Wednesday, January 17, 2018

What Happens to Fronts over the Western US

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

What happens to fronts over the western US?

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

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

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

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

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

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

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

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

Tuesday, January 16, 2018

Global Warming and the Salt Lake City Olympic Bid

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

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

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

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

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

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

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

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

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

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

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

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

Sunday, January 14, 2018

Today Went to an Eleven

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

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

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

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

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

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

Saturday, January 13, 2018

Incredible Northern Wasatch Asperatus Display

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

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

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

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

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

More info at Ogden Nordic's web site

Thursday, January 11, 2018

A Storm with Big Winners and Big Losers

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

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

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

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

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

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

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

Wednesday, January 10, 2018

Big Snows in the Western Alps

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

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

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

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

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

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

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

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