In the Red

Today's basin-wide snowpack snow water equivalent analysis is quite remarkable.  Much of the interior western US is in the red, signifying <50% of the 1971-2000 average.

Most impressive are the low numbers in Utah and Colorado where there are many high elevation basins that are typically just past peak snowpack in late April.  Observations from these states show only a few sites above 50% of average snowpack SWE.

At the Spud Mountain SNOTEL in the San Juan Mountains (10,660 ft), the snowpack was completely gone a few days ago.

The few sites in the San Juan Mountains that are above 50% of average snowpack SWE are all at or above 11,000 ft.  At Wolf Creek Summit (11,000 ft), snowpack SWE was running near average through February, when Mother Nature put on the breaks for storms and flicked the warming switch.  At this high altitude location, the average peak snowpack SWE occurs around May 1, but this year it appears like they are looking at a late March maximum.  

In northern Utah, low elevation locations such as the Parley's Summit SNOTEL (7500 ft) are now snow free.  

At higher elevations, the storms between heat waves have enabled the snowpack to cling to a precarious existence, at least on northerly aspects, as illustrated by the Snowbird SNOTEL.  

This looks like a good year to plan a backpack or peak-bagging trip in places like the Uintas and Colorado Rockies where in many years snow tends to linger and block high elevation routes well into the summer.  Baring a major pattern change, it will be a very early hiking season this year.  

That's All She Wrote

Today was the last day of lift served skiing at Alta.  My son coaxed me up for a few runs as he hates to miss the last day.  It's been a sub par year during which I have struggled mightily with a balky back, but we still had fun and made the best of it.  Here's to hoping next year is better.

A highlight of the day was spotting these old Olin Comp SLs in the lift line.

I actually sold a couple pair of these back in the day (1980s).  At the shop I worked at, we got a $10 commission on these babies, instead of the $1-$2 we got for other boards.  They were the Comp $L instead of the Comp SL for a starving college student.

As the season concludes, I'm amazed that problems persist with the rascally facets (a.k.a. depth hoar) that developed earlier this year.  At least two slides during last week's heat wave failed on the old depth hoar (see here, here, and here).  I didn't bother checking out this slide that pulled out above the entrance to Ballroom, but it was a good reminder that the green light has never really been on this year.  

This may be the slide mentioned in the avalanche report on Friday morning (triggered by control work).   The UAC remains concerned about large, unsurvivable avalanches in steep north facing terrain.  Apparently the green light will remain off until the snow is gone.

Eye Candy of the Day

Beautiful loop above of a developing upper level trough as it exits the Intermountain West and moves over the Great Plains today.  This is an infrared satellite image, but the wavelengths sampled are strongly influenced by the distribution of water vapor and temperature in the upper troposphere (usually above about 6 km).  As a result, it only sees the highest clouds and color contrasts in other areas reflect differences in water vapor and temperature.  The protrusion of red extending into central Kansas at the end of the loop is in the heart of what is typically called the "dry slot," where the upper levels have very low relative humidities.    In addition, flow around the developing low has also produced a beautiful isolated region of what is probably locally dry air in the upper troposphere over western Kansas.  Call it the eye of the storm if you will, although tropical cyclone aficionados will likely grimace.

Is This Lake Effect?

With rain pelting my window and a cool, moist airmass in place, I slept like a baby last night.  In the mountains the storm delivered largely as advertised, laying down about 6" at the Alta-Collins site overnight, although snow water equivalents (SWE) were a bit higher than expected, totaling just over an inch.  

Some of the SWE fell as rain early in the storm.  In fact, as the front moved in, snow levels looked to be near 11,000 feet.  About .35 inches of SWE fell before temperatures at Alta-Collins fell to below freezing.    

The storm is now much colder with snow showers continuing over the Cottonwoods.  Fairly strong west to west-northwest winds are blowing at crest level, with gusts over the last 3 hours on Mount Baldy in the 60–70 mile per hour range.  Looking at the latest radar loop, I can't help but ask the question, "is this lake effect?"

Precipitation is falling mainly over high topography downstream of the Great Salt Lake.  That is an area where the terrain is higher than the terrain to the north, so perhaps this is just the result of the deeper lift.  On the other hand, maybe lake influences are also important.  

