 5 am also showing the decrease in snowfall from east to west. 

Source: https://utahavalanchecenter.org/

This is not something that is unusual for such a flow pattern or that experienced meteorologists wouldn't anticipate, as illustrated by yesterday's blog post.  However, we don't have a good understanding of the physical processes operating during such storm periods, nor do we have models that reliably anticipate such fine-scale snowfall variations.  By reliable, I mean not getting it right every now and then but instead being able to do it consistently. 

This has been the most challenging academic year of my career.  I'm not getting any younger and it has me thinking about what I want to do in my last few years as a researcher.  During my career, I have always been interested in snow and I'm especially interested in understanding and predicting microscale variations of snowfall in areas of complex terrain.  

Last night's case is a good example.  What I wouldn't give to have had a portable radar in the Heber Valley to see what is happening to the cloud and precipitation system on the south side of the Deer Valley Ridgeline and over the Snake Creek area southeast of Brighton.  It would have been so exciting.  More importantly, I'd like to compare this storms to others, as we all like to generalize, but the reality is that there are a lot of variations that we can't anticipate.  

Additionally, what I wouldn't give to have the time and horsepower to improve fine-scale forecasts of these storms using traditional numerical weather prediction or newer Artificial Intelligence/Machine Learning prediction systems.  

The reality is that we do not currently have a high-resolution ensemble that can reliably predict these fine-scale snowfall variations.  If you think the HRRR can do it, think again.  Here's the forecast from yesterday morning's HRR through 1200 UTC 6 March (5 AM today).  The numbers for Deer Valley and Alta-Collins aren't bad, but note how the snowfall maximizes not on the Deer Valley ridgeline but instead on the Alpine Ridgeline near Lone Peak.  We don't have observations up there, but I think that's an unrealistic spatial pattern.  

In part, this may be due to the resolution, or the grid spacing of the HRRR, which is about 3 km, or possibly due to how it deals with cloud processes, including the growth and transport of snow in storms.  The swiss aren't running models at 3 km grid spacing.  They are running them at 1 km grid spacing and trying to get even finer.  That might help.  However, one also has to be able to handle the snow growth, transport, and fallout processes right and this is where observations and clever minds are needed to incorporate such effects into our modeling systems.  

AI/ML is pretty exciting and is going to become increasingly important moving forward, but it's unclear how to do it for such fine scale precipitation patterns in which training datasets are limited.  There are some proposed approaches, but it's going to take careful testing and evaluation to advance AI/ML capabilities for situations like this.  

I guess in the end things haven't changed much throughout my career.  I love snow and winter storms in complex terrain and these are the kinds of problems that I want to work on.  Beyond my own personal interests, advances in these areas would likely help with forecasting for the 2034 Olympic Winter Games, road weather and avalanche mitigation in Little Cottonwood Canyon, and other weather and climate applicatios over northenr Utah.  

A Big Spring Storm

The model runs are pretty spicy and exciting this morning with a significant storm moving later today.  The latest GFS suggest precipitation beginning in the central Wasatch this afternoon or evening and continuing into Friday.  Let's have a look.  

The GFS forecast valid 0600 UTC 6 March (11 PM Wednesday) shows that the early part of the storm is characterized by an deep upper-level trough (upper left panel below) with a lower-tropospheric cyclone (red L in images below) over Nevada.  Associated with this system is an inland penetrating atmospheric river  atmospheric river that approaches northern Utah via the lower Colorado River Basin.  The crest-level 700-mb flow (lower left panel) is southerly or even south-southeasterly in the vicinity of the central Wasatch with 700-mb temperatures near -5°C.

Thus, the overnight storm period has all the hallmarks of a warm, high-density storm period.  Through 9 AM tomorrow morning, the GFS generates 0.77" of water and 8.2" of 10% water content snow. The HRRR is even more excited with water with 1.65" of water and 13.8" of 12% water content snow.  And that's just the start.  For the Wasatch Back crowd, that little bit of easterly flow gets my attention and suggests this could be a period where  you do pretty well.  This could be a period where snowfall is greater at Deer Valley and less at Snowbird, but we'll see if that pans out.  

