How Predictable Are Downslope Winds?

The warning flags have been raised and the National Weather Service has issued a high wind watch for late tonight through Thursday evening for the Salt Lake and Tooele Valleys and northern Wasatch Front for strong and potentially damaging downslope winds.

As we have discussed in earlier posts, there is good reason for issuing such a warning.  The key large-scale ingredients for a downslope windstorm have been anticipated by earlier computer model forecasts and are evident in the latest forecast produced by the NAM model.  These key ingredients are strong easterly to northeasterly flow, a stable layer near crest level, and a level aloft with reverse shear (i.e., winds weakening with height).

NAM model forecast valid 8 AM MST Thu 1 Dec 2011

However, although our computer models today can predict the environment in which a downslope windstorm is likely, they provide us with little useful information with regards to just how strong the peak winds will be.

The National Weather Service Salt Lake City Forecast Office now runs a high-resolution (4-km grid spacing) computer modeling system that better resolves the Wasatch Mountains and can produce downslope windstorms, but it produces a single forecast.  The latest forecast produced by this modeling system shows strong (>50 knot) winds in the Farmington area tomorrow morning.

SLCFO WRF-model forecast valid 9 AM MST Thu 1 Dec 2011

Unfortunately, we can't hang our hats on that 50 knot wind speed.  There are several reasons for this.  First, even at 4-km grid spacing, the details of the Wasatch Mountains and the processes that contribute to downslope windstorm development are not fully resolved.

Second, recent work by Reinecke and Durran (2009) suggests that the magnitude of the winds produced in computer model forecasts are very senstive to small changes in the initial conditions.  Specifically, they ran a 70 member forecast ensemble to examine the sensitivity of two downslope windstorms in the Sierra Nevada to slightly different initial conditions.   In one event, the difference in downslope wind speeds produced by the strongest 10 and weakest 10 members of the ensemble was 28 m/s (56 kt) for a 6-hour forecast.  In a second case, the difference was 22 m/s (44 kt) for a 12-h forecast.

Therefore, in the event anticipated for tonight and tomorrow, high, potentially damaging winds are likely, but the peak magnitude of the event remains uncertain.  Hopefully in the future, when we have high resolution forecast ensembles with many members, we'll be able to provide better guidance with regards to the likelihood of winds that exceed critical thresholds.

Update @ 1:40 PM MST 30 Nov: Prof. John Horel will be leading a "storm chasing" team into the storm late tonight through tomorrow morning.  They will be collecting observations with our portable mesonet stations, mobile truck-mounted mesonet stations, and two portable upper-air sounding systems.    I'd also like to note that the Reinecke of the Reinecke and Durran (2009) paper above, is Alex Reinecke, a University of Utah alum who worked in the Atmsopheric Sciences department and is currently at the Naval Research Lab.  Nice work Alex!

Three Lows, Then a Big Blow?

As we discussed yesterday, the atmosphere is in the process of going into outlier mode, and the next few days should prove quite interesting for weather.

Our explosively deepening cyclone is now just south of the Aleutian Islands.  As expected, another low has formed downstream and is just off the Queen Charlotte Islands.  Those of you who are lovers of cyclones will be pleased to know that there is also a third cyclone over the Sea of Okhotsk.  This is what meteorologists call a polar low, because of it's small scale and formation in cold air over a warm body of water.

1500 UTC (0800 MST) 29 Nov 2011 IR satellite image and sea level
pressure analysis

The cyclone near the Queen Charlotte Islands is expected to dig southeastward into Washington State.  Although it will fill at low levels, the upper-level low is expected to be centered near Las Vegas by 1200 UTC (5 AM MST) 31 Nov (Thursday morning).

GFS forecast valid 1200 UTC (5 AM MST) 31 Nov 2011

At this time, northern Utah is experience strong crest-level (700-mb) flow from the east–northeast.  The GFS forecast sounding for the Salt Lake City airport shows a 700-mb wind of 40 knots (20 m/s) from the ENE, and even stronger flow [50 knots (25 m/s)] at 750 mb.

