Splendid Weekend Ahead

The weekend weather is looking splendid with a high amplitude ridge in full control.

High temperatures in the valley will be in the 60s.  It will be a perfect weekeng for driving home from skiing in flip flops, drink a beer in the sun, and having a BBQ.  'Nuff said.

The Imperative to Ski Now

It's late March, there's deep powder to be had on the north half of the compass, and it is imperative you ski now.  The weekend is looking warm and ridgy, and perhaps some north aspect powder will survive, but probably not a lot.  Get out there ASAP.

Stuff We Know About This Storm and Stuff We Don't

It's always great to be greeted by post-frontal snowfall in the morning.

I've been taking a look at the various model forecasts and I have to say that this is one of the most complicated storms I can remember.  There are a variety of frontal positions, low-center tracks, and precipitation distributions and intensities being forecast by the various models and their ensembles that make forecasting details very difficult.

For simplicity, I'm going to use only the 0600 UTC NAM to illustrate some of the issues at play.  The red dot and yellow plus sign are located at the approximate location of Alta.  One can see the frontal band that is over us this morning in the early part of the loop,  Then a narrow band sets up very close to Alta, followed by more scattered precipitation.

Those precipitation features are very narrow and small in scale and predicting their intensity and position with any accuracy is very difficult.  One can imagine how a slight change in location is the difference between heavy snowfall and no snowfall at any given time, and that's assuming the model forecast has a reasonable handle on what Mother Nature will do.

If we look at the time series of 3-hour accumulated precipitation, we see quite a bit of "pulsiness" with some periods of moderate precipitation and periods of less precipitation.

One can look at the GFS, HRRR, or NCAR ensemble and find all sorts of variations in the timing and intensity of precipitation features over the next 48 hours or so.  Below is the NCAR ensemble.  The box-and-whisker plots in the bottom frame show the wider range of possible precipitation rates being forecast by the ensemble members each hour at Alta.  Total accumulations range from a bit under an inch to a bit over two inches.

So, let's start with what we know.  Winter has returned and we are going to see periods of snow over about the next 48 hour period.

What we don't know is the details of where and when the snow will come (other than some snow continuing this morning).  For Alta-Collins, I think the most likely range of totals by 6 AM Wednesday is 0.7 to 1.4 inches of water and 8-16 inches of snow.  We could do better or worse depending on how things develop.  I expect to see quite a bit of spatial variability in snowfall from this event (Powder Mountain is already reporting 5 inches).  Thus, be patient and go where and when it snows.  I expect there will be some good skiing to be had and a modest event would actually be better than a big one, at least for my skiing interests.    

Pessimism and Optimism

It was abundantly clear today that I was overly pessimistic on Thursday when I assumed that the sun was going to be baking most aspects (see Dribs and Drabs Aren't Enough in Late March).  We found plenty of good snow today on northerly aspects.

Of course, spring is in full force and many areas are baked out, necessitating very un-Wasatch like approaches and departures for late March.  Although I think we have some good low-elevation snow years in the future, this folks is the snow climate of the Wasatch in the future.

Now let's talk about optimism.  The models continue to key in on an active pattern for Monday and Tuesday.

I'm not speculating on details yet, but I'm keeping my schedule clear for Wednesday.

Troughy Uncertainty

Changes are afoot for early next week as a deep upper-level trough and colder air are expected to settle over the interior western U.S.  The 0600 UTC GFS, for example, parks the trough right over northern Utah on Tuesday with a very encouraging pattern for mountain snowfall.

That same GFS run goes big for precipitation, producing about 2 inches of total water equivalent at Alta from roughly noon on Monday through noon on Wednesday.  Wouldn't that be nice.

I'm intrigued, but am keeping my excitement in check.  Much will ultimately depend on the track, position, and intensity of the trough and this is the type of pattern that is typically characterized by considerable uncertainty in precipitation amounts.  Most members of our downscaled NAEFS are generating precipitation during the period, but amounts range from about .75 inches to as much as 3 inches of water.  That's a big range and my impression is that the uncertainty might be even greater than indicated.

