Remarkable Avenues/University Mix Out But Nasty Elsewhere

This afternoon we've had a remarkable mixout of the inversion in the upper Avenues and around the University of Utah.  Check it out and the lens of dirty air remaining along the valley floor.  University of Utah just to the left in the photo below.

Things remain gunked in over the many other portions of the valley, including the area near downtown.

The cleansing agent in this case was a weak surge of easterly flow that pushed into the Avenues and the University area at about 1400 (2 PM) MST, which pushed by the valley cold pool and scoured out the pollution.  The trace below shows the onset and then persistence of this easterly flow at our mountain meteorology lab near the mouth of Red Butte Canyon over the past three hours.

Source: MesoWest

At the same time, the air pollution in the heart of the Salt Lake Valley is still getting worse and PM2.5 is at the highest levels of the event.

Source: Utah Division of Air Quality

The trough brushing by to our northeast is giving us a few raindrops and does appear to be doing some scouring of the upper portions of the inversion even as things get worse along the valley floor.  I'm not optimistic, but keep your fingers crossed that we scour things out more than I expect.  I also suspect the cold pool and the gunky air along the valley floor will surge back into the University area, but will be hoping we continue to stay in the clear.

Addendum @ 5:45 PM

I just learned that they will be doing two fireworks displays in downtown Salt Lake tonight, one at the Gateway at 7 PM and the other downtown at midnight.  THIS IS A HUGE MISTAKE AND THESE SHOWS SHOULD BE CANCELLED.  Fireworks produce large quantities of particular matter.  I don't want to be a party pooper, but fireworks produce bad air quality in the summer.  Here's a post from the Cliff Mass Blog showing what happened this summer in Seattle during their fourth of July fireworks display.  There is no way we should be having fireworks displays when the air quality is this bad.

It Is Getting Worse

Air quality in the Salt Lake Valley is now what I consider to be the worst of the event thus far with the  24-hour average PM2.5 sitting at 49.1 ug/m3 compared to a National Ambient Air Quality Standard (NAAQS) of 35 ug/m3.  Hourly average values last night were not the highest of the event, but close to those seen on the 27th and 28th and certainly more sustained.

Source: Utah Division of Air Quality

Box Elder, Cache, Uintah, and Weber Counties also have 24-hour average PM2.5 above the NAAQS.  Utah County is just below it, but was above it yesterday.  Nastiness all around.

Watching the evolution of this event from the Avenues reminds me that having data from only one site in all of Salt Lake and Davis Counties is unacceptable.  For comparison, the Puget Sound Air Pollution Control Agency provides hourly real-time data from 15 sites.  We should at minimum have coverage on the benches where people often assume the air is cleaner.  Although this is sometimes the case, it's not always a good assumption. 

If you are wondering how the air can be so bad during these inversions, you might have a look at a post we did last year entitled "Your Box of Air During Inversions."  Each of us gets a cube of air about 75 yards on a side.  That's it.  Plus, an article in the Salt Lake Tribune today states that the state added another 46,000 residents last year, so the cube continues to shrink.  

Remembering Snow

Photo during heavy snow from the Bushnell/University
of Utah North Redfield SnowCam.

I know everyone in Utah is missing snow.  I can't help much, but click here and you'll see 30 cm/12 inches in 6 hours from our el cheapo snow cam on the Tug Hill Plateau earlier this month.  Refresh and repeat until you feel better.

Hideous Inversion Lives On

Yesterday's weak trough did drop temperatures above the inversion and does appear to have reduced PM2.5 concentrations somewhat, but hardly to levels we would like.

Yesterday morning, the 5 AM sounding from the Salt Lake City Airport showed a very strong inversion with temperatures increasing from about –8ºC at the surface to about +4ºC at 775 mb (approximately 2250 m/7500 feet).

Source: University of Wyoming

With the passage of the weak trough yesterday, temperatures aloft cooled quite a bit.  This morning, the 5 AM sounding showed a surface temperature of about –7ºC with temperatures increasing only to about -3ºC at 830 mb. 

Source: University of Wyoming

Although the cooling aloft weakened the inversion, it didn't break it.  It has merely helped ventilate the valley a bit and both hourly and 24-hour average PM2.5 levels are lower today than yesterday.  That's good news, but only in a relative sense.  We are still fluctuating around levels that are considered unhealthy for sensitive groups.

