The February Southwest Snowpack Disaster

As far as snowfall and snowpack in the southwest are concerned, February was a disaster.

Below is the percent of average snowpack analysis from February 2nd (when I grabbed it for a blog post) showing very healthy conditions relative to average for that time of year across much of the southwest.

Source: NRCS

Fast forward to today and you can see that we have failed to keep pace with the climatological buildup of the snowpack in nearly every drainage basin in California, Nevada, Utah, Colorado, Arizona, and New Mexico.  The situation in Arizona and New Mexico (as well as the south Sierra not shown here) was especially disastrous.  

Source: NRCS

Let's take a look at a few select stations.  At Snowslide Canyon in Arizona (9730 ft), the peak snowpack was achieved in mid January, after which the snowpack has not only lost some ground to climatology, but also in absolute terms, with a slight decrease in total water equivalent.  

Source: NRCS

If we go to lower (and warmer) elevations, the situation is even worse.  At Baker Butte (7300 ft), the snowpack is gone as of today.  

Source: NRCS

How about a snowier location like Squaw Valley near Lake Tahoe?  It hit the wall at 30 inches of water equivalent at the end of January and has been flatlined ever since.  

Source: NRCS

Areas that didn't do too badly are in the northern tier of the western U.S.  Paradise Ranger Station on Mt. Rainier, for example, is still tracking close to median (lower elevation sites in the Cascades lost some ground).

Source: NRCS

Similarly, one can find sites in the northern interior that were able to preserve or in some cases close the gap relative to the lack of snow earlier this winter.  Good for them.  

We'll have to see if the advertised pattern shift in a few days comes through and if the southwest snowpack can get back on track.  

Aspect Is Everything

This weekend I was able to get back into the mountains after a 3 week hiatus.  The transformation during that time was quite dramatic, especially on south-facing aspects.

The importance of aspect (i.e., the direction a slope faces) cannot be understated in Utah and for skiers, especially this time of year.  Given a few days of sunny weather, it is often possible to find corn (or sun crust depending on the time of day) and dry powder on adjacent slopes.

It was pretty easy to find examples of such stark snow contrasts this weekend.  For example, in the photo below, the snow on the left was corn (technically a stout melt-freeze crust since it was taken this morning after the overnight freeze), whereas the snow on the right was soft, settled, dry snow.  The only difference was aspect.  The slope on the left was southeast facing, the slope on the right northwest facing.  

The graphs below show the amount of direct solar radiation on north, west, east, and south facing aspects at 40ºN (very close to the latitude of the central Wasatch) as a function of the time of day on June 21, March/September 21, and December 21.  I've highlighted with red lines the 30º slope angle and with green lines solar noon.  

Source: Whiteman (2000)

We'll use March 21 as a proxy for this weekend, although the numbers for this weekend would be somewhat smaller.  On a north facing 30º slope, the noontime direct solar radiation is about 450 Watts per square meter.  In contrast, on a south facing 30º slope, it's about 1300 Watts per square meter.  That's roughly a factor of three difference.  

In a dry, interior climate like Utah's most of the energy for melting snow comes from the sun.  Even when atmospheric temperatures are above 32ºF, snow on upper-elevation north-facing aspects this time of year often doesn't melt during clear days because the solar input and heat flux from the atmosphere into the snow are countered by long-wave cooling (the snowpack is always emitting long-wave radiation, but receives very little back when skies are clear) and cooling from sublimation.  If you want to get the snow on those aspects to melt, it often takes thin or broken low clouds, which supply some long-wave radiation and sometimes reflect some solar radiation onto the slope (a phenomenon known for better or worse as greenhousing).  

Here are two additional observations from the weekend.  First, as one might deduce from the graphs above, the sun is high enough now that it is illuminating all but the steepest north-facing slopes at noontime.  As a result, the views are simply spectacular.  

Second, although I was out of town for the intense frontal passage during President's week, I was able to find evidence of it in the form of downed trees.  I saw several scenes like this on my tour this morning.  

