Category Archives: Orographic Precipitation

About Last Night…

If you want blower pow, last night was your storm.  From 5 PM yesterday to 6 AM this morning, the Alta-Collins observing site picked up a paltry .32" of water equivalent, but 10" of snow.  That's a mean water content of only 3.2%!   That's a snow-to-liquid ratio of 31-to-1.  If you want a big dump, but don't have a lot of water to play with, that's how to do it. 

Really, conditions overnight were only marginally better for orographic precipitation generation than they were yesterday.  The 0000 UTC (5 PM MST Saturday) sounding still showed stable conditions at mid level with strong wind shear from 700 to 600 mb. 

The morning sounding is a bit better with northerly flow through depths and colder temperatures aloft, although a weak stable layer remained just above 700 mb. 

But the secret to the overnight dumpage wasn't large orographic precipitation enhancement.  There was some enhancement, but .32" of total water and a maximum water equivalent rate of 0.05" per hour isn't much.  The secret was the huge snow-to-liquid ratio.  If the snow-to-liquid ratio for this storm was Alta's average of 13-to-1, the snowfall would have been 4 inches.  But at 31-to-1 you get 10 inches and the stuff that Kodak moments are made of.  Yup, this was a snowfall that will bring smiles to skiers, but continue to give water managers, who need to see higher water content dumpages, heartburn. 

A Good Front Followed by Bad Orographics

A cold front moved across northern Utah last night bringing much needed snow from the valley floor to the highest peaks.  Powder panic brought gridlock to most (probably all) routes to the Cottonwoods, providing an all-too-frequent reminder that we are loving our canyons to death. 

Those who braved the traffic were rewarded with knee deep powder.  I would describe the skiing as good, not great.  A spongy layer of high-density snow to start would have helped reduce the bottom feeding, but this year, beggars can't be choosers.  

Snowfall produced by the front went largely as advertised by the models, at least in the upper Cottonwoods.  Alta-Collins had .59" of water and 7 inches of snow through 8 AM this morning.  This compares very well with the NAM forecast we discussed on Tuesday, which put out .64" of water and 8 inches of snow (see Frontal Snowfall Event on Tap for Late Tomorrow and Tomorrow Night).  The observed water totals are also near the middle of what was advertised by the University of Utah downscaled SREF ensembles.  The model wizards can be happy about this period.

I suspect that those hoping for a true storm ski day were a bit disappointed, however, with today's offering.  Light snow fell for much of the day, but since 8 am it added up to only .15" and 2 inches of snow at Alta Collins, which is probably a bit more than what we saw where we were ski touring in Big Cottonwood Canyon.  

Quite frankly, the orographic forcing today simply sucked, which we discussed as a possibility yesterday (see Probabilistic Snowfall Forecasting).  The morning sounding shows the situation quite well.  The atmosphere was quite moist, but also generally stable below 700 mb (10,000 ft), with a strong stable just above, which is associated with the front aloft.  The flow at low levels was northerly, but swung to southwesterly in the strong stable layer.  This is simply not a recipe for orographic enhancement.  

Source: NCAR/RAL
During the afternoon, the flow on Alta-Mt. Baldy slowly shifted to northwesterly to westerly, but remained weak and with high atmospheric stability, wasn't generating much at upper elevations.  

Instead, the radar loop below shows the development of the strongest echoes along the east bench and within the lower canyons through 0028 UTC (5:28 PM MST).  

Driving down Big Cottonwood late this afternoon, it was clearly snowing harder in the very bottom of the canyon and along the east bench than it was in the upper canyon.  Note that you can also see this effect in the Oquirrh Mountains where the radar returns, especially later in the loop are stronger on the lower and mid elevation western slopes.  

The devil is in the details.  

Big Snows in the Western Alps

By popular demand, here's a quick post on the big snows in the western Alps.

You may have seen news stories about this.  Huge snows in portions of the French, Italian, and Swiss Alps.  Thirteen thousand stranded in Zermatt, etc.