Plus, the EC has the trough further west than the GFS, further illustrating the uncertainty in the details of the large-scale flow.

Let's see how this all comes together in the coming days.

Dribs and Drabs Aren't Enough in Late March

Readers of this blog are well aware that all snow isn't created equal.  Some storms produce better skiing than others simply because of how the snow is stacked (e.g., right-side up vs. upside down).  Once on the ground, many terrible things can happen to snow due to wind, sun, or (gasp) rain and other factors.

The pattern setting up for later today through early Friday morning will probably produce a few dribs and drabs of snowfall in the central Wasatch.  The NAM generates about .3 inches of water and a bit more than 6 inches of snow for Alta Collins by Saturday morning.

The GFS is more excited, but has proven to be somewhat untrustworthy for precipitation amounts, so I'm blowing it off in this discussion.  Hence, I'm sticking with a forecast of dribs and drabs.  A trace to two inches tonight and maybe 2-4" tomorrow night.

If this was January, dribs and drabs aren't preferable to a larger storm, but they are a nice refresher.

In late March, however, the bang for the buck is more limited because the sun is simply too powerful. Today's filtered sunshine and high temperatures will bake the snow on most aspects, and the dribs and drabs won't be enough to bury the resulting stiff or refrozen uneven snow surface, except perhaps on slopes that are high elevation and due north facing.

For good powder skiing this time of year, significant dumps are needed.  The one from Tuesday and Tuesday night has already been trashed by wind and sun.  Such a pity.

Jaw Dropping Warmth

This is old news for some of you out there, but I thought I would mention it in case you missed it.  Global temperatures in February were mind-bogglingly high, coming in more than 2ºF above the 20th century average thanks to global warming combined with a strong El Nino.  

Source: NCEI

If we look at 12-month periods ending in February, the past year was about 0.3ºF warmer than the previous year and a bit more than 0.5ºF warmer than any year prior to that.  Mother Nature sees your global warming hiatus and raising you 0.5ºF.

As we've discussed previously, we're going to see some relaxation from these high temperatures once this El Nino wanes.  That will not reflect an end to global warming.  The long-term trend is up.  Count on it. 

Ch-ch-ch-ch-changes

There's nothing like a spring trough passage in northern Utah.  As David Bowie said, ch-ch-ch-ch-changes.

The loop below presents the 700-mb (near crest level) temperatures and winds from 2200 UTC (4 PM MDT) yesterday to 1500 UTC (9 AM MDT) this morning.  We've transitioned from warm southwesterly flow with 700 mb-temperatures near +3ºC, leading to a maximum temperature yesterday of 68ºF, to northwesterly flow with 700-mb temperatures near -6ºC.

Those current 700-mb temperatures, when combined with the high precipitation rates we are currently experiencing, are sufficient to bring snow levels down to the valley floor.  We are currently seeing heavy snow comprised of large aggregates (i.e., large snowflakes formed by the sticking of snowflakes together) at the University of Utah.

I took that photo about 10 minutes ago and since then the snow is piling up fast.  In addition, although the ground is warm, snowfall rates are high enough that snow is sticking on some untreated asphalt and concrete surfaces.

Radar shows not a heck of a lot upstream to our northwest (i.e,. over the northern Great Salt Lake), but the band currently over us is moving very slowly.

That makes this a very tricky forecast.  A slow moving band of high intensity can lay down a lot of snow in a hurry, especially on colder surfaces (accumulations today will depend strongly on where you measure).  For the University of Utah, I think accumulations of 2-4 inches on colder surfaces are likely today, with snowfall greatest over the next two hours.  Ultimately, accumulations will depend on how long the band holds on with lesser amounts if it fizzles and greater amounts if it hangs on.

Is Snow Storage Coming to Utah?

Snow storage for the 2014 Sochi Winter Olympics

Every spring I bemoan the loss of winter snowpack.  It seems like such a waste.  If only we could start the next ski season with some of that snow.

