Saturday, January 20, 2018

A Good Front Followed by Bad Orographics

A cold front moved across northern Utah last night bringing much needed snow from the valley floor to the highest peaks.  Powder panic brought gridlock to most (probably all) routes to the Cottonwoods, providing an all-too-frequent reminder that we are loving our canyons to death. 


Those who braved the traffic were rewarded with knee deep powder.  I would describe the skiing as good, not great.  A spongy layer of high-density snow to start would have helped reduce the bottom feeding, but this year, beggars can't be choosers.  



Snowfall produced by the front went largely as advertised by the models, at least in the upper Cottonwoods.  Alta-Collins had .59" of water and 7 inches of snow through 8 AM this morning.  This compares very well with the NAM forecast we discussed on Tuesday, which put out .64" of water and 8 inches of snow (see Frontal Snowfall Event on Tap for Late Tomorrow and Tomorrow Night).  The observed water totals are also near the middle of what was advertised by the University of Utah downscaled SREF ensembles.  The model wizards can be happy about this period.

I suspect that those hoping for a true storm ski day were a bit disappointed, however, with today's offering.  Light snow fell for much of the day, but since 8 am it added up to only .15" and 2 inches of snow at Alta Collins, which is probably a bit more than what we saw where we were ski touring in Big Cottonwood Canyon.  

Quite frankly, the orographic forcing today simply sucked, which we discussed as a possibility yesterday (see Probabilistic Snowfall Forecasting).  The morning sounding shows the situation quite well.  The atmosphere was quite moist, but also generally stable below 700 mb (10,000 ft), with a strong stable just above, which is associated with the front aloft.  The flow at low levels was northerly, but swung to southwesterly in the strong stable layer.  This is simply not a recipe for orographic enhancement.  

Source: NCAR/RAL
During the afternoon, the flow on Alta-Mt. Baldy slowly shifted to northwesterly to westerly, but remained weak and with high atmospheric stability, wasn't generating much at upper elevations.  


Instead, the radar loop below shows the development of the strongest echoes along the east bench and within the lower canyons through 0028 UTC (5:28 PM MST).  


Driving down Big Cottonwood late this afternoon, it was clearly snowing harder in the very bottom of the canyon and along the east bench than it was in the upper canyon.  Note that you can also see this effect in the Oquirrh Mountains where the radar returns, especially later in the loop are stronger on the lower and mid elevation western slopes.  

The devil is in the details.  

Friday, January 19, 2018

Probabilistic Snowfall Forecasting

For decades, snowfall forecasts have typically involved the issuance of a range of accumulation amounts, typically (but not always) based on a factor of two.  For example, 3-6 inches, 4-8 inches, etc.

I have no idea why.  Perhaps it is a convenience thing.  Maybe people like it that way.  I don't know what that range even means.  Does it represent the middle 50% of possible outcomes, with a 25% chance of more and a 25% chance of less?  Does it represent the middle 80%?  Why always use a factor of two?  Sometimes the range needs to be bigger, especially in longer range forecasts. 

And then there is my favorite, "higher amounts in favored locations."  What the hell does that really mean and how do you verify it? 

There was a time when snowfall forecasting was truly guesswork, but things are changing.  Computer models are now capable or will soon be capable of simulating smaller storm details, what meteorologists call "cloud scale."  Ensembles can be used to better estimate the future outcomes.  There remains much work to do, but there is great potential to dramatically improve snowfall forecasting. 

The National Weather Service is now producing experimental probabilistic snowfall forecasts, and they are available at https://www.weather.gov/slc/winter.  They provide much richer information about storm potential than a simple range.  For example, ,aps are provided showing the likelihood of snowfall above several thresholds, an example of which is the probability of 6" of snow or more for the period from 5 AM today to 5 PM Sunday, shown below. 

Source: NWS
They also provide a table with snow amount potentials and probabilities of snow within certain ranges, shown below, as well as above certain thresholds. 

Source: NWS
Readers of this blog are snow lovers.  Start perusing these forecasts and provide feedback through the links on the page. 

