Perspectives on the Wasatch Weather Weenies

The Wasatch Weather Weenies began on October 1, 2010 as an invite-only blog for atmospheric sciences students and professional meteorologists to discuss the weather and climate of the Wasatch Front and Mountains, as well as issues related to the weather and climate in other regions, especially those with complex terrain.

Shortly thereafter, I realized that this should be a public access blog.  There was a great deal of interest in mountain weather, especially in the Wasatch Mountains, and no shortage of material.  As I like to say, weather never sleeps.

Readership is growing rapidly, which is great.  It keeps me motivated and allows me to connect with far more people than I could in a classroom.  We will approach 20,000 page views this month, which is far more than I could have envisioned in October 2010.

After Thursday morning, we enter into yet another long, dry period in the Wasatch Mountains.  Please leave a comment and let me know what interests you.  I'm always looking for good topics for posts.

The Straw That Breaks the Camel's Back

Source: goenglish.com

After a brief flirtation with a more active storm pattern, change is coming to the Wasatch Mountains yet again.

As we discussed on Saturday, we have a chance for a snow on Wednesday night, after which we may be looking at an extended period of well below average snowfall.

The proverbial straw that breaks the Camel's back is a weak cyclone located over the north Pacific just west of the dateline.

The GFS and other computer models forecast this system to move northeastward into the Gulf of Alaska, where it develops into an intense midlatitude cyclone.  As is often the case during major cyclogenesis, strong warm advection and diabatic heating within the cloud shield downstream of the cyclone contribute to the development of a high-amplitude ridge over western North America.  This ridge builds over the western US following the passage of our storm on Wednesday night.

This sets the stage for persistent ridging and flow splitting over the western United States, as discussed in the previous post, and illustrated by the 8–10 day mean 500-mb height forecast from the ECMWF and GFS.

Looks like another tough stretch is coming for powderhounds, but great conditions, at least for a time, for those interested in bluebird corduroy.

A Nightmare of a Forecast

The last couple of days I've been looking at the long-range forecast with grave concern.  A couple of weak systems may give us some snow early next week, but after Wednesday, the dreaded high-amplitude ridge is forecast to return.  Further, this ridge persists in most members of the 0000 UTC GEFS ensemble for a long, long, long time as shown in 144, 240, and 336 hour forecasts below, which cover the period through 0000 UTC 11 February.

Source: Penn State E-Wall

That's essentially a 2-week forecast, which gets us out into "dream-prog land," so there is some hope that the long-range forecasts won't verify.  But a dry period setting up in the 5-10 day period seems pretty likely and the fact that this is a high-amplitude, long-wave ridge has me concerned.  Ditto for the Climate Prediction Center, which gives us >50% chance of below normal period in the 8-14 day time frame.  Those are pretty much the strongest odds they will give at such long lead times.

Source: NOAA/NWS/CPC

I guess the bottom line this year is ski it if you got it.

Banner Clouds

Beautiful bluebird conditions in the Wasatch this morning revealed a spectacular banner cloud downstream of Dromedary Peak.

Banner clouds typically form in upslope return flow on the lee side of a mountain peak, as illustrated below.  

Source: B. Geerts Lecture Notes

Professor Bart Geerts at the University of Wyoming provides more description of the process here.  Perhaps the most famous banner clouds in the world form downstream of the Matterhorn.

Source: Zacharie Grossen/Wikipedia Commons

Headscratcher Snowband

One of the more interesting aspects of last-nights storm was an intense wind-parallel band that developed as the frontal band moved out of the region.

The band developed shortly after 0300 UTC (0800 PM MST) and persisted for about 3 hours.  It formed over the southeast corner of the Great Salt Lake, extended over the mountains north and east of the University of Utah, and into the eastern Uinta Mountains.  Greatest impacts were probably along I-80 between Kimball Junction and Coalville.  Tragically, the band largely missed the central Wasatch, as suggested by the 0419 UTC (0919 PM MST) radar image below.

