Tuesday, May 29, 2018

Last Weekend Had Us Spinning

It ended up being a pretty good weekend for Utah storm chasers with two tornadoes reported yesterday in the state, one at Strawberry Reservoir, the other near Snowville.

There were a number of good videos of the Strawberry Reservoir tornado, including the one below.

If desired, you could call this a waterspout, which is a tornado that forms over water or moves from land to water.  It would probably rate as an EF0 on the Enhanced Fujita Scale, with winds of 65-85 mph.

I haven't seen any video of the Snowville tornado, but the NWS reported that pictures to broadcast media suggested a brief touchdown with no damage or injuries.

The most impressive storm of the weekend was a mesocyclone that was observed by radar near Elko, Nevada, illustrated in the loop below.

A mesocyclone is a rotating thunderstorm and one can see the rotation in the Doppler velocity imagery above.  Note the transition from bright green to dark red over a short distance.  The bright greens are toward the radar, the dark reds are away from the radar, consistent with cyclonic (counter-clockwise) rotation.  This was a really unusual storm in that it is moving southwestward.  Most mesocyclones that I've seen in the Great Basin have been produced in southerly or southwesterly flow.

Such storms can sometimes produce tornadoes, but I did not see a report of one in the National Weather Service reports I sifted through this morning. 

I'm pleased to report that while no tornadoes were spotted at my house, we got a decent dousing of rain.  Anything this time of year is a treat and a blessing.

Saturday, May 26, 2018

Hiking Lookout Peak

I often think that one way to alleviate pressure on the Cottonwood Canyons is to better promote the recreation opportunities that exist in other areas around Salt Lake City.

With the holiday weekend upon us, we opted to hike this morning up Lookout Peak, which sits at the head of upper City Creek Canyon and is most easily accessed from Affleck Park along State Route 65.

In a previous life, I had mountain biked down the hiking route after circling into it from Big Mountain Pass.  Those were the good old days, pre-Bonneville Shoreline Trail and extensive mountain biking in the Park City area, which necessitated riding all sorts of narrow trails and avoiding face plants as we descended with hard-tail bikes with about 2 inches of front-fork travel.

Today we found ideal hiking conditions, with plenty of wildflowers.

It's about 4 miles and 2700 vertical feet to the summit, depending on where you start.  The views today were fantastic and include vistas of Little Black Mountain, Lower City Creek Canyon, and the Great Salt Lake.

Upper City Creek Canyon and Grandview Peak.

Closeup of Grandview.  This might be my next hike once the Skyline Drive is open (supposedly June 1st).  I've always thought it would make an interesting ski adventure in the spring to ride up City Creek and approach it up the canyon.

Hikers on the false summit, with the Uinta Mountains in the distance.

The central Wasatch showed off their coat of white today.  Sadly, it is going fast.

Now is really the time to hike these mid-elevation peaks.  Check out how green it is in Emigration and Parleys Canyons.

The route is not well signed and takes a little sleuthing to start.  It begins in the Affleck Park campground, which was open for camping, but gated today for access.  Thus, we parked at the Quacking Aspen Grove Historic marker and walked into the campground, finding the trail near the northern end.  After that, one ascends to the divide with Killyon Canyon, turns right (north) and works their way to the summit.

Enjoy while the grass is green and the heat is not too stifling.

Friday, May 25, 2018

Memorial Day Weekend Outlook

Mass exodus from northern Utah begins shortly with the annual pilgrimage to southern Utah and other environs for Memorial Day weekend.  I'm not traveling this weekend, but am taking a keen interest in the staycation forecast. 

Today looks to be a hot, dry one, but a closed low lurks over California.  This closed low will drift eastward and be located over Nevada tomorrow.  Thus, tomorrow is a transition day for Utah, with cooler air spreading in from the east.  The southeast part of the state will continue with dry, warm, windy conditions, but the western half of the state should be cooler tomorrow afternoon.  Northern Utah has a slight chance of a late day shower or thunderstorm. 

NAM 700-mb temperature (color contours), 500-mb geopotential height (black contours), 700-mb wind (barbs) and 3-hour accumulated precipitation (color fill) forecast valid 0000 UTC 27 May (1800 MDT 26 May) 2018.

The closed low drifts slowly eastward through the Memorial Day weekend.  By 1200 UTC 28 May (0600 MDT Monday), the low is centered over northwest Utah.