Source: Utah Division of Air Quality

In addition, at least visually, things look a bit worse today than the day before the weak trough passage.

I've been looking at the forecasts and their is no Christmas miracle between now and at least January 2nd (there may be hope for at least a temporary reprieve farther out, but it's too soon to say with any confidence).  We are, however, a bit downstream of the ridge axis with some weak systems passing just to our north and northeast.  If we are fortunate, these systems will stir things up a bit and keep the inversion from getting really ugly.  If we are unfortunate, it is going to get worse, much worse. 

Deeper and Dirtier

The weak upper-level trough and cold front pushing through northern Utah today has stirred the pollution a bit, but has thusfar provided little relief.  I went for a hike this afternoon in the avenues foothills and the pollution is deeper today than it was yesterday.  The photos below show the deepening and thickening of the smog over the 3 days since Christmas.

Here are a couple of additional views, the 2nd photo taken from my place in the Avenues at 4:30 PM for comparison with those in previous posts, although skies were overcast at that time making it look even bleaker.

24-hour average PM2.5 peaked this afternoon at 42.8 ug/m3, the highest of the event thusfar.  Hourly PM2.5 did drop in the past couple of hours, in contrast to the previous couple of days when it became elevated around noon and stayed that way all afternoon and evening.  Perhaps we are seeing a bit of mixing related to the upper-level trough and cold frontal passage, or the cloud cover that moved in this afternoon has lack of sun is affecting the photochemistry (not my area, so maybe others can comment).  It would sure be nice if it were the former.

Source: Utah Division of Air Quality

Mountain Highs, Valley Lows

I was going to wait and do this post tomorrow, but weather never sleeps and it seemed like a good idea to do it now.

Today was the first day I got up to the mountains during the current inversion.  The weather was as nice as you can get in December with blue bird skies, warm temperatures, and barely a whiff of wind.  

I never tire of the view of Mt. Superior

Balmy.  Temperatures as high as 43ºF were recorded at the top of the Collins chair today, but I suspect this was a bit higher than the free-air temperature as the winds were light and this often leads to overheating of the sensor when it is sunny. 

In addition, the ski conditions at Alta were fun too.  The groomers are grippy and carvable.  Fast is fun. 

Then came the dreaded descent into the Salt Lake Valley.  

And for comparison with the photos in the previous post, here's today's 4:30 PM shot from the Avenues.  

As discussed in that post, the buildup of pollution in this event is really quite remarkable.  Observations through 4 PM this afternoon suggest it is a virtual lock that we are going to exceed National Ambient Air Quality Standards (NAAQS) this afternoon (we sit at 34.3 ug/m3, but will surely climb through the 35 ug/m3 threshold in the next few hours).  That's right, from nearly pristine air for Santa's deliveries on Christmas Eve to a likely NAAQS violation 60 hours later.  

Source: Utah Division of Air Quality

I don't know what the fastest transition from clean air to a NAAQS violation is, but this may be one of the fastest in the past decade or two in Salt Lake City (perhaps someone out there can dig through the records).  This is a purely anecdotal observation, but it seems like in the past few years we have shifted into air pollution overdrive and the buildup of pollution is getting more rapid after cleansing frontal passages.  Welcome to Wasangeles.  

Rapid Pollution Buildup

The build up of pollution in the Salt Lake Valley the past two days has been quite remarkable.  Below are photos taken at about 4:30 PM on December 24 (Day 0), 25 (Day 1), and 26 (Day 2) showing how we went from pristine skies to serious smog over a period of about 48 hours. 

PM2.5 data from the Utah Division of Air Quality showed that the 24-hour average PM2.5 for yesterday was 25 ug/m3.  It currently sits at 27.6 ug/m3.  Recall that the National Ambient Air Quality Standard (NAAQS) is 35 ug/m3.  The rise from 0 to 27.6 ug/m3 occurred in about 48 hours during a holiday period, which is both remarkable and disturbing.  

Source: Utah Division of Air Quality

An interesting aspect of the PM2.5 evolution is the dropoff in hourly concentrations during the overnight hours.  At 6 AM this morning, hourly concentrations had dropped to 14.8 ug/m3, but I would expect them to rebound later today.  

