Thursday, January 31, 2013

More Weirdness

Weather is weird, and it frequently does not conform to our climatological expectations.  Meteorologists often say that climate is what you expect and weather is what you get, but this winter seems even wackier than usual.  From the "Mother of All Inversions" to storms that produce more snow in the valley than the mountains, this winter has given us a full range of insanity, which continued yesterday.

In the higher elevations, very low density snow fell during the day on Tuesday.  We're talking 4% water content.  Then higher density snow fell yesterday which, when combined with rime, contributed to the development of upside down snow.  Today's Utah Avalanche Center report pretty much tells the tale.
There was a major change in snow conditions that happened on Wednesday.  Higher density snow fell coupled with strong winds creating an 'upside down' layering situation.  This was capped off with a nice rime event which produced a crust varying in thickness and distribution through the central Wasatch. 
I've heard mixed reports about the quality of the skiing, which perhaps depends on when and where people were skiing.  Conditions likely deteriorated rapidly during the day and may have skied better at the resorts as cut up upside-down powder often skis better than untracked upside-down powder.

Having worked yesterday, we did a late afternoon tour in the Avenues foothills and found the damp powder to be quite pleasing.   Instead of being upside down, the snow was consolidated with a relatively uniform water content, making for fun turns.

Jeff Massey
So, we can add to our weirdness list yesterday afternoon's contrast between upper- and lower-elevation turning conditions.

For those of you from out of town, this will give you an idea of where the Avenues foothills are.  Yes, it is quite a snow year in the lowlands around Salt Lake.

Yours truly enjoys his commute home
Addendum: 12:20 PM

If you have any good photos of rime from this event, I'd be interested in seeing them.  

Wednesday, January 30, 2013

A Waste of Good Powder

Automated weather observations indicate that we are starting to see higher density snow fall in the mountains.  For example, at Alta-Collins, we've seen three inches of snow with 0.38 inches of water over the past five hours.

There are also reports of riming, which will further densify the snow surface.  Note the ice on the southwest facing camera on top of Snowbird.

The rime and high density snow are a recipe for upside down snow in which high density snow is sitting on top of lower density snow.  The UAC reported snow water contents as low as 3-4% from yesterday's storm.  Alta-Collins measured 16 inches with .68 inches of water for 4.25%.  Now we're putting something like 10-13% on top of it.  The resorts were probably tracked out today, but this will probably create punchy, inverted turning conditions in the backcountry and further exacerbate the avalanche hazard.  Let's be careful out there.

Heat Miser Cedes Wasatch Front

Snow Miser and Heat Miser from the Rankin Bass classic,
A Year without a Santa Claus
Heat Miser has officially announced that he has temporarily ceded the Wasatch Front to his brother Snow Miser.  "It was a deal brokered by my mom, Mother Nature, as compensation for global warming.  Snowy gets to sock it to some midlatitude region every now and then just to keep his spirits up."

Meanwhile, a reliable source tells me that the government weather control experts have brokered a deal with Snow Miser to bring this storm cycle to an end.  Some periods of snow through tomorrow morning, then we get a break.  

Tuesday, January 29, 2013

Another Look at the Mountain Effects

The image below shows the radar reflectivity (purple = high) this morning at 1300 UTC (0600 MST) and the dramatic effects of the mountains on the precipitation during this event.  Note the higher returns over the Stansbury, Oquirrh, and most of the Wasatch Mountains (see also previous post).  I have also annotated the outlines of the ski areas on this map in red.  There are issues with radar sampling over and downstream of the Wasatch range (for example, the lack of returns over Snowbasin is due solely to beam blockage by the terrain - it was snowing hard there during this period).  Nevertheless, this image illustrates that in the central Wasatch, the heavier precipitation was found over/near the western slopes, with a dramatic dropoff in radar reflectivity as one moved eastward to the Park City ridge line and the Wasatch Back.    

In addition, the storm was quite shallow in the Cottonwood Canyons.  I just got an informal report from Snowbird and was told there were a few inches on top and a foot and a half near the base.  Apparently Baby Thunder was the place to be.  Wow.

Also evident in the image above is the dramatic enhancement of precipitation near Ben Lomond Peak northeast of Snowbasin.  At 11 am, Liberty, which is in the valley immediately east of Ben Lomond, reported a storm total of 28 inches.