I first heard of snow storage a couple of years ago when Sochi was stockpiling snow through the summer to ensure adequate coverage during the 2014 Winter Olympics.  Then, Outside Magazine ran an article on it today (see The Future of Winter Sports Is Storing Snow), sparking the synapses yet again.

The idea is not as crazy as it sounds.  Let's suppose that you wanted to do this at Alta.  Typically closing day is in late April or early May.  That's just a bit after the time of peak snowpack, but there is typically a lot of snow left, especially on the upper mountain.  Groomers would then push the snow into large piles along Mambo and Corkscrew, which would be covered with insulated, reflective blankets.  This snow would then be used to cover the trails the following ski season.  Some snow would be lost during the summer, but if the piles are deep and well covered, a good chunk of the snow would probably survive (a good exercise for the snow science geeks out there would be to try and determine how much snow would survive for piles of various sizes).  

Economically, I suspect this would be cheaper than making snow the next season.  If not, it would free up water for snowmaking in other areas.  Environmentally, I'm not sure what the impacts would be.  They wouldn't be zero, but much might depend on the melt rate from the piles and where the piles were placed.  

I'm willing to bet that we will see this eventually in Utah. 

Of course, we have something better coming late tonight and tomorrow - natural snow from the skies. I'm traveling today and don't have time to put together a proper forecast, so no predictions from me on this one.  Good luck. 

Still Waiting for El Nino in the Southwest

Chris Farley always delivered, but even if all other "tropical storms" bow before El Nino, as he suggests above, the 2015/16 "Super" El Nino continues to be a bust for the southwest United States.

Let's go back to a few months ago when we knew a strong El Nino was likely for this winter.  At that time, one could find all sorts of graphics like the one below showing the "typical" wintertime pattern associated with El Nino, including the wet southwest.

Source: NWS

Forecasts from the Climate Prediction Center showed the dice loaded for above average precipitation in the southwest.

Source CPC

The fact that this El Nino was going to be strong led some people to believe that the odds of a wet southwest were high, especially since the last two strong events, 82/83 and 98/99 were fairly wet in the southwest.

Let's fast forward to today.  The percent of average precipitation below covers the past 90 days (roughly since just before Christmas).  Northern California has done quite well, which is why reservoirs there have recovered, but most of the southwest has been below average during this period.

If we look at snowpack the numbers in the far southwest are really dismal, but even southern Utah, southern Colorado, and northern New Mexico are slipping below average.

Source: NRCS

And, if we look at forecasts for the next 10 days, they continue to skunk the southwest.  Below for example is the 10-day accumulated precipitation forecast from the GFS.  Plenty of action in the north, but little in the south.  Rather than El Nino, this looks like a good La Nina pattern.

Source: NCEP

It's time to face facts.  First, the skill and utility of seasonal outlooks, whether they show weighting of the odds or pick an outcome (e.g., below average, average, or above average), remains limited for many applications.  I suspect we will see improvement in coming years and decades, but where we are in seasonal prediction today is about where we were in weather forecasting in the 1960s and 70s.  The models are crude and we're relying too much on past analogs and human intuition.  Ultimately, these outlooks are going to improve, but even then, we must guard against converting those probabilities in to deterministic outcomes (e.g., increased odds of above average precipitation equals above average precipitation).  Second, we need to be more cautious about pushing analogs based on past events too far.  A few months ago, many people were using the past two super El Ninos as analogs for this coming winter.  A sample size of two is simply too small.

Finally, if you are skiing in northern Utah, you should just stop looking at seasonal forecasts altogether.  SEASONAL OUTLOOKS ARE UTTERLY AND TOTALLY VALUELESS FOR ANTICIPATING WHAT KIND OF SKI SEASON WE ARE GOING TO HAVE IN NORTHERN UTAH.  Next October, don't waste your time on this stuff.  Wax your skis and be happy.

