The Good Old Days

A friend send me a copy of Grove Gilbert's 1928 USGS report Studies of Basin-Range Structure yesterday. 

Gilbert was scientific giant who did a number of pioneering studies of the geology of Utah and the western U.S.  He named Lake Bonneville, the historical lake of which the Great Salt Lake is a remnant. 

The report appears to have been published posthumously (Gilbert died in 1918).  What caught my attention are the many photos of the undeveloped Wasatch Front, which I've added to the end of this post and which would have been taken probably in the early 20th century.  They are a reminder of how rapidly we have transformed the landscape in the short period of a century (or less). 

What will Utah and our beloved Wasatch Mountains look like in 2050 when we are told the population will be twice what it is today? 

About the Wasatch Weather Weenies

Head Blogger, Jim Steenburgh, Hakuba, Japan

The Wasatch Weather Weenies began on October 1, 2010 as an invitation only blog for discussing the weather and climate of the Wasatch Front and Mountains, western United States, and mountainous regions in general. 

It started out as invitation only because I didn't want discussions to descend to the levels seen, for example, in comments following Salt Lake Tribune articles online.  However, I quickly had so many people asking to read it that I decided to make it publicly viewable and accessible.  Thanks to you, comments have remained substantive and approved for all audiences, and I'm grateful for that.  I'd actually like to see more questions, comments, and suggestions, so feel free to do so liberally. 

I consider the Wasatch Weather Weenies to be an educational blog.  Posts sometimes target a more advanced meteorological audience, others a general audience.  There's plenty of graphs and plots (including meteorological ones like Skew-Ts and time-height sections), not to mention jargon.  If you'd like to learn more, find the subject cloud in the right hand column and click on forecast tools.  There are sometimes pearls of wisdom in there that can be useful for the self-motivated learner (click on older posts when you get to the bottom to delve deeper into the archives).   Alternatively my book, Secrets of the Greatest Snow on Earth, is a good resource.

Please don't consider the Wasatch Weather Weenies to be a weather forecasting service.  I sometimes provide forecast thoughts in my posts, but for the most part, I use forecasting intermittently as a gateway drug to teach people about weather and the challenges of weather forecasting.  In times of hazardous weather, your best resource is the National Weather Service.  When it comes to watches, warnings, and informing decisions to protect life and property, they should be your go-to source. 

Over the past couple of years, I've been using twitter more and more.  It's a great way to share information during high-impact or simply very interesting weather events without having to write up a long post.  Follow me @ProfessorPowder or just take advantage of the tweet scroll in the right column.

The Wasatch Weather Weenies is a free blog, unencumbered by ads.  If you are a regular reader, consider making a donation to our Mountain Meteorology Fund at the University of Utah.  To do so, go to https://umarket.utah.edu/ugive/level4.php?catid=742,  select "make gift", and then "add special instructions" and specify that you would like the gift to go to the mountain meteorology fund.  Donations will then be vectored to the mountain meteorology fund.  The fund is focused on support of student education and research in mountain meteorology and may be used to support international exchanges of students and faculty; to purchase research equipment; to conduct special seminars, workshops or short courses at UU; to fund student travel to national and international conferences, field experiments, workshops and short courses; and to promote other activities or purchases in support of mountain meteorology students. The fund is also used to provide travel funds for scientific visitors who come to the department to present mountain meteorology seminars.

Donation or no donation, thanks for reading.  See you on the skin track, ski lift, or nordic trail.

Best Regards,

Jim Steenburgh
Professor
Department of Atmospheric Sciences
University of Utah

Saturday Night's Rime "Storm"

A thin veil of altostratus clouds led to an upper-elevation riming event Saturday night that likely threw a wrench at some lift operations in the central Wasatch this morning (Sugarloaf was a late start at Alta, although I'm not sure if that was due to rime or a mechanical).

The large-scale setup was typical for these events in the Wasatch Mountains with moist, northwesterly flow spillover over a ridge located near the Pacific coast.

This mornings sounding shows very dry conditions at low levels over the Salt Lake Valley, but saturated conditions, indicated by the colocated dewpoint (green) and temperature (red) traces within the cloud layer from about 725 to 650 mb.  This is consistent with cloud bases near about 10,000 ft.

