A Look Back at April and March

If the snowfall history page on Alta's web site is correct, they've had 102" of snow this April.  During this past week, the Alta-Collins peaked at 161", which I suspect is the highest since the epic 2010/11 season.

The SNOTEL data from Snowbird shows a roughly 8 inch increase in water equivalent for the month.  We now sit just below 60 inches of snow water equivalent, about 15 inches above average.

Source: NWS

Having missed the entirety of our most recent storm cycle, I'm forced to reminisce about 2010/11, which still reins supreme, with a larger increase in snow water equivalent for the month and a snow water equivalent of almost 75 (see cyan line above).   I got a reminder from Facebook this morning of just how great that season and April was.  

Meanwhile, in the valley, the Salt Lake City International Airport is now up to 6.73" of precipitation since March 1st.  That rates as the 5th wettest March–April period on record.

For those of you who were able to enjoy the skiing this past week, you can thank the Steenburgh effect.  You will, however, be disappointed to learn that I'm back in town next week.  Not surprisingly, the computer models call for a high-amplitude ridge to develop over our area by mid week.

Sigh...

Approaching the Big One-Five-Oh

I'm still out of town and you continue to reap the benefits.  Through 4 AM MDT, another 9 inches overnight at Alta–Collins.

Total snow depth is now at 149" and approaching the Big One-Five-Oh.  I believe that puts it at the deepest snowpack of the season, although I haven't done an exhaustive check.  

Speak Well of Me

I hope you people are happy for my sacrifice.  I sit this morning in a coffee shop in central Pennsylvania with a light rain falling and leaves on the trees.  Winter is thousands of miles away.

Meanwhile, in the Wasatch Range, the Steenburgh Effect is working wonders.  Alta–Collins has received 15 inches of snow since yesterday morning, with 9 inches overnight.

Source: MesoWest

With temperatures falling overnight, and now sitting at 23ºF, I bet there are some good turns to be had at upper elevations.

Make some for me.

Steenburgh Effect This Week

I am currently on travel, which is good news for you, because it means the Steenburgh effect is en force.  I haven't had time for a proper look at the models, but did notice that they are looking active for this week, as evinced by the NAEFS ensemble plumes for Alta.

As we mentioned a week or two ago, it ain't over until it's over.  Keep the skis waxed and get on it before it's baked.

Last night I received the Charles Hosler Alumni Scholar Award from Penn State's College of Earth and Mineral Sciences for contributions to the development of science through teaching, research and administrative leadership. Dr. Hosler served his country in WWII and Penn State as Prof. of Meteorology, Dean of Earth and Mineral Sciences, VP of Research, Dean of the Graduate School, and Provost. He also signed my acceptance letter to Penn State in 1985. It was an honor to sit with him and receive the award from Dean Bill Easterling.

I'm pretty shocked about the whole thing as the College of Earth and Mineral Sciences has a ton of excellent alumni and I'm really a ski bum with a Ph.D.   Despite the fact that Pennsylvania is Austrian for "ski purgatory" I was very fortunate to attend Penn State and learn from so many great faculty.  I'm also fortunate to have a wife and kids, family and friends, students and post-docs, and colleagues and collaborators who have helped me go places I could never go by myself.

April 22, 2017

Some Truth about Lake Effect

Another day, another front, another gasp of lake effect.  Radar imagery this morning shows some lake-effect precipitation over the Salt Lake Valley and the central Wasatch.

Alta-Collins is now up to a storm total of 8 inches.  Some of that is from yesterday's and yesterday evening's non-lake-effect convection, but some of it is lake effect.  Might be some decent spring skiing up there today.

The fact we've had a couple of brief lake-effect episodes this month shouldn't be a surprise.  We are in the heart of the spring lake-effect season.  A curious aspect of lake effect produced by the Great Salt Lake is that it's not most common during the heart of the ski season, but instead during the shoulder seasons.  The primary peak in lake-effect frequency (and amount) is in November, and the secondary peak is in March and April (depending on if you are looking at amount or frequency).

Average number and amount of precipitation produced at Snowbird by Great Salt Lake events by month.  Source: Secrets of the Greatest Snow on Earth.

Fall events tend to last longer and produce more precipitation in November than March and April.  One reason for this is that the sun is much lower and the days shorter in November, which allows for stronger, more persistent land-breeze circulations at night and into the morning.  Such circulations play an important role in lake-effect initiation and maintenance during many events.  

