Thank You Salt Lake City Public Lands!

If you haven't been out in the Avenues foothills lately, do it soon so you can enjoy the new trails that the Salt Lake City Public Lands Division has built and is building while the grass is green, the flowers in bloom, and the mountains white.  Below is a shot  this morning of one of the new flow trails put in above Morris Meadow and 18th avenue.

Information on what the are doing is available at https://www.slc.gov/parks/trails-natural-lands/fts/, with the latest information on their blog at https://www.slc.gov/parks/trail-updates-in-slcs-foothills-natural-area/.  Many trails opened last Friday and. as illustrated by the image below, they include multiuse trails (green; hiking and biking in both directions), hiking only (brown), hiking and uphill biking (orange), and downhill biking (black).  

https://www.slc.gov/parks/trail-updates-in-slcs-foothills-natural-area/

The red trails are old trails that are now closed.  I am not sure if that's permanent or temporary.  

The trails are posted with signs like the ones below, indicating the approved usage.  In this case, hiking and uphill biking is permitted, but downhill biking is not.  

The city has also posted more detailed signs, but I've noticed most people don't read those and they are not bilingual.  It is going to take a bit of time for people to get used to these new designations.  I've seen both mountain bikers riding hiking only trails and hikers and runners on downhill only trails.  I've spoken with several and they all confess confusion and I walked on one of the closed red trails a few days ago without recognizing it had been shut down.  Let's obey signage and work on education to avoid conflicts, injuries, and trail decay.  

Beyond that, enjoy!  I look forward to more new trails opening in the Avenues Twin Peaks and Dry Fork areas in the coming weeks.  

The End of the Snow Accumulation Season

In all likelihood, yesterday marked the end of the snow accumulation season on upper-elevation northerly aspects in the northern Wasatch Range (we're past snow accumulation season at other elevations and aspects where the snowpack has been melting and running off for some time).  

Data from the Snowbird SNOTEL shows (9177 ft) shows that as of midnight last night we were at the highest snowpack water equivalent of the water year (which begins October 1) with 32.7 inches.  

Source: NRCS

That will almost certainly be the maximum for the year as the forecasts for the next few days suggest an abundance of sunny, warm weather through Saturday.  The snowpack at this location is ripe or near ripe, meaning it's warmed to near 0˚C and additional energy input into it will likely release meltwater.  A weak system on Sunday may bring a few flakes, but not enough to rebound from these losses.  

As a friend noted yesterday, this is pretty much right on the date of median peak snowpack water equivalent at this site (April 28th).  If you are wondering, the median peak is 42.9 inches, so peak this year is 76% of that.  Alternatively, you could ask what percentage of water years had a higher peak snowpack water equivalent than this one.  The answer to that is a bit over 70%.  

Bets on the last day of snow cover at this site? 

Desperately Needed Spring Storm Today

Rain and upper elevation snow are falling across much of northern Utah today as a slow moving front moves across the region.  We desperately need the precipitation and right now it looks like a pretty good soaker.  

The latest radar imagery shows pretty solid coverage (given the limitations of the radar due to the region's complex terrain).  This includes strong returns over the central Wasatch.  

Surface observations within an hour of 1441 UTC (0841 MDT) show the shallow surface front draped across the central Salt Lake Valley with northwesterly flow at the Salt Lake City airport and in the West Valley region and southerly flow in Draper and Herriman.  

Source: MesoWest

This leading nose of the front is, however, shallow and the flow became southwesterly in the morning (0600 MDT) sounding at an elevation of only 7000 feet.

Source: SPC

Thus, most of the precipitation is being generated this morning in the southwesterly flow, which is quite juicy.  The sounding above shows nearly saturated flow through the troposphere and the analysis below shows a narrow filament of high integrated vapor transport, known commonly as an atmospheric river, extends inland across southern California, Nevada, Utah, and Wyoming.  

Source: CW3E

Snow levels in the mountains are currently high.  Temperatures at 8 am in Little Cottonwood Canyon were 35˚F at 7688 feet, 30-34˚F at observing sites near the base of Alta, and 23˚F at the top of Mount Baldy (11,000 feet).  The web cam image below, taken at about 0855 MDT, suggests wet snow is falling at least down to the tram base as there appears to be slush on the bypass road.  