This case further illustrates my view that we need to be far more aggressive in declaring mandatory air quality action days during which the use of wood stoves and fireplaces is banned (see earlier posts on this subject here, here, and here).  On Christmas, we were in unrestricted action (a.k.a., "green burn").  Why?  The pollution in the Salt Lake Valley when there is snow on the ground simply builds too fast when a ridge builds in.  It appears we may get at least a partial mix out tomorrow as a weak front and upper-level trough moves through.  This may help the state avoid a NAAQS standard violation, but in the interim we are subjected to poor air quality approaching NAAQS levels.  Although it won't solve our air quality problems, reducing or eliminating wood burning probably represents the lowest-hanging fruit that can be plucked to improve our air quality during wintertime inversions. 

Pollution Doesn't Take a Holiday

A truly spectacular airmass pushed into the Salt Lake Valley and gave us a remarkably clear Christmas Eve.  As the sun was setting and Santa approached, one could breath easy for their long winter's nap. 

4:30 PM MST Christmas Eve, 2013

However, it didn't take long for a veil of smog to begin to develop.  On Christmas, air quality was still good, but one could begin to see the effects of all the yule log burning on the visibility over the valley (I'm not sure if wood pollution is the problem, but "yule log burning" seemed to fit well there).  Note in particular the smoggy tint in the right-hand side of the image below.  

4:30 PM MST Christmas

The view from the Avenues Foothills also showed a veil of Christmas smog hanging over the valley on an otherwise unbelievably spectacular day.

A thin veil of Christmas smog over the Salt Lake Valley

The latest observations from the Division of Air Quality shows that the hourly PM2.5 concentrations peaked last night at 33.5 ug/m3 and the 24-h average PM2.5 now sits at about 15 ug/m3.  Although these are only in the moderate air quality category, this case provides an example of just how fast we see our air quality deteriorate after the valley has been flushed out and the inversion builds in.  From good to moderate air quality in 24 hours.  Pollution doesn't take a holiday. 

Source: Utah Division of Air Quality

There is, however, a small Christmas gift on the way.  The inversion will strengthen today and probably reach peak intensity tomorrow.  On Friday night through Saturday morning, the models are calling the ridge to shift just far enough upstream to let a weak front push through Utah.    

Hopefully this will give us at least a partial mix out, if not better.  

Christmas Gifts

For new readers, a link to the now classic post, "The Untold Story of How Santa Claus Really Comes to Town."

For fair weather skiers and resort owners and operators, a big ridge and mild weather for Christmas Day and at least a couple of days thereafter, with forecast 700-mb (near 10,000 ft) temperatures climbing to positive numbers by Thursday evening (i.e., just above 0ºC).  

For powder skiers and water managers, hope that this pattern changes and we get more snow in 2014.  

Merry Christmas and Happy Holidays to everyone.  

When High Density Snow Rocks

Great turns in 10% yesterday at Alta

As some of you know, my favorite days to resort ski are not bluebird powder days but storm days during which it starts snowing high-density, heavily rimed snow in the late morning.  Yesterday was such a day as heavily rimed, high density snow began to pick up in intensity around mid morning.  From 9 am to 4 pm, the Alta Collins stake recorded four inches of snow with a water content of about 9%.  That's perfect for smooth, creamy turns and also good for base, so we were happy to accept it.

Contrary to conventional wisdom, water does not necessarily freeze when the temperature is at or below 32ºF/0ºF.  Typically to convert from liquid to ice, it needs something to help it arrange it's molecules in the proper configuration to form ice.  That something is known as an ice nucleus.  Water that is below freezing but still in a liquid state is supercooled.  Wintertime storm clouds in Utah and many other regions are typically comprised of a mixture of ice particles, snowflakes, and supercooled cloud droplets.  Under some situations, you get a lot of supercooled cloud droplets in the clouds and you get a lot of rime.  Rime forms when these supercooled cloud droplets freeze on contact with trees and other objects.

A lightly rimed tree makes a beautiful scene in the Supreme area

Riming on a Supreme lift tower.  Note how the riming is confined to the windward side where cloud droplets are impacting the tower.  