Global Cooling Is Coming

Many of you have probably read that February produced the most abnormally warm temperatures in the instrumented record, and by a fairly large margin.  The cause of this extreme warmth is the combination of global warming and El Nino.  The trace below shows the global average temperature anomaly for the month of February since 1890, as analyzed by the Japan Meteorological Agency (JMA).  Note the El Nino fueled spike in 1998, toward the end of the last "uber" El Nino, and then in 2016, consistent with the current "uber" El Nino.

Source: JMA

Once the current "uber" El Nino wanes, global cooling is coming, at least relative to where we sit right now.  That cooling, however, will reflect short-term climate variability and will be temporary.

In my post Locally Dense Snow and Globally High Temperatures back in November, I made made the prediction that such a drop in temperatures, as occurred following the last Uber El Nino in 1997/98, you will hear people claiming a global warming hiatus once again.  Ethan Siegel made a similar prediction today in Forbes.  When that happens, don't be fooled.  In terms of long-term trends, global warming is the signal, and the year-to-year ups and downs in global temperatures the noise.

A Major Forecast Bust

The good news is that as of 4 AM MDT (I'm up early), we've picked up 5 inches of snow at Alta-Collins overnight, bringing the storm total to 10 inches and the total snow depth to 100 inches.  The bad news is that the storm total of 10 inches is well below expectations.

My forecast issued on Sunday was probably my worst of the season (see Skinny Skiing Followed by the Storm Forecast).  I called for .7 to 1.4 inches of water and 7 to 14 inches of snow at Alta-Collins through 6 PM MDT Monday.  Only 0.54" of water and 5" of snow were observed.

One has to hand it to the NCAR ensemble.  While the NAM and GFS were going for big water, the NCAR ensemble was drier and the 0.54" that was observed fell within the range predicted.

This is the future folks.  Multi-member ensembles run at cloud-permitting grid spacings (i.e., 3 km or less).  The sooner we get there, the better.

Model Resolution and the Post-Frontal Crapshoot

Alta-Collins picked up about 3 inches of 10% water content snow and we now are waiting to see what the post-frontal crapshoot will do today.

When it comes to post-frontal precipitation, current operational models like the NAM and GFS often have problems.  There is a good reason for this.  The clouds that populate the post-frontal environment tend to be shallow and small in horizontal scale, as illustrated by the photos below which I took this morning.

With grid spacings of around 12-15 km, neither the NAM nor the GFS can directly simulate these clouds.  A meteorologist would say they are unresolved.  In addition, those models are not able to resolve the fine-scale topography we have in northern Utah, creating just a nice gradual slope from the Great Salt Lake basin to the Uinta Mountains and southeast Idaho.  Because of this poor resolution, in post-frontal situations like the one we are in today, the NAM (and GFS) often just generate steady precipitation over the mountains as shown below.

Higher resolution models like the HRRR, with a grid spacing of 3 km, are sometimes called convection permitting models.  At 3-km grid spacing, one doesn't fully resolve the convective clouds, but you are able to directly capture some of their effects.  Below is the HRRR simulated radar reflectivity forecast for 1900 UTC (1 PM MDT).  Notice how convection is occurring in many areas. 

So, we have an issue that the NAM and GFS cannot be fully trusted in situations like this.  Sometimes they do OK, but other times their forecasts are misleading.  The HRRR, however, produces only one forecast and the chaotic nature of post-frontal convection is such that one really needs an ensemble of high-resolution forecasts to properly assess the range of possible outcomes in the forecast period.  

Thus, we'll have periods of snow today, not steady all day stuff like indicated by the NAM and GFS.  The convection could be strong at times.  Powder Mountain is in an operation hold this morning due to high winds and lightening.  If thunder roars, head indoors.  

Skinny Skiing followed by the Storm Forecast

Hoping to get in one last hour of cross-country skiing this winter, I drove up to Soldier Hollow this morning and took advantage of the man-made ribbon of white that still covers about 10 km of trails.

