Another New Product from the Utah Snow Ensemble

By popular demand, we now have a tabular output product from the Utah Snow Ensemble for Alta-Collins available at https://weather.utah.edu/text/ensgefsdslccforecast.html.  This should provide access to the most recently processed run and provides tables of total water equivalent precipitation, total snowfall, 6-h water equivalent precipitation, 6-h snowfall, snow-to-liquid ratio, and wet-bulb 0.5C height.  Times are local (mountain) time.  Cells are color filled by magnitude.  An example of the first two tables is below.

The row labeling for each table goes from the minimum value of the 82 ensemble members to the maximum value.  The P stand for "Percentile" so that P10 means the 10th percentile.  That means that 10% of the ensemble members are below that amount and 90% are above it.  

P50 thus is right in the middle, or what statistical types call the median.  50% of the members lie between P25 and P75 (this was corrected from a typo that said P50 incorrectly), or what statistical types call the interquartile range.  If you want to get an idea of what the range of the middle of the forecasts is, these are the rows to use.  

This is a lot to stick on a web page, so you may need to get used to zooming in and out and scrolling up and down.  I don't have time to program a proper web site so you get what you get and don't throw a fit.  At least that's what my mother told me.  

The table above is from last night's 0000 UTC forecast cycle.  We are finally starting to see some hope of a bit of storminess during the holiday period, with median snowfall at 22.4" by 5 PM Saturday 28 December.  That sets the over-under.  Are you betting over or under?

Do You Believe in Santa Claus?

These are desperate times in the natural snow department.  I supposed it could be worse.  Alta has had 105" of snow so far this year and has a base depth of 37", but these are meager numbers for mid December.  Snowpack water equivalent is the best measure of the robustness of the snowpack and we are well below median for that metric at most high-altitude observing sites in the Wasatch including 50% of median at Snowbird, 56% at Brighton, and 43% at Thaynes Canyon.  Somehow, Mill D North sits at 104% of median. Good for them, but that's still only 5.7" of water.  

The snowguns continue to blaze, but this is Utah, home of the so-called Greatest Snow on Earth.  Is there hope of a Christmas Miracle?

The truth is that the ensembles are not giving us much love. Last night's Utah Snow Ensemble came in with most members under 0.5" of water at Alta-Collins for tomorrow's storm.

A look towards Christmas shows that there are some members giving us the goods, but there are 82 members in the ensemble, and the reality is that half of them are producing 0.5" of water and 6" of snow or less.  That's the glass half empty perspective.  The glass half full perspective is that the half of them are producing more than 0.5" of water and 6" of snow.  If you are an eternal optimist, the wettest 8 members produce 1.33" or more of water and 17.4" or more of snow, which would do wonders.  

The odds are low, but this is the holiday season. Maybe we can get a Christmas Miracle.  Surely you believe in Santa Claus?

A Welcome Storm

The Alta snowstake cam suggests we're getting more action that may have been advertised by the models with perhaps 6-7 inches on the stake earlier this morning before the ski patrol wipe (kudos to my friends at OpenSnow for producing the Alta Snow Stake time laps so I could grab this image). 

And then another inch was added since then (not shown).  

The automated sensors are coming in a bit below that with about a 6" increase in the total snow depth and 6" on the interval board (ignore the spurious 34 reading at 08:00).  

Those numbers are a bit higher than advertised by the HRRR and GFS over the past few runs.  Below are the forecasts for Alta-Collins going back to 0000 UTC 12 December for snowall amounts through 9 AM this morning.  Only one of the HRRR runs went for 6" and the most recent was only an inch.  The GFS was a bit more optimistic overall, but never got above 4.5 inches.  

0Z 12 Dec HRRR: 1.1”

6Z 12 Dec HRRR: 6.0”

12Z 12 Dec HRRR: 3.3”

18Z 12 Dec HRRR: 2.4”

0Z 13 Dec HRRR: 1.0”

6Z 13 Dec HRRR: 1.0”

0Z 12 Dec GFS: 0.0”

6Z 12 Dec GFS: 3.7”

12Z 12 Dec GFS: 2.5”

18Z 12 Dec GFS: 3.4”

0Z 13 Dec GFS: 4.5”

6Z 13 Dec GFS: 4.5”

And a total of say 6-8" is also near the top of the snowiest members of the Utah Snow Ensemble over the runs since 0Z 12 Dec.  The snowiest member I could find from the last three runs of the ensemble was in the forecast below which produced about 7.5" for Alta Collins through 9 AM MST (16Z 13 Dec).  Even that was an outlier.  

Consider yourselves blessed.  Let's hope the snow keeps coming as we need all we can get.

The Future Is Uncertain

The physicist Niels Bohr once said that "prediction is very difficult, especially if it's about the future" and that is very evident when looking at the latest Utah Snow Ensemble (USE) plume for Alta-Collins.  The 82 members of the USE show remarkable spread for total water equivalent and snowfall over the next 10 days ranging from nearly zilch to about 2.5" water/38" of snow.  

The period features three storm systems and the 82 members can't seem to show much agreement in the intensity and timing of snowfall associated with each of them. 

We'll likely to be in a better place at the end of the 10-day period than we are now (it helps that the bar is set so low), but don't try to pin me down on details such as should I get a parking reservation at Alta for Friday, Saturday, or Sunday.  The first system moves through Thursday night and Friday, and the second Saturday night into Sunday, but I don't have any confidence concerning how much each of them will produce.

Your guess is as good as mine.

