Thoughts for New Years Eve

The dribs and drabs are in.  A whopping 2" last night at Alta-Collins.  The drought continues.  The skiing isn't very good, but at least it's cold and windy now too.

Given that there is little reason to get up early for first scratches tomorrow, be careful this New Year's Eve.  As mountaineer Ed Viesturs likes to say, "Getting to the top is optional. Getting down is mandatory."  Getting home safely this New Year's Eve is mandatory too.  One option: taking advantage of this free $15 voucher for a New Year's Eve cab ride from KSL.com.  

Forecast Tools: Ensemble Forecast Systems

In the early 1960s a famous meteorologist at MIT, Ed Lorenz, discovered that if he started his computer model with slightly different initial values it produced results that were drastically different.  A small difference in the initial conditions grew to a much larger difference during the forecast period.  Lorenz puzzled over these results for quite a while, but eventually published a paper entitled Deterministic Nonperiodic Flow, one of the most important papers ever in the atmospheric sciences.  Lorenz then  contributed to the development of a new field in mathematics that examines the behavior of chaotic systems, which are systems that are sensitive to their initial conditions.

Lorenz passed away in 2008, but I was fortunate to meet him when I was a young professor and swap stories about exploring Utah.  He was an avid hiker, climber, and cross-country skier who would have fit in well with the Wasatch Weather Weenies.  

Ed Lorenz.  Source: Realclimate.org.

Lorenz showed that atmosphere is a chaotic system.  It cannot be predicted like the phases of the moon or the tides.  Small differences in the initial state of the atmosphere can yield dramatically different forecasts.  This places a limit on forecast accuracy as we project farther into the future.  However, how predictable the atmosphere is varies somewhat from day-to-day and region-to-region.  Some patterns are more predictable than others.  The idea behind an ensemble forecast system is to produce lots of forecasts with somewhat different initial analyses (or in some cases model configurations) to get a handle on forecast confidence and the range of possibilities during the forecast period.  

The National Centers for Environmental Prediction Global Ensemble Forecast System (GEFS) provides global ensemble forecasts out to 16 days in the future.  The GEFS consists of 21 forecast members, one being the Global Forecast System (GFS) model forecast, the other 20 additional forecasts produced with slightly different initial conditions.  One way that meteorologists visualize the output from all of these models is using spaghetti diagrams that include a couple of selected contours from all the ensemble members on a single plot.  For example, we can plot the 5280 and 5700 m geopotential height contours at 500-mb from this morning's model analyses.

Source: NCEP

The upper-level flow roughly parallels these contours.  Because they lie nearly on top of one another, the initial analyses in these forecasts are quite similar.  The weak northward bulge in the 5700 m contour over the Intermountain West reflects the weak upper-level ridge that sat over us this morning.

As we go forwards in time, the forecasts begin to diverge, just as chaos theory suggests.

    

Add: NCEP

This is a 5-day forecast based on the analyses above and is valid at 0500 MST Wednesday 4 January.  Although there is clear divergence of the forecasts, it varies geographically.  There is good correspondence between the ensemble members over the western United States.  They all produce a ridge of comparable amplitude.  This is one reason why we have high confidence that a ridge will build over Utah following the cold front passage tonight.  Over the northeast United States and eastern Canadian Provinces, however, there is a mess of contours indicating great uncertainty in the timing and amplitude of the upper-level trough.  This is an area where forecast confidence is lower, especially with regards to the timing and amount of any precipitation that might accompany the upper level trough.

Now, lets go way out in the future and examine the 10 and 16 day forecasts.

Source: NCEP

Chaos in action.  There's a mess of spaghetti, although some might argue that there's also some "signal in the noise."  For example, in the 10-day forecast (top), many of the ensemble members suggest a trough over the eastern United States.

If you were forecasting in that area, you might have a closer look by examining postage stamps from the ensembles.  Those below from the Penn State e-Wall provide a glimpse of a subset of 12 of the ensemble members.  Many show a long-wave trough over the eastern United States and a ridge over the west, but there are variations in amplitude in position and a couple favoring a trough that is shifted a bit to the west.