NAM 700-mb temperature (color contours), 500-mb geopotential height (black contours), and 3-hour accumulated precipitation (color fill) forecast valid 0000 UTC 27 May (1800 MDT 26 May) 2018.
By and large, I don't think this is a disastrous forecast for the weekend.  This is not an especially cold trough, although we will see a cooling trend through the period.  It is also not going to produce widespread rainfall, although there are going to be showers and thunderstorms (more on where in a minute) and the possibility that a persistent rainband gets going that might prove to be a real annoyance for a few hours if it sets up right over your location of interest. 

Forecasts from the SREF illustrate where precipitation is most likely for the period through 1800 UTC (Noon MDT) Monday.  Precipitation odds are lowest in southeastern Utah.  Northern Utah has higher precipitation odds, including a small number of ensemble members generating more than an inch of precipitation in a few regions. 

The plume for the Salt Lake City International Airport shows most members generating less than 0.2" of precipitation, but one member puts out 0.7".  Thus, while heavier precipitation isn't likely, it could happen if there's a strong thunderstorm or a slow moving precipitation system parks over you.
Bottom line: Tomorrow is probably the day for long-duration outdoor activities in northern Utah, with a mindful eye on how things evolve late in the day and in the evening.  While there is a chance of showers the rest of the weekend, keeping an eye on the forecast and the radar should allow you to get most activities in, provided that you're not skunked by a slow moving area of precipitation or thunderstorm. 

A quick reminder that lightning is Utah's top weather-related killer.  When thunder roars, head indoors.  Tents and awnings provide no lightning protection.  Move to your car or RV if you are camping.  If caught in a storm hiking, move quickly to lower terrain (avoiding areas that might pose flash flood threat), avoid large trees, crouch low on the balls of your feet, and spread your group out. 

Wednesday, May 23, 2018

A Wee Bit of Snow Left

Pretty disturbing late May snowpack numbers from around the state as a super majority of SNOTEL stations are now reporting zero or near zero snowpack water equivalents.

Source: NRCS
In the central Wasatch, only Snowbird is reporting what I'll call "above the noise" snowpack water equivalent values with 6.2 inches.  The freefall over the past month is quite obvious if you follow the blue trace below.  

Source: NRCS
That gives us about a "weekish" of snow cover duration left at that site.

In the western Uintas, the Trial Lake SNOTEL has maybe a day or two left with 1.0" of water equivalent on the pillow.

Source: NRCS
Bald Mountain Pass, which is a bit higher, looks like it still has about a footish of snow on the ground.  With effort, there's probably some skiing to be had up there, but this is a pretty meager snowpack for the approaching Memorial Day weekend.  

Whose got the most? The Doc Daniel site in the Bear River Range is the statewide leader at present with about 15 inches. 

Source: NRCS
For what it's worth, the Alta web cams suggest we should have reasonable top-to-bottom ski touring this weekend, despite the skimpy Snowbird SNOTEL numbers.  

Source: Alta
The snow depth at Alta-Collins this afternoon sits at 55".  

More Info on the GFS FV3 Upgrade

See http://www.nws.noaa.gov/os/notification/pns18-15gfs-upgrade.htm.

Tuesday, May 22, 2018

Little Cottonwood Terrain Shading

I'm on a short road trip to La Jolla, California where I'm visiting the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography.  There's a lot of great research happening here, so the trip has been quite invigorating.  Morning walks on the beach have been nice as well, although San Diego has been draped in "May Gray" since I got here. 

Flying out of Salt Lake City late Sunday revealed some curiosities about terrain shading in Little Cottonwood Canyon.  In particular, note how the high terrain north of the canyon is casting a shadow on the north facing sidewall. 

This can happen at our latitude between the spring and fall equinoxes when the sun sets north of west and is most pronounced at the summer solstice (late June).

Thursday, May 17, 2018

Uinta Misadventures

My son and I have thought about skiing Kings Peak all season.  We figured we'd wait until the road was melted out and then give it a shot.  Then, work, weather, and other factors conspired to prevent us from doing it, until yesterday.

Our plan was to do it as an overnighter, skiing in, setting up camp at about 11,000 feet, summiting, spending the night, and then skiing out on frozen snow the next morning.

There were, however, two major problems with this plan.  First, there was very little snow.  Second, it was exceptionally warm.

The trailhead sits at 9400 feet.  Bone dry. 

