An Out-on-the-Limb Maximum Temperature

Yesterday's high of 93°F at the Salt Lake International Airport was a record for the day but not really all that exceptional when you consider that the record high on the previous day (September 11) is 99°F and a couple days later (September 15) is 97°F.  Somehow September 13 has never gone out-on-the-limb for maximum temperature compared to the surrounding days.  Such are the realities of climate statistics. 

That said, yesterday's maximum temperature was out-on-the-limb in terms of what was forecast.  Several days ago, we posted that the forecast for tomorrow (Sunday) was uncertain.  Some models, like the GFS, were bringing in a cold front on Saturday, resulting in much cooler weather for Sunday.  Others, like the ECMWF AIFS brought the front in during the day on Sunday.  I'm not sure if any called for the cold front to come in Sunday evening.    

These days one can process the forecast from well over 100 ensemble forecasts to produce a range of possible forecast outcomes.  This is what the National Weather Service does for their National Blend of Models (NBM) forecast.  On Tuesday we posted about the enormous spread for the maximum temperature forecast for yesterday (see An Uncertain Sunday) with the middle 50% of high temperature forecasts lying between 68° and 84°F (see the tallest red box in the left-hand graphic below).  

That middle 50% is sometimes referred to as the interquartile range.  However, half the forecasts lie outside of that box.  The whiskers surrounding the box extend through the middle 80% of forecasts (or from what is called the 10th to 90th percentile).  In this case, the top whisker extended to 92°F.  That means that 90% of the forecasts called for a maximum temperature at or below 92°F.  

I went back to that NBM forecast distribution and it turns out it ended at 92°F.  In other words, not a single forecast member called for a maximum at or above 93°F.  The observed maximum temperature on Sunday was outside of the entire predicted ensemble range.  

Ideally that would not happen except in the most exceptional circumstances, but our ensembles are not perfect.  Forecasts do sometimes verify outside of the ensemble range and that's a sign that we still have work to do.  

Part of what made the forecast for Sunday challenging was the occurrence of something called anticyclonic wave breaking (AWB).  If you have been to the beach, you have experienced wave breaking.  As water waves approach the shore, they decelerate and steepen.  Eventually the waves steepen to a point where they overturn and break down into turbulence, or what scientists call wave breaking.

Breaking wave. Source: NOAA.

The waves you experience at the beach are gravity waves that occur at the interface between the high-density water and the lower-density atmosphere.  Gravity waves also occur in the atmosphere where gravity wave breaking is one important cause of aircraft turbulence. 

In AWB the waves that are breaking are not gravity waves but very large-scale atmospheric waves known as Rossby waves.  Rossby waves are named after Carl-Gustaf Rossby, who did pioneering work on the jet stream and the dynamics of upper-level waves in the middle 20th century.  He was even featured on the cover of Time Magazine in December 1956.

Source: https://content.time.com/time/covers/0,16641,19561217,00.html 

Rossby waves are the large-scale upper-level troughs and ridges that we observe at upper-levels (i.e., jet-stream level) in the atmosphere.  These waves are so big (1000 km or more across) that the rotation of the Earth strongly affects their behavior.  

Like waves at the beach, Rossby waves can also "steepen."  In contrast to water waves that steepen in the vertical, Rossby waves "steepen" in the north-south direction, which is what happened last week over the north Pacific. At 1200 UTC 10 September, there was a ridge over western Alaska and a trough over the northeast Pacific.  This ridge-trough system amplified over the next 24 hours, with the 500-mb height contours "steepening" and turning clockwise (anticyclonically) on the downstream side of the ridge.  Eventually, the 500-mb height contour "broke" in the sense that higher 500-mb heights were north of lower 500-mb heights.  This is consistent with the red contour crossing the purple meridian three times at the end of the plot sequence below.  Think of the ridge pushing over that meridian as an overturning wave with the surfer in the the tube or trough at lower elevations.  

AWB is associated with the amplification of a strong upper-level ridge, which is often related to the release of heat by condensation in precipitation systems just upstream of the ridge.  Although AWB events are often predicted reasonably well in terms of their occurrence, the details (i.e., the "turbulence") can be hard to get right.  Sunday was a hard forecast because the strength and position of the trough and cold front was challenging to anticipate.  The 93°F we hit on Sunday reflects the fact that the AWB produced a stronger trough than anticipated, delaying its progression across the Pacific Northwest.  

Surfers can be surprised by breaking wave behavior.  So can meteorologists.  