MesoWest data shows a very nice confluence zone over the northern Salt Lake Valley.  The flow over the northern Wasatch front has a bit more of a northerly component than the northwesterly flow over the Great Salt Lake.  Note also the westerly flow near the Saltair Marina just north of the Oquirrh Mountains.  

This could be an instance where both lake and mountain effects are working synergistically to produce the precipitation, as suggested in recent research by Trevor Alcott here at the U.  There seems to be a class of "lake-effect" events where both lake influences (which warms and destabilizes the airmass over and downstream of the lake) and convergence related to blocking by the northern Wasatch Mountains (i.e., the northerly flow evident above which is convergent with the northwesterly flow over and downstream of the lake) are necessary for precipitation generation.   Blocking by the Oquirrh Mountains can further contribute to this "funneling" effect.

So, is this lake effect?  Maybe.  I confess that I'm not sure how to best categorize it, but we'll figure it out eventually.  

Thar She Blows

As is often the case over the Intermountain West, we're seeing a nice surface frontal boundary develop during the day over eastern Nevada and southern Idaho.  Although a front was largely indistinguishable this morning, note the clear wind shift and contrast in temperature that currently exists from the southern Snake River Plain to near Elko Nevada.

Frontal passage at Jerome, Idaho was quite apparent in the meteogram as temperatures were climbing in the pre-frontal environment, but dropped abruptly with frontal passage.

I suspect we'll see further frontal strengthening (in terms of the abruptness of the temperature contrast) over the next few hours.

Kudos to MesoWest for collecting and integrating surface observations from all over the western United States.  They are indispensable for frontal analysis!

Valley Rain, Mountain Snow

Things are starting to get interesting to our west.  The regional radar shows scattered showers, some fairly heavy, over Nevada and Idaho.

A close up from the Boise radar shows the linear nature of some of the stronger precipitation features.

At the moment, however, there is no well defined surface front in this area, although one may develop during the day today over eastern Nevada.  I think we'll see a better organized front with a sharper airmass transition and wind shift develop later today.

Looks like we will pick up a decent amount of valley rain with the frontal passage, and it is badly needed.  There's a chance we may get some showers ahead of the front as well if we can bubble up some pre-frontal convection.  Thunder and lightning?  Perhaps.  

Mountain snow is also in the cards, but snow levels look to be quite high.  The forecast 700-mb (crest-level) temperature is around -1ºC as the front comes in this evening, and falls to about -6ºC late tonight as the frontal precipitation tapers off.

Thus, snow levels will probably start out near 9000 feet just ahead of the front and fall to perhaps 5500 feet later tonight (if the precipitation hangs).  Given how productive the storm is upstream, water totals in the mountains tonight might be in the range of .4–.8".  Snow accumulations will depend very strongly on elevation due to the warmth of the storm.  Given that the snow will probably be high density, perhaps 4–8" will fall above 9000 feet in Little Cottonwood.

There is some potential that we will see lake-effect late tonight and tomorrow morning, which would add to these anticipated totals.  The NAM forecast time-height section shows a layer of moist, potentially unstable flow below about 600 mb at 1200 UTC (0600 MST).

Further, the lake is quite warm given our recent "heat wave."  I don't have a recent lake-temperature estimate, but the mean satellite-derived lake temperature analysis for April 15–21 showed a range of about 11–14ºC.

It is probably warmer than that after a few more days of warm temperatures.  The expected temperatures, humidities, and lake temperatures yield a 50–70% likelihood of lake effect forming somewhere downstream late tonight and early tomorrow morning.  Note that is just the likelihood that we see some lake-effect precipitation features develop during that period.  Whether or not these features are persistent and produce accumulations is anyone's guess.  Most lake-effect events do not produce large accumulations, but occasionally they do.  Unfortunately, we have little skill at predicting lake-effect intensity, duration, coverage, and location at these long lead times.

Let's Rock

The next couple of days look to be quite interesting for northern Utah.  After three days of record breaking maximum temperatures (the 86ºF, 88ºF, and 86ºF observed at KSLC on Sunday, Monday, and Tuesday were all records for their respective dates and close to the all-time record high for April of 89ºF), change is coming.

A deep surface and upper-level trough has developed off the coast of California and has tapped into moisture from the tropics and subtropics.  This moisture has streamed northward and is now pushing into southern California and the desert southwest.