That was somewhat expected since the road was open, and we naively thought we would hit snow shortly up the trail, but alas, that wasn't to be.  Instead it was 3 or 4 miles of hauling overnight packs with an additional 12-14 pounds of AT gear up the trail. 

In addition, the Kings Peak trail is nearly pan flat.  So you don't get an abrupt transition from no snow to snow.  Instead, one gets miles of patchy snow.  To ski or not to ski that is the question we asked for some time.  Further, the snow was totally rotten, with postholling aplenty.  This led to rat-maze-like routes on snow patches, frequent ski removal, and lots of skinning across grass, mud, and rocks.

Did I mention swimming?  Well, none of that was done, but it was plenty boggy in many places.  Did I mention collapsable snow?  Plenty of that, even with skis on. 

This continued for some time, but we were hopeful that when we reached Elkhorn Crossing at 10,500 feet, things would surely improve.

Sadly, they didn't.  Look at all that bare ground!

We had hopes that perhaps the western side along the trees would be better, so we crossed the bridge, which thankfully was there as fording the stream here would have been impossible.

We then tried a few options.  One was working our way through the woods above the trail, which proved extremely difficult due to the limited snow cover, downed trees, bare spots, and the like.  We then dropped down to nearer the trail and moseyed along some more, but finally shortly after this photo was taken, we realized we were well behind schedule. 

At that point, we had a few of options.  Reaching the summit that day seemed nearly impossible.  Our pace was much slower than we expected.  If we were to make it, it would be late and we'd be exhausted.  At 25, I would have surely given it a shot, but at 50, it seemed like a recipe for disaster.  Further, at this point we had seen absolutely nobody.  It seemed a foolish move.

Another option was to go a bit further, camp for the night, and summit in the morning on firmer snow.  That seemed doable, but the exit out then would be a painful one involving the same sorts of travel conditions we'd experienced so far.  Two days of this wasn't looking like fun.

Ultimately, we decided it wasn't our day and to pack it in.  We then trudged 6-7 miles back to the trailhead, taking a few photos above the crossing for memories.

Over the past couple of weeks, I had tracked the snowpack at SNOTEL stations in the Uintas.  Several were at zero prior to our trip, but others had 10-15 inches.  I found it hard to believe that there was zero snow along the Kings Peak Route, and really, there wasn't.  There just wasn't enough to piece together a reasonable approach.  Confirmation bias and wishful thinking led me to conclude that there would be more snow than there was. 

Lessons learned: Kings Peak is really meant for light weight backcountry gear on a healthy snowpack.  A time machine to take me back to my 20s might also be helpful!

Tuesday, May 15, 2018

FV3GFS Moving Forward

It is well known that the US is lagging other forecasting centers, such as the European Center for Medium Range Weather Forecasting and the UK Meteorological Office, in global model forecast skill. 

In testing now is an entirely new dynamical core for the US Global Forecast System, developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and known as the Finite Volume Cubed-Sphere Dynamical Core, or FV3 for short.  See https://www.gfdl.noaa.gov/fv3/ for gory details. 

Source: https://www.gfdl.noaa.gov/fv3/
Experimental "FV3GFS" runs are now being produced by NCEP and I've started to download them to weather.utah.edu.  Look for FV3-13km (experimental) in the left hand nav bar.  For now, I'm only downloading wind and precipitation forecasts.  These should be at near-native resolution and hopefully my post-processing of the data is calculating the 3-hour precipitation correctly.  If you see anything odd, let me know.

In addition to a new dynamical core, the FV3GFS is also using the GFDL cloud microphysics parameterization to simulate cloud process.  This should be an upgrade compared to the old GFS cloud microphysics scheme, but I'm not sure if it has been tested much in winter storms or complex terrain. 

From a research standpoint, I'm not that interested in model performance during the warm season (i.e., right now and during the summer).  I am, however, hoping to get my paws on some of the runs from this past cool season so we can see how it does for winter storms over the west.

Whether or not this proves to be the game changer that NCEP hopes it will be remains to be seen.  I suspect we will see the FV3GFS become the operational GFS at some point later this calendar year or in early 2019. 

Monday, May 14, 2018

Disruption Continues in the News Industry

Modern technology is disrupting many industries, including news.  Today (Monday), the Salt Lake Tribune announced it was eliminating 34 of its 90 newsroom staff.  This isn't the first time they have downsized.  It ma not be the last.  I love reading a print newspaper in the morning, but I'm one of the few diehards left in the valley.  Advertising revenue for the Trib is down 40% in two years and circulation has dropped from 85,000 to 31,000 in four years.