Dust-on-Crust Uncertainty

Desperate times call for desperate measures, so today we talk about the dust-on-crust uncertainty for tomorrow.  The loop below shows a sequence of forecasts for tomorrow afternoon from GFS runs initialized from 0000 UTC on the 25th to 1200 UTC on the 26th (this morning).  Note how the area of precipitation shifts southward into northern Utah, then back north, then southward again, then back north.

Basically, this indicates some uncertainty in the track of the storm.  If the northern track verifies, we'll probably see anything from a trace to an inch of snow.  If the southern track verifies, maybe we get 2-3 inches.  For what it is worth, at Alta-Collins the NCAR 3-km ensemble calls for very little precipitation and the NAM is going for an inch, so I'm just hoping the wind doesn't blow too hard to ruin whatever high-north powder remains in the upper elevation backcountry.  

Is Inversion Season Over?

Yes, and no.

If you mean situations in which a shallow, persistent (i.e., multiday) cold pool is capped by a layer in which temperature increases in height, then it's pretty much over.  With no snow on the ground, a higher sun, and a longer day than a few weeks ago, shallow cold pools are now readily burned off during the day.

That being said, we can still have an elevated inversion, as was the case yesterday afternoon.

Source: SPC

Under such conditions, however, pollutants are mixed through a layer that extends through a greater depth of the valley and PM2.5 concentrations don't get as high as they do during shallower cold pool events in winter.  Nevertheless, you will see some smog around, so we're not pollution free.

Does this then mean that elevated PM2.5 days are over for this winter?  Nada.  We are just moving into the time of year when dust storms occur, and I wouldn't be surprised if we saw some events this spring if the dry conditions continue.

Announcements

With the weather fairly benign over Utah this week, now is a good time to make a few announcements concerning items that may be of interest to readers.

1. The National Weather Service will be hosting the Internountain West Aviation Weather Safety Workshop at Westminster College from May 6-7.  This looks like a good meeting for pilots.  More info below and at http://weather.gov/riverton/iwaws.

2. The Utah Avalanche Center has launched a new initiative centered on backcountry ethics.  This is a critical issue for safety in the backcountry and above highways (e.g., SR-210 in Little Cottonwood Canyon).  Catch the video and coverage at TGR.

3. After a few weeks of scarcity, the 2nd printing of my book Secrets of the Greatest Snow on Earth has hit the shelves and is now available from Amazon and other retailers.   If you don't have a copy already, buy it now to tide you over through the dog days of summer.

Where Art Thou El Nino?

As we near the end of February and the home stretch of the snowpack accumulation season, snowpack water equivalents are running pretty close to average across much of the western U.S.

Source: NRCS

There is, however, reason to be concerned in the southwest where the snowpack in the mountains of Arizona and much of New Mexico are running below average.  I flew over this area about 10 days ago and was blown away by the lack of snow.  Similarly, the southern Sierra are now running at only 88% of average for the date (left hand value below, right is percent of the April 1st average).

Source: California Department of Water Resources

And, while one always needs to be cautious with extended-range forecasts, there's really not much being produced by the models over the next 1-2 weeks for the southwest.  Just to underscore that point, below is the 8-14 day precipitation outlook for the first week of March.

Source: CPC

Oh, where art thou El Nino? I see your warm sea-surface temperatures, but where are your storms?

Whereabouts

Some of you have contacted me concerned about my absence.  Thanks for the concern and for checking in.  

My father passed away over President's weekend, so my attention has been elsewhere.  He inspired much of what you read on this blog.  I will write again, but don't have a timetable for a return.  I suspect and hope it will be sooner rather than later.  Until then, take a run for us. 

Best,

Jim

A "Silver Lining" to This Inversion Episode

For the most part, the news during prolonged inversion events is bad, but I did want to share one "silver lining" with regards to the current event, which is the cloud cover and what it is doing to increase mixing in the Salt Lake Valley, especially overnight.

The air is still polluted, but these clouds are actually a help rather than a hindrance

If the solution to pollution is dilution, then the development of cloud cover over the Salt Lake Valley is an improvement over the cloud-free inversion that we had prior to yesterday.  Let me show you why.