Source: Penn State e-Wall

Ensemble forecast systems are useful because they help us assess forecast confidence, examine the full range of possibilities during the forecast period, and better quantify the likelihood of specific weather events.  They aren't a panacea, however.  For example, what actually occurs can lie outside the envelope of possibilities forecast by the ensemble.  There are a number of reasons for this, including biases in the model and an inability of the ensemble to produce solutions that diverge fast enough (sometimes called underdispersion).  Ensemble modeling systems are, however, an area of active research that should yield substantially better medium and long range forecasts in the coming years.

Dribs, Drabs, Cold Front, Ridge

I so look forward to the day I can say a big dump is coming, but that day is not today.  We remain on the southern periphery of the storm track and an impressive plume of moisture that will be bringing the goods to our friends in the Tetons and others to the north, but will only give us some dribs and drabs through tomorrow night when an upper-level trough and a healthy push of cold air surge into Utah. 

We'll get some snow, but the models presently call for most of the action to remain to the north.  Thus, I don't see the this being the big dump that we want and need.  Sharpening the edges on the rock skis is probably a better idea than mounting up those new powder skis.  

Then, a ridge builds in for another extended visit.  

Alta reports a cumulative season snow total of 92", plus the couple of inches fell last night and today.  With luck, we might get to 100" for New Years.  Might.

A Post-Christmas Stocking Stuffer

You wanted precipitation.  You've been begging for it.  Well, we got some this morning.  Some wet snow in the high elevations, rain in the mountain valleys.  Here at the Wasatch Weather Weenies, we try and refrain from product endorsements, but avoid clammy butt cheeks and get yourself a beavertail!  It's a great investment in a warming climate and a perfect gift for that special someone who you overlooked this Christmas.

Source: http://www.beavertail.net/main.sc

Yup, can there be anything more attractive than a beavertail, and it could come in handy this morning.  Temperatures are in the low-to-mid 30's at the bases of Alta and Snowbird (even warmer at the base of Park City). 

We'll only see some periods of precipitation today, with perhaps an inch or two of snow falling on top of the 2 inches that was recorded at Alta-Collins by 9 am.  Temperatures and snow levels will drop somewhat to perhaps 7000 feet by late this afternoon, but get the beavertail anyway as it works well even in wet snow and is a great fashion statement...

Rime

Today if you are in the high country, you might getting a first hand look at rime, which forms as supercooled cloud droplets freeze when they contact solid objects.  

I got a pretty good coat of it on my googles in upper Collins gulch while skiing through the altostratus deck that was draped over the Wasatch Mountains early this afternoon.  

Source: Alta Ski Area

In fact, I even resorted to a maneuver I call the Snoqualmie swipe, which involves rubbing my thumb across my goggles to remove the rime between turns.  I don't use the Snoqualmie swipe much in Utah, but I was a real pro at it when I lived in Seattle and skied frequently at Alpental in Snoqualmie Pass, where rime is practically an every day occurrence.

Unless heated, no object is safe from rime.  One of the Snowbird cameras at the top of the tram was coated with it.

Source: Snowbird Ski Area

Rime is produced by clouds that contain large supercooled cloud droplets or drizzle.  Supercooled means that the droplets or drizzle are below 0ºC but are comprised of liquid water rather than ice.  Water does not necessarily freeze when it is below 0ºC.  To freeze, it needs a particle, known as an ice nuclei, to help it transition to ice.  Without such a particle, it can become supercooled.  

At temperatures just below 0ºC, there aren't many particles that can serve as ice nuclei.  So, shallow clouds that are just below 0ºC are prone to riming.  Today as we have a shallow layer of altostratus hanging over the Wasatch Range.  The temperature at Alta-Collins, which sits right at cloud base, is about -1 to -2 ºC.  

Riming is a concern for aviators.  The buildup of rime ice on aircraft not only adds weight to an aircraft, but also changes the wing aerodynamics.  The National Weather Service Aviation Weather Center produces analyses of icing severity, on which they overlay pilot reports of icing.  Light icing is presently being reported by pilots over northern Utah.  They won't linger long at altitudes where this is a concern.

Source: NWS/AWC

Salt Lake, We Have a Problem

Looking down at the University of Utah from the Avenues
Twin Peaks, 27 Dec 2011

I'm appalled at how quickly our air quality has declined the past couple of days.  This is an emotional reaction that perhaps warrants a more careful scientific analysis, but it blows my mind that we can go from relatively clean air on the 23rd, to a particular matter (PM2.5) levels above National Ambient Air Quality Standards in just four days.