Incredible Mountain Effects

Lots of interesting stuff going on in todays storm.  Check out in the loop below the very persistent echoes over the Northern Wasatch, the Stansbury Mountains, the Oquirrh Mountains, and the area around the Cottonwoods (crudely identified by red boxes).

The Northern Wasatch has really gotten a pounding.  Ben Lomond Peak near Ogden has seen its snow depth increase 20 inches in the past 24 hours.  They will be moving into "too much of a good thing" territory if this keeps up.

Similarly, the Middle Bowl site at Snowbasin has a storm total of 18 inches.

Check out what is happening across the Cottonwoods.  The flow at crest level is westerly.  There is strong enhancement evident along the Wasatch Front, but the strong echoes barely penetrate to the Park City ridgeline.  There are very few echoes by the time one gets downstream of the Cottonwoods.

Here's what automated snow depth sensors show since the beginning of the storm last night through 10 am:

Elberts (7600 ft in Little Cottonwood): 12 inches
Alta-Atwater (8700 ft): 12 inches
Alta-Collins (9600 ft): 8 inches
Deer Valley Ruby (8300 ft): 1 inch

Check out the contrast between Alta and Deer Valley based on web cams

Alta.  Source: Alta Lodge
Silver Lake.  Source: Deer Valley
Snow at Alta, filtered sun at Deer Valley.  It looks like we have a remarkably strong mountain wave in the westerly flow over the Cottonwoods, with the strong lee subsidence preventing much penetration of snow into the lee.  The contrast in snowfall between the sites in Little Cottonwood Canyon and Alta-Collins is sure interesting (yet again!) and in need of explanation.  I have ideas, but need to get back to my day job.  

Monday, January 28, 2013

Cold Fronts and the Orographic Ratio

There have been a few events this year during which the enhancement of snowfall over the mountains has been limited.  Some have started to call these "upside down" storms, although being a powder skier I prefer to use the term upside down to describe storms in which the snow density increases with time.

Meteorologists typically refer to the ratio of mountain to lowland precipitation as the orographic ratio.  In a storm in which the mountain and lowland precipitation is the same, the orographic ratio is one.  If the mountain precipitation is four times the lowland precipitation, the orographic ratio is four.  In an upside down storm, the orographic ratio is less than one.

On average, the orographic ratio between Alta and Salt Lake City during January is nearly 5.5.  This is the highest average orographic ratio of any month during the year and illustrates that on average January features the greatest enhancement of precipitation by the Wasatch Range.  

This is, however, the average, and the orographic ratio can vary dramatically from storm to storm and even during storms.

Most research suggest that the smallest orographic ratios along the Wasatch Front occur during frontal passages.  In part, this is because the front provides most of the lift for precipitation formation and the mountains play a less prominent role.  In some instances, the front can be fairly shallow and the storm dynamics are actually better over the valleys than over the mountains.

For example, during the November 2001 Hundred Inch Storm, there were two storm periods.  In the second, the orographic ratio was smallest (and equal to 1.3) during the frontal passage, indicated by the frontal storm stage below.  Note how Alta received only 1.3 times as much precipitation as Salt Lake during the frontal passage.

Precipitation at Alta and Salt Lake City during the second storm period
of the Hundred Inch Storm.  Stable and UPF indicate pre-frontal precipitaiton
stages, frontal the passage of the cold front, and PF1, lakeband, and PFII
post-frontal storm stages that included lake-effect precipitation.
Source: Steenburgh (2004)
In contrast, larger orographic ratios typically occur during pre-frontal periods if there is a lot of dry air in the low levels (which causes precipitation to evaporate before reaching the valley floor) and during post-frontal periods when lifting by the mountains is essential for generating precipitation.

That being said, these are generalizations and there are a lot of factors that affect precipitation.  Producing models that can better predict the distribution of precipitation is an area of ongoing research and one in which we sorely need advances to help us anticipate everything from deep powder to commuter disruptions.

Sunday, January 27, 2013

Temporary Return of Inversion Smog

It appears that the inversion smog over the Great Salt Lake was dragged back into the Salt Lake Valley with the cold front this afternoon.  Check out this marvelous video of it from George Wilkerson's web cam (, video compilation by Prof. Steve Kreuger, University of Utah Atmospheric Sciences). Note the shallow nature of the cold air, the formation of low level "scud" clouds where the southwesterly prefrontal air is lifted over the frontal nose, and the smog, haze, and fog behind the front.  Don't worry, this is a short-term setback.