What processes led to the development of this isolated, wind-parallel band?  700-mb temperatures were about -4ºC in the 0000 UTC sounding.  Perhaps lower it some to -7ºC for a couple hours later.  Lake temperature perhaps -2ºC.  Thermodynamically, this isn't a situation where we would expect lake effect.  Some digging is needed to understand this event.

Another Warm Storm

A healthy band of precipitation associated with a weak cold-frontal zone presently sits over southern Idaho and will sag into Utah during the day today.

Although this is climatologically the coldest week of the year, this will be yet another warm storm.  Temperatures beneath the band along I-84 and I-86 are presently in the upper 30s and low 40s, including 41ºF in Burley with moderate rain and fog.  Before precipitation moves into the Salt Lake Valley, the NWS is calling for temps to peak out near 50ºF.

As precipitation spreads over the Salt Lake Valley later today, this morning's NAM predicts 700-mb temperatures for 0000 UTC (5 PM MST) this afternoon of about -4C.  That equates to a snow level of about 7000 feet.  This storm will start as rain in the valley.

As the cold air moves in, 700-mb temperatures fall to about -9C by 0600 UTC (11 PM MST) tonight, which will bring snow levels down to the valley floor at the end of the storm.

Valley accumulations should be light since the storm will be primarily rain.  For Alta, the NAM generates about .65" of liquid precipitation, which our snow-density algorithm suggests corresponds to about 7" of snow.  That's an average water content of about 9.1%, slightly above average, but because of falling temperatures, the snowfall should be right side up.

The GFS is a bit drier, but that's not uncommon given it's poorer representation of the topography.  Thus, something in the 6–10" range looks likely for the Cottonwoods through tonight.  This is a quick hitter, and the storm should be over by morning.

2011 Global Temperature Perspective

Source: NASA/Hansen et al. 2012

Jim Hansen and collaborators recently posted their analysis of global temperatures in the instrumented period through 2011.  Be sure to have a look, especially with their analysis of the factors influencing year-to-year temperature variability, including the apparent recent slowdown in the global warming rate.  For those of you preferring the Cliff Notes version, here's the summary.

2011 was only the ninth warmest year in the GISS analysis of global temperature change, yet nine of the ten warmest years in the instrumental record (since 1880) have occurred in the 21st century. The past year has been cooled by a moderately strong La Niña. The 5-year (60-month) running mean global temperature hints at a slowdown in the global warming rate during the past few years. However, the cool La Niña phase of the cyclically variable Southern Oscillation of tropical temperatures has been dominant in the past three years, and the deepest solar minimum in the period of satellite data occurred over the past half dozen years. We conclude that the slowdown of warming is likely to prove illusory, with more rapid warming appearing over the next few years.

- Hansen et al. (2012)

The Holidays Are Finally Here

Time for a quick look at the snowpack.  First, my favorite SNOTEL station, Ben Lomond Peak, which always seems to get hammered in any storm that features southwesterly to westerly flow and subtropical moisture.  Indeed, the water totals there from last weeks storm cycle are staggering with an increase in snowpack SWE from 6 to 16 inches (green line).

I sometimes question the huge water totals that this site observes.  I have visited it many times in the winter and am always amazed at how much snow there is there – typically the snowdepth there (@ 8000 ft) is comparable to Alta–Collins (@9700 ft).  There is little doubt that it is an incredibly snowy place.  I'm not sure I buy a 10" increase, but I'm sure they got plastered.  Let's assume the data is legit.  This puts them at 71% of average, with a snowpack SWE that is comparable with early Jan in an average year (compare green and blue lines).

At Snowbird the snowpack SWE increased from about 9 to 15 inches (green line).  This also puts them at about 71% of average with a snowpack consistent with early January in an average year.  

That being said, this site has changed some, so direct comparison of this year with average could be a bit misleading.  I suspect the snowpack we presently have might be closer to that of late December.  For instance, the current Alta-Collins snow depth of 70 inches is just a bit greater than the average Christmas morning snow depth (1997–2010) of 63".