Conditions were surprisingly good.  The 10 K of trails that were open were well covered and meticulously groomed.  With few people out, it made for fun skiing.  Trail passes were $10, so the price was also right.

By now, you are well aware of the forecast of a storm beginning tonight.  Although we may see a few mountain snow showers today, things don't really pick up until sometime between 9 PM and midnight tonight as an upper-level short-wave trough moves through.  Snow levels through midnight will reach 7000-8000 feet, before falling late tonight.  In the wake of the trough, strong northwesterly flow should yield periods of snow showers through Monday.

For Alta-Collins, most of last night's NCAR ensemble members are going for 0.6 to 1.2 inches of water through 0000 UTC 15 March (6 PM MDT tomorrow).

The NAM is a bit more excited, going for 1.36".  The NAM time-height shows the overnight trough passage and then persistent westerly, potentially unstable flow through tomorrow (and into the overnight period.

There are a couple of concerns that I have about this event.  First, the flow as currently forecast never comes all the way around to northwesterly through tomorrow afternoon, which could limit accumulations in upper Little Cottonwood.  Second, there is always a lot of uncertainty concerning how productive the post-frontal instability showers will be in the Wasatch.

For these reasons, I'm issuing a forecast that is a bit more pessimistic than most you will see.  For 6 PM tonight through 6 PM tomorrow (Monday) I'm going for .7 to 1.4" of water and 7-14 inches of snow at Alta-Collins.  That relatively low 10 to 1 snow-to-liquid ratio reflects concerns about high density snow due to the warmth of the storm overnight and the strong wind tomorrow.  Ultimately, the snow amounts will depend a great deal on what falls tomorrow, when the snow density should be lower.  The NWS is going for 10-20", which I can see verifying if things go bigger tomorrow.

Bottom line: Snow is coming, but keep your expectations tempered.  It's always better to be surprised with too much than too little.

The Bergeron Process in Action

Nearly all of the snowflakes that fall on the Wasatch Mountains grow in mixed-phase clouds.  Mixed-phase clouds are comprised of both ice crystals and supercooled cloud droplets.  Supercooled cloud droplets are below freezing, but are still in the liquid phase.

The beauty of mixed-phase clouds is that the relative humidity for ice is typically higher than it is for liquid water.  As a result, ice crystals grow faster than water droplets in mixed phase clouds.  This preferential growth of ice crystals is known as the Bergeron Process after Tor Bergeron, the Swedish meteorologist who proposed it as a mechanism for precipitation formation in clouds (it is sometimes called the Wegener-Bergeron-Findeisen process since Alfred Wegener and W. Findeisen also made contributions to the theory).

If you were at Alta today (Saturday), you may have seen the Bergeron process in action.  The shallow cumulus clouds that were over Alta were mixed-phase clouds.  Their relatively "hard" bottom and edges suggest they were comprised primarily of supercooled liquid water.  Beneath these clouds, however, one could see falling snow.  The cloud droplets in the cloud could not grow large enough to fall out, but the ice crystals could.

That's today's lesson in cloud microphysics.

GO UTES!

I Miss Winter

Note: Blog post has been updated to correctly identify the warmest 10 Feb – 10 Mar period on record.

For the second third year in a row, the Heat Miser (pictured above) has taken over northern Utah, bringing in a strong early thaw to the detriment of snow lovers.  From February 10 to March 10 (yesterday), the average temperature in Salt Lake City was 44ºF, making that period the 6th warmest on record.  Last year 2014 was the warmest such period on record.

The warmth has ended cross-country skiing at Mountain Dell and along many (but not all) of the trails in the Park City area.  We had a decent corn cycle and a fortunate burst of powder a few days ago, but by and large ski conditions are similar to what we might expect in perhaps 3-4 weeks.

A trough is advertised for early next week and if it brings the goods, I suggest you take full advantage.  