The two seasons of lake effect produced by the Great Salt Lake is unusual compared to larger bodies of water.  For example, downstream of Lake Ontario, there is a single peak in December and January (note change in x-axis from above).  

Source: Veals and Steenburgh (2015)

This difference is closely related to lake depth.  The Great Salt Lake is a very shallow lake and warms rapidly in the spring, enabling a 2nd peak of lake effect.  In contrast, Lake Ontario is a deeper lake and exhibits a strong lag in lake temperature relative to the seasons.  As a result, lake-effect peters out as one gets into the late winter and spring.

The importance of lake-effect for skiing in Utah is massively overstated by many ski writers.  It represents only about 5% of the cool-season precipitation (on average) in the Cottonwoods.  It can, however, occasionally provide enough at the end of a storm to vastly improve the skiing.  I suspect that might be the case today.  Without the lake effect, we were clearly looking at a dust-on-crust event this morning with perhaps 4-6".  Putting 2-4" more on top of that surely will help the skiing a bit.

This Radar Loop Is for the Birds

Meteorological radars are designed to detect precipitation, but any objects of sufficient size and number concentration will give you a return.

This morning's radar loop, for examples, shows returns that are almost certainty from birds leaving their roosts along the wetlands surrounding the Great Salt Lake.  Note in particular the plume-like development and dispersion of echoes from near the southeast shore of Farmington Bay and the Bear River Migratory Bird Refuge.

To the west, you can see some echoes created by precipitation.  Some April showers are on tap for today.  

About Last Night

April showers bring May flowers.  The combination of abundant moisture and good large-scale forcing yielded a solid frontal precipitation band that swept through northern Utah last night.  Below is the KMTX radar image from about 0100 UTC (1900 MDT) yesterday evening.  Something for everyone.

Source: NCAR/RAL

About all we missed out on was severe thunderstorms.  There were some lightning strikes in the area, as indicated below, but I didn't see any strong wind or hail reports on the SPC web site this morning.  That's probably for the best.  It is only in the warped mind of a meteorologist that one is disappointed when severe weather doesn't materialize.

Source: lightningmaps.org

Rainfall reports reported to the National Weather Service show accumulations over .9 inches at several sites along the east bench.  The airport came in with 0.65 inches.  Those are good totals for a relatively brief storm.  

As of 7 am, Alta-Collins has observed precisely 1.00" of water and 7 inches of snow.  I suspect that the first tenth of an inch or so of water fell as rain as temperatures at that location (9662 ft) were in the 40s until 6 PM.  After that, cream on crust.  The snow depth is back up to the 125" US-unit psyche point.  Nice, but for those of you attending the March for Science this weekend, that's 317.5 cm.

A Spring Airmass Best Described as "Juicy"

I got some odd stares this morning carrying my umbrella on the bus, but I know better.  A juicy airmass is in place over the Great Basin and, with good dynamics later today, we're in for a wet period later today and tonight.

First, let's take a look at the moisture that we have in place upstream.  The 1200 UTC (0600 MDT) sounding from Elko shows a near saturated atmosphere through most of the troposphere, with the exception of a dry layer just above 500 mb.

Source: SPC

The total precipitable water in this sounding is 15 mm (0.6 inches).  The sounding climatology shows values of 0.6" are near, but not quite at the upper-limit of prior observations at Elko.  There are a few days in mid-to-late April with values reaching about 0.65-0.67".  Nevertheless, 0.6" is pretty high for April.

Source: SPC

If we look at the return interval of the precipitable water forecast produced by the North American Ensemble Forecast System for 1200 UTC (0600 MDT) this morning, we're looking at something that maximizes between 1 day every 5 years and 1 day every 10 years in a strip extending across central Nevada and northwest Utah.

Source: NWS

By 0000 UTC (1800 MDT) this afternoon, that strip is parked over the Salt Lake Valley.

Source: NWS

A juicy airmass isn't enough, however, to ensure precipitation.  One also needs lift.  In other words, something to initiate clouds and precipitation,  Better yet if the stability is weak.  We have both today along a developing trough.  The HRRR forecast shows this trough centerd over northwest Utah with extensive precipitation, some heavy, over much of the area.

  We will have to see how this all plays out, but rain and thunderstorms look likely to develop during the day today.  Below is a summary issued by the National Weather Service.

I'll add that the Storm Prediction Center has our region in a marginal risk category for severe thunderstorms this afternoon and evening.