Finally, looking to the Park City side at Thaynes Canyon we see an increase in snowpack SWE from about 5 to just over 8 inches.   This gives them 63% of average snowpack, which is similar to late December in an average year.

So, as far as the snowpack is concerned, the holidays have finally arrived in the Wasatch.

Risk and Reward in Little Cottonwood Canyon

Date and source of this photo are unknown.  I've had it for years.
If you know either, let me know.

This weekend brought us a reminder of the risks and rewards of skiing in Little Cottonwood Canyon (LCC).  The rewards are obvious.  LCC offers up some of the best deep powder skiing in the world with an abundant natural snowfall averaging over 500" annually, with an average of 17 deep powder days per season (i.e., days with 10" or more of new snow).  Further, the terrain in the canyon is steep and easily accessible, whether it be by lift or human locomotion.

The risks are also obvious.  There are 34 major slide paths on the south side of LCC, most of which intersect the highway.  These paths have been mapped out beautifully by avalanchemapping.org.

Purchase this and other avalanche maps at
http://www.avalanchemapping.org/Avatlas.htm

As discussed in the Little Cottonwood Canyon SR-210 Transportation Study, this makes the highway of LCC one of the highest avalanche-risk roads in North America.  One of the most problematic stretches of highway is the area surrounding the White Pine and Little Pine slide areas, which lie just below the lower entrance to Snowbird.  

Snow safety personnel in LCC do an unbelievable job at reducing the risk posed by these slide paths.  As noted in the transportation study, "SR-210s great safety record is due to the high level of dedication, training, and collaboration of UDOT, Salt Lake County Sheriff, USFS, and resort and snow safety personnel." 

That being said, the snow safety personnel cannot completely eliminate risk nor can they ensure that the road can be open 24/7 during the ski season.  Closures occur during avalanche control work, the removal of slides that have hit the highway, etc.  This occurred from Saturday night into Sunday, resulting in a log jam at and around the bottom of LCC and a number of frustrated skiers.  

Compounding the challenge is the sheer volume of traffic in LCC during the ski season.  Average daily traffic is ~5,000 vehicles, peaking at 8,000 on high volume days.  It is not uncommon during periods of heavy snowfall for a trip down the canyon at the end of a busy ski day to take more than an hour from Alta.  

The transportation study suggests two avenues for risk reduction, one involving changes to the road and avalanche control.  These involve active measures for avalanche control and passive measures involving structural changes to the road, such as the use of snowsheds, berms, and nets.  The other avenue involves changes in traffic.  

Source: Little Cottonwood Canyon SR-210 Transportation Study

The transportation study includes a discussion of alternatives for the future.  A few minor alternatives have already been implemented, whereas other proposed alternatives have proven highly controversial (e.g., skier compaction enabled by a lift up Flagstaff Peak).  There are many others, such as options for highway realignment or rail.  Have a look for yourself here.

Don't Let This Be You

Snow has finally arrived in the Wasatch and powder fever is at an all time high after months of withdrawal.  In the sidecountry and backcountry, steep, untracked powder is everywhere and, as noted by the Utah Avalanche Center this morning, it's only a matter of time before people start to push it out into avalanche terrain (I suspect it is happening already).  The temptation is going to be great.

However, the weak, faceted snow that formed over the past few weeks is not going away quickly.  Where it exists, it is going to persist.  A Russian roulette snowpack will likely be around for quite a while, as we discussed way back in November.

Now is a good time to remind everybody that the vast majority of fatal avalanche accidents happen on days with considerable or high avalanche hazard (Greene et al. 2006).

Source: Greene et al. (2006)

In most instances, the victim was aware of the avalanche hazard, but elected to recreate in avalanche terrain anyway.  The lure of powder is great.  Don't let this be you.  See McCammon (2004) for a summary of some of the human factors that contribute to avalanche accidents.

Addendum @ 9:30 AM

Just remembered that the UAC has some additional discussion of these issues here, including statistics for avalanche fatalities (I believe for Utah) showing a pronounced peak in the considerable category.