Ski and Avalanche Treasures at the Marriott Library

The role of libraries is changing in the digital age, but their importance isn't.  Librarians stand as protectors of history and conduits to knowledge.  Here at the University of Utah, the Marriott Library is a key cog in the machine of education and innovation.  I use the resources provided by the Marriott Library at least daily if not hourly for teaching and research.

Readers of this blog may be interested in the Marriott Library's Utah Ski Archives, which were created to preserve the history of skiing in Utah and the Intermountain West.  Amongst the treasures in the archives is this historical video of skiing Alta in 1946, which has been posted up on YouTube.

Click on the link and find many more.

Materials and reports produced by avalanche research at Alta are also available at the Marriott Library, including a subset that you can access digitally here.  For example, there's Barry Nielsen's 1966 report on forecasting snowfall rates at Alta

Source: Marriott Library

as well as classics like the 1953 Avalanche Handbook, written by Alta Snow Ranger Monty Atwater and Forest Service Supervisor Felix Koziol.

Source: Marriott Library

Enjoy a surf through these digital archives and remember the vital role that the Marriott Library plays in preserving history and providing access to these archives.

Today's Talk

No time for a post today as I'll be giving the Global Change and Sustainability Center seminar this afternoon at 4 PM in 210 ASB.  Those of you on campus should stop in for the pre-seminar refreshments and, hopefully, hang around for part of the talk.  Details below.

Forecast Tools: The Downscaled NAEFS

Yesterday and last night provided a much needed return of weather to northern Utah.  Wind, thunder, falling snow levels, and a decent pasting of high-elevation snow made for an exciting day.  If I'm adding up the totals right, Alta-Collins came in with nearly 1.4" of water and 11 inches of snow.  I suspect the high-elevation skiing should be decent today.

Although we may see a few hit-or-miss snow showers through Wednesday, the work week ahead looks mainly dry, which means we're back to looking far into the future for the next storm.  As such, now is a good time to discuss the experimental downscaled products from the North American Ensemble Forecast System (NAEFS) that we provide on weather.utah.edu.

Ensemble forecast systems are now de rigueur for medium-range forecasting and are quickly reaching that status for short-range forecasting.  They are run by several major forecast centers.  The NAEFS is comprised of ensemble forecasts produced by the U.S. National Centers for Environmental Prediction (NCEP) Global Ensemble Forecast System (GEFS) and the Canadian Meteorological Center Ensemble (CMCE).

The resolutions of the GEFS and CMCE fail to adequately resolve the influence of topography over the western U.S., especially over the interior.  For example, the GEFS precipitation climatology (left panel below) is far smoother than the climatological precipitation analysis by the PRISM Climate Group (right) over the western U.S.

Courtesy Trevor Alcott

As a result, one needs to develop an approach to account for local effects of the western U.S.  The simplest and most straightforward approach is to use climatological (a.k.a. statistical) downscaling.  The approach is fairly straightforward.  You take a high-resolution precipitation climatology and smooth it to a resolution comparable to the GEFS or CMCE, yielding a low-resolution climatology.  Taking the ratio of the high to low resolution climatologies yields a downscaling ratio (top row below), which can then be multiplied against the precipitation generated by each GEFS or CMCE member to obtain a downscaled precipitation forecast.  

Courtesy Trevor Alcott

Doing this for every member of the GEFS and CMCE yields a downscaled ensemble. 

The NAEFS-experimental forecasts on weather.utah.edu use this approach.  Although both the GEFS and CMCE are comprised of 20 members, we only use 10 from each ensemble system.  Special thanks to one of my former students, Trevor Alcott, who wrote much of the code to do this.  

The plume diagram below shows the NAEFS-experimental forecast produced at 0000 UTC (5 PM MST) yesterday afternoon.  Note the precipitation with the front last night, then a period of mainly dry weather, and then the potential for precipitation toward the weekend.  

One often sees a tendency for clustering amongst ensemble members, especially in the early stages of the forecast period.  For example, the GEFS members in the graph above were wetter overnight than the CMCE members,  yielding strong clustering.  This is a very common problem in ensemble prediction.  Ensembles based on a single modeling system like the GEFS usually are what we call underdispersive.  They underestimate the full range of possible outcomes.  Adding one or more additional modeling systems to the mix typically diversifies the ensemble and yields better probabilities than obtained from a single ensemble system. 