Below is a loop of upper-air soundings collected each morning at the Salt Lake City airport over the past week.  These soundings are plotted as a "skew-t" so a line of constant temperature slopes upward to the right.  Note the two-stage evolution of the inversion event.  First, the upper levels warm dramatically while the morning surface temperatures remain relatively steady.  This occurs in the first 5 frames of the loop (note how the red line shifts rightward from about 800 to 600 mb).  Second, a mixed layer forms very near the surface in the last 2 frames, as indicated by temperatures cooling with height to an level just above 850 mb.  

Morning upper-air soundings from the Salt Lake City airport from 5–12 Feb 2016

Here are a couple of still images to further highlight this point.  On the morning of 8 February, the inversion was based right at the surface.  In such a situation, there's very little vertical mixing of the airmass.  Pollution is trapped right at or very near the valley floor.  

Morning upper-air sounding from the Salt Lake City airport on 8 Feb 2016

This morning (11 Feb), however, the inversion base is elevated and sits at 836 mb (5680 ft), almost 1500 feet above the valley floor.  Below the inversion, there is a shallow mixed layer that is allowing for some vertical mixing of pollutants.  

Morning upper-air sounding from the Salt Lake City airport on 11 Feb 2016

The growth of such a mixed layer is very common following the formation of low clouds during inversions in the Salt Lake Valley.  Radiative cooling at cloud top drives turbulence that mixes the layer in and below the clouds.  The end result is what meteorologists call a cloud-topped mixed layer.  If you are in the Salt Lake Valley this morning, you are not in the inversion, you are beneath it, in the cloud topped mixed layer.

What does this mean for pollution?  Well, it means that at night there's more mixing through a deeper layer, which reduces PM2.5 concentrations.  The presence of the mixed layer also slows the long-term rate of rise (NOTE: Concentrations are still high, so this is no reason to roast marshmallows over a fire tonight).  For example, PM2.5 levels at Hawthorne last night were not as high as they were the previous night.  In addition, there is a flattening of the long term trend.  

 As discussed in the previous post, PM2.5 concentrations at University of Utah sensors are running higher than those aw Hawrhorne, but they also suggest a flattening out over the past 24 hours or so. 

None of this means the air quality is good.  We are still in unhealthy territory.  What it does mean is that the rate of increase along the valley floor will probably slow.  Those of you on the upper benches may see more sustained high PM2.5 levels in this scenario as the base of the inversion lifts and you are more continuously enveloped in the gunk.  Perhaps this isn't a silver lining for you.  

Science wonks can see Pataki et al. (2005) for more info on these cloud-topped mixed layers and their influence on mixing and transport in the Salt Lake Valley.

Above My Pay Grade

Some of you have commented about the large difference between the DAQ sampler at Hawthorne and the observations collected along the Trax line as well as at Neil Armstrong Academy and the University of Utah.

For the most part, these sensors have generated pretty similar results for the first few days of the inversion.  PM2.5 at Hawthorne has risen fairly steadily during the period (with some ups and downs) with a range from midnight on the 9th to midnight on the 10th between about 38 and 54 ug/m3.  PM2.5 at Neil Armstrong Academy (sorry for change of scale) was generally close to Hawthorne through midnight on the 10th, with the range from midnight on the 9th to midnight on the 10th of about 39 to 65 ug/m3.

Source: MesoWest

Source: MesoWest

The divergence begins shortly after midnight on the 10th (last night), with NAA going to much higher values.  Similar, other samplers operated by the University of Utah at the University of Utah and along Trax are also reporting high values compared to Hawthorne.

It is above my pay grade to explain these differences.  I know little about the measurement of PM2.5, other than it is very difficult, or the instruments used, other than the fact that the DAQ and University of Utah samplers are different.  The discrepancy appears to have developed during a period when fog was in the area.  I don't think this is a coincidence, but I lack knowledge of the sensor characteristics and atmospheric chemistry to provide a reasonable hypothesis why.  I'd rather say I don't know than speculate.  

As such, in future posts during this event, I will be referencing both samplers and mentioning the uncertainty at play.  

Note, however, that the lower Hawthorne sampler was in the unhealthy category for several hours today and the 24-h average is now very close to the unhealthy threshold.  Thus, even though there are uncertainty in the measurements, this remains a serious event.