Source: Utah Division of Air Quality

As noted on the DAQ trend chart above, the real-time data they provide is not quality assured, and some of the spiking in the observations seems odd, but visually the transformation in the valley is simply incredible.

This is purely anecdotal, but it is my impression that the air quality situation in the valley has worsened the past couple of years.  When inversions develop, the air quality seems to deteriorate more rapidly than it used to.  The current event has occurred during a period when we don't even have snow on the ground!  

It's a damn shame that the state that promotes itself as Life Elevated, can only show our ski and snowboard visitors who arrive or stay in Salt Lake PM2.5 Elevated.  

Tantalizingly Close

The latest GFS forecast puts us tantalizingly close to the winter storm action over the next few days, but we remain on the periphery of the storm track.  

I try to avoid wishcasting – forecasting what one wants to happen rather than what is most likely to happen – so my sober assessment is that the Cottonwoods might get a bit of snow if this forecast verifies, but not more than a few periods of snow adding up to a few inches.  However, since we sit right on the edge of the storm track, a even a slight shift in position could make a big difference.  If the storm track is south of what's presently forecast, we'll see more snow.  If it is north, we'll see less.

The northern mountains may fare better.  Nevertheless, anything that falls during this period in Utah is likely to be dense and gloppy thanks to wind and high temperatures.  The 700-mb (near 10000 ft) temperatures are forecast to be -2 to -3ºC, which is quite balmy by late December standards.  Such is life on the south side of the jet core.

A Whopper of an Inversion

Looks like about -7ºC on the valley floor and +3ºC at 800 mb (~2000 m) this morning.  Incredible.

Looks like the latest model runs have shifted the jet ever so slightly southward – enough that we could see some snow in the coming days.  Keep thinking those positive thoughts!

Jet Stream to Tease Us

We're finally seeing a shift in the pattern, with a breakdown of split flow and a strong jet stream set to rake the Pacific Northwest and northern Rockies over the next few days.  Unfortunately for us, the jet remains to our north and we are mainly teased by a series of storms in the latest GFS forecast.

Our best shot at something is Thursday night based on this forecast loop.  We'll take what we can get, but we're going to have to hope that jet stream ends up a a bit further to the south if we are to get into a decent storm cycle for the Cottonwoods this week.

Perhaps the northern Wasatch and Bear River Range will fare better, and the Tetons and environs will get a hammering.  Road trip?  Perhaps, but avalanche conditions are likely to be dicey.  Even up there, this will be a series of windy warm storms with snow falling onto a thin, weak snowpack.  Backcountry travelers be careful.

The Untold Story of How Santa Claus Really Comes to Town

Source: free-extras.com

Contrary to popular belief, Santa Claus' main weather concern on Christmas Eve is not snow, sleet, and rain, but wind.

He used to have problems with snow, sleet, and rain, not to mention fog and other visibility obstructions, but these haven't been a concern since he added Rudolph to his team many decades ago.

Source: Rankin-Bass Productions, Inc.

Instead, Santa's primary concern is wind.  To deliver presents to roughly 800 million homes, Santa has to travel 160 million miles (see Fermi Lab analysis).  He has a whole night to do this, so if he can work his way around the globe delivering presents over a 24 h period, he needs to maintain an average speed of 6.7 million miles per hour (1850 miles per second).

Santa travels mighty fast, which is why you can't see him.

Santa has strong reindeer, but you don't want to further stress them by flying into a headwind.  Keep in mind that the sun moves from east to west, so Santa is working his way westward and, since much of the world's population lies in the mid latitudes, he is working against the mean westerly jet stream.  Further, the southern hemisphere mid-latitude jet stream ain't a heck of a lot weaker in summer (which is what they are presently experiencing) than winter, so this is a chore in both hemispheres.

But, Santa has an ace in the hole.  He has some elves who used to work at Google and are incredible programmers.  Using the global weather forecasts from the National Centers for Environmental Prediction and the European Center for Medium-range Weather Forecasting (ECMWF), combined with Santa's naughty and nice list, they have optimized Santa's route to minimize head winds and reduce wear and tear on Donner, Blitzen, Dancer, Vixen, Prancer, Dasher, Comet, Cupid, and Rudolph.