What are the advantages of such an approach?  Well, the resulting forecasts are better than one can obtain without downscaling, especially over the western interior where the terrain is very fine scale. Model validation work by another one of my students, Wyndam Lewis, suggests that a 5-day GEFS forecast with climatological downscaling is as good as a 3-day GEFS forecast without downscaling. 

What are the disadvantages?  Well, you use climatology and you get climatology, no matter what the situation.  Sometimes we see situations where the mountain effects are more limited than climatology (e.g., during a frontal passage), whereas in others it can be much stronger (e.g., post-frontal snowshowers).  The approach also does not correct for model biases.  For example, if the GEFS is too wet in your region, you're going to get a downscaled forecast that on average is too wet.  

So, overall the downscaling helps, but we can do better.  That's what keeps us in business.  

The Early Bird Won't Get the Worm

After many days (seemingly weeks) of little storm action, we are going to get some tomorrow and tomorrow evening.  Details are difficult to project, but it's more action than we've seen in some time, so that has me excited, even if I'm uncertain how it will all play out.

The NAM forecast for 2100 UTC (2 PM MST) tomorrow shows some of the key features of the event.  The first is precipitation associated with moist, southwesterly flow ahead of the approaching cold front and occuring in the forecast below over the Wasatch and Uinta Ranges.  The second is precipitation associated with the cold front, which at 2 PM tomorrow, is forecast to be pushing into western Utah.

The NAM precipitation forecast graph below shows the two part system with a burst of precipitation in th southwesterly flow from about 5 AM tomorrow to 1 PM in the afternoon in the moist southwesterly flow, then a lull in the precipitation, and then a burst with the front late tomorrow and tomorrow evening.  Total water equivalent produced by the forecast is about 1.25 inches, which would be a godsend.

The moist southwesterly flow is always a bit more of a crapshoot, especially at Alta, than the frontal precipitation.  I'm not betting on precipitation starting at 5 am, or a lull at 1 pm, but I do think we will see some periods of precipitation tomorrow ahead of the front.  Further, the pattern is one in which the Park City side should do nearly as well as Alta-Collins (possibly better) for water equivalent given the strong southwesterly flow tomorrow and the strong large-scale forcing accompanying the front.

In addition to the southwesterly flow crapshoot, another wildcard tomorrow is the snow level.  The NAM wet-bulb zero graph below provides some guidance on snow level.  The wet-bulb temperature is a thermodynamic variable used by meteorologists and the snow level is typically about 1000 feet below the elevation of the wet bulb zero level (i.e., the level at which the wet bulb temperature is near 0ºC.  For much of the day tomorrow, the wet-bulb zero is near or above 8000 feet, before it finally falls near and around the time of frontal passage later in the day.  This means what precipitation we get tomorrow ahead of the front will probably fall as rain below 7000 feet (possibly lower in the afternoon as temperatures begin to fall) and as wet snow above 8000 feet.  I wouldn't be surprised to see a bit of mixed precipitation between those two levels depending on the time of day (especially in the morning) and precipitation intensity (heavier precipitation typically lowers the snow level).

Overall, this is a forecast in which the early bird probably won't get the worm.  Most aspects are now crusted up pretty well (I *tried* to ski tour this morning and it was heinous) and it's going to take some snow to bury it.  My plan is to go out tonight to drown my sorrows for today's lousy skiing, sleep in, and decide late tomorrow morning if I should bother heading up for some storm skiing tomorrow afternoon.  If the southwesterly flow is productive, I'll ski in the afternoon (possibly late afternoon).  Otherwise I'll probably work.  Waiting until Monday will probably yield the best conditions, especially in the backcountry, although duty will preclude me from taking advantage.