GFS forecast sounding for KSLC valid 1200 UTC (5 AM MST) 31 Nov 2011

The concern here is the potential for strong easterly downslope winds along the Wasatch Front, especially from Olympus Cove through Brigham City.  These winds are sometimes called canyon winds, but it is important to recognize that a downslope windstorm is not necessarily isolated to in, near, and downstream of the canyon mouths.  Instead, during a downslope windstorm, the flow crosses the mountain crest, plunges downward, accelerates, and produces strong winds along much of the leeward side of a mountain range.  For that to occur, one typically needs a stable layer near the mountain crest or an area where the magnitude of the cross barrier flow weakens with height.  Both are present in the morning sounding above.

The strength, duration, and areal coverage of this event will depend on the positioning of large-scale weather features, the strength and height of the stable layer, and other processes that are difficult to predict with precision at longer lead times.  The potential is there, but we will have to see how this event comes together early Thursday morning.

I Was Wrong...

Yup, I admit it.  A couple of weeks ago I declared the fall single track season over.  There was too much snow, too much mud, and too little sun angle to make the trails rideable again this year.  Well, several days of mild, windy, dry weather have laid that forecast to waste.  The Bonneville Shoreline Trail was in pretty good shape today with just a few muddy spots.  In fact, it was downright FAST in places.

In addition to being a decent substitute for crappy skiing, it's also nice to exercise above the smog.  Ick.

Going into Outlier Mode

The atmosphere is about to go into outlier mode over the western United States, and the foundation was laid upstream over the north Pacific last night.  As shown in the satellite image below, at 11 PM MST, a weak cyclone (note the comma cloud) was just south of the western Aleutian Islands.  

0600 UTC 28 Nov 2011 GFS Analysis and IR satellite image

The surface analysis from the Ocean Prediction Center showed a central pressure of 1000 mb, with two ominous labels near the low center: Rapidly Intensifying and Developing Hurricane Force.  

Just six hours later, at 1200 UTC, the cyclone central pressure was 989 mb, an 11 mb drop in 6 hours.  

Thus, this cyclone is well on it's way to meet the criteria for explosive cyclogenesis, which requires an average deepening rate of 1 mb per hour for 24 hours.  Meteorologists sometimes refer to such rapidly deepening cyclones as "bombs" (Sanders and Gyakum 1980). 

A cyclone like this has major implications for the large-scale circulation.  In particular, it contributes to something known as downstream development, which is characterized by the subsequent development of high amplitude weather features downstream (east) of the cyclone.  For example, the GFS forecasts the development of a strong ridge of high pressure over the eastern Pacific and western Canada, followed by a surface cyclone over the southwest United States, over the next few days.    

The ramifications for the weather of Utah and the southwest United States are substantial.  As shown below, the GFS puts a deep upper-level low over the southwest at 1100 MST 1 Dec 2011 (Thursday morning).  Strong easterly flow at crest level (i.e., 700-mb) extends across northern Utah, and strong northeasterly flow extends across most of Nevada.

These are strong winds from an unusual flow direction, and could lead to downslope (a.k.a. canyon winds) along the Wasatch Front and, if they extend further southwest, Santa Ana winds in California. Climatological leeward areas may see rare cool-season precipitation, including the deserts of southern Nevada, Panamint Range of California, etc.

A wise man once told me to beware when the atmosphere is in outlier mode, as will be the case this week.  It's still far enough out that we can't read too much into the latest model runs, but the next few days bear careful watching.  Some unusual things are likely to happen over the Southwest.  

Why Thin Snow Is Weak Snow

Compared with last winter's snowpack, we're dealing with a far more insidious beast this year.  We have already had one avalanche fatality and natural and human triggered avalanches are still being reported in the backcountry.  Brett Kobernik summarized the situation quite well in today's avalanche advisory (my emphasis added below).

Our main concern remains triggering an avalanche that breaks into old faceted snow. As I see it, we are in a holding pattern right now where I have not seen any significant improvement with the weak layer over the last seven days.

Faceted snow is a type of snow characterized by angular crystals.  Facets can be small, but when they reach large size with steps and striations on their surfaces, they are called depth hoar (see below).