Source: Utah Avalanche Center

Have a look.

Storm Potpourri

Storm totals reported this morning by the Utah Avalanche Center for this weekend's storm (snowfall and snow water equivalent):

Little Cottonwood: 24"/2.39"

Big Cottonwood: 29"/3.35"

Park City Mountains: 15"/1.8"

Ogden Mountains: 22"/2.46"

Provo Mountains: 19"/1.93"

Quite an event.  Avalanche conditions sound like the worst since I moved here in 1995.  Some quotes from the advisory:

"One 20 year patrolman remarked it was [one of] the most impressive snow events he'd ever seen"

"It was probably the diciest day I've experienced in many years"

The latter comment refers to the backcountry avalanche conditions.

This morning the Town of Alta is interlodged and Little Cottonwood Canyon is closed. Estimated opening time is 10:30 am.  Big Cottonwood is open.

My rule of thumb is that 24-hour snow totals of 24" often put you into the "too much of a good thing" category, unless the snow is all low water content (which wasn't the case yesterday).  Looks like we're there.

The powder skiing should be sublime if they can get anything open.

Dumpage Like We Haven't Seen All Year

Old Man Winter is Back, and none too soon.  Over the past hour or two the vertically pointing radar that NC State has at Alta shows the highest, deepest radar reflectivities that I have seen all year (top panel below, time in UTC).  In other words, it is puking snow.

Source: NC State

The Alta-Collins precip gauge shows .24" of SWE from 2–3 PM MST (21-22 UTC).  Based on the vertically pointing radar image above, that has likely continued through 4 PM.  Snowfall rates are likely around 2-3" per hour (possibly greater).  

Snow crystal images from the snowflake camera at Alta show some nice graupel at 2:20 PM.  Graupel is produced by riming, the freezing of supercooled cloud droplets onto snowflakes.  It is favored in warmer storms.

Source: University of Utah, Center for Snow Science at
Alta, and Alta Ski Area

And, things are just getting started.  The flow is veering to northwesterly and KMTX radar imagery shows plenty of action upstream, including near the UT–NV border where the pattern is more cellular, indicative of the sort of instability we want to keep post-frontal precipitation going over the mountains.  As temperatures fall, I suspect we'll see less graupel and more low-water content dendritic snow.  

It's in the bag that this will be the biggest storm of the year in the Cottonwoods.  That's the good news.  The bad news is that the combination of a weak pre-existing snowpack, huge snowfall rates, and graupel during the initial portion of the storm must be pushing the avalanche hazard to the extreme.

440PM Update

A remarkable .29" of SWE fell at Alta Collins from 3-4 PM.  The snow interval during this period increased from 9-14".  There can be roundoff errors and some uncertainty in the snow interval changes, but snowfall rates of over 3" per hour seem likely.  Rock on!

550PM Update

Weird.  Just pulled up the Alta Collins data again and the 4 PM snow interval now says 10".  How bizarre.  I'll be checking with the MesoWest team on this one.  Presumably someone out there can confirm that it was indeed snowing like snot.

A Warm Start to a Good Storm

The wet, warm, and windy pattern continues this morning, but change is coming.  The radar is painted in beautifully with full coverage of precipitation across northern Utah.

Source: NCAR/RAL

Even Wendover is getting rain.  Temperatures are balmy.  It was 30ºF at Alta-Collins at 7am, but fortunately cooled off a smidge to 27ºF by 8 am.

At 8am, the base of Alta was 34ºF.  A hard-shell day for sure.  Down a bit lower, a mix is falling on the hungover Sundance types in Park City where the roads have a veneer of slush on them.  

As discussed on Thursday, temperatures will fall today and we should see a transition in the water content of the snow to give us a nice right-side-up snowfall and some great skiing tomorrow.   Pity that I'm home with the virus from hell.  Get a core shot for me.  There will be plenty to be had.  