Still, it's surely a wet-sponge day out there.  Hard-shell recommended if you are braving the cream-on-crust conditions this morning.  

The models suggest precipitation will continue for most of the day today in the Salt Lake Valley and the central Wasatch.  This agrees well with the slow moving precipitation field evident on radar this morning.  The 0600 UTC GFS puts out 0.76" of water equivalent and 7.6" of snow with a high average water content of 10% high-density for Alta through 5 PM.  Temperatures and snow levels will, however, lower slowly today.  My best guess would be 4-8" of high-density cream above 9000 feet through 5 PM.  Numbers could be higher if we transition to lower density snow this afternoon, but there's too much favoring high density in this one.  

Note also that we could see some thunder this afternoon to spice up the day.  

There could be some snow showers tonight and tomorrow, although the models right now are bringing in some dry air at mid levels and are not bullish on mountain precipitation other than perhaps a couple of inches in favored locations.  

Dust on Snow

The short-lived but frequent post-frontal dust storms we've had over the past several weeks have left their mark on the Wasatch snowpack.  

After it snows, the dust is buried, but it doesn't take long for it to emerge as the surface snow melts and the water trickles down through the snowpack, leaving whatever impurities at the surface.  Instead of white snow, it may be "marbled" with areas of tan or brown.  Some people call this "snirt" for part snow part dirt.

Due to the uneven distribution of fresh snowfall and melt rates, the emergence of the dust-laden snow varies.  Today, on this southeast aspect, one saw a very heterogeneous (diverse) snow surface with everything fro white to dust-laden brown.  

Dust is important for snow hydrology because it affects the amount of solar radiation absorbed by the snow.  Dusty snow is darker and absorbs more sunlight, so there is more energy available for melt.  As a result, a dusty snowpack melts faster than a clean one.  I could feel this today on one of my runs.  At upper elevations, the dusty snow was melting and softer, whereas the white snow was still frozen.  

Sadly, this dust is here for the spring.  It may get buried again by Monday's storm, but as soon as that snow starts to melt, the dust will emerge again. 

I'm not a fan of dirty snow.  Not only does it reduce the length of the snow-cover season, but it also makes for unappealing spring skiing.  

Back in Time...

Many of the products we provide on weather.utah.edu derive from computer models run by the National Centers for Environmental Prediction, or NCEP for short.  Centers is plural because NCEP actually consists of nine National Weather Service centers that deliver national and global products from weather warnings and forecasts to computer model guidance.

We are fortunate in the United States that government weather data from NCEP and other government agencies is for the most part freely available.  This enables groups like mine and others to drive products like those on weather.utah.edu and to develop new forecast techniques.  Many private companies can integrate NCEP model forecasts into their operations, from IBM to Amazon.  This has huge benefits for the nations economy and resilience to weather hazards.  In many other countries, this is not the case as government groups may charge for these products.

When I began my education in the late 1980s, there was essentially no ability to get digital model output from NCEP.  Model forecasts came via difax machine and you only got a few levels.  In my office, I have a small photo of myself standing in front of these difax charts on my graduate day in 1989.  The only way to forecast the weather during this period was to go to the department's map room and look at these charts.  

Around that time, some groups began to explore the possibility of obtaining and using the NCEP model output in novel ways.  One was here at the University of Utah where John Horel (currently our Department Chair), Lloyd Staley, and Tim Barker began to access and display NCEP model forecast data obtained via satellite broadcast.

  

Critical for this effort were the initial products developed by Unidata, a community of education and research institutions supported by the Unidata Program Center in Boulder, CO, which was and is still funded by the National Science Foundation.  This community and the Unidata Program Center exists today and pushes the envelope of what is possible in atmospheric sciences education and research.  

My first foray into really taking advantage of these developing capabilities was in graduate school in 1994 when I began to work with Cliff Mass, Mark Albright, and others at the University of Washington to develop a real-time forecasting system for the Pacific Northwest.  Essential for that effort is the timely availability of model forecasts from NCEP to create the initial analyses and other information needed to run such a forecast system.  