Source: zacstracks.com

Faceted snow is weak, lacks cohesion, and forms where there is a large temperature gradient in the snowpack.  A temperature gradient is a measure of how quickly the temperature changes with distance.  For faceted crystals to form, the temperature gradient needs to be at least 10ºC per meter.  Or, equivalently, at least 1ºC per 10 cm.

The temperature gradient is important because it plays a role in the movement of water molecules in the snowpack.  Water molecules in a snowpack are not locked in the ice phase.  They can sublimate (i.e., change from ice to vapor phase) or condense (i.e., change from vapor to ice phase).  Water vapor can also move through pore spaces in the snowpack.  What happens in a snowpack with a large temperature gradient is that there is a net movement of water molecules from the warmer snow crystals to the colder snow crystals.  The warmer ice crystals lose mass to net sublimation while the colder ice crystals grow through net condensation.  In addition, because of the close proximity of the warm snow crystals, the cold snow crystals grow in a water-vapor-rich environment, which, through the wonders of ice physics, leads to faceted crystals.

Snow crystals in snowpacks with a weak temperature gradient also experience sublimation and condensation.  However, the net transfer of water molecules between snow crystals is much slower, which, through the wonders of ice physics, leads to rounded rather than faceted snow crystals.  These rounded crystals tend to be strong.

In a snowpack, the temperature near the ground is usually near freezing (0ºC), so the temperature gradient is strongly determined by the temperature at the top of the snowpack.  If the snowpack is deep, the temperature gradient in the snowpack will generally not be very large even in cold weather because the difference in temperature between the bottom and top of the snowpack is spread over a large distance (an important exception is right near the snow surface where a strong, localized temperature gradient can develop in cold, clear conditions even in the deepest snowpacks).  However, in a thin snowpack, the temperature gradient can be quite large, especially in cold weather.

What happened in the Wasatch this year is our mid-October snow sat for a few weeks on high-altitude northerly aspects, was fairly thin in depth, and developed faceted snow crystals due to the presence of a large temperature gradient.  Mother Nature has now dropped a load on that weak snow, and we are dealing with a cranky snowpack.  The weak layer is not strengthening because the snowpack remains thin and the temperature gradient is too large for the facets slowly transform into stronger rounded grains.

If we can get several storms to make the snowpack deeper and reduce the temperature gradient, then the snowpack will likely strengthen gradually.  I emphasize gradually because this process doesn't happen over night.  In addition, there will be some places where the snowpack remains thin and may continue to be weak.  I call this a Russian Roulette snowpack because the snowpack can become strong in some areas, but remain weak and "loaded" in others.

Important Clarification: The Wasatch Weather Weenies is an educational blog, not an avalanche advisory service.  We like to take advantage of what is happening in our backyard and mountains, but for professional, up-to-date avalanche advisories for the Wasatch backcountry, please consult the Utah Avalanche Center web site.

Turkey Forecast Leftovers

It's snowing lightly in the mountains this morning, but not enough to get excited about.  I was just taking a look at the medium range forecast and it, like the forecast for this holiday weekend, is a turkey for skiers too.

The medium range forecasts being produced by the National Weather Service and European Center for Medium-range Weather Forecasts are dominated by a ridge over the western United States for the next 5-7 days, with only a couple of weak systems pushing into the ridge, the strongest on Wednesday, which right now the computer models suggest will give us a dry cold-frontal passage.

Global Forecast System (GFS) forecast valid 1200 UTC (0500 MST)
Wednesday 30 November).

The bottom line is that it looks mainly dry through the middle to end of next week.  Having a thin snowpack sit around for so long is simply not good for avalanche conditions.  The buried facets from October's early snow are not healing (see today's Utah Avalanche Center advisory) and we might see facets developing on other aspects if it continues to stay dry (our saving grace thusfar has probably been the warm temperatures – facets grow in snowpacks with a large temperature contrast between near the ground and the snowpack surface, so warmer temperatures mean a smaller contrast).  Avalanche conditions are likely to be nasty when it finally starts snowing again.

This Forecast Is a Turkey for Skiers

We've gotten off to a decent start for the ski season, but conditions remain marginal.  The latest forecasts suggest that our best chance for snow this holiday weekend will be Friday when a weak short-wave trough brushes by us to the north.  As things stand now, one computer model thinks it will be a weak event, the other a non event.