Breath Deep!

Ding Done the Inversion Is (almost) Dead!
The inversion has finally cracked!  At least for most of the Salt Lake Valley, south winds ahead of an approaching cold front scoured out the fog and smog last night.  Weather cameras show fog and smog are lingering in some areas, as you can just make out in the weather cam image below.

Looking NW from Holiday.  Note smog/haze just beyond downtown SLC.
Source: Meteorological Solutions Inc. 
With the inversion nearly behind us, it's time to move onto better things, snow!  The snowpack in the Cottwonwoods has really suffered over the past month.  We now sit at only 79% of average and, in fact, we're almost even with where we were last year at this time.

Water year 2013 snowpack SWE at Snowbird (green line) compared
to the 1981–2010 average (blue) and 2012 (red).  Source: NRCS & NWS.
We have a cold front coming in for later today and tonight that should bring a much needed dump to the entire Wasatch Range.  Let's hope this one holds together, as we seem to have been teased rather than treated by recent storms.  

Friday, January 25, 2013

Mother Nature's Final Insult

A friend described yesterday's freezing rain as the 7th plague.  I'm not sure what the other six are, but I suppose we could include split flow, inversion, and pollution.  Today we can add fog.  We might have to add drought if this pattern continues.  Feel free to comment with your additions.

By Utah standards, there is a remarkable rain crust on the snow here in the Salt Lake Valley.  It leaves a  shimmer on the snow and is several millimeters thick.

Crusts of this type, sometimes far stouter, are not uncommon where I grew up in upstate NY. I swear I still have bruised shins from trying to walk, snowshoe, and ski on days like this.  I had a far lower tolerance for bad snow then than I do now.

It may be foggy and not look too polluted, but an inversion remains in place and a quick look at the real-time charts from the Utah Division of Air Quality shows that Salt Lake, Utah, Weber, Uinta, Tooele, Cache, and Box Elder counties remain above the National Ambient Air Quality Standard for 24-hour average PM2.5.  Most remarkable is that despite yesterday's freezing rain, the PM2.5 concentrations (24-h average) have actually gotten worse at a few sites, including Salt Lake City.

Source: Utah Division of Air Quality
On the other hand, the air quality has thankfully improved in Provo, although the PM2.5 levels are still high.  

Source: Utah Division of Air Quality
Contrasts in the long-term trends and daily variations of PM2.5 at these sites during this event remain unexplained.  My knowledge of emissions, aerosol physics, and atmospheric chemistry is insufficient to provide a good explanation.  Perhaps others can pipe in.  

Mother Nature has at least one final insult.  This weekend, she will probably keep us locked into crappy air.  A storm passes to the south, and one approaches from the northwest, but doesn't make it into Utah by late Saturday afternoon.  As a result, Salt Lake and Utah Counties are in a dry slot between these two storms when one looks at the total accumulated precipitation through 5 PM MST Sunday.  

Source: NCEP
This means we'll probably remain inverted and polluted through the weekend.  There's some hope for Sunday night and Monday, but the models are still showing some spread in the intensity and track of the approaching trough.  It looks like we'll get at least a partial mix out, and probably a full mix out, but we'll have to see how things come together.  

Thursday, January 24, 2013

The Air Is Still Filthy

Don't be fooled by the light drizzle and fog.  The air is still filthy and nasty out there.  The 24-h average PM2.5 levels in Salt Lake City have climbed to near their highest point of this inversion event.  I remain perplexed as to why they were lower for the last couple of days and whether or not that was truly representative of the valley as a whole, but I think we can probably conclude that we are still above National Ambient Air Quality Standards (NAAQS).

Source: Utah Division of Air Quality
There has been some drop in PM2.5 levels at North Provo (thankfully!).  However, they remain well above the NAAQS.

My cup runneth over this week.  I need a vacation, preferably involving either a warm beach or deep powder.  Anyone willing to sponsor such a trip should give me a call.

How Unusual Is This Ice Storm?