Memorial Day Weekend Outlook

Watching from a distance, the official start of summer/Memorial Day Outlook is generally looking good for Utah.  For the most part it looks nice, with above average temperatures, although the models are hinting at a chance of some afternoon or evening showers and maybe even a scattered thunderstorm mainly over the high terrain over the weekend.  One example is below, which is the ECMWF forecast for 2100 UTC (3 PM MDT) Sunday 24 May with some showers over the Uintas, Tavaputs Plateau, and high terrain in southern Utah.  

Models vary on the where and the when and so it's just worth keeping an eye on the forecasts and the sky when recreating.  

Enjoy the long weekend.  

A Front

After about 7 weeks in Austria, I feel very detached from the weather in Utah.  I hear rumors and see evidence of an abysmally bad snowpack and have been sent photos from Little Cottonwood that look like late May rather than mid April.  A look at the SNOTEL data shows every site in the Wasatch, Oquirrh, and Stansbury Ranges at a record low water equivalent except the "plucky" Farmington Lower SNOTEL which is in 3rd simply because of what appears to be a spurious reading of 0.1".  Pretty sad.  

However, I also noticed that there is a cold front coming through northern Utah tomorrow (Thursday) and it should bring a pretty good drop in temperatures.  The 0000 UTC GFS (I'm 8 hours ahead of  you and writing this in the middle of the night Utah time) shows the cold front draped over northern Utah at 1800 UTC 16 April (1200 MDT Thursday).  Salt Lake City is likely to be post-frontal at that time and seeing colder air moving in.  

The front will bring a substantial drop in temperatures.  At 700-mb (roughly crest level), temperatures fall from around +2°C this (Wednesday) afternoon to -12°C by Thursday evening.  The temperature at the Salt Lake City International Airport has not dropped to or below freezing (32°F) since March 15.  We should be able to do drop to at least that by Friday Morning when the forecast low from the National Weather Service is 30°F with snow showers.  

As advertised by the more recent model runs, the precipitation prior to, during, and immediately following the frontal passage is not all that prolific.  For Alta-Collins through 12 AM (Midnight) MDT Friday the 06 UTC 15 April HRRR is producing 0.51" of water and 7.9" of snow.  Through the same time, the 00 UTC 15 April GFS is producing a more pathetic 0.3" and 5" of snow.  Five members of the 0000 UTC RRFS Ensemble have 0.5" of water and 7" or less of snow through 0000 MDT Friday (06 Z 17 Apr), but there is one member with a more enthusiastic total near 0.8" of water and 12" of snow.  

Perhaps such a dust-on-crust event will be enough to get your heart rate up.  If not, one has to hope for one or two things.  One is that the period around and including the frontal passage on Thursday is more productive and comes in higher than most of the models and maybe near or above the wettest RRFS member.  The other is that the cold, post-frontal northwesterly flow is productive Thursday night and maybe Friday morning, with either orographic or lake-effect snow.  It's a bit soon to have confidence in the post-frontal crapshoot, so monitor forecasts while I stare at the Alps. 

Warm, Wet, and Juicy

It felt a good deal like spring yesterday with the sun now higher in the sky compared to its winter solstice minimum and mild temperatures from the valleys to the mountains.  Yesterday's high of 55 at the Salt Lake City airport was not exceptional, but combined with the brilliant sunshine, it was a beautiful day to be outside.  

We have one storm on tap for the coming week and it's looking like a warm, wet, and juicy one.  It is associated with an inland penetrating atmospheric river.  The ECMWF HRES forecast valid 0000 UTC 25 February (5 PM MST Tuesday) shows the atmospheric river (bottom right panel) over the Bay Area curving clockwise through northern Nevada and Utah.  Integrated vapor transport (IVT) values in our area are > 250 kg/m/s, which puts is in the low end of atmospheric river categories, although that's a fairly high value for the western interior.  

It's a relatively slowly evolving pattern. As can be seen above, the ECMWF puts our mountains in precip Tuesday afternoon and that continues overnight.  The forecast valid 1200 UTC 25 February (5 AM MST Wednesday) shows atmospheric river conditions persisting in mild westerly flow. 

Atmospheric rivers in northern Utah can be fickle.  Some are heavy precipitation producers.  Others do little.  This one does have something going for it and that's large-scale confluent flow, meaning that there are two air streams merging together over northern Utah, one with a more northern origin and the other with a more southern origin.  This often creates and environment of large-scale lift that helps to generate a larger-scale cloud and precipitation shield to get precipitation going.  

