Monday, December 31, 2018

Steenburgh Effect Delivers Again!

I'm not out of town, but am unable to ski today.  Hence, yesterday's storm became a clear overproducer, generating 11 inches on the Collins stake through 8 AM this morning. 

If you love cold smoke, this is your snow.  Total water was only 0.56 inches, yielding a water content of about 5%.  Wear a buff or something to cover your face.  Temps are around 10ºF at 8500 feet and below zero at 11,000 feet.

This storm reminds me a bit of New Year's Eve 2010, when a super cold storm produced great touring conditions.

However, it was several degrees colder than today.  I remember that day well as we did over 6,000 vertical feet of climbing and I don't think I ever took my puffy jacket off.  Taking a significant break wasn't an option.  We would stop, chug some water and have a quick bite, and get moving again.

This could be it for several days as the NAEFS shows little action over the next few days, then we will have to see.

Saturday, December 29, 2018

Yesterday's Mountain Waves

Yesterday's MODOS imagery showed widespread evidence of clouds produced by trapped mountain waves over the Great Basin. 

Such clouds are produced in situations when the wind speed increases rapidly with height and the atmospheric stability decreases above a crest-level stable layer.  Such conditions were very apparent in the afternoon sounding form Elko.  Note how winds increase from 10 knots at about 725 mb to 130 knots at 300 mb, the existence of a sharp inversion near 725 mb, and then a decrease in statisc stability from 700 to 500 mb. 

This leads to a sequence of lee waves downstream of the mountains.  Clouds can form in the wave crests and dissipate in the wave troughs if the humidity is right, as was the case yesterday.

Source: Durran and Klemp (1983); COMET
Mountain waves were also evident over the Wasatch Range yesterday.  Note the wave-like undulations in the clouds in the photo below. 

Really, the mountains are generating waves pretty much all time time, but you need clouds to see them.  Their structure, however, varies.  Sometimes trapped lee waves are produced.  At other times, the waves may be confined to directly over the mountain.  Much depends on the characteristics of the flow and the atmospheric stability.

Friday, December 28, 2018

The Low-Angle Sun

Early afternoon with the low-angle sun barely cresting the ridge
I'm a big fan of the low-angle sun as it allows powder to persist for long periods on many aspects.  However, on a cold day like today, it gives little warmth and it allows cold pockets to persist deep into the afternoon.  Chances are you felt them as you skied around today into shady areas.

Sadly, last night's northeasterly flow did some damage to the snow, at least in the mid-elevation area we were touring today.  I heard rumors of great skiing while I was out of town for Christmas, but today's skiing was so-so.  Good in places.  A bit wind affected in others.  Still, compared to last year, we shouldn't complain.

The net chance of a decent refresh looks to be Sunday or Monday.  Forecasts from the SREF vary on timing and amount, with some getting things going in the morning, others later, and most of the members generating 0.2 to 0.6 inches of water at Alta Collins by New Years Eve morning.

For the most part, this looks to be a low density snowfall, so that would be a range of about 4 to 12 inches, although there are a couple of members going bigger than that. Note that while temperatures will rebound tomorrow, they will be dropping on Sunday and New Years Eve (Monday) may feature an airmass a bit colder than todays.

Thursday, December 27, 2018

Shutdown Insanity

Anyone who works in the weather business knows just how critical government weather services are for the protection of life and property.

As I write this, we are now in day 5 of the government shutdown, which now looks to last into the beginning of 2019.  To illustrate the challenges ahead for public servants, here's a tweet issued by the U.S. Office of Personnel Management this morning.

Despite the financial pressures of not knowing when they will receive their next paycheck, National Weather Service meteorologists continue to work through and provide critical forecast services.  You won't notice much of a change in their products, just some reductions in social media outreach and the like.  However, if you use other NOAA services, you might just be completely out of luck.  For example, go to the web site of the National Centers for Environmental Information, a critical provider of climate data for the nation, and you'll find this.

Need some of their climate data for forensic work, an algorithm for providing weather products to agricultural customers, or maybe your thesis?  Good luck with that.

The annual meeting of the American Meteorological Society begins on the 6th of January.  This is the largest meeting of meteorologists in the United States (probably the world) with attendees including academic, private sector, and government meteorologists.  Many of the latter may not be able to attend, which will put a serious damper on the meeting since some of the best weather and climate scientists in the world are in government positions.  Admittedly this is trivial stuff compared to the financial pressures that individuals are facing without a paycheck, but it is a real drag on the future weather and climate capabilities of the nation. 