Bottom line: Keep and eye on the sky and the radar.  Things might just get interesting...

The End of an Era

Today marked the last day of lift served skiing at Alta and the end of an era for the resort as General Manager Onno Wieringa retires after this season.

Onno was a strong supporter of weather research and forecasting efforts in Little Cottonwood Canyon and the Wasatch Range.  Alta was the first ski area to begin supplying weather observations to the "Utah Mesonet," predecessor to MesoWest (special thanks as well to Alta Ski Patrol, especially Howie Howlett and Titus Case).  Before the advent of MesoWest, weather observations were sparse to non-existent across most of the Wasatch Range.  Today, we get observations from nearly all of the ski resorts and they make a huge difference.  The Alta-Collins site, in particular, provides very high quality snowfall and snow water equivalent measurements that are used by everyone who forecasts in the Wasatch Mountains including the Utah Avalanche Center, National Weather Service, UDOT, utahskiweather.com, opensnow.com, and wasatchsnowforecast.com.  During storms, I probably look at those observations 50 times a day (even though they come in only once an hour!).

Onno also generously provided the wonderful snow observations collected by Howie, Titus, and the Alta Ski Patrol to develop algorithms for forecasting snow density during winter storms in the Wasatch Range, observations that we ultimately used not only for science, but also to set the record straight on why Utah snow is so great (er, Alta snow) in an article for the Bulletin of the American Meteorological Society and also my book, Secrets of the Greatest Snow on Earth.

Much thanks to Onno for all he has done for the weather forecasting community of Utah.

Onno's retirement isn't, however, the only one at Alta this year.  The Supreme Chair is being replaced (as is Cecret) with a high-speed quad that will run from near Alf's Restaurant.  If I remember right, the old triple was the Germania triple prior to the installation of the Collins high-speed quad, so it has a long history at Alta.

In my view, a perfect closing day is sunny, warm, and corn filled and that was the case today, and the Frank World Classic really packed them in.  You don't see lines like that for Wildcat every day.

I elected to lap Collins.  I missed out on the Frank debauchery (such a shame), but did enjoy some entertainment at times from Collins.

Looking forward to the Frank YouTube video...

What Is Greenhousing?

Greenhousing is a commonly used word in the snow and avalanche business.  It may be 12 letters long, but for powder skiers, it is essentially a four-letter word because it leads to snowmelt and melt-freeze crusts.

Although I didn't do an exhaustive search, it's actually quite difficult to find a good definition or explanation of greenhousing.  It is not defined, for example, in the National Avalanche Center Encyclopedia, and isn't in the indexes of the avalanche books that I have on my shelf.  One can, however, find discussions of the importance of the greenhouse effect, such as this passage from The Avalanche Handbook by David McClung and Peter Schaerer:

"Conditions leading to the formation of wet-snow avalanches by radiation in the spring are governed by the net radiation balance of the snow surface. The net balance includes both long-wave (terrestrial) and short-wave components (sunlight). Direct sunlight warms the snow cover, making wet-snow avalanches possible, but thin cloud cover may provide a greenhouse effect to prevent long-wave cooling" (my underline).

Let's take a look at what McClung and Schaerer are talking about.  The net radiation balance of the snow surface involves both shortwave and longwave radiation.  As illustrated below, the sun provides shortwave radiation during the day, some of which is reflected back to the sky.  The remainder is absorbed by the snow and serves as an energy input.  Atmospheric gases (especially water vapor) and clouds provide long-wave radiation, while the snowpack emits long-wave radiation.  The former is an energy input, the latter an energy output.  The net radiation balance is sum of the absorbed solar radiation, plus the longwave in, minus the longwave out.

There are other processes that affect energy flows to and from the snowpack.  For example, if the temperature of the snow surface and the air are different, there is an exchange of heat known as sensible heat flux.  Latent heat fluxes involve snowpack sublimation (energy lost by the snowpack) or vapor condensation onto the snow (energy gained by the snowpack).  There can also be exchanges of energy within the snowpack and ultimately with the ground.  These can be important, but for the purposes of this discussion, let's focus on radiation.

The so-called greenhouse effect noted by McClung and Schaerer involves the longwave in part of the net radiation balance.  Under clear skies and relatively dry conditions (low atmospheric water vapor), the incoming longwave radiation is small.  It increases, however, if clouds are present and, since the amount of longwave radiation emitted by clouds is dependent strongly on temperature, is greater for warm clouds than cold clouds.  Low clouds are typically warmer than high clouds and usually result in a larger input of longwave radiation to the snowpack.