Can Mother Nature Crack This Terrible Inversion?

The situation in the Salt Lake Valley continues to deteriorate and is becoming quite serious.

Here on campus, we are currently fogged in.  Although depressing, it doesn't look as ugly as the pollution does under clear skies, but don't be fooled.

PM2.5 concentrations this morning are sky high.  The University of Utah's Trax-mounted PM2.5 sampler is measuring concentrations this morning in the 85-115 ug/m3 range (maroon filled circles below).

I did a quick inspection of the data and found a maximum concentration of 111 ug/m3, which is well into the unhealthy category.  

The terrible situation we are in is one of our own doing.  Persistent cold pools (a.k.a., inversions) are naturally occurring phenomenon in the western United States during the winter.  One shouldn't equate inversions with air pollution.  Inversions happen all over the west.  Pollution happens in those areas that experience inversions and where emissions are concentrated.  

At this point, we're up to our eyeballs in alligators.  We have absolutely terrible air quality and things are not going to improve until Mother Nature cracks this inversion.  What will that take?

The situation we have right now is one where dense, cold air is pooled in the Salt Lake Valley (and other basins of northern Utah).  From March to October, there is typically enough energy provided by the sun each day to warm that airmass and allow it to mix with the air aloft, limiting pollution concentrations.  This time of year, however, we don't have enough solar energy to mix out the cold, dense airmass.  We're basically mired in an oil and water situation.  I did a quick calculation and found that the air near the valley floor is about 25% more dense than that at crest level.  That's a big number.  

Thus, there are only two ways to get rid of this inversion.  One is to bring in even denser, colder air at upper levels, allowing what meteorologists call buoyancy driven turbulence to scour out the valley.  The other is to increase the winds near the top of the cold pool, allowing what meteorologist call mechanically driven turbulence to scour out the valley.  

One can do another quick calculation and find that for a valley temperature of 0ºC one needs to bring in an airmass with a crest-level temperature of about -17ºC to remove the cold pool with no help from the wind.  You can get a way with a higher crest-level temperature if the valley is a bit warmer (such as might be found in the afternoon), or if clouds are present (the release of heat in clouds helps invigorate turbulence).  So, perhaps we could do some damage if we got down to say -12ºC or so. 

Of course if you add wind, you can sometimes get a bit more bang for the buck.  It's very hard, however, to determine how wind will influence a cold pool unless it is very strong.  

Which brings us to our sole glimmer of hope, the trough that is forecast to brush by Utah on Saturday evening.  This will drop our crest-level (700-mb, 10,000 ft) temperatures to about -6ºC to -8ºC.

That by itself is not enough to crack the inversion, but there is also an increase in flow aloft and at low levels.  

We really don't have the tools today to determine if this will yield a full mix out (I doubt it, but can't rule it out), partial mix out at all elevations, a scouring out of the pollution from the top down, leaving a lens of pollution near the valley floor, or no change.  This is in my view a critical area for research as it is forecasting of these weaker trough passages and their influence on inversion and air pollution strength that is the hardest part of forecasting these events. 

As things stand now, all we can do is hope for the best, but I'm concerned that even if we mix out some, we're still going to see this event persist into early next week.  

Strengthening Inversion, Worsening Air Quality

A comparison of soundings from yesterday morning and this morning shows further strengthening of the inversion over the past 24 hours.  Yesterday morning, the surface temperature was 27ºF (-2.8ºC), with temperatures aloft maxing out at 1.4ºC just below 700 mb (10,000 ft).

Source: SPC

This morning, the surface temperature is once again 27ºF (-2.8ºC), but temperatures aloft have increased at all levels up to about 650 mb, with a maximum of 3.6ºC at 773 mb (8000 ft).

Source: SPC

Above the valley floor, the freezing level isn't hit until just over 12,000 feet.  You'll find some below freezing air in cold mountain spots this morning, but with the exception of shady north-facing spots, temperatures throughout the Wasatch Range will be above freezing today.

Air quality in the Salt Lake Valley continues to deteriorate.  Observations from DAQ sensors and from Trax-mounted sensors operated by the University of Utah are generally between 35 and 60 ug/m3, which is in the unhealthy for sensitive groups category.