In my 25+ year career at the University of Utah, model data from NCEP has been integral for my teaching efforts and the products we produce on weather.utah.edu.  In the case of the latter, we not only use that model data directly, but we also generate derived products that add considerable value to the raw model output and push the envelope of what is possible from a weather forecast standpoint.  Probably the most popular are the "forecast plumes" that we produce for Alta and many other sites and involve the application of downscaling techniques and snow-to-liquid ratio algorithms to produce higher-resolution forecasts.  The one below is for Alta and suggests we may see a return to powder skiing early next week.  Much thanks to Trevor Alcott and Mike Wessler, former students who developed much of this capability.  

However, every now and then we go through a rough patch and we're going through one now.  There is often a mismatch between the demands of the users of NCEP model products and their ability to provide them. Although some NCEP products are now available on the cloud, some of what we use for our ensemble products is not.  On April 20th, to avoid their servers being overwhelmed, NCEP throttled access on some of their servers, limiting user access to 120 hits per minute.

That sounds like a lot, but to generate the NAEFS products, we need to download 1508 files.  Prior to their April 20th "speed limit," we parallelized the downloading to speed things up.  

When NCEP throttled access, everything broke.  Our code was not built to deal with blocked access.  It took me some experimenting and testing to figure out how not to invoke the wrath of NCEP and how to deal with things if we do.  I've had to basically rewind the clock, going from parallel to serial downloading of the data and also building in delays to slow the code down.  Sadly, this isn't progress and it will probably result in the NAEFS products being ready about 15-20 minutes later than previously, although that's a guess as I never saved runtime information from the parallelized code before NCEP broke it.  

For those of you who use this data, my apologies for the delays.  If I was on the ball and monitoring things, I could have probably seen it coming and perhaps dealt with this in advance, although I confess that the time I have to actually program is pretty limited.  It may be a few days until things are running smoothly again. 

Mid April Snowpack Update

 Despite the fact that I've had some decent skiing the past few weeks and conditions on "closing day" at Alta were pretty damn good, the snowpack situation across much of the western United States remains below median at many sites and in many drainage basins.  

Source: NRCS

The best situation relative to normal is in the Cascades of Washington, the interior northwest, and areas near and east of the Continental Divide.  Southern Oregon, southern Idaho, and much of the southwest is at 75% of median or lower.  

Even with the recent snow, Snowbird is 76% of median with 31.8" of snow water equivalent as we approach the climatological maximum of snowpack at the elevation of the SNOTEL site (9177 feet).  

Source: NRCS

For those of you hoping for a refresh this week, the odds are low.  The best chance will be Thursday when we are tickled by an upper-level trough.  Most of the ensemble members are calling for light accumulations, although there are a couple (out of 52) that push 10 inches.  The odds favor 4" or less.  

After the weekend, we see the return of some possibilities as a deep trough and associated cold front moves into the western US,  Below is the GFS forecast valid 1200 UTC (0600 MDT) Monday. 

As indicated by the spread in the forecast plume, however, it's too soon to say whether or not that will give us a decent round of freshies.  It does look like the upper elevations of the Sierra may bet something out of this, and they need it even worse than us.  

What a Difference a Year Makes

Today was Alta's "closing day", which I put in quotes as they will be open three more bonus days next weekend.  

Last year there was no real closing day.  The season simply ended due to COVID.  This year, we were treated with truly spectacular weather and ski conditions for April 18th.  

Although the lot was full by 9:15 am, the morning tailgates meant that we had a few runs with few people around. It's always great to have the top of high boy to yourself.  

Piste was fantastic.  Off piste varied.  Baldy was open, so we schlepped to the top and enjoyed the views before a run down Main Chute.  

Many others were enjoying the hike as well.

I don't know if you could have a nicer closing day than today and it was appreciated given the turmoil of the past year.  

That being said, I may have a few days left in me this season...