Source: SPC

The key for rime is that the clouds need to be below 0ºC, but not have cloud top temperatures much below about -8 to -10ºC.  Indeed, this mornings sounding suggests a cloud-top temperature around -10ºC.  At such temperatures, the cloud droplets can exist in large concentrations in liquid form despite the below "freezing" temperatures.  These drops then freeze on contact with whatever object that they meet.  Note also that the winds at 700-mb are about 30 knots.  Flow is needed for significant riming to occur since it's the main mechanism for transporting the cloud droplets.  Without it, you'll probably get mostly hoar frost.  If cloud tops are colder, enough ice can form in the cloud to "glaciate" (i.e., freeze) enough of the supercooled cloud droplets that riming is limited.

In some rime events, larger cloud or drizzle drops can form.  That may have been the case last night.  The Supreme Chair, for example, was coated in a fairly clear layer of ice that suggests to me some larger drops may have around.

The situation should be better tomorrow.  Overnight, as the ridge shifts eastward, the flow weakens and the mid levels dry, as can be seen in the time-height section below.

That should make for a pretty day too.

PyeongChang 2018 Olympic Weather Preview

With the Olympic Winter Games beginning on February 8, it seems like a good time to take a look at the climate of PyeongChang and the sorts of weather that we might expect in a couple of weeks.

PyeongChang is a county in the Gangwon province of South Korea.  On the broad scale, it lies on the eastern edge of Asia, east High Mountain Asia, including the Tibetan Plateau, and between the Yellow and East Seas.  The East Sea is known to many Americans and the Japanese as the Sea of Japan, but the Koreans as the East Sea, so for this post, I will stick with East Sea since they are the Olympic hosts.

Olympic venues are concentrated in the Gangneung Coastal Cluster, along the coast of the East Sea, and just inland in the PyeongChang Mountain cluster.  The topography of this region is a mumble jumble of small ranges and mountains.  In a broad sense, the Teabbek Mountains run from south-southeast to north-east, paralleling the coast, with some higher-elevation branches that extend southwestward.  The PyeongChang Mountain Cluster is near the crest of the Teabbek Mountains within one of these spurs.  Most venues in the mountain cluster are near the triangle, but the Jeongseon Alpine Center, site of the men's and women's downhill, super-G, and combined, are in the mountains about 15 km (10 miles) to the south.  The top starting altitude is for the men's downhill at 1370 meters (4495 feet), with finish altitudes for all races at the venue around 545 meters (1788 ft). The highest peak in the region is Seoraksan at 1707 meters (5,603 feet), about 50 km (33 miles) north of the mountain cluster.

Perhaps the closest (but imperfect) analog for the terrain of this region from past Olympic cities would be Lake Placid.  The photo below of the Yongpyong Alpine Center, site of the men's and women's giant slalom and slalom, reminds me a great deal of the high peaks region of the Adirondack Mountains.

The Gangneung Coastal Cluster includes hockey and curling centers, the ice arena for figure skating and ice dancing, and the speed skating ovals.  The PyeongChang Mountain Cluster includes the Olympic Stadium, biathalon and cross-country skiing center, ski jumping center, sliding center (skeleton, bobsled, luge), Phoenix snow park (snowboarding, freestyle skiing), Yongpyong alpine center (alpine skiing gs, slalom), and Jeongseon alpine center (alpine skiing dh, sg, combined).

The weather of PyeongChang and the Korean Peninsula during the Olympic period is strongly influenced by the Siberian high, characteristic of the Asian Winter Monsoon (monsoon means season).  The average sea-level pressure for February is shown below and illustrates the arm of the Siberian high that extends southeastward across eastern China and the Korean Peninsula.

Source: ESRL

The February climatological 1000 mb (near sea level) temperatures show that Korea, like the eastern seaboard of the United States, is relatively cold due to it's location downstream of a major continent.  Pyeongchang is located at a remarkably low latitude (37.3ºN) and altitude (500-1400 meters), but can host an Olympics because of the dominance of cold, continental air.  If you take a close look at the map, the Korean peninsula and nearby eastern China have the lowest average 1000-mb temperature at their latitude.  Amongst past Winter Olympic cities, only Nagano (36.6ºN), which similarly benefits from a location downstream of Asia, is further south.