Global Forecast System (GFS) forecast valid
11 AM MST Friday 25 Nov 2011.

North American Mesoscale (NAM) model forecast valid
11 AM MST Friday 25 Nov 2011.

After the weak event on Friday, a high-amplitude ridge builds over Utah.

GFS forecast valid 11 AM MST Sunday 27 Nov 2011.

That means it will be sunny and warm in the mountains.  In other words, beautiful weather and great travel conditions, but I'd like to have some fresh powder for which to give thanks this weekend.

A Wasatch Cold Pool

Southwest winds picked up in the mountains overnight, bringing in much warmer temperatures.  It is presently in the 30s at most locations, including the top of Mount Baldy (11,000 ft) where it is 31F.

1425 UTC (0725 MST) 23 Nov 2011 temperature and wind observations
in the Cottonwood Canyons.

The coldest temperatures are presently at the Spruces Campground in Big Cottonwood Canyon where it is 21F.  The area around the Spruces is the refrigerator of the Wasatch as it is a flat basin in which strong cold pools can form.  That is certainly the case this morning.  In fact, it is 19F warmer at the top of the Moonbeam lift at Solitude.  Now that's what I call an inversion.

The cold air pool should scour out this morning, and temperatures at the Spruces will rocket to near 40F.  Be sure to dress in layers!

Trib Article

A nice article appeared on the Salt Lake Tribune web site this evening about our activities with the DOW.  I especially liked that our graduate students were prominently featured as they are the real brains behind the operation and they took full advantage of every opportunity this past month.  Weather never sleeps and, on a few nights, neither did they.  This past month with the DOW was easily the best teaching experience of my career.

Peering into Wasatch Range and Little Cottonwood Canyon with the DOW

What Is That?

1500 UTC (0800 MST) 22 Nov 2011 IR Image

That is Hurricane Kenneth, a category 4 hurricane that is now the strongest late season eastern North Pacific hurricane on record.

It is perhaps a sick and demented dream of mine to have a surge of hurricane-related moisture into Utah in November.  Imagine the high-density, heavily rimed snow and graupel that would result in the upper elevations, just what is needed to build a snowpack.

Alas, the moisture from Kenneth is presently forecast by the Global Forecast System (GFS) to surge up through western Mexico and then into New Mexico.

GFS sea level pressure (contours) and integrated water vapor (color fill)
for 0600 UTC 22 Nov – 0600 UTC 25 Nov 2011

Close, but not quite close enough, although this does bear watching to see if the moisture can creep up into the four corners area.  And, in case you are wondering, hurricanes have produced high-mountain snow over the southwest, including the infamous Hurricane Lester in August 1992.

Au Revoir Mon Ami

The folks from the Center for Severe Weather took the Doppler on Wheels away this afternoon.

However, I didn't let it leave without a fight!

DOW Data from Friday Night

Here's a sneak peak at some of the Doppler on Wheels data we gathered Friday night.  Below are vertical scans taken looking up Little Cottonwood Canyon from Daybreak.  The reds are where we are getting strong returns from the terrain, which means it's ground clutter and not precipitation.  The oranges are areas where the precipitation rates are the greatest.  At this time, the frontal band is right over Little Cottonwood and the Salt Lake Valley and it is snowing hard everywhere.  

That being said, we see some of the same features that we saw during the storm the previous weekend.  In particular, the peak depth of string radar reflectivities (organge colors) occurs at about mid canyon.  Farther up canyon, the storm shallows.

Later, after the frontal band moves through, this structure persists.  

In particular, note how the strongest/deepest returns are mid canyon.  In this instance, precipitation rates are probably greatest from about the White Pine Parking lot to the High Rustler ridge and much weaker in Albion Basin and the Supreme-lift area.  

It will be fun to diagnose the processes responsible for this storm structure and see if our computer models can replicate it.

A Big Anniversary

Tomorrow marks the 10th anniversary of the start of the 2001 Hundred Inch Storm, a multiday storm cycle that laid down 108 inches of snow at Alta, including 100 inches during a 100 hour period.  If memory serves me correctly, there was little snow leading up to Thanksgiving weekend that year, so the snow answered the prayers of desperate Utah skiers who were burning their rock skis as a sacrifice to Ullr to bring snow.