I've witnessed massive change in regional and local science reporting during my career.  When I began as a professor 23 years ago, most reporters I interacted with were science specialists or reporters.  This was true for television as well as print.  Television suffered first.  Instead of science specialists, I mostly interact today with general reporters or camera people, the latter with a list of questions handed to them by a supervisor.  Science is complicated.  Reporting science is a specialized talent.  Unfortunately, regional and local science specialists appear to be going the way of the dinosaur. 

I've been interviewed by many Salt Lake Tribune editors and reporters over the years including Judy Fayhs (now with KUER), Jennifer Nappier-Pierce, Brian Maffly, George Pyle, Mike Gorrell, and Emma Penrod.  All are consummate professionals.  If you think reporting is easy, let me assure you that it is not.  I recall giving some terrible interviews that somehow contributed to great articles.  That takes talent. 

Sadly, these layoffs are the continuation of a downward spiral.  Let us hope that digital media develops in a way that turns things around.  There are some bright sides to digitalization of news media.  In my area, the Capital Weather Gang is one such bright side, providing outstanding weather and climate reporting, much of it by meteorologists. 

I don't envision paper newspapers arriving in my driveway for much longer.  Let us hope that some great digital science reporting on our region is in our future.  There are many great stories to tell.

Saturday, May 12, 2018

Cascadian Skiing in the Wasatch

You just have to love a rainy day like this when we reach May.  Anything from here on out is a blessing and loved by all the vegetation.

Not willing to give up on the ski season, however, we had a Cascadian experience in Little Cottonwood Canyon this morning.  The drive up the canyon was entertaining enough.  There are probably only a couple of days each spring where everything is green and cloud base is near the canyon mouth to give you views like those below in the lower canyon. 

The Alta web cams suggested that we might break out and be in only occasional murk above 8500 feet and indeed we broke out, temporarily, at the base of Snowbird.

The view from the parking lot was beautiful when we arrived, although the clouds were in and out during our tour.  Sadly, Superior is quickly losing it's white coat this spring. 

As I've mentioned several times this year, the key to a good ski day is low expectations.  Ours were pretty much rock bottom as we drove up the canyon, but sliding on snow combined with the usual workout endorphins seemed to lift the spirits by the time we finished our first climb. 

It is easy to think that spring snow is safe snow, but if there's enough snow to glide, there's enough snow to slide. We noticed a pretty healthy slough that penetrated down into the Ballroom between our first run (pictured above) and second (close up below). 

You'd probably have to work pretty hard to bury yourself today, perhaps initiating something that goes into a terrain trap, but a wet slough like that could toss you over a cliff or into obstacles.  The best skiing is often low angle when we get just a few inches of cream on crust anyway, so why push it.

Ski conditions were decent on top with a few inches of cream on crust.  It skied fairly well, perhaps because the underlying surface wasn't frozen hard and was fairly pliable.

Roller balls abounded in many areas.  Usually you wan't to avoid them at all costs, but they were amazingly soft today and we ended up just pretending the weren't there. 

About half way down, we hit the rain line and the high water content snow skied like sandpaper.  You can almost see the friction in the photo below 😃.

For about 1000 vertical feet, you could just point your skis right down the fall line, do shallow turns, and barely move.  Strange, but surprisingly fun. 

Precipitation was pretty limited while we were skiing through about noon.  The Alta-Collins gauge reported only .02 inches from 7 am to noon.  It was strange to descend back down into the "rain forest" of the Salt Lake Valley. 

By and large a surprisingly fun day of skiing.

Friday, May 11, 2018

May Powder?

It's never over until it is over.  At 1200 UTC (0600 MDT) this morning, and upper-level trough was centered just northwest of northern Utah. 

Local radar shows precipitation filling in across the northern half of the state.  Most of this precipitation is still light and scattered, but there is a band of heavier precipitation across the northern Great Salt Lake and a few stronger showers scattered about.

If you think it feels humid and sticky out there, that's because it is (by Utah standards).  Dewpoints at the airport this morning were near 50ºF. 

Turning out attention to the mountains, it's balmy up there, with a 700-mb temperature of about +3˚C.