Of course, clouds also reflect shortwave radiation back to space, which reduces the amount of shortwave radiation that reaches the snow.  However, in a cruel irony for skiers, a thin cloud is very effective at emitting longwave radiation, but only reflects a portion of the solar radiation.

Which brings us back to greenhousing.  The greenhouse effect is always present on this planet.  Always.  24/7.  There is always some longwave radiation input to the snowpack from the atmosphere and, if present, clouds.  In this sense, greenhousing is always occuring.  Greenhousing, as used colloquially however, typically describes situations in which the longwave input to the snowpack is larger than than observed under clear skies, contributes to a positive net radiation balance, and leads to or enhances warming or melting of the snow surface.

Here's an example.  Imagine a nighttime situation where skies are initially clear and the snow surface is well below 0ºC.  However, low clouds spread over the area.  In this instance, even in the absence of sun, the snow will warm due to the increase in longwave radiation input to the snowpack.  This happens frequently in the arctic and even on nights where cloud cover increases in Utah.

Now lets try another extreme.  A warm, juicy airmass with temperatures above 0ºC moves in overnight.  Fog forms, then the sun rises.  The fog, however, is relatively thin, enabling most of the solar radiation to reach the snow.  In this case, you have considerable net solar radiation, but also a great deal of longwave radiation input into the snow.  The net radiation balance is strongly positive and contributes to snowmelt (as do sensible and latent heat fluxes, the latter from condensation).

One "aspect" we haven't talked about is aspect.  The net solar radiation is strongly dependent on aspect, time of year, and time of day.  However, there are many situations in which clouds are present over all aspects.  In such a situation, the greenhouse effect is an equal opportunity offender, affecting all aspects.  On aspects that don't receive a lot of solar radiation, the presence of clouds can tilt the net radiation into positive territory and contribute to warming and melting of the snowpack.

Strong Fronts: Timing is Everything

Spring is the time of strong cold-frontal passages, as defined based on large, rapid temperature changes, especially in the Intermountain West.  This is quite clear if we look at the frequency of strong cold-frontal passages across the western United States by month, which shows a peak in the late spring (May).  There is a similar peak in the Intermountain West, but it is more pronounced and reaches a maximum in June.

Source: Shafer and Steenburgh (2008)

There are two major reasons for the spring peak.  One is that it is still a synoptically active period, with frequent trough passages.  The second is that surface heating is also quite strong, which leads to daytime frontal intensification.  As a result, the frequency of strong frontal passages is highest in the late afternoon (~1800 local standard time) and shifts to later in the day as one moves from winter to summer.

Source: Shafer and Steenburgh (2008)

Intermountain fronts strengthen during the day because in our part of the world the pre-frontal environment to be cloud free or feature thin, high clouds, whereas the post-frontal environment typically features deeper, often precipitating clouds.  As a result, during the day, there is a contrast in surface heating across the front, with the pre-frontal environment heating faster than the post-frontal environment.  The direct effect of this heating contrast is to increase the cross-front temperature difference.  An indirect effect is that it produces a thermally driven from from colder to warmer air, which helps sharpen the temperature contrast.  In many events, post-frontal cooling from precipitation further augments these effects.

An example of this daytime frontal sharpening is provided by forecasts of today's frontal passage.  The GFS 700-mb (10,000 ft) temperature forecast for 1200 UTC (0600 MDT) this morning shows a frontal zone over Nevada with temperatures on the warm edge of the frontal zone around 4ºC.

By this 0000 UTC (1800 MDT) this afternoon, however, prefrontal temperatures have warmed to 6ºC and the front entering northern Utah has sharpened significant, with the isotherms (lines of constant temperature) packed much closer together.  This is an example of the frontal sharpening process.

Precipitation behind the front will likely play an important role in the frontal sharpening of this event.  Shifting to the high-resolution rapid refresh (HRRR) forecast for 2100 UTC (1500 MDT) this afternoon shows the expected precip just behind the well-defined frontal shift that is entering northwest Utah at this time.  Lovers of wind and dust will be pleased to see strong southwesterly flow ahead of the front.  Yup, another dust layer is likely for the mountain snowpack.

