Wednesday, March 27, 2024

The Goods on Graupel

Ski conditions in the Wasatch backcountry continue to be excellent with cold weather and cloudy skies enabling pretty good snow preservation despite it being late March.


Over the past couple of days, a lot of the snow that has fallen has been in the form of graupel, which is an opaque, white snow pellet that sometimes takes a lump, hexagonal, or conical form. Graupel often causes the snow surface to have a dippin' dots appearance.  


Catch some on your sleeve and take a close look you'll find that graupel particles are aggregates of a bazillion tiny frozen cloud droplets.  


Graupel forms in strong updrafts when supercooled cloud droplets freeze on falling snowflakes.  Supercooled means that the cloud droplets are unfrozen, despite being colder than 0˚C.  These droplets freeze on contact.  This process is sometimes called riming or accretion.  If the riming is light, you can still distinguish the original snowflake.  However, at the extreme, the flake is completely coated and you get graupel.  

One of the reasons why we have had so much graupel the past few days is that it's been very unstable.  Graupel requires strong updrafts to suspect a snow crystal until it is fully rimed.  Snowflakes typically have a fall speed of about 1 meter per second.  Graupel is about 3 meters per second.  So, if you want big graupel, you typically need updrafts of at least 3 meters per second.  

Graupel is important for thunderstorm electrification.  In thunderstorm updrafts, smaller ice crystals are often carried upward by updrafts, whereas the larger graupel particles can fall out.  This contributes to the charge separation process within the cloud and in some cases lightning and thunder.  This happens even in summer thunderstorms, which extend well above the freezing level, although in those small hail can also form and be a contributor to electrification).  This has been happening some in Utah the past couple of days.  Snowbasin closed early yesterday due to lightning.  

People are often surprised by thundersnow, but the process is the same as in summer thunderstorms. It's just less common because you need strong updrafts and those are less common in many winter storms. Lake effect snowbelts see thundersnow more frequently because lake-effect storms often have strong updrafts.  In northern Utah, the daytime heating if post-frontal cold airmasses in the spring can also lead to strong updrafts and thundersnow.  

Because graupel is dense and has a higher fall speed, it can penetrate farther below the melting level than most snowflakes.  Thus, sometimes you see graupel falling at higher temperatures than snowflakes.  

Graupel should not be confused with sleet.  Sleet is a translucent ice pellet.  Unlike graupel, which is opaque and typically a bit pliable if you squeeze it, sleet is hard.  Sleet also forms through a different pathway.  Sleet forms when snow falls into a warm layer aloft with temperatures above 0˚C and melts into rain or droplets that are predominantly water with perhaps a small particle of ice in it.  It then falls into a colder layer near the surface that is below 0˚C and freezes.  

Sleet is basically a frozen raindrop.  Graupel is a snow particle formed by riming.  Very different processes.  Graupel skis much better than sleet.  You would definitely notice the difference.  Sleet is exceptionally rare in northern Utah (I'm not sure if I've ever seen it here). 

Enjoy the graupel skiing while it lasts. 

Monday, March 25, 2024

Just What the Doctor Ordered

The Saturday night frontal passage seemed to provide just what the doctor ordered for transitioning from spring skiing back to proper powder skiing.  I found good skiing yesterday with just a hint of bottom feeding at mid elevations if you kept the slope angles reasonable.  


Storm totals per the Alta-Collins site are now over two feet.  Elsewhere, a healthy lake band developed overnight, but raged over the western Salt Lake and Utah Valleys.  

It has since broken up some but scattered lake-effect snowshowers continue this morning, albeit a bit farther west than central Wasatch skiers would like. 


The unsettled post-frontal weather will continue through Tuesday evening.  Although not in the lake-effect now, expect periods of snow showers throughout this period in the Cottonwoods and perhaps even a thunderstorm or two in the afternoons.  For the period from 6 AM MDT this morning through 6 PM 12 AM MDT Wednesday March 27, the GFS produces .79" of water and 14.7" of snow at Alta.  The HRRR was less excited (.44"/7.6").  Regardless, it will come in fits and starts, which is my new phrase for the week.  It's great skiing for late March.  Hopefully the sun will not do too much damage when it appears at times.  

Enjoy.

Sunday, March 24, 2024

Storm Update

Pretty good delivery of the white in the upper Cottonwoods yesterday afternoon and last night. As of 6 AM, Alta-Collins is already at 13 inches with 1.32" of water.  That's a mean water content of 10%, although I suspect it's right side up.  We need some high density snow to bury the coral reef anyway.  All in all this is good news.  

Radar early this morning showed a fairly active northwesterly flow pattern.    

Expect periods of snow in the northwesterly flow to continue today.  The HRRR really lights it up with .66" of water and 11.1" of snow at Alta from 7 AM to 5 PM MDT today.  The GFS is not as excited puts out .19" of water and 3.6" of snow.  A big reason for this discrepancy is the HRRR really gets things going in the northwesterly flow, including some lake effect, as illustrated by the forecast radar image for 2000 UTC (2 PM) this afternoon.  


And for the 6-h period ending at 0000 UTC (6 PM MDT) the HRRR is generating 7.5" of snow for Alta and even higher amounts along the Alpine ridge to the west in the Lone Peak area.  

A real question is how much to buy into the HRRR forecast?  I'm inclined to expect another 4-8" at Alta-Collins for 7 AM this morning through 5 PM this afternoon, which sits between the GFS and HRRR.  I'd put the odds of going above that higher than below.  How bout we throw in the chance of a T-storm too?  Welcome to spring.