Ice crystals and snowflakes also experience riming in clouds.  Sometimes this riming is light, but yesterday it was fairly heavy.  The three large ice crystals in the photo below could have fallen to the ground as beautiful six-armed snowflakes, but instead they were rimed, leaving a high-density hexagonal structure with a consistency similar to styrofoam.    

We saw lots of hexagonal ice crystals like that yesterday, as well as an assortment of lump, cone, and ball shaped crystals.  Some of these were almost large enough to be called graupel, but I thought they were best classified as heavily rimed crystals.

When you get heavily rimed crystals like this and it is cold enough that they don't stick together, they make for great skiing.  They have a lot of body and only a few inches smooths out the hill nicely.  Don't reveal your ear-to-ear grin, however, to those people on the chairlift complaining about how heavy the snow is or that it hurts their face.  Simply remind them that it would probably be best if they head down the canyon early.  Once they are gone, the skiing will only get better until they close the lifts at 4:30.  

An Early Christmas Present

On Friday I received four new snow measurement tubes from SnowMetrics, which was a nice early Christmas present thanks to generous support provided by many individuals to our Mountain Meteorology Fund.

We've been getting by with just a couple of these, but have needed more for some time.  In particular, I will be putting the new measurement tubes to good use in upstate NY in January when I will be visiting some of the local schools.  They will allow us to let the kiddies go out and make some of their own snow and snow density measurements, which is far better than just having them stand around and watch one of us do it.  In the long run, it will allow us to expand our snow sampling efforts during field programs.

SnowMetrics makes all sorts of great tools for snow and avalanche geeks and hobbyists.  If you order today, they claim they will have them in the mail tomorrow morning.  I'm not sure that guarantees a delivery by Christmas, but your special someone is not going to complain about having to wait a day or two for these snow study tools.  Plus, there will be plenty of time to put them to good use during and after the holiday period as we have a really wild snowpack in the Wasatch that is not going to heal up anytime soon.

Snow Metamorphism and Settlement

We have a couple of weak storm systems passing through northern Utah today that will bring some light snow to the mountains where it is sorely needed and ensuring a white Christmas for the Salt Lake Valley.  Drive and ski carefully.  There are plenty of slick spots on the road and booby traps in the mountains.  I haven't ventured into the Wasatch backcountry since returning from New York, but the list of backcountry avalanches is really long, with numerous close calls, including by people sporting air bags.  Let's be careful out there.

Over the past couple of days I've been having the first good look at some of the data we collected on the Tug Hill Plateau and thought I would do a comparison of the weather and snowpack evolution there over the past two weeks with that at Alta Ski Area in Utah.  To do this, I'm going to use data from ultrasonic snow depth sensors at the two sites.  These are extremely useful instruments and they tell us a lot about how much it has snowed and how the snowpack is evolving.

We begin with our North Redfield site on the Tug Hill Plateau.  Late in the day on December 7th there was no snow on the ground due to a thaw event the previous day.  A weak storm on the 8th boosted the snow depth to about 4 inches.  Then came the big lake-effect cycle with four major events, the first on the 10th and 11th, the second on the 13th, the third on the 15th, and the fourth on the 18th.

Source: University of Utah/MesoWest

There are two things to note about this graph.  The first is the remarkably rapid increase in snow depth during each storm.  This reflects the intensity of these lake-effect storms during which we observed snowfall rates of as high as 4+ inches per hour.

The second is the decrease in snow depth following each storm.  This is not a consequence of melting, but of something known as settlement.  Snow flakes and ice crystals are remarkably dynamic.  From the moment they land, they begin to change.  This change is often called metamorphism.  Fluffy, low-density dendritic snowflakes lose their angular, tree-shaped branches and become rounder.  This leads to a settling of the snow.  The settlement after each storm at North Redfield was quite dramatic and a consequence of the fact that the storms were comprised largely of dendrites and dendritic aggregates (flakes made up of multiple dendrites that have stuck together) that have lots of angular branches.  These types of flakes tend to settle the most as they transform into more rounded shapes on the ground.  