The HRRR has the front entering the northern Salt Lake Valley at 0200 UTC (2000 MDT) this evening, perhaps a little late for a truly colossal temperature change, but it still should be a significant drop.  Note that the HRRR calls for the surface front to outrun the precipitation with time, something we often (but not always) see.  Those of you hoping for a dump like last weekend will be disappointed to know that the models pretty consistently weaken the post-frontal precipitation band as it moves into our area.  This time, Mother Nature says, NO SOUP FOR YOU!

Curiously, there are parts of the world where daytime surface heating can weaken cold fronts.  Portions of Australia, for example, observer stronger cold-frontal passages at night than during the day.  These are areas where the climate favors dry conditions both ahead and behind the front.  Because the post-frontal airmass is shallow, daytime heating is more confined vertically behind the front, leading to more rapid heating in the cold air than the warm and frontal weakening.

It's Not Over Until It's Over

This weekend's storm provided a wonderful exclamation point for the ski season.  Really, I was feeling depressed about how the season was going out with a whimper rather than a bang.  From March 5th to April 8th, there was only one day during which Alta recorded 10 inches of snow (March 27th) and we suffered through the hottest March on record.  Corn is fine, but I'd rather be skiing powder.  For April, Sunday was as good as it gets, and I liked it so much that I did a dawn patrol yesterday morning to get in some more turns before class.

My season now feels complete, although it's never over until it's over.  I've skied good (not great) powder on Memorial Day weekend in the Wasatch.  It happens.

And, while we're talking about it not being over until it's over, we turn our attention to California.  The models continue to show this prolonged period of southwesterly large-scale flow over the eastern Pacific and western US in which California continues to see storm after storm.  For the next seven days, our downscaled NAEFS ensemble product is producing mean water equivalents reaching over 3 inches.

Such numbers are not remarkable compared to wet periods this past winter, but they show winter hasn't yet quit in the Sierras.  In addition, it will likely do the job for breaking the wettest water year on record in that region.

Major record may fall this week. Less than 1" of precipitation needed to break Northern #SierraNevada wettest year on record: 1982-83 #cawx pic.twitter.com/4VPNgyUQWp

— NWS Sacramento (@NWSSacramento) April 10, 2017

I really need to take a trip over there to check out the snowpack.

The Greatest April Snow on Earth

I had the good fortune to be able to ski tour with my son today.  We found some of the best April snow that I've ever skied.  Bottomless, right-side up, hero snow.  My personal favorite, especially in April.  A few shots of the kid getting the goods.

Cold-Front Double Whammy Update

Morning has broken and the GOES-16 visible imagery is simply beautiful and shows clearly the two frontal bands that will influence our weather today and tonight, the first over northern Utah and the Salt Lake Valley right now (as if you needed a weatherman to tell you that) and the second moving across northern Nevada.  

Source: College of DuPage

Talk about well defined.  Really, you aren't going to see two more distinct frontal systems than that in the Intermountain West.

Radar imagery through 1406 UTC (0806 MDT) shows the frontal band progressing across northern Utah with a well defined back edge.  Perhaps I will be able to get in a bit of gardening today after all.

One thing that I noticed in the overnight model runs is how colossally bad the 0600 UTC 3-km NAM forecasts were for this morning.  The end of the loop above shows a very wide and broad frontal precipitation band over the Salt Lake Valley and surrounding area, but the 3-km NAM has nothing of the sort.

In fact, that forecast is so bad that I thought there might be an error in my retrieval and processing software.  Thus, I surfed around and found another site that serves up the 3-km NAM, Pivotal Weather.  They have 3-hour accumulated precipitation plots (above is 1-hour), but it's still enough to confirm a massive forecast bust.

Basically, the 3-km NAM was completely clueless.  Resolution is worthless and even harmful if you can't get the basics of the large-scale flow, and it clearly didn't in this case.  The 12-km NAM wasn't much better.

Getting back to reality, the Alta-Collins site began to record precipitation at just before 6 AM and has observed 0.18" of water and an inch of west snow through 8 am.  It's a nice start, and they should see snow with the frontal band for the next 2-4 hours.  After that, how about scattered snowshowers and thunderstorms.  Yup, that nice clear gap this morning between the fronts could allow for good destabilization this afternoon and with strong flow and and approaching second front, we could see some stronger convection.  In fact, the Storm Prediction Center has us in a marginal risk category for severe thunderstorms.