As described in Steenburgh (2003, 2004), the Hundred Inch Storm provides a great example of what Mother Nature can do when the atmosphere and Great Salt Lake are in outlier mode.  It was produced by two storm systems that were strongly augmented by the Great Salt Lake Effect.  Each storm system featured a period of stable, pre-frontal precipitation, a period of unstable pre-frontal precipitation that followed the surge of cold, dry air aloft, heavy frontal precipitation accompanying the cold front, and then intense lake-effect snowfall.

Radar imagery from storm system I showing stable prefrontal
(top left), unstable prefrontal (top right), frontal (bottom left),
and lake-effect (bottom right) stages.

Radar imagery from storm system II showing stable prefrontal
(top left), unstable prefrontal (top right), frontal (bottom left),
and lake-effect (bottom right) stages.

Most remarkable about this event was the intensity and duration of the lake-effect precipitation, which produced 2.18 inches of snow water equivalent at Alta-Collins.  Kristen Yeager shows in the MS thesis that she will defend next week that the two lake-effect periods are extreme outliers during the 1998–2009 radar record.

The bottom line is that the 100-inch storm was a whopper of an event, especially with regards to lake-effect, and we haven't had anything like it since.

A Good Early Season Start

After spending much of the last month storm chasing, I was finally able to get out and sample the early season snowpack myself yesterday.  The meteorologist inside of me said that this is a good start to the ski season, but on the climb through the aspens, my tibias were screaming that I'd better keep my tips up on the descent.

Let's look at some numbers.  Data from the Natural Resources Conservation Service (NRCS) SNOTEL stations show that on a basin-wide scale, northern Utah is sitting about 110–125% of average snowpack snow-water equivalent (SWE).

Not too shabby.  At the Snowbird SNOTEL, it appears we are running just a shade behind average and   where we were on this date last year.

That being said, during the early season, we deal a bit with the so-called statistics of small numbers.  The average this time of year is quite low and the difference between a good or bad early season is really just a reflection of whether or not we get a storm or two in late October and early November.  Thus, anything from practically nothing to several inches of snowpack SWE is within the typical snowpack range for this time of year.

Thus, with enough snow to ski, I think this is a good start to the ski season.

IOP8 Concludes

Our Doppler on Wheels activities for the night have concluded, bringing an end to the eighth intensive observing period (IOP8) of the Storm Chasing Utah Style Study (SCHUSS).  Our overnight crew tells me they shut down around 4 am.  What does a storm-chasing crew do at that time of morning?  Head to Denny's of course...

Today will be the first Saturday in 3 weeks that I've had off.  I'm going to relax and head up for a short backcountry ski tour in the afternoon to check out the snowpack and start getting into shape for the winter.  

Midnight Snack

It was quite a pounding of snow this evening when the frontal band came through.  Alta-Collins picked up 6" of snow with .27" of water in 3 hours, including a 4" uptick on the snow interval stake from 9-10 PM.

Things are looking interesting for later tonight.  The precipitation to the west has some embedded stronger cells.

We're hoping we might see some deeper orographic convection over the mountains once this comes in.  We will be adjusting our radar scanning strategies more heavily on the Cottonwood Canyons area to allow for more frequent scanning if this turns out to be the case.  Keep your fingers crossed.

Incredible Storm!

It's nuking out there and we are seeing some incredible stuff with the Doppler on Wheels (DOW) including the highest radar reflectivities of the entire field campaign.  I just heard one of the people in the cab say, "we are the kings of stratiform snow." 

The screen shot below shows a vertical scan across Lone Peak.  Check out the waves at the top of the precipitation band. 

More later, if we can find the time...

Here We Go

The Doppler on Wheels (DOW) should be getting setup as I type this.  Everything thusfar is proceeding according to plan.  The shallow surface front has pushed through the Salt Lake Valley, but the heaviest precipitation remains to the north where the front is deeper.  Things are, however, beginning to fill in and I suspect things will start to get exciting by perhaps 7-8 PM.