There is also the long-term impact of these now frequent shutdowns on recruiting high-quality scientists into government service.  We need scientists to follow such a path to ensure we have the best numerical weather prediction systems, radar and satellite capabilities, and ultimately forecasts.  Beyond the weather, do we want to settle for mediocrity when it comes to the people who secure our nation's nuclear arsenal and lead cyberdefense and counter terrorism efforts?

Heartfelt gratitude to everyone in the National Weather Service and at other government agencies working through the holiday period during this shutdown.

Update @4:42 PM

Tweets from the American Meteorological Society shortly after I finished this post.

Wednesday, December 26, 2018

Steenburgh Effect Coming to an End

I've been out of town with family for the holiday, but am returning to Salt Lake tonight.  It appears that you have all benefited from the Steenburgh effect.  A friend reports 26 inches and 1.72 inches of water since I left several days ago. 

Not surprisingly, with my return tonight, the SREF is taking pity on you and giving you one last gasp for late tonight and tomorrow.  It's not looking like a big event, but it will provide a refresh. 

Enjoy it as the odds of snow will go down once Mother Nature gets wind of my return.

Wednesday, December 19, 2018

Holiday Reading

Here are a couple of books that might make good holiday gifts for friends, family, or yourself. 

In December 1968, propelled by the most powerful machine ever built, humans escaped Earth's gravity for the first time and traveled to the moon. Nobody had ever before travelled beyond low-Earth orbit. Apollo 8 is probably the riskiest endeavor ever undertaken by NASA, and this book tells the story in an exciting and compelling way.  This Christmas Eve marks the 50th anniversary of the mission orbiting the moon 10 times.  I get goosebumps just thinking about it. 

A friend recommended this book to me and I liked it so much that I read it in a day while traveling.  Those who believe the government does some great things will like it.  Others may not.  It gets a nod here because it contains a major section on the importance of NOAA and National Weather Service and the need to integrate social science and improved messaging into their weather products. 

I read this a couple of years ago, but it is worth a mention here as it is a mind-boggling tale of exploration and survival that will make your backcountry ski adventures seem like a picnic in the park. 

This is a free blog with no advertisements, so allow me an occasional shameless self promotion.  That being said, it does make a good Christmas present.

Monday, December 17, 2018

A Dirty, Stinky Lake Breeze

It was nice to walk to the bus early this morning, enjoy the mild temperatures, and breath clean air.  For a while, south winds ahead of an approaching upper-level trough had pushed back the valley cold pool to the north valley and over the Great Salt Lake.  The transition was especially strong at the Neil Armstrong Academy where around 6 AM MST the winds increased to over 10 knots with a SSE component.

Source: MesoWest 
At the same time, the temperature rose dramatically, from 27ºF to 46ºF. 

Source: MesoWest
However, a look at the two traces above shows that the wind eventually flopped around again to NW and the temperature dropped all the way down to 34ºF.

Further, the PM2.5 concentrations dropped to near zero with the temperature rise, but then crept back up to 35 ug/m3 with the shift to NW flow, although they have dropped back down to 20 ug/m3 at the moment.  

Source: MesoWest
The mesowest station plot for 1718 UTC (1018 MST) shows the situation really well.  In the southern part of the Salt Lake Valley, the flow is southerly and temperatures are in the 40s.  In the northern half, there is northwesterly flow with temperatures in the 30s. 

Source: MesoWest
Sadly, that cold, stable, polluted air has pushed its way back to the south.  This is not uncommon.  The air over the relatively cold Great Salt Lake in a situation like this is the densest, stingiest, and hardest to remove.  Right now there is a battle between the larger-scale southerly flow and the lake breeze which wants to push that cold air southward. 

The air quality where you are today is dependent on who wins that battle.

Sunday, December 16, 2018

Ski Report and Pollution Views

It was a spectacular weekend in the mountains with clear skies and mild weather producing widespread smiles.  Ski touring on Saturday was pretty good, although it required effort to get to untracked powder.  The groomers at Alta today were really fantastic, at least in the morning, when the snow was grippy and you could really lay trench.  Although we need a refresh and another 2-4 feet in the backcountry would really help open up some brushy terrain, the situation is a major improvement over last season.