Current surface temperatures include 39˚F at Alta Base, 36˚F at Alta-Collins, and 32˚F at the summit of Mt. Baldy (thank you Alta Ski Patrol for keeping the observations going in the off season).  That gives us a freezing level of about 11,000 feet and a snow level of perhaps 10,000 feet.

The models call for the upper-level trough to move southward into Nevada today and tonight.  Expect unsettled weather through at least tomorrow, with some thunderstorms.  The valley will see rain. 

Our Upper Cottonwoods meteogram from the 0600 UTC NAM forecast illustrates the situation for the mountains.  The forecast is holding up well so far with a temperature of 32˚F on Mt. Baldy, similar to observed.  Temperatures decline slowly through this evening, rise a bit overnight, and decline again tomorrow morning through early afternoon.  This complex temperature evolution reflects the fact that the trough is both digging to our west and intensifying, which results in a situation whereby we transition back into warmer southeasterly or southerly flow early tomorrow.  Basically, it's a mess. 

Thus, I expect snow levels to drop through this evening, perhaps to about 8000 feet, but rise again late tonight or early tomorrow morning.  There are large error bars on this, however, and here's why.

Model soundings for Alta such as the one below, valid 0800 UTC (0200 MDT) tonight, show a very deep layer in which the temperature is 0ºC.  This is the sort of thing that can happen in the Cascades when the precipitation rates are high.  Think of it as the ice cube effect.  Mother Nature dumps a ton of snow very quickly, and the atmosphere cools to 0ºC, much like throwing ice cubes into your cocktail.  I'm never sure whether or not to count on these sorts of details in the model forecast as they are dependent on many factors.  However, if it were to happen, the snow level will lower perhaps more than expected.  If it doesn't, well it will be high.  Regardless, the snow in such a layer tends to be wet and sticky .  
The NAM is quite generous with water totals, producing 2 inches through noon Saturday.  Our algorithms suggest that equates to about 12 inches of snow at 9700 feet.  Cascade concrete for sure.

This is, however, a very difficult precipitation amount forecast for several reasons.  One is that it is dependent on the track and intensity of the wobbly trough.  Another is that we're going to see a lot of pop-up showers and thunderstorms which can't be pin pointed.  As a result, the SREF shows remarkable forecast plume spread at Alta-Collins. 

Thus, this might be the most wishy-washy post I've done.  I expect precipitation, possibly heavy at times, in the central Wasatch.  I also expect the snow to be high density.  I'm not sure about precipitation totals, other than ruling out low amounts, or snow levels.  The best option in a pattern like this is to be ready to go in the morning, set the alarm, pull up the Alta-Collins ob (http://mesowest.utah.edu/cgi-bin/droman/meso_base.cgi?stn=cln) when it goes off, and either go back to sleep or jump in the car depending on what you see.  May powder?  I don't think so.  Creamy crud?  Maybe, if the trough delivers enough snow tonight.  Otherwise, there's always yardwork.

Wednesday, May 9, 2018

Foothills in All Their Splendor

If you haven't been out for a hike, run, or bike in the Avenues foothills or along the east bench, stop what you are doing and go now. 

May is by far the best month for lowland adventures in northern Utah as the hills green up, but snow lingers on the mountains.  I nearly had to pinch myself yesterday when I was out for a ride on the Bonneville Shoreline Trail with a carpet of grass and wildflowers covering the lowlands and a coating of snow in the mountains (see above).

How quickly the hills turn brown (or if you prefer, "golden") depends on whether or not we can continue to get the occasional storm as we approach the dog days of summer.  Let's hope the trough that will affect our weather later this week and weekend can deliver. 

Monday, May 7, 2018

How Can Snow Freeze When Temperatures Are Above 32˚F?

If you were skiing early in the morning this past weekend, you may have noticed that the snow surface was frozen despite temperatures being above 32˚F.  Conversely, there are times when the snow surface is melting but temperatures are below 32˚F.  How can this happen?

The reason is that there are several energy sources and sinks for the snowpack besides the direct exchange of heat with the atmosphere.  The direct exchange of heat with the atmosphere is known as sensible heat flux.  When the atmosphere is warmer than the snow surface, the sensible heat flux is typically positive, which by itself would yield either a warming snow surface or melting snow if the snow surface was at 32˚F, but there are other sources and sinks that can also play a role, as depicted below.