Views of a Highly Stratified Atmosphere

During the most recent inversion event, fog developed over the Great Salt Lake, eventually expanding to cover most of the lake and environs.  Below is a series of images from the NASA Terra satellite, which flys over roughly mid morning on December 4th, 6th, and 8th, the latter being yesterday (Sunday).  

It was on the 4th that fog first became evident, with a couple of regions of fog extending westward from near Antelope Island over the southern bay of the lake.

By the 6th, fog covered most of the lake.  

And finally, by yesterday (Sunday, 8 December), the lake was nearly entirely covered and spread into populated areas that included the northern Salt Lake Valley and portions of Davis County.  

Given the limited lowland snow cover in this instance, it is likely that the evaporation of water from the lake played an essential role in fog formation in this event, leading to fog development and persistence mainly over and near the lake.

I often tell students not to conflate the inversion with the pollution or the fog layer.  In part, this is because the stratified nature of the atmosphere during inversion events is such that neither the fog nor the pollution are perfectly colocated with the inversion.  Instead they are embedded in it or trapped beneath it.  The former was the case yesterday.

Temperatures in the morning sounding from the airport yesterday increased with height from the surface to 817 mb, or 6010 feet.  

Source: SPC

However, the fog was much shallower than that.  In fact, one was above it at about 5200 feet elevation in the Avenues foothills.

And at times the LDS office building and other tall buildings downtown rose above it.  

There was also evidence of pollution above the fog layer.  This might be inferred from the photos above but is better illustrated by one taken towards the sun and University of Utah.  In it you can see the fog, as well as complex layering of aerosols (i.e., pollution) in the atmosphere.

During inversion events, the atmosphere is often highly stable and "inverted" through a layer that is much deeper than either the fog or pollution layer.  Emissions and the transport of pollution by atmospheric flows and turbulence often occur within or beneath the inversion.  In these instances, the atmosphere is often quite stable (and often "inverted") even above the fog or pollution.  Variations in the timing and location of emissions, and the transport of them by atmospheric flows and turbulence, yield layers of "haze".  Natural emissions of water vapor from the lake are also trapped at low levels, resulting in fog formation over and near the lake, but not necessarily through the same depth as the pollution.  

In this most recent event, we ended up with a highly stratified atmosphere with fog and pollution layers.  

Thankfully, we had something to drive more mixing in the atmosphere yesteday and last night, and that was increasing flow and the decrease of temperatures aloft which were associated with an approaching trough.  Ultimately this mixed out the atmosphere, to everyone's great relief.  

Breathe easy.

Intricacies of this Inversion

We are currently mired in the first major inversion and air quality episode of the season, although I don't think it's been as bad as it could have.  

First, the valley is mostly snow free.  This likely leads to a bit more energy input into the atmosphere (due to less sunlight being reflected back to space and increased ground heat flux) and perhaps a bit of a decrease in particulate matter generated by photochemical processes (although this is not my area so I'm speculating).  

In the case of the former, the soundings released at the Salt Lake International Airport at 0000 UTC each afternoon are too late to capture the structure of the convective boundary layer that develops each afternoon.  The convective boundary layer is the part of the atmosphere in which surface heating by the sun leads to vigorous vertical motions (the updrafts are sometimes called thermals) and mixing.  The depth of the afternoon convective boundary layer varies from meters to kilometers, with the former happening during the strongest inversions and the latter on hot days in the summer.

HRRR model forecasts have suggested that the afternoon convective boundary layers during this afternoon have been about 50 mb or 500 meters deep.  An example is below. 

That's deeper than we would see with snow on the ground and during our strongest inversion events, which might feature a boundary layer that's only a couple hundred meters deep, leading to a bit more dilution of this event.  

For those in the northeast Salt Lake Valley, this event has also featured some enhanced easterly flow.  This is a result of the structure of the "Rex Block" pattern over the western United States.  A Rex block features an upper-level high pressure system to the north and an upper-level low pressure system to the south.  Salt Lake City has been wedged between these two features, contributing to broad easterly to northeasterly flow in the northeastern part of the valley each morning, including this morning.  

Source: Mesowest

Such a flow pattern is not entirely unusual on clear mornings, but it seems like it may be a bit deeper and more effective at diluting the pollution each night.  PM2.5 estimates from the PurpleAir network this morning, for example, show considerable variability along the northeast bench, but several stations with low values west (and downstream in the morning) of Emigration Canyon and in the Olympus Cove Area.  In the case of the former, low values extend farther west to at least State Street.  In contrast, PM2.5 estimates are uniformly high in the western valley.  

Source: purpleair.com (screenshot at 0708 MST 4 Dec 2024)

The nocturnal "flushing" of the air pollution during this event is well illustrated by the measurements from Tracy Aviary in Liberty Park.  There is a clear see saw of PM2.5 concentrations, which drop each night and then increase each morning at about 9 AM.  

Similar but even more pronounced behavior is observed at our Mountain Meteorology Lab near the mouth of Red Butte Canyon on the University of Utah campus where we also have colocated meteorological observations.  Yesterday, for example, PM2.5 concentrations increased rapidly at about 10 AM.  Prior to that, the winds were ENE and coming down canyon.  At 10 AM, they became calm and then shortly thereafter shifted to SW, bringing in the gunk.  Then, at 5 PM, the PM2.5 began to drop.  This corresponded with the return of the ENE down valley flow.  That ENE downvalley flow persisted all night long, with PM2.5 levels dropping eventually to 5-10 ug/m2.

These trends cannot be explained by local emissions or photochemistry.  They are related to meteorology and the diurnal variations of winds in the northeastern Salt Lake Valley.