Unfortunately, this storms has something that we don't want and that's warmth.  HRES forecast 700-mb temperatures over the central Wasatch are around -1°C Tuesday afternoon through about 5 AM Wednesday, afterwhich they decrease slowly.  That's a recipe for high snow levels and low-to-mid elevation rain.  Some models are warmer than that.  The 12Z HRRR for example has a temperature of about +1°C at 700 mb at 0500 UTC 25 Feb (10 PM MST Tuesday).  That would mean rain above the base of Alta, possibly to mid mountain. 

Indeed the HRRR-derived Little Cottonwood forecast is really pumping up the warmth.  It has the wet-bulb zero above 9600 feet, roughly mid-mountain at Alta, from 2 PM MST Tuesday to 2 AM Wednesday.  The snow level is usually several hundred feet below that, but I would not be surprised if it briefly got to 9600 feet or higher Tuesday night if this forecast verifies.  

The 12Z HRRR is one of the warmer models, but even the 06Z RRFS ensemble has some members pushing the wet-bulb zero to 10000 feet or higher Tuesday night (bottom left panel).  Water equivalents in that ensemble vary from 1 to 2.25" for Alta-Collins, but one member has only 3" of snow, which tells you that at least one member producing rain or mixed rain-and snow at Alta-Collins.

So expect a wet, warm, and juicy storm to develope Tuesday afternoon and persist into Wednesday, although temperatures and snow levels will slowly decrease on Wednesday.  Snow levels could be quite high Tuesday night, probably getting above the base of Alta and maybe getting to 9600 feet or even high if the warmer model forecasts verify.  Hopefully the cooler forecasts will verify, but even those are fairly mild.  

Storm Cycle Update

 Pretty good round of snow or should I say graupel overnight and this morning.  Accumulations in the mountains vary substantially and I'm not sure if Alta wiped their interval stake this morning or not as it looks like 4" on it, but the total snow depth is up 5 to 6 inches and the resort is reporting 8" as of 10 AM so I'll go with that.  

Gauge measured water total is about 1.2".  I'm hearing rumors that's high, but anyway you cut it, it appears to be a high-density graupelfest up there.  Sounds like a good day of skiing in my book.

Forecasts are still on for a colder storm tomorrow that will bring the Greatest Snow on Earth to both the mountains and the valleys.  Timing varies, with the HRRR bringing in a commuter special and the GFS a bit slower.  Strongly recommend that you check forecasts later today if you are commuting in the morning.  Quoting this morning's 5:23 AM issued Area Forecast Discussion from the NWS: 

"Locally higher snow rates will come with the front. Precise timing is lower confidence, with models ranging from roughly 5-11 a.m. for it to track through Salt Lake City. This will be capable of affecting valley travel Wednesday for some commutes, depending on timing."

I have more good news.  I am leaving for Austria on the 24th.  The models are already going for the Steenburgh effect to kick in, meaning snow when I'm out of town.  The Utah Snow Ensemble shows a break in a couple of days, but then things picking up again on the 24th.  

Coincidence?  I think not.  

Next Week Looks Active

This post is dedicated to friend and long-time Wasatch Weather Weenies reader Kevin Moore who passed away yesterday after a multiyear battle with pancreatic cancer.   Kevin was a devoted husband and father, avid biker and skier, and lover of life.  He set an example of how to live life to the fullest when facing the greatest of challenges and uncertainties.  

Another Warm Storm

Clouds and precipitation are back in the forecast, but unfortunately it's yet another warm event.  

Below is the ECMWF HRES forecast for 1800 UTC 11 Feb (1100 MST Wednesday).  A weak, decaying atmospheric river extends across the northern Baja Peninsula and up the lower Colorado River Basin.  Elevated (but atmospheric river level) integrated vapor transport extends into northern Utah with south-southwesterly flow at 700-mb.  The 700-mb temperatures at that time are forecast to be about -3.5°C, which would likely a snow level near or just above the base of Park City (7000 ft).  

A look at the HRRR-derived guidance for upper Little Cottonwood shows some dribs and drabs today, but periods of precipitation overnight, tomorrow, and tomorrow night totaling a bit over an inch of water and just under 10 inches of high-density snow through 5 AM Thursday at Alta-Collins.  The middle-left panel shows the wet-bulb zero height and our estimated melting layer in green.  The wet-bulb zero makes it up to the base of Alta, which means snow levels likely pushing 7500 feet or so before lowering Wednesday night (and lowering further on Thursday).

A look at the RRFS ensemble shows  four of the members very close to an inch of water and in the 10-15" range for snow through 11 AM Thursday (18Z 12 Feb).  One member is more excited.  Root for that one. 