Now, to clarify some of my scattered comments to yesterday's post about the situation on Saturday.  Although we have a front pushing through tonight, it is a very slow mover.  As a result, this is not a frontal passage in which we quickly get into deep, unstable, northwesterly flow on Saturday morning. 

This is evident in the NAM time-height section for Alta below.  The front at Alta is a late arriver (light blue line), in this case moving through at or just after 0600 UTC (11 PM MST tonight).  Then, look at the winds behind the front on Saturday (circled).  They are NNW at low levels, but NNE near 700 mb (10,000 ft) and then SSW at 600 mb. 


This reflects the slow movement of the front through the area. 

If we look at the sounding for 1800 UTC (11 AM MST) Saturday morning, we see the low level northerly flow, but note how the winds shift to NNE and then SSW with height.  The temperature and dewpoint traces show a sharp inversion just above 700-mb, or 10,000 feet.  
This is not a recipe for our classic northwesterly instability snow showers over Alta for two reasons.  First, the flow direction isn't right.  Second, the instability is too shallow.  

However, if you look at the sounding for 0000 UTC (5 PM MST) tomorrow afternoon, the low level flow is NNW through a deeper layer, although a capping inversion remains based just below 600 mb.  This is closer to what is needed for the NW instability showers, but the capping inversion height is right on the edge of what I would like to see.  Tough to say if it's high enough that Alta can benefit, or just a bit too low so that the mid and lower canyons and east bench do better.  
And that's just one model run.  There are variations in the timing of these changes, wind directions with height, etc., if one looks at other models.  

All of this illustrates what a complex mess this is for Saturday and why probabilistic forecasting is necessary.  The good news is there's enough going on that in the end, this will be a decent storm for the mountains and even the mountain valleys after snow levels lower today and this evening.  

Thursday, January 18, 2018

Frontal Snowfall Event On Tap for Late Tomorrow and Tomorrow Night

After another 10-day or so stretch with limited to no accumulations, our next storm will be served up late tomorrow and tomorrow night. 

Most of the precipitation for the central Wasatch looks to be primarily frontally forced.  The large scale setup is shown below and features an upper level trough that is initially tilted from southwest to northeast (referred to as "positively" tilted by meteorologists) that closes off and becomes more north-south oriented as it moves inland across the western US.  




This has both pluses and minuses for snowfall prospects in the central Wasatch.  The plus is that the front may slow as it drags through northern Utah, extending the period of frontal snowfall, as depicted below in the 1200 UTC NAM forecast.  At 000 UTC 20 January (5 PM MST Friday), the surface front is over Utah County with precipitation over the northern Wasatch.  


Frontal precipitation fills in, however, as the front phases with moisture sneaking around the southern end of the Sierra Nevada over the next 3 hours. 


That precipitation continues for another 3 hours as the front makes slow progress into southern and eastern Utah. 


By 0900 UTC 20 March (2 AM MST Saturday) the main frontal band is just downstream of the central Wasatch, with some post-frontal snow showers persisting. 


The minus for snowfall prospects is with the low closing off, the post-frontal winds shift very quickly to northerly, when we would prefer a period of northwesterly flow for better orographic forcing.  Note in the Salt Lake City time height section below that the post frontal flow is predominantly northerly and deepens gradually from about 0Z Saturday through 6Z Sunday.  



Actual numbers derived from the 12Z NAM show the wet bulb zero dropping during the day Friday (snow level is usually about 1000 ft below this level), with values low enough that most of the precipitation produced during this event should fall as snow in the mountain valleys.  Perhaps Mountain Dell might see a bit of rain to start, but then turn over to snow.  Total water equivalent at Alta is 0.64" through Saturday at 8 AM, with snow densities decreasing during the storm for a right-side up snowfall.  



Looking more broadly at the ensembles shows that the NAM is roughly in the upper half of the SREF plume for Alta.  Through 18Z 20 January (11 AM) the SREF members put out anywhere from 0.3 to 0.8 inches of water, the former being a slightly better than dust on crust event adding up to perhaps 4 inches of snow, the latter representing a lower end deep powder day with perhaps 10-12 inches of snow.  