Latent heat flux is produced by the exchange of moisture with the atmosphere.  If the air is dry, and ice is sublimating (or water evaporating if the snow surface consists of both water and ice), this has a cooling effect on the snow surface.  Alternatively, if the air is moist, it is possible for water vapor to condense on the snowpack and release heat, which has a warming effect on the snow surface.

Sensible and latent heat fluxes are related to the difference in temperature and moisture content, respectively, of the snow surface and atmosphere, but are also affected by wind speed and turbulence.  For example, a dry, windy day typically yields stronger sublimation and cooling of the snowpack than a calm day.

But there's more to the story than sensible and latent heat fluxes.  Absolutely critical are the roles of longwave and shortwave radiation, the former emitted by the snowpack, clouds, atmosphere, and other "terrestrial" objects and the latter emitted primarily by the sun.

Downwelling longwave radiation from the atmosphere and clouds is absorbed by the snowpack, which also emits radiation at a rate dependent on the snow-surface temperature.  If the snow absorbs more longwave radiation than it is emitting, the net longwave radiation is positive.  By itself, this would warm the snowpack or melt it if the snow temperature were 32˚F.  Conversely, if the snow absorbs less long-wave radiation than it is emitting, the net longwave radiation is negative.  By itself, this would cool (or freeze) the snowpack.  A snow surface can freeze when atmospheric temperatures are above 32˚F if the the longwave energy loss exceeds sensible heat flux.

On Saturday morning, my son and I ski toured in White Pine Canyon.  In most areas, the snow surface was initially frozen, despite air temperatures in the mid-to-high 30s.  At that time, the sensible heating of the snowpack by the atmosphere would, by itself, act to warm and melt the snow surface, albeit slowly since temperatures were just above the melting point.  However, overnight skies were clear or covered by just a thin veil of high clouds.  Under such conditions, the incoming longwave radiation was small and the net longwave radiation was strongly negative.  The longwave energy loss exceeded the gain from sensible heat fluxes, enabling the snow surface to freeze.  Also assisting was the dry atmosphere, which resulted in a small amount of cooling from sublimation.

We noticed that the snow was sometimes not frozen but soft near the trees.  In those areas, the snow surface received significant longwave radiation from the trees and the net longwave energy loss was likely much smaller and unable to offset the sensible heating sufficiently to freeze the snowpack.

During the day, there is an additional energy source, shortwave radiation.  The amount of solar radiation incident on a slope depends on the time of day and the slope aspect.  At our latitude during the winter, south aspects receive more incident solar radiation than north aspects.  East aspects see a morning peak in incident radiation, west aspects an afternoon peak.  Total incident solar radiation from sunrise to sunset is largest on south aspects.

There are, however, some important seasonal differences.  At our latitude, after the spring equinox, the sun rises and sets north of east and west, respectively.  For a time after sunrise and before sunset, the north aspects actually receive more incident solar radiation than the south aspects.  This effect is largest on the summer solstice (around June 21).  In addition, the difference in incident solar radiation at solar noon between a north and south aspect is largest on the winter solstice and smallest on the summer solstice.  For example, at solar noon on December 21st, there is about a 1200 W/m2 difference in incident solar radiation between a 30º south aspect and a 30º north aspect at our latitude.  On June 21st, the difference is only about 400 W/m2.

One of the reasons why I like ski touring in White Pine in the spring is that there are many aspects and slope angles and you can chose your route depending on what the sun and snowpack are doing on any given morning.  I am a big fan of choice when ski touring for reasons related to safety, creativity, and intellectual stimulation.

Finally, it is worth noting that the amount of radiation absorbed by the snowpack, and thus available to warm or melt the snow, is dependent on the ice crystal types, whether the snow is dry or wet (the latter consisting of both frozen and liquid water), and the presence of impurities like dust.  New snow absorbs less shortwave radiation than old snow.  Clean old snow absorbs less radiation than dusty or dirty old snow.

There is another source or sink of energy for the snowpack that I've ignored here and that is the transfer of heat with the ground.  In the midlatitudes, the ground beneath the snowpack is typically very near 32˚F.  In a deep snowpack during the spring, this is not a major player.  In a shallow snowpack during winter, however, the transfer of heat from the ground can create a strong temperature contrast between the base of the snowpack and the snow surface that leads to the development of faceted crystals known as depth hoar.

To conclude, there's a lot going on to affect the snow than just the air temperature.  Often it is the radiation, either the net longwave or shortwave, that is playing a dominant role in the snowpack evolution.