So the good news is snow for the upper elevations.  It should help a good bit with ski conditions as we've had 3.5" of snow at Alta since January 8.  However, low elevations will suffer once again.  Mid elevations near 7000 feet will probably see a net gain through Thursday, but possibly with some rain on Wednesday.

It is what it is.

Will the Storm Track Finally Make an Appearance?

Our run of dry weather looks to continue through this coming weekend.  Since January 9, Alta Ski Area has recorded only 2.5 inches of snow.  If we make through Sunday, February 8 without any more snow, that will be 2.5 inches in a month (31 days).  

The models are showing some signs of life next week.  The Euro, for example, has a cold front moving into northern Utah Monday night and Tuesday. 

That's still a ways away so I'm not betting the farm on it, but a look at the Utah Snow Ensemble shows some signs of life in a number of the members.  

If you are a glass half-full type, half the members produce 1.02" of water and 14" of snow by 11 PM Wednesday 11 February  If you are a glass half-empty type, half themembers produce less than that.  More than 90% of the members produce more than 2.5" of snow, so there's a better than 90% chance that we will better what we got over the past month!  So exciting!

But for now and through the weekend, it's God bless the groomers.

Challenges Forecasting the Snow Level

The snow level is a critical variable for mountain weather forecasting, but it is both difficult to define and predict.  

In fact, there isn't even an agreed upon definition of snow level amongst meteorologists or the public.  One might say it is the level above which precipitation is snow and below which precipitation is rain, but its just not that simple.  Precipitation doesn't just change from snow to rain at a given altitude.  Snow takes time to melt as it falls.  Different types of snow crystals melt faster than others.  Sometimes it is snowing but not accumulating on the ground due to melt.  What do you do when there is a mixture of snow and rain or drizzle?  Where do you put the snow level? 

To begin, perhaps it is easier to begin with a description of the transition zone, the layer in which snow is warming, melting, and turning to rain as it falls.  

The schematic below provides a simple conceptual model of the transition zone.  The freezing level marks the top of the transition zone and is the highest level at which the temperature is 0°C.  Temperatures above this level are below 0°C.  

The transition zone (h/t Ron Stewart)

The freezing level and the snow level are not the same because snow does not melt instantly when it falls below the freezing level.  Instead, it begins a transition as it falls, gradually changing from dry snow to wet snow to slush (part ice part water) and finally rain.  The distance over which this occurs depends on many factors including the profiles of temperature and relative humidity below the freezing level, type and size of the snow crystals, and fall speed of the snow crystals. 

For example, if the relative humidity is low, a given snow crystal can penetrate farther below the freezing level since the crystal will warm more slowly because it is also cooled by sublimation.  A dense graupel particle can generally penetrate farther below the freezing level because they have a high fall speed and take longer to warm and melt fully compared to a small dendrite.  

In addition, because the melting of snow cools the atmosphere to the melting point (0C), there is often an isothermal 0C layer in the transition zone, as depicted above.  This layer can be quite deep, especially if precipitation rates are high.  Because of this effect, the snow level may lower some when the precipitation rate increases and rise some when the precipitation rate decreases. 

In the above schematic, I have denoted the level at which the precipitation is half snow and half slush as the snow level, but that may not conform to what you think is the snow level.  Perhaps it should be lower and at the level where precipitation is all rain or all rain or slush.  Alternatively, perhaps it should be higher and at the level where precipitation is all snow?  

Or, perhaps instead of focusing on the type of precipitation falling, we focus instead on the level at which snow is accumulating on the ground.  That's sometimes called the snow line.  But if we do that, what exactly is the ground?  Grassy surfaces?  Asphalt surfaces?  

There are many ambiguities.

As discussed in Van Cleave et al. 2011, about 15 years ago the National Weather Service Western Region actually established a definition of snow level for their applications:

"The snow level is the elevation above which snow will fall, and below which rain will fall. A mix of rain and snow may be observed at elevations within a few hundred feet of the snow level. Snow will not accumulate on the ground below the snow level and may not even accumulate at elevations above the snow level."

Even that definition is somewhat ambiguous, but they also defined techniques for calculating and forecasting the snow level and identified the wet bulb 0.5°C level as the best proxy for snow level in the western region. That said, there can be variations in precipitation type and accumulations on the ground around this level.  