That spread represents variations in the strength and speed of the front.  Increases in precipitation after 18Z 20 January occur in some model runs that are more bullish on the post-frontal precipitation.  

I continue to keep expectations low and hope for the best.  

Wednesday, January 17, 2018

What Happens to Fronts over the Western US

It's a question that fascinated me 30 years ago and continues to fascinate me today.

What happens to fronts over the western US?

The short answer is a lot, and it varies from case to case.

Let's begin with a loop of the front making landfall later today and tonight.  This loop presents sea level pressure in black contours, 850 mb (1500-m) temperature in red contours, and either radar or 3-hour forecast accumulated precipitation in color fill.  In this loop, note how the front appears to decelerate as it pushes inland across Oregon and northern California and ultimately takes on an "S" shape with the inflection near the trough that develops downstream of the Sierra Nevada. 


These and other effects are more apparent if we look at individual times.  At 1800 UTC 17 January, the system has the appearance of a classic occluded cyclone with a relatively smooth and continuous cold front extending from just off the Washington coast into the subtropics. 


Eighteen hours later, the front is pushing into Oregon and northern California (I've given up analyzing the fronts north of this location as they have become difficult to track).  The front at this time is beginning its inland transformation and is decelerating over southern Oregon and northern California.  At the same time, there is a temperature contrast just ahead of the front over California and the eastern Pacific (circled with a thin light blue line).  This is an important feature as a new front is beginning to form at the leading edge of this zone of temperature contrast.   


By 1800 UTC 19 January, the front and the temperature contours have taken on an "S" shape with the inflection over northern Nevada.  This S-shape is the result of several processes, including the deceleration of the front over northern California and Nevada, and the development of a new front ahead of the old Pacific cold front over the eastern Pacific.  Note in particular that the precipitation band accompanying the old Pacific front is well behind the leading edge of cold air at this time, consistent with a new front forming ahead of it. 


By 1800 UTC 19 January, the front has a strongly distorted into an S-shape.  One can quibble with the precise position of the cold front in my analysis over southern California and Baja California, but the S-shape is very clear in the temperature contours.  Again, note the separation between the cold front and the model precipitation. 


The processes responsible for this evolution are complex and not well captured by simple models of frontal evolution.  Where to position a front in an analysis is always a subject for debate, but the development of the S-shaped appearance is common as cold fronts make landfall in this part of the world. 

If you are wondering what all this means for snow, tough luck.  I've already spent too much time on this post and need to get some work done!  Maybe tomorrow.  Maybe. 

Tuesday, January 16, 2018

Global Warming and the Salt Lake City Olympic Bid

Soldier Hollow Olympic Venue, March 13, 2016
Today's Salt Lake Tribune features an article discussing how global warming could affect future Winter Olympic Games.  Salt Lake hosted the 2002 Olympic Winter Games and is considering a bid for 2030.  It is my view that solely from a climate perspective, Salt Lake City is one of the best places to host the Olympic Winter Games as the vulnerability of our snow climate to warming is considerably lower than many other locations that have hosted the Winter Olympics.

I have had the good fortune to attend two Winter Olympics (Nagano and Salt Lake City), contribute to the weather support effort for the 2002 Salt Lake City Olympics, and participate in meteorological training or consulting activities for several others (Turin 2006, Vancouver 2010, Pyeongchang 2018, and Beijing 2022).   Weather-risk management is critical to the Olympic effort, from ensuring spectator safety to executing fair and safe competitions.

Today, as many competitions as possible are held indoors to reduce weather vulnerability.  Figure skating was once an outdoor competition, something that seems laughable today.  In 1980, Eric Heiden won 5 speed skating gold medals on an outdoor track in Lake Placid.  No contender for Olympic Games host would ever propose an outdoor speed skating competition today.  Even if the climate would support it, indoor facilities allow one to tune knobs and produce faster ice, something that is a high priority for Olympic competitions.