Then we have the front later today and tonight, and I'm hoping for a powder day tomorrow.  The NCAR ensemble is optimistic.  One member is a bit under an inch of total water, but most lie in the 1-2 inch range, which would probably be enough for decent turns in many areas, especially if we end up in the upper end of that range.

I'm now jazzed enough about the two fronts that I'll go for a total of 1–1.75" of water and 10–20" of snow for upper Little Cottonwood from 6 am this morning through tomorrow morning.  The NWS numbers are a bit higher than that.  I'm a little reluctant to go so high because the latest forecast flow directions never quite come around to the coveted northwest direction and I'm unsure about precipitation during the hit-and-miss convection between fronts.

One thing is for sure, I'll be enjoying the weather the next 24 hours or so.  

Wind, Dust, Snow and All That...

I love spring storms, so I'm feeling like a kid in a candy shop today.

Winds picked up yesterday and gusted strongly overnight in advance of a developing trough and surface front over Nevada.  After midnight, peak gusts at upper-elevation locations in the northern, central, and southern Wasaatch are 82, 77, and 70 mph, respectively.  In the valleys, the Great Salt Lake Marina hit 60 mph and a sensor near the juncture of UT-201 and I-80 in the Salt Lake Valley hit 63 mph.

Winds as I write this have actually slackened just a bit.  Obs from the juncture of UT-201 and I-80 show two periods of strong winds overnight, on prior to midnight, the other from about 1:30-4:30 AM.

Source: MesoWest

With the development of the front and surface trough over Nevada today, as well as daytime surface heating, we will see strong winds today.  Given the prolonged nature of the event, dust is likely as well.  

The models are still calling for two fronts to move through northern Utah this weekend, the first late tonight or early Saturday morning:

the second late Saturday or early Saturday night:

 The NAM has backed off a bit in both instances for precipitation at Alta, especially the first front tomorrow morning, and ultimately produces a storm total by Sunday morning of 0.82" of water and 13.5" of snow.

The SREF continues to show a large spread from only 0.25" of water to over 2.5", with strong clustering based on model core (ARW or NMB - essentially, two different models are used for the SREF).  The really wet members all produce considerable precipitation early tonight, and thus get things started early.

So, here we sit, having looked at this storm for about a week, and still no guarantees!  Sunday still looks like the better ski day as snow piles up during the weekend, but how good it is will depend on whether or not we are in the upper-half of these forecasts.  Finding a smooth underlying surface or getting enough snow to fully bury the frozen coral reef will be the key to good skiing on Sunday.  One plus is that we are looking at cold temperatures Saturday night, with 700-mb tempreatures currently forecast to drop to about -14ºC, which should yield a right-side-up snowfall. 

Cold Front Double Whammy?

The latest model forecasts are advertising what I'll call a "cold front double whammy" for the weekend with two pronounced frontal passages.

The 0600 UTC initialized NAM shows precipitation associated with the first front over northern Utah at 1200 UTC (0600 MDT) Saturday morning.  

And then the precipitation associated with the second front over northern Utah merely 18 hours later at 0600 UTC (0000 MDT) Sunday. 

Time-height sections show each of these features quite well.  Keep in mind that by convention, time increases to the left in these time-height sections.  Thus, you can see the pre-frontal southerly flow and low-level dry air that will predominate through late tomorrow on the right side of the diagram.  Tomorrow could be quite windy with blowing dust and perhaps some elevated convection and a slight chance of a thunderstorm.  The bottom falls out with the frontal passage.  Note how the freezing level (blue line) drops, bottoming out at just about 850 mb (5300 ft) early Saturday morning, bringing prospects of snow down to bench level.    

Then there is a rapid rebound in advance of the next front, with winds becoming southerly again and the freezing level rising.  Then, the second front ushers in even colder air Sunday night, with snow levels dropping to the valley floor.

We spoke a few days ago about the medium-range forecasts and the need to treat those with some caution.  We are close enough now that it looks like we will get some action this weekend, but far enough out that specifics regarding timing and accumulations are still hazy.  The first front comes in late enough that even if it drops a decent amount of water, it's probably going to create cream-on-crust conditions rather than true powder skiing.  The second system, assuming it comes in as advertised above, could yield some pretty good late-season skiing on Sunday, since right now it will be adding to Saturday's totals, may be a bigger storm, and will be right side up and colder.