Source: MesoWest

Observations from Hawthorne Elementary along 700 East reached 55.2 ug/m3 overnight, just shy of the dreaded red "unhealthy" threshold.  They have relaxed some overnight, but remain in the unhealthy for sensitive groups category.



Although there are short term ups and downs in PM2.5 concentrations, with values teaching their highest values late in the day through midnight followed by a decline, the long-term trend is upward.  That upward trend is actually not due to the inversion getting stronger (once the valley is capped off, further strengthening doesn't matter).  Instead, we're in a situation where emissions continue to accumulate in the valley.  Further worsening of the air quality will occur through at least the end of the week.

The models are flirting with a trough passage this weekend.  I say flirting because much depends on what model and ensemble member you look at.  Some have a brush by, others a more direct hit.  At this point, it's too soon to say if it will crack this thing, give us at least a partial mixout, or leave the air pollution fully intact.  The Euro provides a brush-by to the north, but perhaps enough to stir us up a bit.

Source: Penn State e-wall

That being said, there's no guarantee we'll get much relief yet and we'll have to see how the forecasts evolve in the coming days.  Steel yourself for a long ordeal, reducing driving, and hope for relief this weekend.

In the Grips of a Monster Ridge

After a run of active weather and regular powder refills from about mid December through early February, the wheels have come off our winter.

We are now fully in the grips of a monster upper-level ridge that will keep the mountains dry and the valley inverted.  On the positive side, that means warm, sunny days in the mountains.  On the negative side, we are staring directly down the barrel of what will be a very poor air quality episode.

Meteorologists use a diagram known as a Skew-T to examine upper-air soundings collected by weather balloons.  In these charts, lines of constant temperature are "skewed" (blue dotted lines below).  Yesterday morning's sounding showed a shallow inversion at low levels.  The surface temperature was 26ºF -3.3ºC and the temperature at the top of the inversion near 800 mb was about 0ºC.  Above this level, at 700-mb near the crest of the Wasatch Range, the temperature was about -4ºC.

Source: SPC

The big change overnight was warming at crest level as the ridge moved further eastward.  Temperatures at 700-mb are now around 0ºC.  A close look at the sounding reveals a temperature increase from -2.9ºC at the surface to 1.4ºC just below 700 mb.

Source: SPC

Those are the gory details.  Bob Dylan would simply say that you don't need a weatherman to know that we're in an inversion.

PM2.5 levels prior to midnight at the Hawthorn air quality site climbed well into the unhealthy for sensitive groups category.  They decreased some overnight, but popped up in the past hour to the unhealthy for sensitive groups threshold.  24-hour averages (blue dots) are now above EPA Clean-Air Act standards.

Source: Utah Division of Air Quality

Temperatures aloft will continue to rise through late Tuesday when the ridge axis is centered over the Intermountain west and the ridge is at maximum intensity.  At that time, forecast free-atmosphere 700-mb temperatures (near 10,000 feet) are near 4ºC.  

Such temperatures are very near the outer edge of anything previously observed in February over Salt Lake City.  The graph below shows the record maximum (red line), median (black line), and record minimum (blue line) 700-mb temperatures observed in soundings collected either in Ogden or Salt Lake City over the past few decades.  In February, the all-time record is 5.8ºC on Feb 1.  For the periods we are heading into (Feb 8–12), the highest previously observed is 4.4ºC.  

Source: SPC

Bottom line: This is going to be an outlier inversion event in terms of meteorological strength and duration, a situation further exacerbated by the extensive snow cover on the valley floor.  From an air-quality perspective, it will easily be the worst event of this season, with the situation worsening through the week. 

This Inversion Is Going to Be Ugly

Smog in the Salt Lake Valley this afternoon as viewed from the Mt. Olympus Wilderness.  Antelope Island sticking up in the distance.

It's time to get real about the air quality this week.  The system that passed by yesterday, and the strong crest-level winds today, have done essentially nothing to stir up the air in the Salt Lake Valley.  We are now facing a situation where the air quality is already poor as a remarkably strong upper-level ridge builds over northern Utah.  With snow on the ground, this is essentially a worst-case scenario for air quality in February and it needs to be treated as such.