Trevor Alcott provided me with an analysis of hourly surface observations from the Salt Lake City International Airport that help illustrate the unusual nature of our ice storm:

  • Since 1940, there have been only 9 measurable freezing rain events (0.01 inches or greater) at the Salt Lake City International Airport
  • None of those events occurred with temperatures lower than 26ºF.  Today's began at 20ºF.  
  • The latest accumulation (0.08 inches as of 11 am) is the largest freezing rain accumulation since 31 Dec 1983. 
  • There are only three events that top this one in terms of freezing rain accumulation:
    • 0.13 inches that fell at 32ºF on 9 Feb 1976
    • 0.16 inches that fell at 29–32ºF from 26–27 Dec 1983
    • 0.21 inches that fell at 30–32ºF from 30–31 Dec 1983 (.11" of this fell as heavy sleet)
So, it appears this is the coldest measurable freezing rain event in the period of record and it is the fourth largest (so far) in terms of accumulation.  


"The ice storm is an event, and it is not an event which one is careless about"
- Mark Twain

The winter of our discontent continues.  We have not only failed to mix out the cold pool and pollution, but temperatures last night remained warm enough aloft that we have light freezing rain falling across the Wasatch Front.  Freezing rain is very rare in Utah, and this event is shaping up to be a doozy.  

The culprit in all this is the cold pool/inversion combined with a wedge of warm air aloft.  Temperatures in the shallow cold pool at the surface are below freezing (18ºF at the airport when the sounding was taken at about 4:30 am).  Aloft, temperatures are well above freezing within the warm wedge (in this graph, known as a Skew-T, the 0º line is slightly skewed and indicated by a solid black line – everything to the right of this line is above freezing).  Snow falling from the cloud deck aloft melts in the warm wedge, turns to rain, and then freezes on contact when it hits cold surfaces in the cold pool.  

Skew-T source: NOAA/SPC
The current situation is probably a worst-case scenario for northern Utah.  If precipitation rates were higher, the warm wedge aloft might cool enough that the snow would be able to reach the valley floor.   The blue line in the image above shows the temperature that the atmosphere would cool to from evaporation and you can see that except in a very shallow layer it is at or below freezing.  However, with light precipitation, there's not enough water evaporating to fully cool that layer and we're stuck with freezing rain.  The National Weather Service deserves a lot of credit for calling for a winter mix and getting the word out last night, although the impacts are still severe.  

Looking at the radar, I don't see this event ending soon.  By Utah standards, this may shape up to be one of the worst ice storms in many many years.  I already can't remember anything this bad.  In 1000+ blog posts, I don't even have a label for "ice storm."  I just had to add it!

Wednesday, January 23, 2013

Like Waves on a Beach

One of our professors here at the U, John Horel, and several students are participating in a wintertime ozone study over the Uintah Basin this winter.  Amongst the tools that they are using is a laser ceilometer, which sends a pulse of radiation upwards and measures how much is scattered back. The concept is similar to a radar, except the wavelength is shorter, so it is sensitive to small particles (e.g., particulate matter) and it takes a series of vertical profiles rather than horizontal scans.

The data being collected is quite remarkable.  One can see not only layering of the pollutants, but also waves.  Most of these are probably internal gravity waves, which are the atmospheric equivalent of the waves that you might see on the ocean.  They are the very short waves apparent in the top image below.
Source: John Horel (click to enlarge)
However, if you look carefully, you can see longer waves.  These are also gravity waves, but they may be more related to the sloshing of cold air within the basin.

If you wish to geek out, you can monitor the weather and air quality action in the Uintah Basin here.

Inversion Snippets

There's much to talk about today.  We'll do this rapid-fire style.

• As discussed in the previous post from last night, the inversion is now at its maximum strength.  In this morning's upper-air sounding from the Salt Lake City airport, the temperature increased from -15.5ºC (4.1ºF) at the surface to 7.6ºC (45.7ºF) at 2130 m (6988 ft).  Within the large-scale inversion, which is elevated just a bit above the valley floor, the temperature increases from -12.7ºC (9.1ºF) at 1623 m (5325 ft) to 7.6ºC (45.7ºF) at 2130 m (6988 ft).  The loop below shows the lowering and strengthening inversion over the past 8 mornings, concluding with today.  

• With the inversion lowering, higher elevations on the east bench are getting tantalizingly close to clean air.  