I couldn't find a good map of recent Alpine snowfall, so I snagged just a few notable numbers from ski reports online.  On Monday, 8 January, Tignes (France) reported 31" of snow, Sestriere (Italy) 31", Cervina (Italy) 10", and Zermatt (Switzerland) 20".  The next day, Tignes got another 31", Sestriere 39", Cervina 20", and Zermatt 0.  I'm not sure exactly where those obs are taken, and if the 0 at Zermatt might indicate no report, but there were significant snows in the western Alps.  There can be huge contrasts in snowfall across and along the Alps, but I haven't bothered to detail these small scale variations here.  I will say that snow-depth forecasts from for the period ending late on 9 January showed a pronounced max along the Alpine chain at the upper reaches of the western Po Valley catchment.  This would include Sestriere and Cervina.  Tignes and Zermatt sit just across the Alpine crest.

The setup for the event is shown below and features a digging upper-level trough that develops over western Europe and closes off over Spain and the western Mediterranean Basin before moving downstream.

In the interest of time, I've thrown together a map of the event, valid 1200 UTC 8 January, that includes sea level pressure (black contours), 925 mb (~750 m above sea level) wind vectors, and 925 mb temperatures.  At this time, the surface cyclone was centered roughly over the idland of Menorca in the western Mediterranean.  The cold front was ahead of the low center, passing over the island of Sardegna.  As a result, low level southeasterly flow over the Tyrrhenian Sea and west of the Italian boot impinged on the western hook of the Alps.  At the same time, there was strong flow blocking on the southern side of the Alps, resulting in a "u-shaped" sea level pressure ridge and easterly flow over the Po Valley.  As a result, southeasterly flow over the Adriatic Sea curved cyclonically and became easterly over the Po Valley, where it ran into the formidable western Alps.

This pattern persisted for a significant period of time and I suspect that the convergence of these two airstreams and strong orographic ascent of the easterly flow over the Po Valley were important contributors to the heavy snowfall.  In pink shading, I've added the are of strongest 700-mb (3000 m above sea level) ascent and not surprisingly there is a big bull's eye over the east side of the western Alps.

Below is a sounding from Milan, in the Po Valley, 12 hours later at 0000 UTC 9 January.  You can see the low-level southeasterly flow, which is remarkably strong, featuring a 25 m/s (50 knot) lo-level jet at about 950 mb.  The temperature and dewpoint traces show saturated or near-saturated conditions through the depth of the atmosphere, with conditions that meteorologists would describe as "moist neutral."  This is a recipe for strong low-level moisture transport to the western Alps, strong orographic ascent on their eastern slopes, and in all likelihood some embedded convection to juice things up and crank up precipitation rates. 

Source: University of Wyoming
A case like this deserves a more detailed investigation than I can provide here.  I'm sure the event undergoes important evolution over the period that I haven't had time to look into, and that the terrain effects are probably very interesting and impressive if one has access to higher-resolution precipitation/snowfall data.  If you dig in, please feel free to comment and share your insights.

A Day and Night of Great Utah Storm Chasing

Finally, a decent storm!  Our storm chasing activities this month were somewhat curtailed by Mother Nature.  Storms were limited, but we made the most of the three major opportunities we had, including our efforts yesterday and today.

Yesterday, we set up camp in the South Jordan Train Station parking lot for the frontal passage and post-frontal precipitation.

We operated here in 2011 when it was in the boondocks.  Although development is spreading westward (note the building behind the DOW), we were able to use it effectively yesterday, although we may need to find an alternative for future deployments.

There was a great deal going on and I suspect we have a great dataset for both an MS thesis and at least one paper.  Here are a few snippets.