After the Wednesday night storm, it has sadly taken only a couple of days for the pollution to return.  The current situation is quite a bit different from that earlier this week because we have an incredibly strong inversion in place right now very near the valley floor.  This morning's sounding showed a surface temperature at the airport of -2.7˚C with the temperature increasing with height to 6.4ºC at 828 mb (5800 ft).

Source: SPC
Compare that to the sounding from last Tuesday when the inversion was elevated and we were actually mixing through a deeper layer that was capped by clouds. 

Source: SPC
With a strong inversion in place near the valley floor, spectacular conditions are found in the mountains, such as the view below from the top of the Supreme Chair at Alta.  

However, a shallow layer of smog lies over the Salt Lake Valley as can be seen looking NW from SR-210 just north of Little Cottonwood Canyon. 

The shallowness of the pollution layer is also evident this afternoon in the upper Aves.  

I suspect the smog will deepen some this afternoon as the lake breeze pushes southward into the Salt Lake Valley.  That is a common occurrence during these shallow inversion events.  

A splitting trough coming through tomorrow will try to stir things up.  The southerlies in advance of the system were evident on the ridge tops today.  It's not a lock, but let's hope it comes through.  

Thursday, December 13, 2018

Yesterday's Radar Oddities

It's worth looking back at yesterday's radar loop because it exhibits several phenomena, some meteorological, some non-meteorological.  In particular, note in the loop below the persistent radar echoes over the Wasatch Range, then the development of a very pronounced north-south oriented band south of the Great Salt Lake, and then finally some north-northwest to south-southeast oriented bands at the end of the loop over northeast Nevada.  

What is all that stuff?

Let's start with the easy stuff.  The persistent echoes over the Wasatch Range and to some degree the Great Salt Lake are associated with orographic precipitation and possibly some lake-effect precipitation features.  I'll cherry pick a couple of times to highlight some characteristics of the storm during late afternoon yesterday.  The first was the tendency for some precipitation features to form over the lake (circled by a green line).  The second was the persistence of precipitation features over the mountains.  This was especially true in high terrain surrounding Little Cottonwood Canyon (circled in red).  In contrast, the Park City Ridgeline saw weaker and less persistent radar echoes.  

This storm stage is one reason why snowfall yesterday was greater in Little Cottonwood than along the Park City Ridgeline, as suggested by snowfall reports this morning (Snowbird 14", Alta 12", Brighton 8", Park City 5").  

Now lets increase the degree of difficulty a bit.  At 0102 UTC (6:02 PM MST), a pronounced band of extended from the south shore of the Great Salt Lake across the Tooele Valley, and over the Rush Valley.  Some might expect a lake-effect snowband, but it wasn't.  In all likelihood it was produced by birds, possibly eared grebes.  

This is the time of year when eared grebes tend to migrate southward, taking off in large concentrations for southern climates.  They typically become detectable on radar about 30-90 minutes following sunset, as was the case yesterday.  For more information, see our previous post Birdbrained Meteorology.  The loop below shows this southward migration very well.  Ambitious readers could check radars along their flight path to southern California.  In some instances, they can be detected on those radars as they continue southward.  

Finally, ratcheting up the degree of difficulty further, there are the bands over northeast Nevada.  

I suspect these are chaff, countermeasures used by the military to confuse radars and sometimes used for military training at various bases in the western U.S.  We see it frequently drifting around on western radars.  This is, however, an educated guess requiring further investigation.

National Weather Service radars are polarimetric and provide information that help discern birds and chaff from precipitation.  I lacked the time this morning to have a look at that.  Perhaps others can investigate.

Tuesday, December 11, 2018

A Classic Cloud-Topped Mixed Layer Followed by Snow

There's a fantastic view looking west this morning from the top of Snowbird's Hidden Peak, with a sea of stratus clouds over the Salt Lake Valley and the Great Salt Lake Basin.

Source: Snowbird
The scene is not unlike one might see from a hill above San Francisco in the summer as those clouds reflect what is known as a cloud-topped mixed layer, which dominate the weather along the coast of California in summer.  Below is the sounding from the Salt Lake City airport.  I have added a grey bar to denote the cloud layer.  From the surface to the top of the cloud deck, the temperature decreases rapidly with height and the atmosphere is well mixed.  Right at the top of the cloud layer is an inversion layer in which the temperature increases about 1.6ºC (about 3ºF) over a depth of about 60 m (200 ft).  That doesn't sound like much, but it is sufficient to keep a lid on the valley atmosphere.  Further aloft, a series of stable layers extend to nearly 600 mb, well above the crest level of the Wasatch Range. 