The Parley's Snow Anomaly

Picking up with the end of the previous post, how do we explain the distribution of snow in the photo below taken on Wednesday, especially in the circled area where there appears to be a decrease in snow with elevation?  

Photo: Mike Freeman

My best hypothesis is that this is the result of two mechanisms.  The first is the cold outflow that occurred that morning from Parley's Canyon.  The image below illustrates the locally cold nature of this outflow based on station observations from the region at around 1500 UTC (0900 MDT).  The station immediately upstream (along I-80) of the mouth of Parley's Canyon is reporting a temperature of 31˚F, considerably colder than the station south of the mouth of Parley's Canyon where it was 40˚F.  These two stations are nearly at the same elevation.  

I suspect the outflow from Parley's was colder than the air south of the Canyon, resulting in locally lower snow levels and colder ground temperatures, enabling accumulating snow at lower elevations.  You can see this contrast in snow level quite clearly in the photo above.  

Such effects are very common in the Cascade Mountains of Washington and the coastal mountains of British Columbia and Alaska where they are even more pronounced due to the large temperature contrast between marine air and continental air.  During the winter, continental air channeling through passes and Fjords frequently leads to locally low snow levels.  When I was living in Seattle, this was a critical forecast consideration as the snow level in Snoqualmie or Stevens Pass would often be lower than at Crystal Mountain.  I even wrote a paper on it if you want gory details (The Influence of Terrain-Induced Circulations on Wintertime Temperature and Snow Level in the Washington Cascades).  

However, if you look carefully in the circled area, you can actually see that there is snow coverage at lower elevations below a mid-elevation band where there is no snow.  WTH is that all about? Precipitation processes are such that I doubt that one was seeing snow forming and falling in that low elevation area when it was precipitation free just a few hundreds of meters aloft.  

Instead, I think we need to consider a second mechanism, although it is also related to the Parley's outflow and its ability to transport snow horizontally.  Unrimed snow crystals have a fall speed of 0.5 to 1 meter per second (1.5-3 feet per second).  This is much slower than the strength of the wind during a downslope windstorm.  Even a 10 mile per hour wind equates to almost 15 feet per second.  An unrimed snowflake in such a wind is transported 15 feet for every 3 feet it falls.  If the wind is stronger, the contrast becomes even larger.  

A good example of this process is a phenomenon known as a "snow foot."  In the photo below (taken on a different day). A convective cell is producing heavy snowfall in the left side of the image.  It is also producing a downdraft, which is flow that is descending rapidly toward the ground.  When it reaches the ground the flow spreads out creating strong winds near the surface.  As a result, the snow is transported rapidly away from the convective cell, resulting in snow at the ground in an area where there is little or no snow aloft.  

This process can also occur on the lee side of mountains.  Below is a radar cross section (vertical slice) through a winter storm in the Snowy Range of Wyoming.  The left side of the image is the windward side of the mountain, and there are radar returns in that area as the flow ascends the range.  The right side is the leeward side and there are few radar returns in that area.  However, there is a very narrow protrusion of radar reflectivity that extends near the surface into the lee.  Here, strong downslope flow is carrying snow generated over the windward side and crest of the mountain into the lee, creating a snow foot.  The black lines with arrows are the estimated paths of snowflakes and you can see the transport.  

My hypothesis for the circled area is that you are seeing the effects of low-level snow transport by the strong winds fanning out at the mouth of Parley's Canyon.  This is leading to snowfall accumulation at low elevations at a time when either there was no precipitation falling aloft or it was actually raining in that mid-elevation region where the air was warmer (some of this rain would have mixed with the snow as it fell into the snow foot.  The precipitation that you observe at the ground doesn't always form directly overhead.  

There are a few other possibilities and you should share your observations and views as hypotheses are subject to testing and revision in the face of new data or perspectives.  

Storm Update

Pretty amazing storm yesterday as the "eastern band" discussed in yesterday's post intensified and delivered heavy snowfall to the central Wasatch Range yesterday morning.  Snowfall rates peaked from 11-12 MDT with several automated observing sites from Little Cottonwood Canyon to Park City Mountain Resort recording more than 4" of snow in that hour.  Alta-Collins ticked off 5.  