The photo below from U undergraduate Spencer Fielding is taken facing ESE shows the situation shortly after the surface cold front has passed and was located just south of Point of the Mountain.  A pronounced cloud rope was evident above the frontal interface with the cloud structure suggesting ascent in the strong southwesterly flow over and downstream of the front.  It was black as coal over the Cottonwoods, but we saw very little on radar during this period.  Instead, the precipitation was falling further downstream (relative to the flow aloft) in the area east of Mill Creek Canyon.  This appeared to be a classic situation of precipitation growth, transport, and fallout with the growth happening in the ascent region, but the particles needing sufficient time to grow big enough to fall out, which happened further downstream.  It was unclear if the mountains really mattered at all in this period.  Making all of this easy to see was the fact we seemed to be in a rain shadow east of the Oquirrh Mountains.

That Oquirrh rainshadow was a prominent feature for a couple of hours after frontal passage.  The image below is a vertical slice taken facing northwest toward the Oquirrh Mountains (Grey region marked with an "OM."  The color fill is Doppler velocity, which measures the speed of the flow toward (cool colors) or away (warm colors) from the radar.  The northwesterly flow is clearly evident.  Note how it descends into the Salt Lake Valley southeast of the Oquirrh Mountains.

That descending flow is quite consistent with surface observations at the time which showed WNW flow at most sites along and east of the Mountain View Corridor (highway 85) in the western Valley.  This was about the time of the frontal passage at our location.

An hour later, the front had pushed through Point of the Mountain.  There was a clear boundary over the southwest Salt Lake Valley between the downslope westerlies and the along-valley northerlies.  Curiously, the downslope flow was colder than the northerly flow to the east.  Downsloping air warms compressionally, so for that to occur, precipitation must have cooled the airmass.

The photo below was taken shortly after the MesoWest analysis above and was taken facing NNW.  Here you can see quite well the dark, low-level clouds that accompanied the front into the Salt Lake Valley.  To the left, however, one can see snow (fibrous clouds) that is spilling over into the lee of the Oquirrhs.  The sublimation of that snow presumably contributed to the cooling of the airmass.  Note also how the subsidence has eroded the low-level cloud away on the west side of the photo.

Looking west, you can see this spillover really well in the radar reflectivity.  Note in particluar the shallowing of the echoes toward the radar due to the downslope flow.  One might think of this as a "snow foot."

Eventually, the cold air deepened and the flow aloft veered (turned clockwise) to westerly and eventually westnorthwesterly and we got quite a treat as the area around Little Cottonwood Canyon finally lit up.  One example is below, which is a vertical slice taken facing east directly up Little Cottonwood Canyon.  The bright yellow/orange colors are ground clutter produced by the sloping canyon floor.  Above that clutter, there's not much happening in the lower canyon, but from mid canyon up, there are strong returns, indicating heavy snowfall.

The photo below was taken about 2 hours earlier when it was still light, but shows perhaps what was happening with little or no precipitation falling at the base of the mountains and in the lower canyon (v-shaped notch in background to left of radar dish), with heavy snow in the middle and upper canyon.  

At times, away from the mountains, we also saw some beautiful fall streaks.

Google it and look at the photos and you'll know what I mean.

We eventually deployed to a site we could scan further north for the orographic and lake-effect precipitation that fell overnight.  I went home and tried to sleep, but it was largely a night spent looking at the radar and hoping we were getting great data, which we were.  You'll probably hear a lot of talk about the snow being lake effect, but in the mountains, most of it wasn't.  It was just good old mountain lifting doing the job until early this morning when we got a bit of lake effect.

We surveyed the lake-effect showers for most of the morning.  At one point, we decided to forget about doing anything except two rapid fire vertical slices taken through the lake-effect convection as it moved inland.  A still is below, but these slices were taken every 16 seconds.  I can't wait to process the data and see a video.

All of this brings to an end the OREO field phase.  Although storms were limited, we actually have some great data from three major storm cycles, and the students enjoyed a smorgasbord of precipitation and wind phenomenon.  

Special thanks to the National Science Foundation for sponsoring this visit and the Center for Severe Weather Research (CSWR) for making it happen.  The CSWR deserves extra special thanks for letting us keep the DOW a few extra days to catch this latest storm.