Much depends on how much snow the two fronts put down.  I've largely given up on using the GFS, which has an overforecast problem and has produced about 2000 inches of snow this season at Alta.  If you want to have hopes consistently dashed, go with the GFS.  The 12-km NAM has generally been more reliable (although it was upgraded in March and we don't have a good sample on the new version yet) and generates 0.5" of water and about 5" of snow at Alta with the front on Saturday, and then goes crazy Saturday night and early Sunday, tacking on an additional 1.23" of water and 19" of snow by 9 AM Sunday.  What a treat that would be.  

The downscaled SREF shows quite a bit of variation in both the timing of the front (earlier in the ARW members) and the amount of precipitation produced, especially by the first front (more in the ARW members).  Typically members based on the ARW dynamic core are wetter, and that's the case for the weekend.  The spread here does sober up my excitement as there are quite a few members producing less than an inch of water, which is probably the minimum required for decent powder skiing given the current spring snowpack.  

So, I'm guardedly optimistic for a good day on Sunday.  Saturday looks to be a day of transition, with good skiing more likely late than early and the need for the first front to come through big to get the spring snow buried.  

Ode to Physics on a Manure Heap

When I was an undergraduate at Penn State, Craig Bohren was my physical meteorology professor and my advisor.  He had two major influences on my career.  First, sometime perhaps late in my junior year, he told me that I had pretty good grades in math and physics and that I should think about going to graduate school.  Up until that time, I hadn't considered it.  In hindsight, it was only a brief comment, but one that completely changed my life path.

Second, Craig taught in a very unconventional way.  The usual approach in physical meteorology is to subject students to long derivations and heinous interpretations of mathematical relations for cloud and radiative processes.  The approach can be very dry and unappealing.  In contrast, Craig almost always began with an example, typically a photo taken on his walks to, from, and around campus, and then insisted that we explain the subject.

One day he came in with photos of manure heaps, and asked us to explain the processes producing the steam (an example of what is known a mixing cloud) and its optical characteristics.   The topic forms a chapter in his classic science-geek book Clouds in a Glass of Beer.  He first taught us how to think and explain observations of the natural world, and then brought in more quantitative, mathematical explanations, which is an approach that I favor and try to employ as an instructor today.

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I often think of Craig when I am out walking (I have mentioned him in previous posts) and that was the case this morning.  Back in the day, he had to wait for his film to be developed overnight (or eventually in 1 hour), but we get instant gratification with smart phones.  I find days with spotty frost to be especially interesting as I try to surmise why the front forms in some areas and not in others.  What caught my attention initially this morning was simply the existence of front in patches on my lawn.

Why was there frost at all when temperatures at the airport did not fall to 32ºF overnight (the low was 33ºF)?  One possibility is that it was colder at my house than at the airport, but I didn't have a thermometer to test this hypothesis.  A nearby observing site above 18th Avenue reported a minimum temperature of 30ºF, as did sites near the mouth of Red Butte Canyon, so this is a possibility.  Another possible contributor is that the temperature at or near the ground is actually colder than the "air" temperature measured at the airport or surrounding observing sites.  Air temperatures are typically measured at 1.5 to 2 meters above ground level or even on buildings.  At night, when a nocturnal inversion is present, the temperature is typically even colder near the ground.  Thus, frost can form at times that the temperature at 1.5 to 2 meters above ground level is just above freezing.

But then we can dig in a bit further and try to explain the patterns of frost on plants and grass.  This is where things start to get really wild.  Here are two interesting patterns that I found.  The first pattern features frost in the middle and at the top of bushy low plants.

In this instance, I suspect that the entire bush has cooled to below freezing, but frost is forming only where there is both a sufficient source of water vapor (the ground below the plant which has been protected by the sun and experienced less drying yesterday) and weak flow (due to protection from the surrounding leaves) to allow the humidity to rise high enough.  This is, however, pure speculation subject to debate by Wasatch Weather Weenies readers.

The second pattern is a bit more perplexing and features a line of frost parallel to and a few centimeters from the sidewalk.  Does the existence of the deep edge along the grass provide a conduit for deeper soil moisture to escape, but frost only forms when it gets to the grass that is a bit removed from the concrete and thus a bit colder?  I the grass just off the curb slightly higher and thus elevated just enough off the ground to cool sufficiently for frost formation?

A high-resolution infrared camera would be great in circumstances like this as it would shed light on the temperature part of the problem.  More difficult are issues related to sources and variations in water vapor, which are also very important.

Feel free to share your thoughts and hypotheses.  If this sort of thing interests you, you are a true Wasatch Weather Weenie.