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. 

Book Review: The Starting Zone

If you are looking for a good gift idea this holiday season, one that will expand the snow knowledge of your friends and family (or maybe yourself), look no farther than Karl Birkeland's recently published "book" The Starting Zone, available from avalanche.pressbooks.pub ($49.95).

I put book in quotes because The Starting Zone is an e-book available from the Friends of the Colorado Avalanche Information Center.  The Starting Zone is a living document, with tons of figures and videos and more references than you can shake a stick at (these are footnotes, so they don't clutter up the reading).  Everything is hyperlinked, allowing you to watch videos or access supporting materials with the click of the mouse or a tap of the finger (I read it on my tablet).  

I am not an avalanche professional or researcher, so my perspective is of a scientist in a complimentary field (meteorology) who has been backcountry skiing for more than 30 years but has little formal avalanche training.  I loved that the book was written in plain English with many opportunities to dig deeper into subject matter.  This appeals to my practical and sciency sides.

Karl is an excellent writer and does a great job of simplifying complicated subject matter.  I found the chapter on Dry Snow Metamorphism to be especially insightful.  I finally understand the differences between near surface facets and surface hoar and can now better explain the physical processes that distinguish metamorphism that leads to facets vs. rounds.  Karl utilizes laboratory experiments to illustrate many concepts, with extensive use of videos and images from micro-CT scans.  This also appealed to my sciency side, but also my bias toward visual learning.  I much prefer to be shown something rather than be told something or forced to infer from equations. 

There is one oddity with The Starting Zone.  It is still in the runout zone.  Sections I (Laying the Foundation) and II (The Mountain Snowpack) are finished and available except for one section (Spatial Variability at Multiple Scales).  The final section (Avalanche Release, Mitigation, and Forecasting) will be released by Fall 2025, although all three sections of the book are included in the purchase price.  Think of this as an opportunity to advance your eduction as you creep out on the slab but before you feel the collapse, hear the whumpf, and are carried down the hill by the snowy torrent.  Perhaps what you learn from Sections I and II will keep you from creeping out on that slab in the first place. Not everyone in that situation gets a second chance.

Will It Ever Snow Again?

December arrives with the dreaded "Rex Block" firmly in charge over the western United States. 

A Rex Block is a large scale pattern characterized by high pressure "over" low pressure as is evident in the GFS forecast valid for 0000 UTC 3 December (5 PM MST Wednesday) with an upper-level ridge centered over Oregon and a weak trough off the coast of Baja California.  

Rex blocks are named for meteorologist Daniel Rex who published an early article on blocking in 1950. 

Given the stability of this pattern, all members of the Utah Snow Ensemble are snow free at Alta-Collins through 0600 UTC 6 December.  After that, you can find a few members that try to bring a storm or two to Alta, but most keep us dry.   

It's easier to forecast the onset of a block than its demise, so perhaps the odds will ever be in our favor as we approach 10 December, but for now anticipate dry conditions, valley inversions, and faceting snow on shady aspects.