DOW7 Deployed in Huntsville for IOP1

DOW7 is now deployed in Huntsville for OREO IOP1.

The weather is perfect right now.  Radar has shown a quasi-stationary precipitation cell immediately downstream over the Wasatch crest and over our area.  For a while, it was noticeably clearer west of the Wasatch from our vantage point.

This is just the sort of thing that we're looking for and trying to better understand. Very few studies have examined lee-side precipitation processes.

Lessons from Orographic "Cumulus Patheticus"

After yesterday's dustpocalypse, the weather this morning seems relatively benign, but there's always something to be learned. 

Overnight and early this morning, orographically (i.e., mountain) forced cumulus clouds developed over the Wasatch Range. 

The unofficial name for such shallow cumulus clouds is "cumulus patheticus" as they are pretty wimpy compared to their cumulonimbus (towering clouds associated with rain and thunderstorms) brethren. 

Officially, they are stratocumulus clouds.  In this instance, lifting by the mountains appears to have been essential for their formation.  The GOES 16 satellite image for 1512 UTC (0912 MDT) showed the clouds were confined primarily to very near and downstream of the Wasatch Range, with lee waves generating wave-like clouds further downstream. 

This morning's satellite imagery shows that the clouds were confined to a layer between about 750 mb (about 8000 feet) and 550 mb (about 16,000 feet).  A pronounced stable layer with a base at 550 mb (16,000 feet) prevented penetration to greater heights. 

Source: SPC
The cumulus patheticus did produce some small snow pellets in the Avenues overnight.  I suspect there were a few snow showers in the central Wasatch as well.  

Shallow cumulus clouds like these can produce prolific mountain snowfalls under the right circumstances.  This morning, we appeared to be moisture limited given the large difference between the temperature and the dewpoint at low levels.  This, combined with the stable layer aloft, limited cloud depth and updraft strength.  More moisture at low levels would have likely enabled deeper clouds, stronger updrafts, and more rapid growth of ice crystals.   

Under such a scenario, snowfall rates of two ore three inches an hour are possible if the cloud exists at temperatures favoring the growth of dendrites, those wonderful 6-armed snowflakes that we all love.  

Orographic clouds do not necessarily need to be deep to be prolific snowfall producers, but they do need the right ingredients. 

Think of this the next time you're skiing, it's snowing hard, and yet you can make out the sun when you look up.  

Storm Chasing Doppler on Wheels Radar Coming to Utah!

A storm-chasing Doppler on Wheels (DOW) radar will be coming to the University of Utah in November for the Outreach and Radar Education in Orography (OREO) field program.  

The visit of the DOW, operated by the Center for Severe Weather Research and supported by the National Science Foundation, will give University of Utah students a hands-on education in radar operations and interpretation, mountain and lake-effect precipitation processes, and the use of mobile observing platforms for field research. 

The DOW last visited campus in fall of 2011.  We had a field day, observing everything from intense fronts pushing through the Salt Lake Valley to lake-effect bands pushing into the Wasatch Range. 

During the visit, atmospheric sciences graduate students will plan and lead several DOW field deployments and educational activities for majors and non majors.  We are also planning a major public display of the DOW.

More information on these activities will be forthcoming in the next couple of weeks.  Stay tuned!

Snowpack Extraordinaire: North Ogden Valley

As many of you are aware, the microclimate of the North Ogden Valley holds a special place in my heart (see Pound for Pound the Snowiest Place in Utah).

Source: Secrets of the Greatest Snow on Earth
Every time I go there in the winter or spring, I'm blown away by the snowpack.  Today I needed to get some work done, and battling the weekend crowds in the Cottonwoods wasn't very inviting, so my son and I shot up to the Ogden Valley for some skinny skiing at the North Fork Park trail system maintained by Ogden Nordic.