One situation that can be particularly challenging involves stable layers in which wet-bulb temperatures are very near 0°C through a deep layer.  An example is provided by last night's GFS forecast for 1000 UTC 01 January 2026 (3 AM MST New Year's Day).  At that time, a precipitating cloud layer has spread over northern Utah in southwesterly flow aloft, but stable conditions remain at low levels as the cold air currently in place over the Salt Lake Valley is slow to mix out.  The freezing level in this case is located just above 750 mb, or near about 2500 m MSL (~8200 ft MSL).  Because the atmosphere is saturated at that level, the freezing level in this case also represents the wet-bulb zero level. However, below that level, the layer is very close to 0°C down to almost 800 mb.  This might enable snow to penetrate farther below the freezing level than if temperatures were increasing with decreasing altitude below the freezing level.  

Layers in which temperatures are relatively constant with height are called isothermal, which means having a constant temperature. Having a deep isothermal layer near 0°C is one way for the snow level to penetrate to low elevations.  One needs to keep a close eye on these layers as they can make a big difference for snow level forecasts.    

Whither the Start of Ski Season

Sigh...what a start to December.

First let's have a look at the northwest storm.  Some bit water totals so far.  For the 3-day period ending at 6 AM PST/1200 UTC today (11 December), more than 10" of liquid precipitation equivalent at many sites in the Washington Cascades including 14.2" at Paradise SNOTEL (agreeing well with the expectations from the previous post) and 18.6" at the Lynn Lake SNOTEL between Paradise and Snoqualmie Pass.

Source: https://www.weather.gov/wrh/hazards

Crystal Mountain is reporting this morning that WA-410 is closed between Enumclaw and Greenwater, essentially cutting them off given that WA-410 is also closed over Cayuse and Chinook Passes to the south.  Most of their web cams are down but sadly, even at 6800' at the top of the Green Valley Express where a cam is working, it looks like a net loss from this storm.

Source: https://www.crystalmountainresort.com/the-mountain/mountain-report-and-webcams/webcams

Could be worse.  Below is the Summit West cam from Snoqualmie Pass.

Source: https://www.summitatsnoqualmie.com/webcams

Meanwhile in northern Utah, December continues to be the new October with yesterday's high of 61 at the Salt Lake city International Airport the average high on October 25.  

I keep thinking how fortunate we are that this low-amplitude ridge developed in a way that we stayed relatively mild at low levels.  If there had been a strong cold pool in place over the valley earlier this week, we would probably be much cooler.  If we can't have snow, then we may has well have a mild conditions in the valley.  We were also fortunate to have that storm last Friday Night and a few days of good snowmaking conditions before the warmth returned.  

A look at the ensembles shows a few members flattening out the ridge just enough to bring some precipitation into northern Utah the middle to end of next week.  An example is the ECMWF HRES. Forecast below for 1800 UTC Thursday 18 December.  

Indeed, for Alta-Collins the Utah Snow Ensemble has a few enthusiastic members, although most of the forecasts are still in the low-end snowfall range and most are warm through the 10-day forecast period.  

Median total water equivalent and snowfall are 0.91" and 8.0" through 5 PM Saturday 20 December, so we will set that as the over/under.  

Now is the time to burn skis.  I'd suggest human sacrifice, but some of you might just take that quip seriously. 

The Europeans and Americans Simply Cannot Get Along

The overnight Utah Snow Ensemble forecast for Alta-Collins may be the crazies I've ever seen.  Most of the European Ensemble (ENS) members are producing little to no precipitation over the next 10 days including during during the period from 10-11 December (more on those dates in a minute).  The wettest ENS member puts out about 0.8" of water and 11" of snow.  

Some members of the American Ensemble (GEFS) have very different ideas.  There are 31 GEFS members and 10 of them put out more than 1.5" of water just through 00Z 12 December.  Several more put out between 1 and 1.5" of water.  There are some that track with the ENS (hard to see but they are there), but many GEFS members are much wetter.

What gives?

I don't plot or attempt to look at forecast plots like our four panel synoptic diagnostic for all 82 members of the Utah Snow Ensemble.  Nobody has time for that.  So I'm going to start here with the ECMWF HRES forecast that serves as the ENS control and is valid 0000 UTC 11 December.  This forecast captures many of the salient details of the pattern that is going to prevail over western North America the next few days.  A broad, low-amplitude upper-level ridge is centered near the Pcific coast (upper left) with an atmospheric river rolling over this ridge and across the northern US Rockies (lower right).  Heavy orographic precipitation occurs where the atmospheric river crosses major western mountain barriers.

So in the HRES the central Wasatch is just to the south of the action.  There are a few dribs and drabs that make it in, but accumulations are scant.  Most ENS members have a similar forecast.

Some of the GEFS members, however, shift the storm track far enough south to get us in on the action.