Sounding source: SPC
Although it makes for depressing skies, we are better off with a cloud-topped mixed layer than a strong inversion based near the valley floor.  That's because in a cloud-topped mixed layer, radiative cooling at cloud top drives turbulence and keeps the atmosphere relatively well mixed within and below the cloud layer.  Hence the term cloud-top mixed layer.

Source: Pataki et al. (2005)
As a result, our pollution is mixing through a layer that is about 850 meters (2775 feet) deep, which is right up to the base of the inversion.  That's much better than when the inversion is based very near the valley floor.

The development of this cloud topped mixed layer is one reason why PM2.5 levels have remained at moderate levels the last two days.  Basically we are seeing pollution dilution due to its development.
Source: Utah Division of Air Quality
All else being equal, a cloud-topped mixed layer with a deep cold pool and elevated inversion results in lower PM2.5 levels on the valley floor than a shallow cold pool with a near-surface inversion.  On the other hand, if the benches or perhaps places like Emigration Canyon see higher PM2.5 levels.  

Looking toward the future, the overnight model runs are showing a vigorous trough passage tomorrow (Wednesday) that should crack this inversion.  Although quick hitting, the system will generate snow down to the valley floor.  Students with finals tomorrow should consult National Weather Service Forecasts and plan on leaving early.  No excuses!  The official forecasts of 6-12" in the mountains are looking pretty good.  Pro Tip: Lift-served skiing tomorrow will be better late than early.  

Monday, December 10, 2018

Greed Is Good

Gordon Gecko should have been a powder skier
Last night I thought of Gordon Gecko as I was bemoaning the lack of snow over the past several days.  

We had a good run through last Monday.  The snowpack is much better than last year at this time.  But we need more.  Greed is good. 

We certainly had some fun, creamy snow ski touring on Saturday.  But skiing it involved pinballing off a few rocks and a bony brushwhacking adventure at the exit.  I kept thinking that another 3-5 feet would be nice.  Greed is good.

The skate skiing at Round Valley has been great so far this year, but on Sunday, things were starting to feel a little worn out.  I kept thinking that a few inches of high-density refresher would be nice.  Greed is good.

I haven't skied at the resorts in a while and have heard a mixture of reports, but surely a major dump or two before the holidays would be nice.  Greed is good.

However, there will be no hostile takeovers or union busting here.  Only the strait skinny.  Looks like 3 troughs will be moving through the west over the next week, but none of them look like a sure thing.  You might recall the storm cycle a couple of weeks ago when the NAEFS ensemble was tightly clustered around a significant storm, but last night's forecast for the next seven days shows huge spread being produced by both the GEFS and the Canadian Ensemble.  For snow at Alta, the mean is about 10 inches, but the range is 0 to 22.  How's that for uncertainty?  

We'll have to see how this all shakes out.  Let us hope the inversion cracks and that we can at least get a bit of a refresher.

Sunday, December 9, 2018

Intricacies of This Inversion, Part II

Building on the previous post (Intricacies of This Inversion), it is worth taking a look once again at some of the intricacies of the our current inversion as the evolution is quite interesting from a meteorological perspective.

First some definitions.  An inversion is a layer in the atmosphere in which temperature increases with height.  Within a valley or basin, a cold pool is the layer of cold air beneath that inversion.  Inversions vary in strength, depth, and altitude.  The current inversion has been characterized by an elevated inversion rather than one that is very near the valley floor.  In addition, the cold pool beneath the inversion has featured high relative humidity.  This has led to a cold pool that has not only filled with pollution, but has featured haze (small water droplets that form in a high humidity environment), and clouds. 

This morning those clouds largely fill the Salt Lake Valley, and the contrast between valley and mountains couldn't be larger.  I awoke this morning to a depressing scene, as bleak and grey as you will see in Salt Lake City. 

Meanwhile, the weather at Snowbird was stunning, with the view from the top of Hidden Peak showing a sea of stratocumulus over the Salt Lake Valley. 

This morning's sounding shows a multi layered structure from the valley floor to crest level.  From the valley floor to about 814 mb (about 6500 feet) the atmosphere was well mixed.  That means pollution is not confined to a very shallow layer near the valley floor, but is actually mixing through a depth of about 2000 feet.  At 814 mb is the base of a weak inversion.  This inversion sits at the top of the stratocumulus layer.  Then there is a second, much stronger inversion, based just below 700 mb (10,000 feet), which is being produced by the upper-level ridge that is building in. 