This is the real miracle of Christmas.  Santa has been able to leverage numerical weather prediction to continue to deliver gifts to an exploding global population.

Merry Christmas from the Wasatch Weather Weenies.

Snomageddon Over, What Next?

We got the anticipated freshening of snow in the Cottonwoods yesterday and last night.  Bruce Tremper's advisory on the Utah Avalanche Center site sums it up well: "New snow amounts are mostly 2-5 inches. A couple automated stations have reported 7 inches but I suspect wind drifting."  The Alta-Collins site has what we'll call using football ling, a short 6" or a long 5".

I'll jokingly call this event Snomageddon because it may have produced the largest 24-hour snowfall this month based on a quick look at the Alta snowfall history.  Thusfar, the largest 24-hour total reported on that site this month is 4.5", so we may have topped it depending on what they use for their 24-hour period.

So, we remain desperate, what are the prospects for the future?  Let's start with Christmas Day.  Unfortunately, there's no hope for a white Christmas in the Salt Lake Valley.  You'll need to go to the mountains for holiday sledding or skiing.  It will, however, be a beautiful day (other than some valley haze) as we'll be under the influence of a high amplitude ridge.

GFS Forecast Valid 0500 MST Christmas Day

There is some hope for later in Christmas week.  The past couple of days the models have been hinting at a weakening of the split flow with a more progressive (i.e., stronger westerly flow) pattern over the western United States.  The latest GFS teases us by keeping the main storm track just to our north, but it is close enough that we could get some action from a couple of storms.

GFS forecast valid 1100 MST 27 Dec 2011

GFS forecast valid 1700 MST 28 Dec 2011 

The first event is presently a brush-by, whereas the 2nd event has more potential.  That being said, these are long range forecasts, prone to errors in amplitude and timing, so we'll need to see how things come together the next few days.  I hate to say it yet again, but keep your fingers crossed.

It's Snowing!

Looking west over the Albion Base Lodge this morning.
Source: Alta Ski Area

Yes Virginia, there is a Santa Claus, and yes Utahns, there is snow falling in the Cottonwoods.  It's just a weak band of precipitation that extends from the Stansbury to Wasatch Mountains, but we'll take every flake we can get.

Not much has changed from yesterday's forecast, although the NAM forecast has shifted somewhat toward the GFS forecast, as always seems to happen.  Recall the GFS was the more bullish of the two models for snow, so that's good, although it's still a brush-by event for us with most of the action passing to our east and the winds shifting quickly with the trough passage so that we don't have a prolonged period of cross-barrier flow with orographic enhancement.

Thus, I think we'll get see some periods of snow today and tonight adding up to a few inches in the Cottonwoods to freshen things up, but that's about it.  If we get more than that, we'll have to consider ourselves fortunate.  Keep your fingers crossed.

Diverging Forecasts

We have an interesting forecast scenario for tomorrow night - one that will test the nerves of most meteorologists.

A few days ago, the computer models were keeping storms to our north this week, but suddenly, on Sunday, they decided to drop an upper-level trough into Utah late tomorrow (Wednesday) and tomorrow night.

The two major computer models run by the National Centers for Environmental Prediction for the National Weather Service are still bringing that upper-level trough into Utah, but with drastically different predictions for how much snow will fall.  The NAM model  pushes the trough axis across the Wasatch Range by 11 PM tomorrow night, but with only light, scattered precipitation.

On the otherhand, the GFS model has similar timing, but a much deeper, wetter trough that would give us a period of snow in the Wasatch Mountains.

The one thing that is "clear" from these forecasts is that we can count on much of the gunk from our ongoing inversion episode to be scoured out.  What is less clear is how much snow the Wasatch will get.

My rule of thumb is to lean to the GFS in situations like this as it seems to better handle the evolution of large-scale weather systems like the upper-level trough.  That being said, even the GFS model isn't calling for a huge snowstorm in the Wasatch Mountains.  Perhaps a few inches if we are lucky as we simply lack a prolonged period of moist cross-barrier flow to give us a big snowstorm.

The bottom line is we need to hope Mother Nature is more generous than these computer model forecasts!  Anything we get now will help as we head into the holiday week.