Diagnosing the why this occurs in this case is beyond my abilities.  I'll just say that I lean heavily toward the view that the central Wasatch may see some dribs and drabs over the next week but it's unlikely we'll see the game changing major accumulations that we need. 

Mother Nature Delivers the Concrete

Twenty-four hour totals at the Alta-Collins automated observing site added up to 1.79" of water and 15" of snow, yielding a mean water content of 12%.  Snowbasin-Boardwalk came in with 1.26"/8"/16%.  As evidence of the wet, windy, and rimey nature of the storm, I couldn't verify our 70 mph forecast for Mt. Baldy because the sensor rimed and is no longer spinning.  The Utah Avalanche Center report this morning says that "at 11,000 feet the wind is absolutely ripping, with sustained speeds at 50–60 mph with gusts pushing into the upper 70s."  That's good enough for me.  

The Alta-Collins snowstake photo this morning is one of the most comical I've ever seen.  Dense snow apparently carved by wind?  Perhaps with the flow wrapping around the Alta sign?  Who knows.  Come up with your own explanation.  

Snowfall at lower elevations is certainly lower and probably a soggy mess between 6000 and 7000 feet (or higher), but that's just a guess.  

The Cottonwoods might get a bit before things taper down later this morning.  The extended forecast shows that you should continue to root for America.  The European Ensemble (ENS) is giving Alta practically nothing after today.  The American (GEFS) is more generous around the middle of the work week.   

USA. USA. USA.

A Pretty Splitty Pattern

The mild first-half of November is now in the books.  The Salt Lake City International Airport came in with the warmest on record with an average temperature of 53.0°F.

Source: https://xmacis.rcc-acis.org/

Last night's storm dropped about 4" on Alta Collins.  Not much to get excited about.  I haven't looked carefully, but the big winner appearred to be Ben Lomond Peak which got about 2.3" of water and had a total snow depth of 10" at 5 AM this morning.  

The pattern continues to look splitty for the next few days.  Just an an example, below is the GFS 500-mb height (contours) and wind speed (color fill) forecast valid 2100 UTC 20 Nov (2 PM MST Thursday).  The Pacific Jet is forecast to be well consolidated near 150 W but split upstream of the North American coast into two branches, one moving through western Canada and the other over northern Mexico.  

The various ensemble forecasts either keep us dry or give us some light amounts as systems in the southern branch of the jet brush by.  The Utah Snow Ensemble forecasts for Alta-Collins from last night show a bit with last nights storm through about 25 November, most members give us scant precip. 

A few push the southern track storms far enough north to give us some decent water and snowfall totals, but there are only a few of these.  At least 75% of the members give us less than 10" of snow and less than an inch of water.  

Paraphrasing Thomas Paine, these are the times that try ones soul.  The summer lover and sunshine worshiper will, in this crisis, shrink from the mountain climates; but one that stands by it now, deserves the love and thanks of powder when it returns.  

Dust on Dirt

A few days ago we discussed how the GFS was an outlier of the many forecasts for the storm that will come through northern Utah tonight (see Some Thoughts on Using Long-Range Forecasts).  Specifically, it was producing a good deal more precipitation than most members of the Utah Snow Ensemble, which is derived from the US Global Ensemble Forecast System (GEFS) and ECMWF Ensemble (ENS).  

Sadly, the latest forecasts continue to produce light precipitation and just a frosting of snow for Alta-Collins.  As an example, the six members of the Rapid Refresh Forecast System (RRFS) ensemble produce 0.25" of water equivalent or less and 3" of snow or less for Alta-Collins.  

The latest HRRR and GFS are also below those thresholds.  The 0600 UTC GFS is down to 0.08" of water and about an inch of snow.  Dust on dirt if it verifies, although the snow might be somewhat wet and not cold smoke.  

Sometimes an outlier forecast like the one from the GFS verifies.  Low probability doesn't mean no probability.  It can be helpful to recognize there is the possibility of something happening and many of the products we have on weather.utah.edu are designed to illustrate the potential of outlier events such as high precipitation intensity periods.  But knowing the odds puts that potential into context.  

One question I was asked about that post about using long-range forecasts is shouldn't we weight the ensemble members?  For example, lean toward the ENS control and the ENS ensemble over the GEFS since the ECMWF model is "better." Being an academic, whenever I'm asked a question, I think of the goblin Griphook in Harry Potter and the Deathly Hallows: Part 2.  

The skill of any forecast model decreases with increasing lead time (model skill might increase in the first few hours of a model forecast due to spin up, but we'll ignore that here).  Eventually, even the best model in the world has no skill relative to climatology, meaning you're just as good forecasting with the long-term mean or probabilities as using the forecast model.  