A photo of a dendritic aggregate in freefall during a lake-effect storm at North Redfield.  The photo was taken by the University of Utah HYVIS crystal camera.  Note the angular tree-like arms.  This type of snow is very low density and is great for skiing, but tends to settle a lot after reaching the ground.  

Of course, there is a limit to how much the snowpack can settle.  At some point, the crystals are rounded and densely packed and settlement occurs much more slowly.  This can be seen in the snow depth evolution at Alta Ski Area in Utah during the same period.  Most of the snow on the ground at Alta fell before this time period and had already undergone settlement and densification.  In addition, there were only a couple of weak storms, one on the 7th, the other on the 19th.  As a result, the snow depth was pretty steady during the period, with only a very gradual decrease of perhaps 7 inches.  

Source: Alta Ski Area/University of Utah/MesoWest

There is one thing that I've glossed over here and that is that the evolution of the snow crystals can vary with depth, especially in shallow snowpacks.  For instance, although after a storm the new snow may be settling and densifying overall, very near the ground where the ground is warm one can actually see more angular faceted crystals (sometimes called depth hoar) forming or persisting in the old snow.  One sometimes also sees angular faceted crystals form at the surface (sometimes called surface hoar).  

Surface hoar 

Both depth and surface hoar can be very problematic for snow stability and avalanche risk as faceted crystals tend to be quite weak and lack cohesion (in the case of surface hoar problems arise when snow falls on it during a storm).  In addition, snowpack weaknesses associated with depth and surface hoar often persist long after the new snow has settled and strengthened.   As a result, backcountry travelers in avalanche terrain need to be cautious about what is lurking in the basement, and this is going to be especially important in the coming days in the Wasatch as buried instabilities persist.  

Freshies for All

Yesterday was a great day for the Utah team in New York and Wasatch snow lovers.

In New York, the University of Utah snow study team was finally able to enjoy some of the fruits of their labor.  The final numbers are in and, from 7-18 December, we recorded 252 cm of lake-effect snow at our North Redfield observing site.  That's 99 inches, which falls just short of the coveted 100 inch barrier, but we would have easily been over that if we had included some of the non-lake-effect snow that also fell during the period.  Very impressive.

With freshies to be had, the team hit Snow Ridge ski area, which is on the east side of the plateau and offers up 450 vertical feet of the tugs finest.   Apparently they had a pretty good day, although they would not share with me any photos of their "secret stashes."  Maybe they are pulling my leg, but having skied Snow Ridge when I was in high school many years ago, it would not surprise me if they found some pretty good powder turns.

Advertised base 36-72 inches, whatever that means.  I suspect the settled snow depth is comparable to or better than Alta right now.  

Few on the trails

First in the field, first on the hill.  Looks like a mixture of twigs and fatties for boards.  

And food is easy on the expense account, making me happy.  One of my students
remarked that you can't get a packet of ketchup in Vail for what they sell burgers
for at Snow Ridge.

Meanwhile in Utah, we finally got some much needed snow.  The Collins stake at Alta measured about 10 inches overnight and during the day yesterday, with about an inch of water.  We sorely needed it, but be careful out there if you are venturing into the backcountry as that snow has fallen on a house of cards.

Tug Hillers: I will be giving a talk on lake-effect, snow, and our field program at the Barnes Corners United Methodist Church at 10 am Saturday 11 January.  A $5 donation is requested and space is limited, so sign up in advance.  For more information, click here.

Cage Match: Tug Hill Plateau vs. Wasatch Mountains

Many of my regular blog readers who have a great interest in Wasatch weather are probably wondering if I have gone off the deep end given all the posts I have been doing from northern New York.  What can I say.  We're having a great time.  Yeah, there's some good weather in northern Utah right now, but my mind is still locked on the incredible storms we observed over the past couple of weeks on the Tug.  So, let's have a comparison or, dare I say, a cage match.  This will also introduce new upstate NY readers to the remarkable snow climate of the Wasatch Mountains.

Topography

The Wasatch Mountains extend roughly north-south through northern Utah with the highest and broadest portion of the range rising up to 7000 vertical feet above the Salt Lake Valley to the southeast of the Great Salt Lake.  This high portion of the Wasatch Mountains, which contains many world-famous ski resorts such as Alta, Snowbird, Solitude, Brighton, Canyons, Park City, an Deer Valley, is called the central Wasatch.