Saturday, March 23, 2024

Winter to Return

After about a week of splendid spring weather, winter will be returning to the Wasatch Range today with the arrival of a cold front this evening.  

Although there will be some periods of high-elevation snow and low elevation rain ahead of the front, the main action will be associated with the frontal passage which will bring a few hours of steady snowfall overnight.  

The cold-front is very apparent in the latest HRRR run with an abrupt drop in the wet-bulb zero height at between 7 and 8 PM.  


We base the wet-bulb zero level estimate on the model forecast from the airport (if we used data from Alta we would not be able to estimate snow level most of the winter since there's no data below ground), so expect that drop to be more like from 8-9 PM in Little Cottonwood.  Snow levels are usually a bit below the wet-bulb zero, so expect them to be in the area of 7000-7500 feet today, possibly higher in the afternoon with heating, and then drop to near bench levels after the frontal passage.  

The HRRR is calling for about 0.85" of water and 8.6" of snow at Alta Collins through 9 AM Sunday.  A bit less than half of this is high density snow that falls in fits and starts ahead of the front, including this afternoon, and the remainder is with the frontal passage.  Densities will be decreasing behind the front.  

The HRRR then shuts things down late tonight through tomorrow afternoon when things begin to pick up again.

The GFS is less enthused about the pre-frontal precipitation today but is in rough agreement on the frontal passage.  It's also a bit more active during the day tomorrow.  Through 9 AM Sunday it's at .55" of water and 7.6" of snow.  

So, expect some fits and starts of wet snow at upper elevations today and then steady snow with the frontal passage this evening and tonight.  I'm inclined to go close to the model water numbers on this with 0.5 to 1" of water and 6-10 inches of snow at upper elevations through 9 AM tomorrow.  

Given the warmth of the past week, I'm not overly optimistic that the frozen coral reef will be buried by tomorrow morning.  Things will need to go above those numbers probably to prevent a lot of bottom feeding.  If that doesn't happen, the better skiing tomorrow morning will probably be in lower angle terrain where the snow surface is currently fairly smooth.

That said, behind the front, unstable northwesterly flow looks to predominate through at least Tuesday and possibly Tuesday night.  The GFS time-height section shows this well.


Thus, expect more periods of snow Sunday through Tuesday, with the coral reef becoming more and more submerged and a return to more winter-like ski conditions in the upper Cottonwoods.

Wednesday, March 20, 2024

Beautiful Spring Weather

If you look carefully, you can see a couple of clouds in the Mt. Baldy web cam from Alta Ski Area this morning.

Source: Alta Ski Area

Those are not enough, however, to mar the incredible run of beautiful weather we have had since the demise of the easterlies on Saturday.  

Indeed, that run will continue today with clear skies and temperatures into the upper 60s in the valley.  If we can't have powder, this is a pretty good alternative.  

A weak short-wave trough will come through tomorrow and bring in a few more clouds and maybe a spritz of a shower or upper-elevation snow shower for the mountains tomorrow.  Significant weather will hold off until the weekend when a cold front is expected to push across northern Utah on Saturday.  Current forecasts suggest that the front will arrive in the afternoon.  Below is the ECMWF forecast valid 2100 UTC 23 March (3 PM MDT Saturday) with the front approaching Salt Lake City.


A real question mark for skiing will be whether or not that storm produces enough snow to bury the coral reef from the warm sunny weather this week.  We shall see.

I used the ECMWF above because I just upgraded the graphics produced for it on weather.utah.edu.  While I was in Austria, ECMWF opened up more of their data for free access.  We are now getting higher resolution data (0.25 degrees instead of 0.4 degrees), radiation fields that allow me to put together a synthetic cloud image (see upper right), vertical velocity (see upper left...smoothed to show the large-scale vertical motion), and additional wind and humidity information that allows me to better calculate integrated vapor transport (lower right).  

Sunday, March 17, 2024

Sastrugi

Along with graupel, sastrugi is one of my favorite weather-related words and I saw plenty of it ski touring yesterday.  

Sastrugi is defined by avalanche.org as "heavily wind eroded snow with wavy textures." Sometimes it looks rough or pockety.  In the photo below, it appears there is avalanche debris on this slope, but in reality it is all sastrugi.  

In some areas, the sastrugi was dense wind board and generally supportive of a skier. 

In others, it was actually somewhat soft and didn't ski to bad on the descent. Each turn was a mystery!

Although I like sastrugi as a word, it's not my favorite snow surface to ski.  We can blame the sastrugi in this case on the multiday easterly wind event that has been affecting the Wasatch Range and Front since Thursday.  Observations from Alta's Mt. Baldy show the winds veering (turning clockwise) from southwesterly just prior to 1200 MCT 12 March (Tuesday) to north by 0000 MDT 14 March and then locking in with easterly flow with gusts reaching over 50 mph on the 15th (Friday) when most of the damage was done. 

The large-scale setup for these winds was something that meteorologists call anticyclonic wave breaking in which a high-amplitude ridge develops in the high latitudes and leads to the formation of a closed low downstream and to the south.  Below is the GFS analysis for 1200 UTC 15 March during the period of stronger easterly flow on Mt. Baldy.  Note the ridge off the Pacific Northwest coast and the deep closed low centered along the CA-MX border, resulting in strong easterly 700-mb (crest-level) flow over the Wasatch. 


This is the same pattern that produced heavy snowfall along the Colorado Front Range. Basically, this is a complete reversal of the climatological westerlies.  They get upslope and we get downslope.  

The pattern has been very persistent and this morning enhanced easterlies ares still being observed along the east bench of the Salt Lake Valley north of Holladay, although they are weaker than at the peak.