As a result, the performance of a better modeling system over another modeling system is maximized at shorter lead times and decreases with time.  I have illustrated this conceptually below.  

How quickly model system skill degrades is dependent on the variable being forecast.  One can skillfully predict temperature or geopotential height farther into the future than precipitation.  In addition, forecast skill in northern Utah is lower than in many other regions of the US, so the rate of decline in forecast skill is higher here. 

The last comparison of GFS and ECMWF model forecasts for precipitation at mountain sites in the western United States that I am aware was done by my group (see Caron and Steenburgh 2020).  For the modeling systems available at that time, the 12-36 hour precipitation forecast produced for SNOTEL sites by the ECMWF was clearly superior to the GFS.  However, by 60-84 hour lead time, the gap was much smaller and not statistically significant (based on a 95% confidence interval).  So for the GFS and ECMWF available at that time, at about day 3, the overall skill of their precipitation forecasts at mountain sites in the western US became somewhat similar.  

So, how much you should weight one modeling system over another when looking at an ensemble likely decreases with time.  It might make sense to weight the ENS over the GEFS at short ranges, but it's unclear is such weighting makes sense at longer time ranges, at least for precipitation in the mountains of the western US.  Rather than basing the Utah Snow Ensemble entirely on the ENS, we included the GEFS because studies have show that doing so results in better probabilities.

The discussion above is informed by experience, but ultimately the only way to know is to do the math.  Once again, I leave this for you to do.    

A Dry Weekend

It's a rarity here in the Great Salt Lake rainforest, but it looks like we have a dry spell settling in that will last through the weekend.  

The trough that brought us rain over the past few days is exiting the area today.  On Saturday, a low-amplitude upper-level ridge looks to be moving in, with no monsoon moisture or Pacific storms to be found in our area.  

There is a brush-by system that flirts with northern Utah late Sunday but right now the precipitation from that looks not push into Utah through 0000 UTC 20 Oct (1800 MDT Sunday).

That system could bring a bit of precipitation to northern Utah, mainly in the mountains, on Sunday night. For Alta Collins, for example, a bit over half of the Utah Snow Ensemble members produce no precipitation Sunday night, but a fee give us some light amounts Sunday Night, with a couple doing a bit better than that. 

That's the main threat for precipitation over the next few days if you are hoping to add to aour already record-breaking October totals.  The next major system that could flirt with us comes in around October 26th. 

Mid May Powder?

Spring powder has been limited this season with April and early May coming in relatively dry, warm, and at times windy and dusty.  

In April, Alta recorded 53.5" of snow, but 37.5" of that was on the first three days of April.  Automated obs suggest about 10" on the 6th of May and we gotten perhaps 4" yesterday.  

So if you are still looking for a late-season powder fix, Sunday has some potential.  The models are hinting at a digging trough and front to be moving through Sunday morning.  The GFS, for example, puts the front right over nothern Utah at 1500 UTC 18 May (9 AM MDT Sunday).  

Some of the models then call for unstable northwesterly flow thereafter, although the moisture, wind direction, and instability do vary some.  

For Alta-Collins, the GFS puts out 0.94" of water and 9" of snow from Saturday evening through Sunday evening, most of which falls with the frontal passage Sunday morning.  

The median forecast from the Utah Snow Ensemble for that period is a bit more optimistic with 0.99" of water and 11.5" of snow.  A look at the distribution shows that most members (about 75%) are more than .75" water and 9" of snow.  

If this timing holds, there could be a bit of a battle between getting the underlying snow surface buried as the snow stacks up and the potential for sun to trash the snow should it appear or simply do damage through the clouds once the front goes through if the snow doesn't keep coming.  The 4" or so of snow we got yesterday may help a bit with the former.  The latter is a crap shoot currently and wintertime aspects that are often friendly to powder even in warm weather are now getting a good deal of solar radiation because the mid-day sun is so high in the sky.   

You can't catch a fish without a line in the water.  It's probably worth baiting the hook and being ready to cast your line for this one depending on how the forecast plays out.   

Where Are the April Showers?

It's been a dry start to April with only 0.16" of precipitation at the Salt Lake City International Airport, 0.15" of which fell on April 1st.  We won't see precipitation today, so that will be the tally through the half-way point of April, which will tie it for the 9th driest first half of April on record.  If you are wondering, there are five Aprils in which we had no measurable precipitation in the first half of the month, most recently in 1992.  

The April Fools storm was a bigger producer in the Avenues Foothills than the airport, but my full-sun gardens are now starting to dry out.  We could use some April showers soon.  