However, there are some competitions that still must be held outdoors.  They include alpine skiing, biathlon, bobsleigh, nordic skiing, freestyle skiing, luge, nordic combined, skeleton, ski jumping, and snowboarding.  Even here, technology is brought to bear to ensure high quality competitions.  Bobsleigh, luge, and skeleton tracks are refrigerated and snowmaking is used to ensure adequate coverage at other venues.  Sochi resorted to stockpiling of snow from the winter preceding the Winter Olympics in case things really went south.  I suppose it is only a matter of time before Abu Dhabi proposes to host a completely indoor Olympics with refrigerated facilities, but for now, some competitions need to be held outdoors.  In case you are wondering, this would be a "tall" order, since the minimum vertical drop of an Olympic downhill is 800 meters (2,625 feet).

The vulnerability of potential Olympic hosts to global warming varies.  Salt Lake City benefits from a relatively cold, continental climate, high-altitude venues, and abundant natural snowfall.  This year provides a stress test of sorts, with an extreme drought and mild temperatures.  Although non-ideal if this weather were to persist into February, the traditional month for Winter Olympics competitions, the Games could still be held.  In 2030, the planet will likely be warmer than today (baring a major volcanic eruption or decrease in solar output), but the odds of not having snow at the outdoor venues in Salt Lake are very low, with one possible exception, discussed below.

Geography and tourism researchers have examined the vulnerability of past Winter Olympic hosts to global warming.  The analysis relies upon climate sites near to outdoor venues rather than specific climate information for each outdoor venue, but it provides a reasonable intercomparison of host vulnerability.  It also attempts to account for snowmaking.

Below are their estimates of the probability of having a snow depth with snowmaking of 30 cm (12 inches) on 1 February under the "current" (1981-2010) climate and low and high greenhouse gas emissions scenarios through the 2050s and 2080s.  30 cm is considered the minimum needed in smooth terrain for many competitions.  The most vulnerable sites are Sochi, Garmish-Partenkirchen, and Squaw Valley.  Salt Lake is one of the more resilient sites with this model predicting a 100% likelihood of 30 cm or more snow with snowmaking even in 2080.

Source: Scott et al. (2015)
The study above is based on a number of models, and thus I would be cautious saying Salt Lake City has a 100% chance of being able to host an effective Olympics in 2080.  However, the view that Salt Lake City is one of the better sites to stage an Olympics giving a warming climate is accurate.  Note that this does not mean that the Olympic weather would necessarily be ideal or that we wouldn't be attending events in shorts and t-shirts.

But let's think about the more immediate future, 2030.  What happens in 2030 should Salt Lake City host the Olympics will be strongly dependent on the whims of the jet stream.  Global warming is loading the dice for warmer winters and perhaps less abundant low-elevation snowfall and snowpack, but February 2030 could still be cold or cold and snowy.  On the other hand, what if it were warm and dry?  Well, the odds of not having enough snow at the venues is nearly zero due to the capacity to make snow during colder periods leading up to the games.  This season we're skiing despite an exceptionally dry and warm winter so far.

The one concern might be Soldier Hollow.  It is a relatively low elevation venue in the Heber Valley with courses spanning 5,500 to 5,900 feet.  This low elevation reflects current requirements of the International Ski Federation and makes Soldier Hollow, which already receives scant natural snowfall, somewhat more vulnerable to global warming than higher altitude venues.

However, Soldier Hollow does have a few things going for it.  It is on the north side of a ridge, providing shade when the sun-angle is low, and it tends to see strong cold pools develop at night and, during winter, persist at lower elevations.  This creates good snowmaking potential for its elevation.

Currently, however, Soldier Hollow only has 1-km of trails open.  It is my understanding that they have had some problems with their snowmaking system, which would presumably remedied with no expense spared in an Olympic year.  The other concern might be the Paralympic Games, which are typically held in March. The photo at the top of this post was taken on March 13th, 2016, providing some perspective.  However, I suspect Soldier Hollow will be able to stage the Paralympic competitions, although they need a functional snowmaking system and won't want to skimp on stockpiling snow for insurance.

Sunday, January 14, 2018

Today Went to an Eleven

As far as weather is concerned, today was a Spinal Tap day, because on a scale from one to ten, it went to an eleven.  