It's hard to believe that North Fork Park is at an elevation of about 5800 feet.  I check the map every time I visit there.  The snowpack is remarkably robust, blowing away anything at a comparable elevation on the back of the central Wasatch.  The scene today was a winter wonderland.

I didn't measure snow depth anywhere, but the Ben Lomond Trail SNOTEL site is nearby and at an elevation 5820 ft.  For 24 February, the median snowpack water equivalent is a robust 17.8 inches and we are running well above that this year with 27.9 inches (this is not a record for the date).

To put those numbers into perspective, they are not that different from the Mill-D North SNOTEL at 8967 feet in Big Cottonwood Canyon, which has a median on 24 February of 19.1 inches and currently sits at 25.6 inches.

If free and clear from traffic, the drive to North Fork Park from downtown Salt Lake City is an hour and five minutes, not that long at all.  I usually take the slightly longer Trappers Loop route up the backside of Snowbasin to enjoy the views.

Of course, despite opting to go up to North Fork Park in part to avoid the Cottonwood congestion, we still got snarled thanks to a bad accident along I-15.  I can't win!

On the plus side, it did give us some time to check out some beautiful cloud formations over Snowbasin. Note in particular the transition from orographic convection at low levels, indicative of unstable air, to wave clouds aloft, indicative of more stable flow.  Note also how those wave clouds were only produced over the higher terrain and are not evident over Ogden Canyon.

How those differing clouds interact to produce the snow that fell today over Snowbasin and the Ogden Valley is a question I'll ponder tonight.

Weak Eastward Penetration during Yesterday’s Refresh

Snow totals in the upper Cottonwoods were generally in line with expectations with about 5 inches falling.  An exception was the Park City Ridgeline and Brighton area which came in with perhaps 3 inches according to the Utah Avalanche Center.

For much of the day yesterday, radar echoes were strongest over western portions of the central Wasatch, with echoes weaker along the Park City Ridgeline and in the Brighton area.  You can see this fairly well in the radar plot below.  I suspect this reflects the relatively weak flow associated with the trough yesterday.  Easy to diagnose in hindsight.  Much more difficult to anticipate in advance, mainly because such precipitation distributions are highly sensitive to many factors, including storm depth.

Overall, the NAM comes out looking good again.  No change in my view of the NCAR ensemble, which I think is extremely valuable, but for the most part, it overdid it yesterday.

We may see a few snow showers today, but accumulations will minimal.  Another modest refresh tonight.

For yesterday's refresh, Snowbird gets my hype-of-the-year award for the tweet below.

Wonderful Orographic Effects

This morning's radar loop shows some wonderful orographic effects over northern Utah.  First, let's get oriented with a terrain map showing the areas of interest which include the Oquirrh Mountains (lower left box) northern Wasatch (top box), and central Wasatch (lower right box).

With the large-scale flow out of the northwest, one can see persistent and strong radar returns over the Tooele Valley to the northwest of the Oquirrh Mountains, upstream of the northern Wasatch, especially the Bountiful Bench Area, and upstream of the central Wasatch.  Note the lack of echoes ("rain" shadowing) downstream of the Oquirrhs over the southwest Salt Lake Valley, and downstream of both the northern and central Wasatch (note: there is some blockage of the radar beam in these areas too, but there are also clear "rain" shadow effects).  

Looking at a single image can sometimes help.  If you look carefully at the central Wasatch, you can see enhancement on the high ridges surrounding the Cottonwood Canyons, including Lone Peak, Twin Peaks, and the Mt. Olympus areas.  It is probably snowing harder in those areas than the upper Cottonwoods when that scan was taken.  

I suspect that this morning we have a case of an "upside down" storm in which snowfall rates (in terms of water equivalent) are higher along the base of the mountains than on the summits.  Snowfall rates might also be higher in the lower canyons than the upper canyons.  That wasn't the case for the entire storm, as the mountains certainly got the goods yesterday and overnight, but it appears to be the case this morning.

Ho Ho Ho...