Long-term climate averages for each cluster during the Olympic and Paralympic periods are provided below.  Unlike Japan, which benefits from being downstream of the East Sea, snowfall in the mountain cluster is relatively scant.  The 41.3 cm average for the 17-day Olympic period scales to a 30-day monthly mean of 72 cm (28 inches).  This would be somewhat higher than Lake Placid (19 inches).  

Source: http://pc2018.kma.go.kr/Climate_of_Pyeongchang_ENG.pdf

The coastal cluster being at lower elevation and east of the Teabbek Mountains might be expected to be drier than the mountain cluster, but it isn't.  It's actually slightly wetter, although due to the lower elevation, precipitation more frequently falls as rain.  The reason for this is the close proximity of the East Sea and the blocking effects of the Taebbek Mountains during periods of easterly flow, which drives the regions heaviest wintertime precipitation events.  An example is provided below.  The sea level pressure analysis (right) shows low-level easterly flow impinging on the Korean Peninsula.  The satellite imagery shows shallow sea-effect clouds streaming toward the Korean Peninsula with clouds and/or snowcover over the eastern peninsula, but clear skies and no snow cover further west.  

Source: Kim et al. (2016)

Such an easterly-flow precipitation event is one of the bigger major weather concerns for Olympics, although there are others. A summary of weather extremes at both clusters during the Olympic and Paralympic Periods is provided below.  

Source: http://pc2018.kma.go.kr/Climate_of_Pyeongchang_ENG.pdf

We were fortunate to host several meteorologists for the PyeongChang Olympic Games at the University of Utah approximately two years ago and have Byung-Gon Kim of the Gangneung-Wonju National University visit us while on sabbatical.  They appear to be very well prepared for the games.  Korea has an exceptional weather observing network of surface stations and radars, as well as many other platforms to help with specialized local prediction.  The best outcome for an Olympics is benign weather, but in the case of inclement weather, they will have great meteorological guidance to help with logistics and planning.  

Rest in Peace Warren Miller

Forget about the weather today.  I just learned of the passing of Warren Miller at the age of 93.  I find myself surprisingly emotional considering I never met the man, but he had a profound influence on my life.

It's probably difficult for younger skiers to understand the anticipation for the screening of a Warren Miller Film in the pre-internet days.  As a youngster in upstate New York, we would travel to Proctor's Theater in Schenectady each fall where Warren would personally narrate that year's movie, shot on location in some of the most spectacular places on the planet.  Today you see footage from many of these places on YouTube after some person's run of a lifetime, but video of skiing in the Selkirks or New Zealand was like seeing skiing from another planet back in the day.  The ONLY way to see it was to go see a Warren Miller movie.

Wikipedia provides a chronological list of Warren Miller movies, beginning with his first, Deep and Light, in 1950.  I can't remember the name of the first Warren Miller Movie I saw, but I suspect it was Snowonder.  I certainly remember aspects of Ski Time, Ski Country, and Steep and Deep.  The plots of these moves are all the same, but that didn't matter.  You could count on videos of humorous chairlift incidents, hero worship of god- and goddess-like bronzed skiers, lines like "we're in the Monashees with Mike Wiegele skiing with <insert ski god and goddess names here>", followed by some incredible ski footage.

In the early 1980s, Warren Miller had no peer.  Greg Stumpf's groundbreaking "Blizzard of Ahhhs," which probably did more to push the ski-movie genre to the "extreme" than any other movie, wasn't released until 1988.  There was no Teton Gravity Research.  There were no GoPros.  You want Red Bull?  Buy cherry Kool-Aid.  Everyone you rode on the lift with at Alta didn't claim to be a pro skier.

It's worth a look at the trailer to Warren Miller's Ski Time from 1983, reflecting the mixture of what one would find in a Warren Miller movie at the time.  Andrew Mclean and other kite skiers will enjoy the sailboarding scene.