I am attaching a guestimated terrain-clutter map from our last observing period.  This one is oriented so that west is up, east is down, and our the red line, which indicates the position of one of our vertical radar slices (i.e., RHI) cuts directly up Little Cottonwood Canyon and as close as we can get to over the snow-crystal imager and vertically pointing radar that Tim Garrett and Sandy Yuter have installed at Alta.  The purple line approximates the canyon floor.

We'll be scanning this area extensively to examine the mountain effects during the storm.  Keep your fingers crossed Mother Nature brings the goods.

Last Dances with the DOW

This is it for Utah storm chasers, our last weekend with the Doppler on Wheels before it returns to the Center for Severe Weather Research.  We will be out in the storm tonight and may opt to go down and look at Mt. Timpanogos on Sunday.

The latest models suggest a strong frontal precipitation band will develop over northern Utah between 5 and 8 PM tonight.  

The event then transitions into an event where mountain enhancement plays an increasingly important role during the late night hours.  

We will be deployed where we can examine the frontal precipitation processes and orographic enhancement in and around the Cottonwood Canyons.  So looking forward to it.

I figure tonight might be our last dance, but the latest model runs are calling for some precipitation to develop in southwesterly flow on Sunday afternoon.  

If this forecast holds, we may head down to Utah County to examine enhancement on the 7000 foot southwest face of Mount Timpanogos.  

Short Paper on Atmospheric Rivers

One of my students, Jon Rutz, is working to better understand the inland penetration and impacts of atmospheric rivers over the interior western United States.  Atmospheric rivers are corridors of moisture transport that contribute to a large fraction of the flooding events in California and appear to play a role in major precipitation events over the interior, as we have discussed in previous posts.

Mike Dettinger, Marty Ralph, Paul Neiman, and several compadres have done a great job illustrating the importance of atmospheric rivers in the weather and climate of the west coast.  In Dettinger et al. (2011) they present a fantastic analysis of the contribution of atmospheric rivers to the hydroclimate of the western United States.

Contribution of precipitation on the day of and day following
atmospheric river landfall on the west coast of the
United States to the wet season (Nov–Apr) precipitaiton.
From Dettinger et al. (2011).

Jon and I were both surprised that the precipitation on atmospheric river days was not higher over Arizona, which we believed was a result of their analysis not considering ARs crossing the west coast of the Baja Peninsula.

Thus, we have just submitted a short paper to Atmospheric Sciences Letters expanding the Dettinger et al. (2011) analysis to include those atmospheric rivers.  Indeed, this does result in a substantial increase in precipitation related to atmospheric rivers over the southwest especially southern Arizona.  Compare the results of our gridded analysis below to their analysis above based on station data (for details, see the paper).

Rutz and Steenburgh (2012) analysis including
atmospheric rivers making landfall on
the west coast of the Baja Peninsula.

An underlying issue in this work is that not all precipitation that falls on a day with an atmospheric river can be attributed to the moisture flux in that river.  This becomes more problematic as you extend the length of the coastline for the analysis.  We'll be looking into this in the future, along with issues related to atmospheric river penetration into the interior.

Another All Nighter?

I have this dream in which we have the Doppler on Wheels, but all the precipitation falls during the day and early evening and we're able to sleep at night.  I don't think that will be the case tomorrow.

But, we should be thankful that we do have a storm on tap.  The latest forecast from the North American Mesoscale (NAM) model calls for a front to develop and intensify over northern Utah tomorrow.  By late tomorrow, the forecast front is sitting pretty much over the Wasatch Front, but precipitation is still confined to the northern Wasatch.

The NAM calls for the frontal precipitation to fill in nicely tomorrow evening, with the front sitting right over the Salt Lake Valley at 11 pm.

The flow gradually veers to northwesterly as the front moves downstream, with precipitation continuing overnight over the Wasatch Range.

Thus, it looks like an all-nighter is in store for the Doppler on Wheels, but also that there will be some freshies for skiers on Saturday morning.

For our Doppler on Wheels operations, we'll have some difficult decisions to make.  First, do we want to sample the stuff to the north early in the day tomorrow?  There is also a chance of lake-effect late tomorrow night.  Do we want to take a chance on that or concentrate on the mountain-induced precipitation in the Wasatch Mountains.