It's Getting Deeper

Source: Snowbird

Fog and stratus has pushed up Little Cottonwood and is flirting right now with the base of Snowbird.

This is evidence of a special type of cold-pool (a.k.a. inversion) event that features what is known as a cloud-topped mixed layer.  In many of our cold-pool events, the inversion sits pretty much right on the valley floor, but in long-lasting events that develop clouds, the inversion moves upward with time.  This occurs because the radiational cooling occurs at the cloud top rather than on the valley floor.  The cloud-top cooling drives weak downdrafts and turbulence beneath the inversion.  Small amounts of air from the free atmosphere above are entrained into the cold pool beneath the inversion as this occurs, and slowly but surely the inversion climbs upward.

Schematic of the cloud-topped mixed layer in
the Salt Lake Valley (Pataki et al. 2005).
Adapted from Houze (1993).

This can be seen if you compare the morning soundings from yesterday today.  The shift is subtle, but the inversion is higher this morning.

Source: NOAA/NWS/SPC

Note also that below the temperature decreases fairly rapidly with height from the surface (-4 Cº) to the base of the inversion (-9ºC) where the top of the clouds are.  This layer in which the temperature is decreasing rapidly with height is the so-called cloud-topped mixed layer.  The clouds sit at the base of the inversion, cool, and drive weak downdrafts and mixing beneath.  Because of this mixed layer, the pollution concentrations are actually lower than they would be if the inversion were sitting on the valley floor (yes, things could be worse).  

Snowshowers in the Salt Lake Valley occur at night when the cloud topped cooling and turbulence are strongest.  What goes down, must go up, so when you have some weak downdrafts, you also have some weak updrafts elsewhere that help to fuel the snowshowers.  That's why it's snowing in Salt Lake but clear at Alta.

Ski It If It's White

That was the family motto today as we adjusted our attitude and had fun running laps at Alta.  The skiing remains below Utah standards, but was fun for a couple of hours and better than expected.  Kudos to Alta's snowmakers and groomers for a truly outstanding job this year.  It was also great to see blue sky and enjoy a Bucket o' Fries at Watson Shelter.  French fries consumed at Watson Shelter do not count against a person's bad cholesterol.

There is a glimmer of hope for the future.  The models have suddenly decided to drop an upper-level trough into Utah for late Wednesday and Thursday.  This trough was previously forecast to pass to our north, but if Mother Nature wants to bring it through Utah, so be it.

The trough is not the harbinger of a major pattern change, but any freshening of the snow will help.  Further, the trough should punch out the inversion and clear up the air.  The view from Alta today...

and from the 6200 South park-and-ride...

Disgusting.

Finding Snow

Data collected by NRCS SNOTEL stations can be incredibly valuable for planning ski adventures.  I like to access it either via the NRCS web site or the Colorado Basin River Forecast Center and Pacific Northwest River Forecast Center web sites.

Regular readers of this blog already know that this has been a down snow year thusfar for much of the western United States.  That is readily apparent when looking at the percent of average snowpack snow water equivalent at SNOTEL stations this morning.

Source: NRCS

The snow situation is below average across much of the west and is incredibly grim in the Sierra Nevada.  The few regions with several stations at 125% or more above average include the Mogollon Rim of Arizona and adjoining mountains, Big Horn Mountains of Wyoming, and central Cascades of Washington.

Percent of average has it's advantages, but can be deceptive.  For instance, the Promontory SNOTEL is one of the snowier sites on the Mogollon Rim.  It presently has 6.6" of SWE compared to an average of 2.9.

That is not substantially different than Snowbird, which sits at 6.4" of SWE, but that equates to a much lower percent of average (63%) because it snows so much at Snowbird on average.

So where is the southwest snowpack deepest?  A quick look suggests Wolf Creek Pass in the San Juan Mountains, which sits at 13" of SWE.

Wolf Creek Pass is one of the snowier locations in the San Juans, so don't assume that deep snowpack applies throughout the range.  SNOTEL locations further to the west, including those in the mountains around Silverton and Durango, have much less SWE.  For example, Lizard Head Pass sits at only 4.8".

Thus, the Wolf Creek Pass area appears to be the southwest sweet spot right now.