The beauty of January is that you can have a warm airmass, abundant sunshine, and snow that on north aspects just doesn't want to melt.  The reason for this is while a warmer atmosphere transfers energy to the snowpack, the snowpack is also losing energy because it is emitting far more long-wave radiation than it is receiving from the clear skies.   As a result, despite the warmth, the snow remains dry except in areas where it is receiving a lot of solar radiation (e.g., south aspects).  In March, the story would have been much different, with snow melting on all but the steeper northerly aspects due to the higher sun angle and greater incoming solar radiation.

We spent the day with out of town guests at Alta.  For someone from the east coast, today was pretty much as good as it can get. 


Surely the resorts were happy.  Nothing is better for business than clear skies and warm weather on a holiday weekend. 


And, while we have a mid December snowpack, the skiing remains better than expected.  Everyone at Alta deserves a pat on the back this year for great work.  With the High-T open, it was fun to ski some steep lines.  Once off the ridge, cover was quite good and the chalky conditions fun, especially in the afternoon. 


The holiday weekend concludes tomorrow with another warm, mostly sunny day.  Enjoy.

Saturday, January 13, 2018

Incredible Northern Wasatch Asperatus Display

A trip to the Ogden Valley is almost always worth it in winter and that was especially true today when we were blessed with an incredible display of asperatus clouds over Ben Lomond Peak.



Rather than call them asperatus clouds, I've always thought they should be called "dementor clouds" after the dementors in Harry Potter. 

The drive home wasn't bad either with a good display over Thurston Peak.


If you are looking for snow for cross-country skiing, North Fork Park is often the place to find it as they get a remarkable amount of snowfall.  They were able to get out and do some grooming last night and while conditions were variable, they were fun.  Even in areas where they didn't groom, the conditions were tolerable. 


For $6 you can't go wrong, and we saw only a few people once we left the yurt.  Plus, the views of Ben Lomond and Willard Peak never disappoint. 


More info at Ogden Nordic's web site

Thursday, January 11, 2018

A Storm with Big Winners and Big Losers

I can't recall a storm in which the winners and losers were more defined than the one that hit us Monday night through Wednesday.  In part, this reflects our meager snowpack, which makes the vulnerability to warmth and rain even more obvious than when we have a more robust low-elevation snowpack.

The winners were clearly the upper-elevations of central Wasatch, above perhaps 8500 or 9000 feet.  The central Wasatch have a great deal going for it, including elevation, and it paid off in spades this week.  Data from the Snowbird SNOTEL shows a nice uptick in snowpack water equivalent. 

Source: NWS
The storm represented only the fourth since mid November, but it finally brings us to something close to mid December snowpack for this aspect and elevation

The losers?  The storm was pretty much a disaster near or below 7000 feet where almost all the precipitation fell in the form of rain.  I was glad to find late yesterday that Mountain Dell still had enough snow to skate ski, but the situation there and along trails in the Park City area is now critical or beyond critical.  Such a shame. 

The Ben Lomond Trail SNOTEL is at 6000 feet and it illustrates the dire situation down low.  Keep in mind this is an extremely snowy location — typically with a much deeper snowpack than found at comparable or even higher elevations in the central Wasatch or Park City area.  There was a bit of a bump in snowpack water equivalent at this location during the storm, with no net change.  I suspect what happened is that the snowpack soaked up the rain initially, causing an increase, afterwhich it experienced net melting and loss.  The site now sits at about 25% of median. 

Source: NWS
Looking forward, we will see some snow showers at times through Friday.  Then, the zombie apocalypse returns with a high amplitude ridge developing over western North America.  


The positive?  It looks like a beautiful Martin Luther King weekend.  For many out-of-towners, and what should be excellent conditions on the groomers, it will be the stuff that dreams are made of.  I expect there will be big smiles at the Cottonwood Canyon resorts.  

Wednesday, January 10, 2018

Big Snows in the Western Alps

By popular demand, here's a quick post on the big snows in the western Alps.