University of Utah students are welcome to join us at 1:30 in 711 WBB as we attempt to hash out these issues and lay initial plans for Friday and Friday night.

Remembering Kyle Tietze

Last November, University of Utah Atmospheric Sciences graduate student Kyle Tietze passed away suddenly in his sleep.  Kyle was a popular graduate student, well known for his love of travel, passion for science, and skills as a brewer.  There are two empty bottles of his Synoptic Stout and Forecaster's Friend High CAPE Beer that I keep in my office as his home-made labels were perfect for this Wasatch Weather Weenie.  He was also an exceptional athlete and skier.  

Kyle's family recently established a development fund in the Department of Atmospheric Sciences to support travel expenses for student exchanges between the University of Utah and institutions in other countries.  Kyle worked at the University of Lille in France prior to his passing.  

Donations can be made by clicking here.  Simply note that the gift is in honor of Kyle Tietze and add a special instruction that it is for the Tietze fund.  

Sneak Peak at Sunday's Storm

We are finally getting a chance to look at some of the data we collected during Sunday's storm.  We haven't had a chance to georeference it, so I need to hand wave a bit in this discussion.

On Sunday, we were taking vertical slices up Little Cottonwood Canyon from our observing site near Daybreak.  An approximate position of one of these vertical slices is shown below.  It is oriented up the canyon and crosses over Snowbird and Alta.  Keep in mind that the radar beam spreads with distance from the radar, so the beam is actually somewhat wider than indicated by the red line.

Below is the radar reflectivity from this slice.  Warm colors indicate higher radar reflectivities that likely indicate greater concentrations or sizes of ice paticles and snowflakes.  The bright red stuff near the bottom is ground clutter.  I've indicated where I believe we are getting returns produced by the Baldy Shoulder (separates Alta from Snowbird), High Greeley (the ridge between Collins and Suguarloaf at Alta), and the Mount Wolverine/Patsy Marley ridge to the east of Albion Basin.

Note how the radar reflectivities increase and deepen over the Wasatch Range, an indication of mountain enhancement of precipitation.  You might also notice that that the storm shallows before it crosses Mount Wolverine.  In fact, it's greatest depth is actually upstream of Snowbird and Alta, which may be related to ice-crystal fallout or perhaps the mountain wave induced by the Wasatch Range tilting upstream with height.  It will be interesting to see if our computer models can replicate this structure as it is likely important for limiting how much snow falls out as one moves eastward across the Wasatch crest.

The lack of returns at low levels east of the Baldy Shoulder occur only because the beam is fully blocked by the terrain.  We know from the Collins data that it was dumping fairly hard at this time at Alta.  Bummer that the radar can't see through the terrain!

Live Long and Powder Ski

It was a terrible day in the Wasatch backcountry yesterday.  The Utah Avalanche Center reports there were about 10 human triggered avalanches, one leading to the death of Jamie Pierre.  Our thoughts are with the victim's family and friends.  Get well wishes to others who were injured.

In late September, we discussed how early snow usually becomes weak snow.  This is what has happened to the mid-October snowfall on northerly aspects during the past couple of weeks, leaving a weak foundation for the snow that came this weekend.  In fact, the weekend snowstorms put down more than 1.8 inches of snow-water equivalent at Alta-Collins, with the heaviest precipitation rates early Sunday morning as people were venturing out.  Even more water weight would have been added to the snowpack in wind-deposited areas.

I'm thankful I was busy with the Doppler on Wheels as the temptations of powder fever must have been great.  It's early season (we're stoked to ski), there was fresh snow (hard to pass up), there were tons of people out (false-security of safety in numbers), and most of them were skiing in familiar, typically avalanche-controlled terrain at the resorts (but it's really backcountry right now).  As discussed by Fredston et al., such human factors play a major role in most avalanche accidents.

Given that we will likely be dealing with instabilities from the rotten early season snowfall for the next few weeks, we need to channel Mr. Spock and think like a Vulcan.  Give Fredston et al. a read and be aware of these human factors as you head into the backcountry.