The Tug Hill Plateau rises very gradually from the shores of Lake Ontario (~250 feet above sea level) to a maximum elevation of about 2000 feet.  Slopes on most of the Tug are pretty gradual and do not support downhill skiing.  The exception is the eastern side which drops sharply to the Black River Valley.  Snow Ridge ski area on the eastern side has a vertical drop of 450 feet.

The topography of the Wasatch Mountains and the Tug Hill Plateau.  Note color scale change.  Shaded elevation
is in meters.

Snow Climate

Snowfall in the central Wasatch Mountains increases very rapidly with elevation.  Average snowfall on the western side of the central Wasatch increases from about 100 inches at 5000 feet to more than 500 inches a year at 8500 feet in Little Cottonwood Canyon.  It is likely that the high terrain surrounding Little Cottonwood receives more than 600 inches a year above 10,000 feet.

Snowfall in the central Wasatch Mountains.  LCC=Little Cottonwood Canyon.  Red numbers are based on records from the Western Region Climate Center.  Yellow are estimates.  The 404 inch average in upper Big Cottonwood is probably lower than recent climate averages.  Background photo from http://sharewhat.blogspot.com.

Very few stations have reported snowfall observations over an extended period on the Tug Hill Plateau, so to get some idea of the distribution of snow in the area for our field program, I concentrated on a three year period with the greatest coverage of observations reported to the Buffalo, NY National Wather Service Office.  Three years is too short for a climatology, but I suspect that these numbers are probably within 20% of the long term average at most locations.  Here you can see why we were so keen to put our instruments up at North Redfield, which had an average annual snowfall during this period of 254 inches, followed by a 236 inch average in North Osceola.  Note that neither of these sites are in the center of the upper Tug, so snowfall amounts in that area remain undocumented.  I suspect that if one wants to argue for the snowiest location in the east, that some of the higher peaks of Vermont, Maine, and New Hampshire, or locations on the Keweenaw Peninsula might top the snowiest portion of the Tug, but in all likelihood the Tug Hill Plateau has the largest continuous area with an average snowfall of more than 200 inches in the eastern United States.

Finally, we could do a comparison of lake-effect in the two regions.  The Tug wins hands down here as most of the snow that falls in the Wasatch is of the non-lake-effect variety.  Based on our estimates, I would say something like 25-50 inches a year (on average) in the Wasatch is lake effect, whereas a big chunk of the Tug snowfall is lake effect.

Storm Intensity

Utah's 24-hour snowfall record is also an impressive 55.5 inches, set at Alta Ski Area, which ain't too shabby.  But sorry Utah powder snobs, the Tug has you beat here.  How about 77 inches in 24 hours on Jan 11-12, 1997 (more than the US record for 24 hours, but it wasn't accepted due to some irregularities in the measurement, but we can be reasonably confident that a boatload of snow fell).   Snowfall rates of more than 5 inches an hour occur in lake-effect storms on the Tug (we observed two periods with 4-5 inch an hour snowfall rates just during our brief stay), so when it comes to intensity, my view is that the Tug beats the Wasatch hands down.  Even hardened Utah powder snobs in our group, including a long-time Alta ski patroller, were blown away by the intensity of snow.

Water Content

Utah is known for the Greatest Snow on Earth.  Many people believe this is because our snow is unusually dry, but really the water content of Utah snow is pretty similar to that of the Tug Hill below.  The map below shows the average snow-to-liquid ratio of snow in the United States, which basically tells you how much snow you get out of an inch of water.  A 15 means you get 15 inches of snow for every inch of water.  Higher values thus indicate drier, fluffier snow.  The average for Utah is somewhere around 14 or 15 to one.  Similar values are found in the lake-effect snowbelts of the east, including the Tug Hill Plateau.

Source: Baxter et al. (2005)

Summary

So, in this cage match we have a draw.  The central Wasatch win for average annual snowfall, the Tug wins for storm intensity, and we have a tie for average snow water content.  The Tug gets more lake effect, but the Wasatch have stronger contrasts in annual snowfall.  Bottom line is that residents of both regions have good reasons to take pride in their snow.  Now go and collect some good observations so that we have better climatologies in the future!