• Overnight low at the Salt Lake City airport 4ºF. Overnight low on Mt. Baldy (11,000 ft) 34ºF.  

• The 24-hour average PM2.5 peaked overnight at 127.8 ug/m3 in North Provo.  That is almost 4 times the National Ambient Air Quality Standard.

Source: Utah Division of Air Quality
• Hourly PM2.5 at the Salt Lake site continues to show massive variations with a peak in the afternoon.  The 24-hour average PM2.5 has declined from a peak a few days ago.  I'm not really sure if I buy this as representative of the long-term air quality trend along the valley floor.

 • Although there are more sensors collecting data for retrospective analysis, we have a desperate need for more real-time air quality sensors along the Wasatch Front.  We have access to only one sensor in each county.  For comparison, the Puget Sound Clean Air Agency provides hourly real-time data from 16 sites, including 8 in the Seattle area.   Here's a wild idea.  Put samplers on every Trax train and we can paint out the spatial structure of PM2.5 along and across the Salt Lake Valley at regular intervals.  Such sampling wouldn't be complete, but it would be a major step forward in knowing the quality of the air we breath in real time.  

• Not much change in the forecast through Friday.  A weak trough moves through tomorrow and should give us a stirring and weaken the inversion by warming the low levels and cooling the air aloft.  I still think it won't fully mix out the gunk.  Looks like some wet snow for both the mountains and the valleys.  

• A series of brush by systems followed by a stronger trough will probably bring an eventual end to the pollution by early next week.  It will be interesting to see what happens to PM2.5 levels over the next few days.  

Tuesday, January 22, 2013

Crazy Warm, Except in Valleys

I can't help but share this crazy stuff tonight.  The inversion has strengthened further as temperatures have climbed aloft with the ridge moving eastward.  Check out the sounding from 0000 UTC (5 PM MST) this afternoon.  There's a 17ºC temperature inversion.

Source: NCAR/RAL
The depth of the cold pool seems to be getting shallower.  Perhaps this explains the modest increase of visibility on the east bench this afternoon.

Notice that the temperatures in the sounding are pushing 5ºC at 700 mb.  It's running near 40ºF on Mt. Baldy (11,000 ft) and we hit 52ºF at our Alta-Atwater observing site on the south facing aspects across from Alta this afternoon.

The Alta-Atwater site might be overcooking, but it is safe to say that it is very warm in the mountains, as it will be tomorrow too.

Stirred, Not Shaken?

The big question on everyone's mind is when will we get rid of this inversion and the pollution (especially in Utah county where PM2.5 levels this morning are at 120 ug/m3, nearly 3.5 times the National Ambient Air Quality Standard).  There is a much advertised trough coming in on Thursday and it may help, but I'm not sure if it will give us complete relief.

The problem is that the trough, at least as forecast, is not very strong and brushes by to the north.  We get the tail end of the system, which brings our 700-mb (near-crest-level) temperatures down to about -6ºC and gives us some precipitation, but that's about it.

To fully mix out the cold pools that are present over most of the valleys and basins of northern Utah likely requires colder temperatures aloft and/or stronger winds.  Thus, this may be a case where the inversion is stirred, but not shaken.  We will probably see a drop in PM2.5 levels (precipitation will help with this too), but not a complete mix out.  Thanks to the precipitation, there's a chance we could get fogged in.    

Of course, inversion mixout is extremely difficult to forecast, so maybe we will do better than I expect.  If not, my advice is that you prepare a vodka martini, 3 measures of Gordon's gin, 1 measure of vodka, 1/2 measure of Kina Lillet vermouth, shake it very well until it's ice cold, pour it into a deep champagne glass, and garnish with a slice of lemon peel.

Monday, January 21, 2013

Mank and Murk in Salt Lake County

Just to be an equal opportunity offender, here are a couple of photos shared by our readers of the mank and murk in Salt Lake County.  

Looking north from Suncrest area Sunday 20 Jan.  Courtesy David Yorty.
Similar view, Monday 21 Jan. Courtesy Jim Edman
And a great one here showing Mt. Timpanogos and the Utah Valley smog.

Looking south from Suncrest area Monday 21 Jan.  Courtesy Jim Edman.
As fascinating as all this is meteorologically, I'm looking forward to this episode being over.