How bad is the air qualty?  Well if you wanted to know this afternoon and had to rely on the Utah Division of Air Qualty, the answer is unclear.  Here's what I got when I tried to go to their web site this afternoon. 

This is simply unacceptable, because the air quality is indeed poor this afternoon.  We know because the University of Utah operates a PM2.5 sampler at the Neil Armstrong Academy in West Valley City and it shows PM2.5 concentrations near or above 35 ug/m3.  That puts it into the unhealthy for sensitive groups category.

The graphs above show that the PM2.5 climbed quite dramatically today, more than we typically see.  I'm not sure if that might be due to photochemistry or perhaps an especially dirty airmass over the Great Salt Lake (there was a bump when the flow switched to west).

The situation is only going to worsen from here.  We will be in the grips of the upper-level ridge through at least Friday.  After that, we'll have to see.  There are some weaker systems being advertised as perhaps influencing things next weekend, but it's too far out to say how things will evolve.

The bottom line is that this is a very serious situation in which we will see poor air quality for several days.  

Today's Three Seasons and Air Pollution

It's a spectacular day in the mountains today.  One that will surely infect some with spring fever.  Visibility was outstanding and I was quite happy to share the day with my son.

We really experienced three seasons today.  The first season was on south-facing aspects where the February sun was causing  dendritic destruction and made it feel like spring.  I was glad we did our south-facing climb early in the morning as it was like a microwave by mid day.

The second season was on other aspects where you could find some nice, settled, mid-winter powder.  This was essentially my son's first taste of pure, unadulterated, Wasatch backcountry powder.  No doubt he'll be back for more.

Of course, I also taught him some other critical skills required for backcountry skiing.

The third season we experienced today was after we returned to the smog of the Salt Lake Valley.  Yes, inversion season is still here.

Unfortunately, the system passing to our north today hasn't done much to stir up the valley, although it has dropped PM2.5 levels some.  

I guess that's better than nothing.

Although we may scour a bit more through tomorrow, this coming week will probably bring the worst air quality of the winter. 

Bad Air and Important Survey

I am swamped today with the demands of my day job.  I wish to take a moment to highlight two important items.

First, a monster ridge will be in full force next week.

We are already in moderate air quality and it's still unclear if we're going to get much relief from the system that will be passing to our north tomorrow.  A poor air quality episode is likely next week.  Now is the time to reduce trips and cut emissions.

Second, the Mountain Accord is doing a transportation survey for the Cottonwood Canyons.  Put in your $0.02 at http://mountainaccord.com/mountain-accord-transportation-survey/.

Surprise Delivery

After yesterday's bummer of a trough passage, which produced all of one inch at Alta-Collins, I confess that I was in quite a pessimistic mood when I went to bed last night.  Thus, it was a pleasant surprise to wake up to a fresh coat of some of Mother Nature's finest dendrites.

The real surprise, however, was in the mountains.  The Little Cottonwood snowmaking machine was really cranking last night with Alta-Collins picking up 11 inches of cold smoke.  That appears to be quite a bit more than fell elsewhere in the Wasatch, as well as that predicted by the models and by operational forecasts.  The National Weather Service Little Cottonwood forecast issued yesterday afternoon called for a 20% chance of a trace and an 80% chance of 1-3".

Source: NWS

And just to show I'm not picking on them, I last did a blog post on Wednesday and while it examined the high-amplitude ridge that will be building over us the next few days, I did mention that I expected 2-4" from yesterday's trough passage.  Some of that I expected to fall during the day yesterday.  If I had produced a forecast yesterday, it probably would have been no better than the NWS forecast and more likely worse.  Other ski weather forecast sites also crashed and burned.  I'd be drinking, but I've consumed my stock of Forecaster's Friend following previous forecast busts.

We still have some work to do, especially regarding what I like to call the "post-frontal crap shoot." It is during these periods of northwesterly flow, when snowfall is produced by shallow convective clouds that are highly sensitive to the depth and strength of instability and moisture, as well as with flow interaction with the mountains and sometimes the lake, that we struggle the most.  Last night provides a good example.