Radar imagery at just after 11 am showed strong returns over the Cottonwoods and Park City Ridgeline.  Even east of the crest, the reflectivities were quite high.  Because of partial blockage of the outgoing radar energy by the Wasatch Range, there's less energy available to reflect back to the radar, so those are pretty healthy returns for east of the crest.  

Consistent with the radar, we saw something that we haven't seen much of this winter: heavy snowfall in Park City and Summit Park.  National Weather Service spotters reported 13" at 6824 feet in Park City through 2 PM yesterday and 17.5" in Summit Park through 6 PM.  

Several factors probably contributed to these big amounts east of the crest.  As discussed in yesterdays post, the upper-level flow was from the south-southwesterly, so shadowing effects were limited.  Expanding on that point, the low-level, near surface flow at 11 am was either light or easterly at many locations in the central Wasatch.  For example, Parley's summit reported a 15 knot east wind, Empire Peak at Deer Valley a 10 knot east-southeast wind, and sights in Brighton and Alta both reported winds with an easterly component.  

So the situation was one with heavy snowfall being generated in a band in the south-southwest flow aloft, but the low-level flow producing upslope on the eastern slopes of the Wasatch Range.  Snowfall was still very heavy west of the crest due to the influence of the band, but there was no shadowing effect on the east side and there may have even been an upslope affect to aid precipitation generation.  

I also received an interesting photo from one of our readers, Mike Freeman.  It was taken looking toward Parley's Canyon and Grandeur Peak at 3:32 PM MDT.  Note how it appears that there's no wnow at mid elevations in the circled area, but snow at low elevations.  

Is this simply an optical illusion?  I suspect if you were walking up ridge on the right in the oval you'd go from snow to no snow. 

I have a few hypotheses for why this happened, but encourage you to share your ideas for how this occurred in the comments.

What a Mess!

Quite a mess of a storm right now over northern Utah.  I'm not sure where to start, but maybe a look at some of the observations is a good place. 

Easterly flow extends from southwest Wyoming to the northern Wasatch Front where strong downslope winds persist from the area around Parleys Canyon northward.  One can also find strong easterlies in local canyons further south, such as Provo Canyon.  In the western Salt Lake Valley and along the US-40 in the Wasatch Back, the flow is northerly.  The former reflects what we sometimes call the "Sandy Eddy", a counterclockwise circulation that often forms over the Salt Lake Valley during downslope windstorms and is centered near Sandy. 

Source: MesoWest

A look at the flow on Hidden Peak and Mount Baldy in the central Wasatch, however, shows southerly flow.  Indeed, the sounding from the Salt Lake City airport this morning is simply amazing.  

Source: SPC

The surface flow out at the surface was actually light and from the northwest, with a layer of strong easterlies between about 800 and 750 mb.  This might seem odd, but often during downslope windstorms there is a hydraulic jump downstream of the mountains where the strong easterly flow becomes elevated, with a rotor beneath with reversed flow at the surface.  

Source: Whiteman (2000)

This appears to be the case at the Salt Lake City airport.  Not the sort of thing that you want for air travel as rotors often are associated with strong turbulence. 

Above the easterlies, the flow veers (turns clockwise) rapidly with height to south southwesterly.  There's also a sharp inversion, which marks the top of the cooler air from the east.  

Satellite imagery shows the counter clockwise circulation around the upper-level low that is centered over western Nevada. We are in the south-southwesterly flow ahead of the upper-level low. Note that despite the easterly flow at the surface in many areas, all of the clouds are being generated in the south-southwesterly flow aloft.  In addition, there are two major cloud bands, one over western Utah and one over central Utah.  

College of DuPage

Finally, if we turn to radar, we see that each of those bands is associated with precipitation.  

Automated snow depth sensors this morning show overnight accumulations in the central Wasatch through 7 AM this morning of about 2 inches at both Solitude Summit and Alta-Collins.  The interval sensor at Canyons-Daybreak appears to be down, but the total snow depth has ticked up a bit over 2 inches , so let's call it about 2 inches in the central Wasatch through 7 am.  