There are some interesting things happening, however, in the Salt Lake Valley on smaller scales.  When looked east this morning, I saw a bright spot along the east bench where the clouds and haze were thinner.   

A look at the surface observations showed light easterly flow at several east bench locations.

That easterly flow appeared to be disrupting the clouds and possibly contributing to the entrainment of drier and cleaner air into the valley cold pool.  Although the PurpleAir PM2.5 values are sometimes too high when the relative humidity is high, they are very useful for identifying pollution patterns over the valley and show lower PM2.5 indicies along the east bench of the northern Salt Lake Valley, especially north of Big Cottonwood Canyon.  These lower values extend eastward across downtown Salt Lake City.  This is consistent with the extension of easterly and southeasterly flow to the Salt Lake City airport in the station map above. 

Consistent with this easterly flow and associated entrainment of cleaner air, the air quality at Hawthorne Elementary has dropped significantly, all the way down to about 12 ug/m3. 
Source: Division of Air Quality
On the other hand, values remain high in other parts of the Salt Lake Valley to the south and to the west.  One disturbing aspect about this is that the DAQ uses the Hawthorne observations for their web site and is listing the air quality as good (i..e, green) on their web site, despite the fact that it is clearly not good in many parts of the Salt Lake Valley. 

Source: Utah Division of Air Quality
The Governor recently announced $100 million for air quality in his 2020 budget.  Whether or not this survives the legislature remains to be scene, but if it does, perhaps some of it can be used to put together a proper air quality monitoring and reporting system for the Salt Lake Valley.  Community (PurpleAir) and University of Utah ( networks are now providing valuable information that could be mined to improve air quality information and alerts.  The challenge is how to blend higher cost but higher fidelity instruments with lower cost but lower fidelity instruments.  In any event, we can certainly do better than a county-based air quality sensor based on one site. 

Friday, December 7, 2018

Intricacies of This Inversion

As far as dismal grey goes, the past few days take first prize in the Salt Lake Valley due to frequent mid-level clouds, occasional low clouds, plenty of haze, and smog.  I was surprised to hear that we reached PM2.5 levels yesterday that were unhealthy for sensitive groups as the air was still relatively clean on Tuesday and it usually takes four days to build up to that level.  However, the PM2.5 trace from Hawthorne (below) shows it took three, although it was a spike around noon that got us to over 35 ug/m3.
Source: Utah Division of Air Quality
A curious aspect of the current inversion is that it has been relatively elevated.  Yesterday's morning and afternoon soundings from the Salt Lake City Airport show the inversion base is at about 825 mb, which is about 1500 feet above the valley floor. 

Source: SPC

Source: SPC
Often, the inversion is right down near the valley floor, but in this instance, the smog is actually being mixed some.  It's just not able to mix through a deep enough layer to get out of the valley. 

The relatively elevated nature of the inversion and the deep layer of haze and pollution over the Salt Lake Valley is very evident in the Snowbird camera image from this morning.  While it is not uncommon for haze and pollution to be found in the Salt Lake Valley, the depth extends relatively far up the Oquirrh Mountains. 

In some respects, that is good news as it means the pollution is diluted a bit by the deeper layer of mixing.  On the other hand, it's still very depressing and the air quality is a best moderate and at worst into the unhealthy for sensitive groups category.

Looking through the weekend, it appears our goose is cooked, at least in the valley.  The models show an upper-level ridge building in with temperatures warming aloft and in the mountains.  The 3-km NAM sounding for Sunday Morning shows a still-elevated inversion with a shallow, moist mixed-layer near the valley floor.
Source: Tropical Tidbits
 The models are notoriously bad at getting the details of inversions right, but there's really no hope of mixout through the weekend.  Further, given the moisture in place in the valley and the forecast sounding above, I wonder if we will see more widespread fog or stratus (i.e., elevated low clouds) developing through the weekend. 

Finally, it is worth pointing out that blaming the pollution on the inversion, as I hear so often, is simply incorrect.  The inversion is simply a meteorological phenomenon that caps a pool of cold air in the valleys and basins of the intermountain west.  The pollution is caused by emissions due to the burning of fossil fuels and wood, as well as from agricultural and other sources.  As I like to say, we have met the enemy, and it is us.

Wednesday, December 5, 2018

Remarkable Overnight Temperature Rises

If you are into microclimates and the impact of radiation on temperature, the last two days have been incredible.