For upper Little Cottonwood, the National Weather Service forecast issued this afternoon called for  .7 to 1.3" of water and 6-12" of snow for a storm total.  I think I might go for a bit more than that.  Say .9 to 1.5" of water and 8-14" of snow.  I suspect that the upper mountain of Park City will do just as well, but the lower mountain will see far less.

No-Change Forecast and Arctic Sea-Ice Record

No-Change Forecast

Not much change in the forecast from yesterday to today.  The models are still calling for a front to move through northern Utah on Sunday.  Most of the NAEFS ensemble members, for example,  are calling for a significant event, although I think the totals might be a bit overdone (e.g., Alta below).

After that, most (but not all) of the NAEFS ensemble members are keeping us mainly dry with only a few snow showers here or there through the middle of next week.  The European has a similar solution.  After that I'm not speculating.

Arctic Sea-Ice Record

As most of you know, globally average surface temperatures have really jumped through the roof over the past several months due to the combination of long-term global warming with a strong El Nino event.  Much of the focus has been on the tropical Pacific and projections of impacts on the western U.S., but this has also been a remarkably warm winter in the Arctic.  This warmth, along with other factors, has contributed to the lowest average February sea-ice extent in the satellite record.

Source: NSIDC

Further discussion this record minimum is available from the National Snow and Ice Data Center (NSIDC).

A Look at the Changes Ahead

When suffering through a dry spell, it's easy to read too much into the extended range forecasts, but we will take a peak today, keeping in mind that we are still 3-4 days from seeing a possible storm moving in late Sunday.

In the short term through Saturday, the warm-weather corn harvest will continue, with just a few clouds at times (like this morning).  For you weekend warriors, Saturday will be characterized by warm southerly flow with 700-mb (10,000 ft) free atmosphere temperatures reaching nearly 2ºC by afternoon.

Given that we've had virtually no snow now for some time, treat Saturday like an April day and work the aspects.

Right now, Sunday looks to be a transition day with a possible frontal passage late in the day.  The 0600 UTC GFS put the front just downstream of the central Wasatch by 0000 UTC 7 March (5 PM MST Sunday).

It's a bit too soon to have confidence in the exact timing of the front, but it does look like we will finally see some precipitation during the day Sunday or Sunday night during and around the time of frontal passage.  Snow levels are likely to be high ahead of the front (8000-9000 feet), but drop quickly with the frontal passage.

The NAEFS precipitation plume shows a range of water equivalents for Alta between 1.0 and 2.7 " for the frontal passage period from Sunday morning through Monday morning (1200 UTC 7 March - 1200 UTC 8 March).  Based on what I'm seeing right now, I'd probably shift that downward a bit and go for a most likely range of about 0.5 to 1.75 inches of water.

As noted earlier, it's easy to read too much into an extended forecast when you are suffering in a dry spell and I confess that I'm fairly uncomfortable discussing such details at these long lead times. With that CYA statement in mind, plan on checking the forecasts in the coming days.

As noted in the previous post, the pattern-shift hype machine has been in overdrive the past few days.  One of the biggest risks for the central Wasatch is the potential for us to transition to a big split, and I'm seeing evidence of that in the latest runs from the GFS and ECMWF.  For example, the GFS forecast valid 1200 UTC 8 March (5 AM MST Tuesday) puts the main trough well to our south, although we get a bit of frontal precipitation.

So, the good news is this is a pattern change to cooler weather.  We are going to get some precipitation Sunday and Sunday night, although it's a bit early to say how much, or to read into the details.  After that, much will depend on the storm track and the strength of the split.  Again, avoid the tendency to read too much into these extended range forecasts.  They are very alluring, especially when one is in a dry spell, but ultimately, one needs to be respectful of the chaotic nature of the atmosphere.

Announcement:

For those of you on the University of Utah campus, I will be giving the Global Change and Sustainability Center seminar this coming Tuesday.  It will be based in part on my book, Secrets of the Greatest Snow on Earth, but delve a bit more deeply into orographic and lake-effect precipitation.