In the 1980s, Miller began shooting footage of Scot Schmidt, who was unlike anything seen previously, at least in the United States.  Everyone wanted to copy his signature "smear turn" and the footage of Schmidt skiing was the highlight of any Warren Miller movie at the time.  Here's some footage of Schmidt from those 1980s Warren Miller films.

Early Scot Schmidt from JC Ski on Vimeo.

What Schmidt did on the equipment of the time was quite staggering.  In the early 1980s, there were no all-mountain skis.  For a while, I believe Schmidt skied on a pair of K2 Comp-910s, basically K2's downhill racing ski from about 1980.  If you look closely at the footage, you can make out the "COMP" on the bases and the signature dark-to-light green striped top-skin graphics.  That ski was sold at lengths starting at 210 cm.  Think you're a good skier?  Swap your Fat-ypus skis for a pair of World Cup Super-G or downhill boards and tell me how you do skiing the Baldy Chutes.

Ultimately, Warren Miller had a profound influence on my life.  He got me excited about snow and skiing, which has had huge ramifications for me personally and professionally.  His was a life well lived.  Rest in peace my friend.

A Primer on Sea-Effect Snows of Japan

Utah may have the Greatest Snow on Earth, but Japan has the Greatest Snow Climate on Earth.  Nowhere in the world does it snow with such intensity, at such low latitudes, at such low elevations, with such prolific mountain snowfalls.

The driver of all this snowfall is the so-called Asian Winter Monsoon, which is characterized by a cold, northwesterly flow from Asia over the Sea of Japan.  This produces copious sea-effect snowfall that inundates northwest Honshu and Western Hokkaido Islands. 

Source: Secrets of the Greatest Snow on Earth

The snowfall accumulation is relatively punctuated, but incredibly intense.  Compare, for example, the average monthly snowfall at Kutchan, a Japanese Meteorological Agency observing site near the base of Hokkaido's Niseko resorts to that at Alta.  Alta features a broad peak from December through March (the minimum in February largely reflects the smaller number of days), whereas Kutchan features a pronounced maximum, exceeding 140 inches, in January.

The Japanese Meteorological Agency observing site at Sakayu Onsen, in the Hakkoda Mountains south of Aomori averages 181 inches of snow in the month of January and 695 inches a year and may be the snowiest regular observing site in the world.  This is why I say that there is no surer bet for deep powder skiing anywhere in the world than in the mountains near the western coasts of northern Honshu and Hokkaido Islands in January.

The Sea-Effect machine is currently raging in Japan and the satellite imagery is marvelous.  One can see the two primary modes of sea-effect snowbands in the modis imagery from yesterday.  The first are longitudinal mode bands (or "L-mode"), which are oriented roughly parallel to the low-level wind.  The second are transversal mode bands (or "T-mode"), which are oriented roughly perpendicular (or across) the low-level wind.  Upstream coastal geometry and coastal geometry often affect the initiation of the L-mode bands.   It is curious that both types can be observed simultaneously over the Sea of Japan for reasons that perhaps we will discuss sometime in the future.

As I often like to say when it comes to sea- and lake-effect precipitation, "there are a lot of critters in the woods."  L-mode and T-mode bands aren't the only sea-effect features observed during the winter monsoon.  There can be areas of larger-scale convergence, such as the Japan Sea Convergence Zone that affects Honshu and convergence zones that form downstream of the Sikhote-Alin Mountains of eastern Russia that affect Hokkaido.

What happens to these sea-effect snowstorms as they approach and interact with the coast and topography of Japan depends on a number of factors.  There are events that sometimes produce heavier snowfall in the coastal lowlands, for example, rather than in the mountains, which is more common.  In a classic study on ice crystals and sea-effect snowfall at Hokkaido University, Magono et al. (1966) referred to storms with heavier lowland snowfall as "Satoyuki" type and heavier mountain snowfall as "Yamayuki" type.  Their conceptual model for Satoyuki-type storms is below, and features a strong inversion layer near mountain top that likely prevents the flow from surmounting the barrier.

Source; Magono et al. (1966)

With support from the National Science Foundation, we have been working with researchers at the Snow and Ice Research Center in Nagaoka, which is part of Japan's National Research Institute for Earth Science and Disaster Resilience, as well as scientists at the University of Hokkaido, to better understand the processes affecting the distribution and intensity of snowfall along the Japanese coast and inland mountains.