Reloading

Two cyclones are interacting over the eastern Pacific to "reload" the atmosphere for this weekend's storm.  As shown in the loop below, there is a slow moving cyclone over the Gulf of Alaska and a second, faster moving cyclone moving across the Pacific further to the south.  The merger of these two systems enables the penetration of cold air from the high latitudes into the mid latitudes, as indicated by the southward penetration of deep cumulus clouds (a.k.a. open cellular convection) behind the cyclone late in the loop.  That kind of airmass is the skiers friend and it should begin to move in during the day on Saturday and give us some lower water content snow.

1600 UTC 18 Jan – 1600 UTC 20 Jan 2012 IR satellite and sea
level pressure (contours every 4mb) loop (3-h increments)

The two cyclones also contribute to the advection of high precipitable water air into the western United States.  In this regard, they are also contributing to some of the higher water content snow we will receive between now and Saturday.

1600 UTC 18 Jan – 1600 UTC 20 Jan 2012 IR satellite and precipitable
water (contours every 5 mm) loop (3-h increments)

The large-scale perspective above helps to place what we will see between now and Sunday into context.  Meteorologists typically use something called the forecast funnel to predict the weather.  That funnel starts with the largest scales and then zooms into smaller scales.  An analysis of the large-scale context, such as done above, is the first part of the forecast funnel.

Return of the Greatest Snow on Earth

Surely you remember why we ski in Utah?

I'm not up skiing today, but suspect those who are are scratching their heads and wondering who transported them from the Wasatch Mountains to the Cascades.  Actually, the snow in the Cascades is probably better as they've had a streak of cold, snowy conditions over the past few days (culminating with a debilitating ice storm today in the lowlands of western Washington).

Here's a statistic for you.  Based on the snowfall history at Alta.com, last night's storm was the first to produce a daily (24-h) snowfall of 10" or more since 31 October.  Now I consider 10" to be the minimum amount of snow required for deep powder skiing since you need about that much before you start getting true ski floatation.  Given that the 13" we got overnight was Cascade Concrete, that essentially means that there hasn't been a good deep powder day in almost 3 months!

That will change this weekend when the Greatest Snow on Earth finally returns.  Through Friday night, the wet, windy, and warm regime will dominate with some periods of higher density snow in the Wasatch and rain in the Salt Lake Valley (benches may see some snow mixed in at times).  The weekend storm, however, looks great.  In the latest GFS forecast, it starts out warm with 700-mb (crest level) temperatures around -5ºC at 11 am Saturday.

Then the trough goes through and temperatures fall, reaching -14ºC by 5 am Sunday morning.

That's a perfect recipe for a right-side-up storm.  The decline in temperature during the storm means some higher water content snow to start, but a transition to lower water content snow over time, which is ideal for deep powder skiing.  The Alcott snow density algorithm presently calls for 13-14% water content snow on Saturday morning, declining to ~5% by Saturday night.

Things could start out manky on Saturday, but if these forecasts hold up, Sunday will be the first good powder skiing of the year.  I know I'm going to regret saying that...

The Big 50

Thanks to 13" of new snow, the Alta-Collins site finally hit the big 50 snow depth this morning.  Ah, the start of early season ski conditions!

The 13 inches includes 2.19" of water, which equates to a water content of 17%.  Yup, Cascade Concrete.  Let's look at how the NAM and Alcott snow algorithm verified at Alta based on yesterday morning's forecast.  The NAM called for 2.06" of water by 7 am this morning, very close to observed.  That is a remarkable forecast.  I figured we'd come up a bit short of that, but am not going to complain about coming in on the high side.  The Alcott snow algorithm called for an hourly snow water content of 14% for most of the night, decreasing for to 9% this morning, yielding 15.6" of snow.  It had the right idea, but clearly did not go for 17%, although the Utah Avalanche Center (UAC) suggests that most reports were in the 13–15% range.  Settlement might be inflating the water content numbers at CLN some as we're now getting well into the storm.  Note how the snow interval observation has hung in there at 13" even as precipitation continued.