Mank and Murk in Utah County

Looking north over the smog of Utah County.
Courtesy Jeff Massey.
The most remarkable (and depressing) PM2.5 readings so far during this "Mother of All Inversions" event come from Utah County.

Online data from the North Provo site operated by the Division of Air Quality show that 24-hour average PM2.5 concentrations reached 104.3 ug/m3 this morning.  That is almost 3 times the National Ambient Air Quality Standard.

Source: Utah Division of Air Quality
I thought I would try and take a look at how this compares to recent events.  To do this, I went to the Utah Division of Air Quality Air Monitoring Center Data Archive for PM2.5.  There you can find a Byzantinian array of data.  I ended up grabbing the highest maximum 24-hour PM2.5 observation from any Utah county station during each year in the Yearly Percentile Frequency Report.

Plotted below is a comparison with our current event (the data for 2012 was not available).  Assuming I'm using the data in the archive correctly (this is an important caveat to consider as the data tables on the DAQ web site contain limited information), current readings at the North Provo site are higher than any measured in Utah county since 2000.

We will have to see if this stands up to a more careful analysis, including quality control of the North Provo data, but this does appear to be an exceptional event by recent standards.

Addendum 11:25 AM: suggests that North Provo has the highest air quality index (hourly PM2.5) in the US, with Logan a close second.


Sunday, January 20, 2013

Pollution Oddities

We remain mired in the inversion, but there are some curious oddities with the PM2.5 concentrations that warrant analysis by people who know more about emissions and air chemistry than me.

From a stability perspective, there's been little change over the past couple of days.  The inversion remains very strong, with a temperature increase of about 13ºC sitting on top of the valley cold pool.

During this period, the 24-h average PM2.5 concentrations at the DAQ sites in Logan and North Provo  climbed fairly steadily.  Utah County is now approaching a putrid 100 ug/m3.

Source: Utah DAQ
In contrast, the 24-h average PM2.5 concentrations in Ogden and Salt Lake have declined somewhat. 

Source: Utah DAQ
Perhaps someone can chime in and explain these declines.  Yes, there are differences in the magnitude and timing of weekday and weekend emissions, but why would Logan and North Provo keep climbing steadily while Salt Lake and Ogden decline?  Chew on that while I get back to football.  

Saturday, January 19, 2013


Lowering Lid

The changes are subtle, but over the past three days, the inversion has gradually lowered and strengthened, reducing the depth of the valley cold pool through in which pollution is trapped.

A quick look at the air quality data shows we have exceeded the National Ambient Air Quality Standard for 24-hour average PM2.5 in several counties including Utah, Salt Lake, Davis, Weber, Cache, and Tooele.  This is a regional scale event with a large fraction of Utah's population impacted.

In Salt Lake, our 24-hour average PM2.5 presently sits at about 60 ug/m3.  Despite a drop in hourly PM2.5 overnight, there seems to be little reason not to expect the longer term upward trend to continue.

Source: Utah DAQ
Our first opportunity for a bit of stirring will be mid week.  The models still lack consensus, however, concerning how much of a mixout we may get.  Keep your fingers crossed.

Friday, January 18, 2013

And Then Depression Set In....

That quote, uttered by Bill Murray character John Wingo in the Oscar-snubbed film Stripes, pretty much sums up the situation this morning.

The upper-air sounding from this morning shows little change compared to yesterday.  We are capped under a very strong inversion in which temperatures increase from -13.1ºC at 5933 ft to 0.0ºC at 7534 ft.

In the Salt Lake Valley, particulate matter concentrations continue to climb.  The eclipsed the National Ambient Air Quality Standard for PM2.5 yesterday and now sit at 47.5 ug/m3.

Source: Utah Division of Air Quality
The one silver lining in this event (if there is one) is that the base of the inversion is still elevated.  As a result, emissions are mixing through a layer that is about 1700 ft deep, which probably helps keep concentrations a bit lower than they would be if the inversion where based at the valley floor, as sometimes occurs during inversion events.

We're stuck with this until at least Tuesday.  Forecasts for mid week vary in the intensity and timing of an approaching trough, so we're just going to have to see in a couple of days if there's any hope of busting this thing out.

As is usually the case during inversions, the weather is spectacular in the mountains.  High temperatures yesterday at the base of Alta were in the 40s.  Escape and enjoy.