We have one paper currently in review that examines the transformation of T-mode bands as they interact with the Shakotan Peninsula of Hokkaido Island, generating a region of more persistent and intense precipitation over the Ishikari Plain, which includes the city of Sapporo.

Source: Campbell et al. (in review)

We have also been using the Japanese radar network and space-borne cloud radars to examine sea-effect storms over the Sea of Japan and their interaction with Honshu, focusing on the area around Nagaoka.

I have had to good fortune to travel to Japan twice, the first for the 1998 Olympic Winter Games and the second to meet with scientists in Nagaoka.  I consider both trips to be career and personal highlights.  It is worth going over for more than just skiing.  Seeing how the Japanese deal with such prolific snowfall is quite interesting.  In Nagaoka, for example, much of the snow falls at temperatures at or just above 0ºC.  Warm water is sprayed on the roads to melt the snow, which then flows through the cities water system.  

Here's an example of another warm-water-based snow removal system at the Snow and Ice Research Center.

Then of course there is the skiing.  Japan is often described as deep, not steep, and indeed you can find plenty of low-angle powder skiing there.  However, there is also some incredible terrain for backcountry skiing.  A challenge is finding an opening with good weather and stable snow.  We were fortunate to get some on last year's trip.  Below one of my graduate students, Peter Veals, gets the goods in the backcountry of the Hida Mountains.

With big relief and a relatively poor start to the snow season that year, the exit of that tour involved a walk through bamboo forests, fueled by cold pot stickers.

What could be better?  

The More Things Change, The More They Stay The Same

"Pattern change" is currently on my list of banned words and phrases for good reason as it was being thrown around repeatedly for weeks despite the fact that nobody really knows what it means. 

As I've been watching the large-scale pattern for the last several weeks, there has certainly been variability.  Ridges and troughs have formed and dissipated.  There have been some major cyclogenesis events.  However, one thing has remained constant.  The large-scale pattern has been very high amplitude, meaning a wavy jet, especially from the Pacific Ocean to Europe. 

That situation looks to continue for the foreseeable future.  Below is the 10-day GFS forecast for the northern hemisphere dynamic tropopause.  The dynamic tropopause separates the troposphere, or the lower atmosphere in which we all live and reside, from the stratosphere and basically sits at jet-stream level.  Note in particular the high-amplitude, wavy nature of the pattern, with strong ridges and anticyclones (high-pressure systems) forming in several areas including over the Behring Sea and North Atlantic.  Basically, the large-scale flow is highly disrupted. 

This is essentially what we have seen now for weeks.  High amplitude patterns like this can be good for snow if you are in the right place (check out the Alps), but we haven't.  The tendency in Utah has been for us to be under the influence of high-amplitude ridging or just to the south of the storm track. 

So, the more things change, the more they stay the same.  We get the occasional storm, but a real storm cycle is hard to come by.  Unless the storm track shifts southward more than currently predicted, that looks to be the case over the next seven days.  Our best bet for snow, as indicated by the NAEFS forecast plume below is Thursday and maybe Thursday night, with maybe a bit here or there thereafter. 

The NAEFS plume above may even be a bit optimistic for Thursday and Thursday night as the water equivalents advertised by the NAM, GFS, and Euro fall in the low end.  Most members of our downscaled SREF are generating only .1 to .5 inches of water equivalent.  The optimistic Canadian model (CMCE members in the plot above) appears to be an outlier. 

As usual, keep expectations low and hope for the best.  A small storm will be appreciated, but recognize that I have yet to see the whites of the eyes of a real pattern change.  However, my crystal ball only sees out about 7 to 10 days.  Let's hope February is better. 

So Much Snow, So Little Water

It was wonderful to see the white stuff return to the valley and the Wasatch Front this weekend.  Everybody got some.  Here are some of the bigger totals as reported to the National Weather Service (water equivalent in parentheses).