Totals decrease on the Park City side of the range.  This likely reflects precipitation shadowing from the strong westerly flow and the strong gradient in precipitation with elevation, especially early in the storm when there was a lot of dry air at low levels.

It looks like a hard shell day today.  It is balmy this morning, with a freezing level near 8750 feet and temperatures of 33ºF at the base of Alta and 34ºF at the base of Solitude.  Flakes will be wet below 9000 feet.

Starting to Crank at Alta

Things are finally picking up at Alta.  North Carolina State University has a vertically pointing microwave precipitation radar installed at Alta as part of a collaborative project with the University of Utah, Alta Ski Area, the Center for Snow Science at Alta.  Unlike a conventional weather radar, this radar just shoots straight up, collecting profiles over time that can be plotted as a time height section.  As shown in the image below (time is in UTC), things were fairly quiet through about 2200 UTC, when it really picked up.  BTW, you can see in this image that most of the snow growth today is happening below about 4200 m.

Source: NC State

If you don't trust the radar, there's always the Collins gauge.  .10" and .14" in the hours ending at 5 and 6 PM MST, respectively.

Evening Update

Precipitation rates have finally picked up at Alta.  The Alta-Collins site picked up .14" from 2-5 PM with .10" falling from 4-5.  This is a far cry from the .50" forecast by the NAM, but it's a start.  From what I've seen thusfar today, I think the models have overdone the SWE for this event as one our readers, David, suggested.  We'll see how things verify in the morning as even if the models went a bit too berserk, we should see mountain snows for much of the night as there's a fairly solid precipitation shield upstream.  The mountains northeast of Boise must be getting hammered.

The cold air also mixed out enough around campus for the snow to turn over to rain.  Pity!  Snow continues, however, in the northern Wasatch Front.

Update:

Ah, I found the snow.  Ben Lomond Peak Snotel has observed .80" of SWE.  You can always count on Ben Lomond to come through.

Here It Comes

The transition over the next 12–24 hours should be impressive.  Right now, we have your classic "cloud storm" underway in which snow is falling from a layer of stratus clouds aloft, but quickly sublimating as it falls into a dry low level airmass.  Note the ragged cloud base and only slight obscuration of the highest peaks in the Wasatch at 9:20 am.

Looking southeast toward the Wasatch Mountains @ 9:20 AM

The morning sounding from KSLC shows quite well the remarkably dry airmass below 600 mb, which features dew-point depressions exceeding 20ºC at some levels.

Source: NCAR/RAL

The moist stable layer with a base at about 600-mb reflects the upper-level portion of a warm-frontal zone that is approaching from the west.  This warm-frontal zone will descend over time and eventually bring the Wasatch much needed snow.

In fact, the models are going berserk later today and tonight in the Wasatch Mountains.  The 1200 UTC NAM, for example, generates more than 0.5 in of SWE in portions of the Wasatch Mountains during the 3-h period ending at 5 PM.

NAM forecast valid 5 PM MST 18 Jan 2012

Point data from the NAM for Alta shows 1-h SWE rates exceeding 0.1" per hour from 3 PM this afternoon to 3 AM tonight.

As we discussed yesterday, this will likely be Cascade Concrete with a high water content.  The Alcott algorithm is presently calling for a water content of 14%.  All this heavy snow will be falling on an incredibly weak snowpack.  Not surprisingly, the Utah Avalanche Center has issued an Avalanche Warning beginning at 2 PM for the Wasatch backcountry.

Perhaps the most difficult aspect of this forecast is the snow level.  Here's why.  We have a ton of warm air streaming in aloft, but cold, dry air is entrenched at the surface.  Yesterday it appeared that cold, dry low-level air would scour out enough for snow levels would rise, but it is stingier in the latest model runs.  This leads to a deep stable layer in the latest NAM forecast that is below freezing and would enable snow to fall even in the lower elevations of the Wasatch Mountains.

NAM forecast sounding for KSLC valid 8 PM MST 18 Jan 2012

Hopefully something like the sounding above will verify as we need every flake we can get.