The model guidance this morning continues to be a head scratcher.  The latest HRRR has the eastern-most band decaying this morning, while the westernmost persists.  

It then fires up some more precipitation with surface heating and the approach of an upper-level front around noon.  

And then scattered showers (and thunderstorms perhaps) this afternoon.

It's pretty clear from radar that the eastern band is going to persist longer than advertised by the HRRR, but how long I'm not sure.  

The GFS has the heaviest heavy precipitation west of the central Wasatch (and in the current dry slot) this morning, then jumps it to east of the Wasatch this afternoon.

However, it keeps the central Wasatch in precipitation for the period.  Nevertheless, the 0600 UTC initialized GFS numbers for 1200 UTC (6 AM MDT) today through 1200 UTC (6 AM MDT) Thursday are lower than yesterday's 0600 UTC run discussed in the previous post.  It's putting out 0.70" of water and 8.1" of snow for Alta-Collins.  

Finally we have the SREF.  Through 1200 UTC (6 AM MDT) Thursday it is producing a mean of about 6" at Alta-Collis and a range of 2.5 to 10 inches.  

My best guess 6-12 inches for Alta-Collins from for 1200 UTC (6 AM MDT) today through 1200 UTC (6 AM MDT) Thursday, with a good chunk of that coming today.  The snow may come in waves as band position changes or the showers become scattered.  It's worth noting that thunderstorms are a possibility you should move inside or into your car if this occurs.  

Dirty Harry Forecast

Yes, I know the wind is blowing, but my time this week is limited and I'm more interested in whether or not I can have another powder day this season.  

The forecasts through 1200 UTC (6 AM MDT) Thursday vary tremendously depending on what model you look at.  Some promise dust on crust for Thursday, others a deep-powder day.  

Below is the 1200 UTC initialized GFS total accumulated precipitation over the 48-hour ending at that time.  For the mountains of northern Utah, this is a wet forecast with over an inch of water equivalent for the Alta area.  

Source: Tropical Tidbits

The 0600 UTC GFS was even wetter.  Below is the meteogram for Alta from that run showing heavy precipitation starting tomorrow and continuing tomorrow night, with over 2" of water and 25 inches of snow by Thursday afternoon.  Whoot whoot!

On the other hand, the 1200 UTC HRRR is much drier.  Our area appears to be in a dry slot with the heaviest precipitation to the north.  Water totals at Alta through 1200 UTC (6 AM MDT) Thursday are less than a half inch.  Dust on crust.  

The majority of downscaled SREF members lean towards the HRRR-like solution and produce about 0.5" or less of precipitation, although there are 3 that go for bigger amounts of an inch or greater.  

On the other hand, most of the NAEFS members are wetter like the GFS and suggestive of deep powder.  

With model guidance like this, you must ask yourself one question...

Spring Ahead

Forecasts for the coming week look to be spring like, but are uncertain in terms of precipitation details.  The GFS forecast valid 0000 UTC 15 April (6 PM MDT Wednesday) shows a cold, closed low parked right over northern Utah.  

That would favor colder, unsettled weather, but other models have different ideas concerning the position of the low and associated precipitation features.  

Let's start with the easy stuff.  Tomorrow (Monday) will be dry and cool.  After that, the possibility for showers increases on Tuesday as the low digs southward into the Intermountain West. Wednesday may be unsettled, with the possibility of lingering showers on Thursday.  

It's difficult, however, to pinpoint details.  The 0900 UTC initialized Short Range Ensemble Forecast System forecast through 0000 UTC 15 April (6 PM MDT Wednesday) shows enormous spread with a several members producing little or no precipitation at the Salt Lake City Airport, but some others producing more than 0.4 inches.  

An additional complication for Wednesday (and Thursday) is that snow levels could be down to the benches and potentially the valley floor.  It's too soon to say if things will come together for bench accumulations.  Much is going to depend on the track of the low and the associated precipitation features.  

The bottom line is this is a spring-like pattern with the potential for precipitation.  Best to keep an eye on forecasts over the next few days as forecasts are likely to evolve.