You may have seen news stories about this.  Huge snows in portions of the French, Italian, and Swiss Alps.  Thirteen thousand stranded in Zermatt, etc.

I couldn't find a good map of recent Alpine snowfall, so I snagged just a few notable numbers from ski reports online.  On Monday, 8 January, Tignes (France) reported 31" of snow, Sestriere (Italy) 31", Cervina (Italy) 10", and Zermatt (Switzerland) 20".  The next day, Tignes got another 31", Sestriere 39", Cervina 20", and Zermatt 0.  I'm not sure exactly where those obs are taken, and if the 0 at Zermatt might indicate no report, but there were significant snows in the western Alps.  There can be huge contrasts in snowfall across and along the Alps, but I haven't bothered to detail these small scale variations here.  I will say that snow-depth forecasts from wxcharts.eu for the period ending late on 9 January showed a pronounced max along the Alpine chain at the upper reaches of the western Po Valley catchment.  This would include Sestriere and Cervina.  Tignes and Zermatt sit just across the Alpine crest.

The setup for the event is shown below and features a digging upper-level trough that develops over western Europe and closes off over Spain and the western Mediterranean Basin before moving downstream.


In the interest of time, I've thrown together a map of the event, valid 1200 UTC 8 January, that includes sea level pressure (black contours), 925 mb (~750 m above sea level) wind vectors, and 925 mb temperatures.  At this time, the surface cyclone was centered roughly over the idland of Menorca in the western Mediterranean.  The cold front was ahead of the low center, passing over the island of Sardegna.  As a result, low level southeasterly flow over the Tyrrhenian Sea and west of the Italian boot impinged on the western hook of the Alps.  At the same time, there was strong flow blocking on the southern side of the Alps, resulting in a "u-shaped" sea level pressure ridge and easterly flow over the Po Valley.  As a result, southeasterly flow over the Adriatic Sea curved cyclonically and became easterly over the Po Valley, where it ran into the formidable western Alps.


This pattern persisted for a significant period of time and I suspect that the convergence of these two airstreams and strong orographic ascent of the easterly flow over the Po Valley were important contributors to the heavy snowfall.  In pink shading, I've added the are of strongest 700-mb (3000 m above sea level) ascent and not surprisingly there is a big bull's eye over the east side of the western Alps.

Below is a sounding from Milan, in the Po Valley, 12 hours later at 0000 UTC 9 January.  You can see the low-level southeasterly flow, which is remarkably strong, featuring a 25 m/s (50 knot) lo-level jet at about 950 mb.  The temperature and dewpoint traces show saturated or near-saturated conditions through the depth of the atmosphere, with conditions that meteorologists would describe as "moist neutral."  This is a recipe for strong low-level moisture transport to the western Alps, strong orographic ascent on their eastern slopes, and in all likelihood some embedded convection to juice things up and crank up precipitation rates. 

Source: University of Wyoming
A case like this deserves a more detailed investigation than I can provide here.  I'm sure the event undergoes important evolution over the period that I haven't had time to look into, and that the terrain effects are probably very interesting and impressive if one has access to higher-resolution precipitation/snowfall data.  If you dig in, please feel free to comment and share your insights.

The Warm of the Storm

Let's start with the good news.

If my math is right, storm total water equivalent at Alta-Collins through 7 am (9662 ft) is 1.20", producing 9" of base-building Cascade concrete.  Snow, possibly heavy at times, will continue through much of the morning.  The HRRR has things winding down late morning.  The NAM keeps a few snow showers around in the afternoon.  I'll go with another 2-5", mainly from 7 to 11 AM this morning.  We'll do a bit better than that if we can a good burst going or snow showers in the afternoon are more productive than expected.  These numbers are lower than the NWS Cottonwood Canyons forecast, but looking at the radar and the HRRR, I don't see us doing much better than that.  Even the "5" in 2-5 was a stretch.  Hope I'm wrong and they are right.

Now, let's shift to the bad news.

The prefrontal southwesterly flow was even warmer than I anticipated and temperatures overnight have cooled more slowly than expected.  A look at the temperature and precipitation graphs for Alta Collins shows temperatures hovering from 31-32ºF until yesterday evening and then only cooling very gradually to the current (7 am) value of 29ºF.