The GFS drags the main cyclone very slowly across our area, resulting in a prolongued period of moist southerly to southwesterly flow that continues through the day tomorrow,  By 0000 UTC 7 March (5 PM Thursday), the storm is so wrapped up that we actually have cooler air moving into northern Utah from the southwest, as illustrated by the 700-mb (crest-level forecast at lower left).  By this time, the AR has moved downstream, but there's fairly deep instability and it's March, so things might get a bit convective tomorrow afternoon.  

I'm not sure if we might get a break in there for a bit as sometimes there can be a dry slot that sets up in a pattern like this, but by and large I suspect we'll see periods of snow tomorrow that will continue to stack up.  

Eventually the system moves through and we get into colder, unstable, northwest flow for Friday.

I've summarized the storm phases in the time-height section below.  Time increases to the left.  First there is the warm and juicy AR period Thursday night.  Note the low-level southeast flow that might favor the eastern side of the central Wasatch.  Then on Thursday, colder air wraps around the system and moves in aloft from the south and southwest.  Call it an upper cold front if you like, but it will destabilize things.  Temperatures drop further on Friday evening with a transition to colder, post-frontal flow.  

For Alta-Collins, storm totals in the GFS through 7 PM Friday are 1.77" of water and 24.1" of snow.  Expect that to be high density snow to start and lower density Thursday night and Friday.  The HRRR only runs through 5 AM Friday but it has 2.22" of water and 20.2" of snow.  It's less bullish on the post-frontal period whereas the GFS produces snow more continuously through the period.  Such model inconsistencies are one of the joys of being a meteorologist!

I'm inclined to go for a storm total of 1.5-3" of water and 20-36" of snow for Alta-Collins through Friday evening.  There are uncertainties in timing and details, but this looks like a pretty good storm.  The early part could be a big producer on the Wasatch Back.  It's a warm storm and perhaps snow levels will flirt with the lower Jordanelle/Mayflower area during the atmospheric river phase tonight and early tomorrow.  At upper elevations, I suspect the snow will be fairly dense everywhere before trending drier late Thursday and Friday. 

Buckle up and monitor forecasts.  There's a lot of moving parts in this storm. 

Meteorological Winter Ends

Meteorological winter, which includes December, January, and February, ended yesterday.  In the Salt Lake Valley, it wasn't much of a winter.  The average temperature at the Salt Lake City International Airport was about 36.4°F, making it about the 7th warmest on record*.  The asterisk is because there appear to be two days missing from the records I can access, so those numbers could change slightly.  

Snowfall was scant with only 10.4".  A day is missing for the snowfall records too, although I believe the total is probably right (the missing day was likely snow free).  This is the third lowest on record behind 2014/15 (6.0") and 1962/63 (8.8").  

Source: https://xmacis.rcc-acis.org/

The SNOTEL data in the central Wasatch has been a bit squirrelly this year, but for end of meteorological winter snowpack, most sites are a bit below average including Snowbird (90%), Brighton (88%), Thaynes (73%), and Parley's Summit (95%).  Mill D North sits at 118% and is the high outlier.  

All of the data discussed above is collected by the NOAA/National Weather Service and National Resources Conservation Service.  Media reports suggest firings occurred in both of these agencies prior to and during this past week.  Additional employees in both agencies opted for the "Fork in the Road" buyout.  I haven't seen official numbers, but I've seen some estimates suggesting that the NWS lost about 10% of their workforce.  It is my impression that nearly everyone in the weather, water, and climate enterprise, which spans the government, private, and academic sectors, believes this will reduce the Nation's capacity to anticipate, prepare, and respond to weather and related hazards as we enter severe weather, hurricane, and wildfire season.  It will also affect the Nation's ability to advance the prediction of weather and water related hazards in the long term.