The likelihood of valley precipitation will finally be on the increase late Wednesday into Thursday as an upper-level trough digs into Utah from the Pacific Northwest.  The GFS shows the trough over central Idaho at 1200 UTC 17 April (6 AM Thursday) with some showers across northern Utah with the accompanying cold front. 

Right now this doesn't look like a supersoaker, except if maybe you are lucky enough to be one blessed with a more intense shower or thunderstorm, but it will cool things down and give us some showers.  We'll call it beneficial rains for the valley (maybe even mixed with some flakes for the benches on Thursday and Friday) and a return of mountain snow.  

Given the unsettled nature of this spring pattern, the spread for water equivalent and snowfall in the Utah Snow Ensemble for Alta is enormous and about as big as I've seen all winter.  

Odds are probably best on Thursday.  After that, it will probably come in fits and starts if it keeps coming.  The dendrites will have a real battle with daytime heating.  Good skiing will probably require high end accumulations and getting on it right away.

Mainly Dry Cold Front

Pickin's are slim this week for weather entertainment, although I'm hoping you are getting out for some time in the sun.

On tap for tomorrow is the passage of what looks to be a mainly dry cold front, which is due to arrive tomorrow morning.  The overnight GFS forecast shows the front at 700-mb (10,000 ft or crest level) pretty much right over northern Utah at 1500 UTC 12 April (0900 MDT Saturday).   

By 0000 UTC 13 April (1800 MDT Saturday), the flow has finally come around to west-northwesterly and 700-mb temperatures have dropped to about +4C over the Salt Lake Valley from +9C today.  So tomorrow will be cooler, but still mild, with valley highs in the low 70s and 9500 foot highs in the mid to high 40s. 

Sunday the 700-mb temps will be down even lower and our machine-learned forecast for Little Cottonwood is calling for temps at Alta-Collins  and on Mt. Baldy to be near or just above 20°F.  

Thus, although there may be some clouds around, it looks like we may get a pretty good hard freeze Saturday night. Given the lack of snow, I'm expecting coral reef conditions on Sunday morning.  Patience and playing solar aspects right will be the key to finding corn. 

For now, there's nothing major on the horizon.  We may go deeper into April without a big storm since the April Fools Powder Surprise.  On upper-elevation north aspects in the central Wasatch, peak snowpack water equivalent is often in late April.  I'm starting to wonder if that will be the case this year.  

Dry Post April Fools

April got off to a good start with the April Fool's storm, but since then, it's been dry.  That dry streak looks to continue through the work week with all members of the Utah Snow Ensemble flatlined for Alta-Collins until 1200 UTC 12 April (6 AM MDT Saturday).  

A few members are excited about snow on Saturday night and Sunday, but most are producing snowfall amounts in the low single digits.  The reason for this is a compact upper-level trough expected to move across the northern Rockies over the weekend.  At 1800 UTC 13 April (Noon MDT Sunday), it's centered over Montana in the latest GFS forecast. 

The trough is fairly dry on its south side, so most of the model runs are giving us a good hard freeze but not a lot of snow.  Basically a good recipe for bone-rattling coral reef conditions, perhaps with a skiff of snow on top on Sunday.  Sounds bad.

If the trough can dig more than currently advertised by most of the members, perhaps we can do better.  About 10% of the ensemble members produce 10" or more by 6 PM Sunday.  Those odds are long, so yardwork is looking like a good option unless things change.

Dry Spring -> More Typical Spring

The work week has proven spectacular with mostly sunny skies and mild temperatures.  "Anything season" is here, meaning that you can do pretty much any type of recreation you desire, sometimes on the same day.  The lower-elevation trails have dried out now in many areas and are quite passable for hikers and bikers. Meanwhile, we still have a deep snowpack at upper elevations.  

Today's sunrise from the Avenues foothills.

We will, however, be transitioning from the dry spring pattern we've experienced the last few days to a pattern that will bring the occasional mild spring storm system to northern Utah.  This transition won't necessarily be abrupt as a couple of weak systems move through Friday and Saturday.  After that, there's a series of troughs that move through.  It's not easy for me to summarize my expectations for the timing and strength of these storms as there's a good deal of variability in the Utah snow ensemble.  Just look at the spread at the end of the 10-day period.  

The best way to summarize this is perhaps simply to say changeable, with the occasional mountain snowstorm interspersed with breaks.  It's a time for adaptation to what Mother Nature brings rather than having strong expectations for what's to come.  Fortunately, spring in northern Utah means a plethora of options.