On Monday night, conditions were ideal for temperatures to fall and fall dramatically in lowland regions.  Tuesday night, things were different and clouds resulted in an increase in overnight temperatures overnight.

To illustrate this, let's take a look at observations from the Peter Sinks, a limestone sinkhole in the Bear River Range of northern Utah known for incredibly low overnight temperatures.

On Monday night, temperatures exhibited some ups and downs after sunset with some wild swings in temperature from about 1800 MST 3 Dec through about 0400 MST 4 Dec.  Those swings occurred in conjunction with gusty winds, which occasionally mixed out the cold pool that was forming in the Sinks.  However, after about 0400 MST 4 Dec, the wind died down and temperatures fell nearly continuously, reaching a minimum of about -35˚F at about 0900 MST 4 Dec when I suspect the low angle sun finally began to warm the sink, after which temperatures rose until the early afternoon. 

Source: MesoWest
Source: MesoWest
Last night, however, things were different.  The air was mainly calm all night and the temperature fell nearly continuously from about sunset through midnight (0000 MST 5 Dec), when it reached an incredible -40˚F.  

However, after midnight something remarkable happened.  The temperature increased 35˚F, reaching -5˚F by 0700 MST 5 Dec.  This happened despite it being night time and the air being calm or nearly calm.  Similar temperature traces can be found at many stations in northern Utah, although as you move southward, the temperature increase occurs earlier.  For example, at the Clover Site in the Rush Valley southwest of Salt Lake City, which is also known for very low overnight temperatures, the minimum temperature occurred at about 2000 MST 4 Dec, after which the temperature rose until this morning . 

Source: MesoWest
What gives?

Overnight several things happened.  First, cloud cover spread over the area, as can clearly be seen in the overnight satellite imagery.

Second, cloud base lowered.  At the Salt Lake city airport at midnight (0000 MST), a few clouds were reported at 20,000 feet above ground level, but by 0400 MST, the broken clouds covering 6/8 of the sky were reported at only 3700 feet above ground level.  

Finally, those clouds accompanied an elevated warm front that moved through the area and increased temperatures aloft.  Yesterday morning, the 700-mb (about 10,000 feet above sea level) temperature was -14.5˚C.

However, this morning the 700-mb temperature was -7.5˚C.  Note also that while there was an inversion yesterday morning from the surface to 850 mb, the atmosphere was relatively well mixed in that layer this morning (although the inversion based at about 800 mb and associated with the warm front still puts a lid on the valley atmosphere).  

So, overnight free-atmosphere temperatures aloft and at mountain elevations increased, but so did the cloud cover.  I suspect that the latter played a critical role in rising overnight temperatures in cold spots like the Peter Sinks.  With the air calm, it would be very difficult to mechanically mix out the -40ºC cold pool that had formed by midnight.  Instead, the cloud cover was probably critical.  

Clouds are important not because they "act like a blanket" (they don't), but because they emit infrared (longwave) radiation.  On a clear night with dry air in place, there is very little longwave radiation received by the ground (or snow) from the atmosphere, and temperatures plummet.  However, on a cloudy night, the incoming longwave radiation is greater.  How much greater depends on the temperature at cloud base, with low, warm clouds emitting more longwave radiation than high, cold clouds.  This radiation is then received by the Earth's surface or the snowpack, which cools at a slower rate than would occur under clear skies.   

Last night provided an extreme example of this in which clouds actually produced increasing temperatures.  In Peter Sinks, temperatures were -40˚F (which equates to -40˚C) before the clouds spread in.  Based on this morning's sounding above, the temperature at cloud base by this morning probably reached at least -10˚C and perhaps as high as -6˚C.  The amount of infrared radiation emitted by the clouds and also by the snowpack strongly increases with temperature.  Thus, with much warmer clouds spreading over a cold snowpack, the snowpack would begin to receive more longwave radiation than it was emitting, and would begin to warm.  That energy subsequently is transferred into the atmosphere, resulting in an increase in the near-surface air temperature.  

This process really has nothing in common with how blankets work.  Blankets keep you warm primarily by reducing the mixing of air near your body with the air farther away.  This keeps relatively warm air near your skin, reducing the transfer of heat to the atmosphere.  Clouds don't directly affect the transfer of heat from the Earth's surface to the atmosphere.  They affect the temperature by changing the radiation balance and providing more longwave radiation than clear sky.