Bountiful Bench (through 5 PM Saturday): 13.5" (0.87")
Bountiful (5 PM Saturday): 10.8" (not reported)
Cottonwood Heights (7 AM Sunday): 17.5" (1.64")
Holladay (7 AM Sunday): 15.5" (0.91")
Summit Park (12 PM Sunday): 20" (not reported)
Alta Collins (4 PM Sunday): 23" (1.16")
Canyons 8800 ft (3 PM Sunday): 20" (0.90")

It was a complicated storm featuring a frontal phase Friday night, a period Saturday and Saturday evening with strong enhancement along the benches, and a period Saturday night when the Little Cottonwood magic kicked in. 

There are, however, a couple of key observations to be made.  The first is that the water-equivalents were largest along the east bench of the Salt Lake Valley, not in the highest elevations of the Wasatch Mountains.  Alta-Collins, Snowbird, and Alta-Guard were the three wettest sites, each coming in just over an inch of water.  In contrast, two sites in Cottonwood Heights reported over 1.5 inches of water. 

A big reason for this is the first two storm phases, which featured strong frontal forcing and then a period with weak low-level northwesterly flow in the Salt Lake Valley, but light flow aloft with strongly stable conditions.  One can't always assume the climatological increase of precipitation (water equivalent) with elevation, and Friday night and Saturday provided a good example of that. 

The second key observation is that this storm really didn't add much to the snowpack.  The 23 inches of snow that fell at Alta-Collins sounds like a lot, but it had an average water content of only 5%.  The 10 inches that fell Saturday night had a water content of only 3%. 

So much snow, so little water!

Basically, the storm put smiles on skiers faces, but water managers still have heartburn.  The Snowbird SNOTEL snowpack water equivalent didn't go up as much as one might expect from the precipitation gauge observations (this is not uncommon), but even if one jacks up the snowpack water equivalent to account for this, we're still losing ground to average.  At the end of December, we were 6.1" below average snowpack water equivalent.  Today, even with the weekend snow, we're 9.1" of water below average. 

Source: NWS

This time of year, we should be adding about 1.5 inches of water to the upper-elevation snowpack in Little Cottonwood Canyon every week.  In the first 3 weeks of January, we added about 1 inch per week.   We're still losing ground. 

This trend looks to continue for the next week.  Other than some snow showers today, our next change of a storm is in the Thursday-Friday time range.  The downscaled NAEFS ensemble generates anywhere from about 0.25 to 0.9" of water with that storm at Alta Collins.  Some members produce a bit more after that event, but not much.  Only one NAEFS member reaches the coveted 1.5" water mark for the week.   

The end of January roughly marks the midpoint of the winter snow accumulation season at upper elevations.  At Snowbird, for example, the snowpack water equivalent averages 22.1" on February 1, roughly half the average peak of 44 inches on April 27.  With 9.9" currently on the ground, we look to be about 50% of average at the end of the month, with somewhere between 10 and 11.5 inches of snowpack water equivalent.  

We need a real storm cycle and we need it now.  

About Last Night...

If you want blower pow, last night was your storm.  From 5 PM yesterday to 6 AM this morning, the Alta-Collins observing site picked up a paltry .32" of water equivalent, but 10" of snow.  That's a mean water content of only 3.2%!   That's a snow-to-liquid ratio of 31-to-1.  If you want a big dump, but don't have a lot of water to play with, that's how to do it. 

Really, conditions overnight were only marginally better for orographic precipitation generation than they were yesterday.  The 0000 UTC (5 PM MST Saturday) sounding still showed stable conditions at mid level with strong wind shear from 700 to 600 mb. 

The morning sounding is a bit better with northerly flow through depths and colder temperatures aloft, although a weak stable layer remained just above 700 mb. 

But the secret to the overnight dumpage wasn't large orographic precipitation enhancement.  There was some enhancement, but .32" of total water and a maximum water equivalent rate of 0.05" per hour isn't much.  The secret was the huge snow-to-liquid ratio.  If the snow-to-liquid ratio for this storm was Alta's average of 13-to-1, the snowfall would have been 4 inches.  But at 31-to-1 you get 10 inches and the stuff that Kodak moments are made of.  Yup, this was a snowfall that will bring smiles to skiers, but continue to give water managers, who need to see higher water content dumpages, heartburn. 

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.  

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.  

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.