Thus, while we've added base builder in the upper elevations, the mid and lower elevation snowpack has taken a serious hit.  Further, the snow level remains high.  Even now, it is 34ºF at 8500 feet and the highest traffic camera in Big Cottonwood Canyon, which is near 7500 feet, shows only wet roads. 

I hate to say it, but get out and enjoy the high elevation snow today.   We may get a bit of windy "dirty ridge" snowshowers Thursday and Friday, but it won't add up to much.  Then the ridge returns.

Beggars can't be choosers.

Addendum @ 10 am

5 inches and .48" of water at Alta Collins since 7am.  Storm just exploded.  Wonderful! 

Tuesday, January 9, 2018

Lessons in Snow Level Forecasting

It's worth taking a look at temperatures this morning as the overnight southwesterly flow has brought in mild air.

At about 1440 UTC (0740 MST) temperatures at Snowbasin were around 35ºF at the base, 35ºF at the Middle Bowl observing site, and 29ºF at the top of Mt. Ogden.  This puts the freezing level, the level at which temperatures are 32ºF/0ºC at about 9000 feet. 

Source: MesoWest
In the central Wasatch, temperatures are above freezing at the base of all the ski areas.  It's currently 38ºF in town at Park City, 36ºF at the base of Alta, and 36ºF at the bases of Solitude and Brighton.  Here, the freezing level sits at right around 9000-9500 feet depending on the local topography. 


The snow level is typically lower than the freezing level for a number of reasons.  As snow falls and begins to melt, it extracts heat from the atmosphere.  This often results in a layer of constant temperature that is near 0ºC.  The evaporation of snow also can cool the atmosphere some or slow the melting of snow.  Eventually, the snow turns into a mixture of snow and wet snow (or slush) and eventually wet snow and rain, before becoming all rain.  The layer in which this occurs is called the transition zone


Because of the effects of melting and evaporation, the snow level (and freezing level) can yo-you depending on precipitation rate, lowering when precipitation rates are high, and there is greater cooling due to evaporation and melting, and rising when precipitation rates are low, and there is less cooling due to evaporation and melting.  This may be noticeable if you elect to don the garbage-bag look and are skiing today. 

To estimate snow levels, meteorologists use soundings extracted from numerical forecast models.  In addition to temperature, humidity (or dewpoint) are also used, along with estimates of precipitation intensity.  Typically, one uses the temperature and humidity profiles to estimate the height of what is known as the wet-bulb-zero level, the height at which the wet-bulb temperature is 0ºC.  The wet-bulb temperature is the temperature of the air if it were cooled by evaporation to saturation and helps to account for some of the cooling effects noted above.  In Utah, one often lowers this level by 1000 feet for an estimate of the snow level, although there are times when one might fudge it by less or more.  For example, in very high precipitation rates, the one might lower it more.

We extract the hight of the wet-bulb-zero level from the NAM and provide them from the latest forecast at http://weather.utah.edu/text/COTTONWOODS.txt.  Below is an example of the tabular output from last night 0600 UTC initialized NAM forecast.  The wet-bulb-zero level was forecast to fluctuate between 8700 and 9000 feet through 8 AM this morning.  It remains between 8700 and 9300 feet through 5 PM this afternoon.  Thus, expect snow levels to be near or around 8000 feet today. 


Lowering of the wet-bulb zero begins slowly after about 10 PM tonight, with a more sudden drop very early Wednesday morning with the approach of the cold front.

Also available in that table are temperatures and winds for Mt. Baldy, estimates of snow-to-liquid water ratio/water content based on an algorithm developed by Trevor Alcott and myself, water equivalents produced by the NAM, and estimates of snowfall based on the water equivalent and the snow ratio estimates.  Handy, if you keep in mind it is just guidance from the NAM model.  Through 7 AM this morning, this NAM run produced 0.35" of water and 2.5" of snow at Alta-Collins, which compares fairly well to the 0.34" and 4" observed.  I can assure you that most forecasts aren't that good!