2011/12 vs. 1976/77

How bad is the skiing in Utah right now?  Actually, this isn't
from Utah, but it is a great photo.  Source: dailydawdle.com

The 1976/77 winter brought drought conditions to most of the western United States.  Amongst my friends who were skiing the Wasatch at the time, it is known simply as The Drought Year, although The Year That Must Not Be Named might be better to communicate to new-school riders just how bad of a ski season it was.

According to the snowfall history at alta.com, 84" has fallen so far this season.  This includes 18" in October, 58.5" in November, and 7.5" so far in December.  Averages are 30.9" in October, 62.8" in November, and 40.6" for half of December, for a total of 134.3".  Thus, we've had about 62.5% of the average snowfall for the season to date.

That's pretty bad, but it ain't nothing compared to The Year That Must Not Be Named.  According to data records at the Western Region Climate Center, the 1976/77 ski season was a disaster.  Only 7.5" fell in October, 12.5" in November, and 14" in all of December.  Can you even fathom that?  Seasonal snowfall through the month of December of only 34".

In The Year That Must Not Be Named, snowfall for the entire season was only 280", although there were three days with missing data in January, so actual snowfall could have been somewhat higher.  In any event, that's a pretty lousy year by Utah standards, which I'm sure was made worse by unbearable pollution in the Salt Lake Valley.

A Foggy Mess

Fog.  The bane of every meteorologists existence.  We hate it as much as you do.

What a complicated mess we have over northern Utah right now.  Check out the satellite loop below.  We have a patchy mid-level clouds moving through the area, as well as areas of fog, including around the Great Salt Lake and in the Uinta Basin.

Those with a careful eye might actually see the shadow from the mid-level cloud moving across the low-level fog over the southeast corner of the Great Salt Lake and Tooele Valley.  Cool stuff.

Web cams are incredibly valuable in a situation like this.  You can't see fog in satellite images when mid-level clouds are present, so having ground truth from a live camera is really helpful.  Here's a quick tour.  First, looking northwest from Olympus Cove, which is down in the fog and gunk as I write this.

Source: Meteorological Solutions, Inc.

The air, however, is nice and clear at Snowbird, which is sandwiched between the fog and mid-level cloud deck.

Source: Snowbird

Because of the shadow cast by the mid-level cloud deck, it's tough to see the fog (which some might call "undercast) in the latest image.  You could see it better this morning when the sun was on it.

Source: Snowbird

The fog coverage is not total along the Wasatch Front.  Weather camera images available from the National Weather Service web site show some areas that are fog free, but still somewhat gunky from haze and smog.  The winter doldrums are officially here.

Perspectives on the Early Season Snowpack

With only 10 shopping days left until Christmas, I thought I'd take a look at what the typical snow depth is on Christmas morning at Alta.

The MesoWest group at the University of Utah has received real-time weather data from the Alta-Collins site since 1997 (special thanks to everyone at Alta who makes this possible!).  Since then, the average Christmas morning snow depth is 63", with a minimum of 44" in 2002 and a maximum of 104" last year.

Those are fairly healthy early season numbers.  The Christmas snowpack at Alta is one of the surest bets in North America.  This is especially true in an era with snowmaking and grooming (although most people ski Alta for other reasons).

Where do things stand this year?  The Alta-Collins snowdepth sensor has not been reporting reliably the past couple of weeks, but the ski report presently puts the base at 32".

I've been looking at the medium range forecast guidance and think that we will be taking a run at the lowest Christmas morning snowdepth since 1997.  We might see a few flakes at times through tomorrow, but it's not going to add up to much.  Then it looks dry for a few days.  While we can't bank on it, the long-range forecast guidance is not suggesting a major pattern change before Christmas.  I can't rule out a storm, but the odds are low.  We will have to hope that Mother Nature doesn't listen to the models (sometimes she doesn't) and punches a storm into Utah if we're to get to even 40" by Christmas morning.  Otherwise, it could be tough sledding and skiing for Santa.

Snow!

Yes, a whopping 3 inch storm total as of 8 am at Alta-Collins.  The Utah Avalanche Center advisory this morning was pure comedy and tragedy all rolled into one.

Source: Evelyn Lees, Utah Avalanche Center

I can't recall the last time an exclamation point was used by a UAC report for such a small snow event, but beggars can't be choosers!  Let's hope we squeeze all we can from this storm.