1200 UTC 22 Apr – 1200 UTC 25 Apr 2012 IR satellite loop with GFS
925 mb (~750 m MSL) wind vectors and integrated water vapor

We're near the end of the wintertime "rainy season" for that part of the world, but they will get some, especially later today and tonight as the upper-level trough sweeps across the region.  

Places like Los Angeles and Las Vegas will see rain tonight, and panic will surely ensue amongst the locals...

Northern Utah should get into the action as well.  We have high clouds draped across us this morning in advance of the system, but as can be seen in the images above, moisture will be on the increase tonight and tomorrow.  By late tomorrow a cold front is encroaching on Salt Lake City.

This looks to me like a good recipe for showers and thunderstorms.  Snow levels will drop following the frontal passage, and there will probably be accumulating snow in the Wasatch Mountains Thursday night.  

I'm looking forward to some interesting spring weather.  All drought and no rain makes Jim a dull boy.  Let's Rock!

2012 v. 2011

What a difference a year makes.  The contrast between last spring and this spring is staggering.  At Alta-Collins for example:

23 April 2011: Snow depth 181 inches, maximum hourly temperature 35ºF
23 April 2012: Snow depth 90 inches, maximum hourly temperature 59ºF

Need I remind the skiers out there that the Alta-Collins snow depth reached a maximum of 207" on April 30th last year.  207"!  Further, the snowpack didn't drop to 100" until July 2nd.  In fact, there was more snow at Alta-Collins on July 4th last year than there is today.

Utah skiers have experienced close to the full range of the Wasatch snow possibilities in just two years.

A Look at Snowpack Trends

We've seen bad and good snow years over the past decade in the
Wasatch Mountains.  Is there a trend?

Detecting and determining the cause of snowpack trends over the western United States is a difficult task.  There are a number of reasons for this including:

• The lack of reliable, continuous snowpack and snowfall records in the early 20th century.  As a result, most analyses begin in about 1930 or 1950, which is a relatively short period.  Further, throughout the historical record, snowfall and snowpack observations have their warts, which complicates quality control and trend attribution.  

• There are large, natural year-to-year and slowly varying decade-to-decade variations in climate and snowpack characteristics, which complicate trend analysis.  

• Non-climatic factors can contribute to the observed trends and variations, including changes in observational techniques or locations, land cover (e.g., increased tree cover), and land use (e.g., increased snowmobile traffic).  

• Long-term trends and variations can also be related to anthropogenic factors besides greenhouse gas emissions, including land-surface change and dust loading.  The latter increases the absorption of solar radiation, leading to an earlier and more rapid snowmelt.    

Recently, the Salt Lake Tribune reported on a soon-to-be published study by Gillies et al. (2012, paywalled, so only the abstract may be available for free to some users) examining snowpack trends in Utah, which was followed by an hour long discussion on KUER (podcast available here).  Given the recent interest in this topic, I thought I would review current understanding in this area.  I'll be wading into shark infested waters, but I will try and summarize the science as best as I can.  

The Trends

For simplicity and because it should be available for free public access, I'm going to concentrate on Pierce et al. (2008), which is probably the most comprehensive paper on western US snowpack trends and their causes.  They examine trends in the ratio of the 1 April snowpack snow water equivalent (SWE, a measure of how much water is in the snowpack) to the water-year-to-date (October to March) precipitation (P).

Using this SWE/P ratio has several advantages, but perhaps the most important is that it reduces the effect of variations in precipitation on the results.  Essentially, the SWE/P ratio tells you how much of the precipitation that has fallen since October is still present in the snowpack on 1 April.  

Results are presented for various regions and elevations for the contiguous western United States and concentrate on sites where "an appreciable fraction of the winter precipitation is retained in the snowpack on 1 April."  

Source: Pierce et al. (2008)

For comparison purposes, Pierce et al. (2008) divide each year's SWE/P by the long-term average for the study period (called fractional SWE/P).  A fractional SWE/P of one indicates that an average mount of water year precipitation is retained in the snowpack on 1 April.  A fractional SWE/P greater than one indicates that an above average fraction is retained, whereas a fractional SWE/P less than one indicates than a below average fraction is retained.

One of the more important figures in their paper presents the fractional SWE/P trends by elevation band, which shows the largest declines in the lowest elevations.  The downward trend, however, decreases with elevation and, above 1910 m, the trends are small and no longer statistically significant.   Keep in mind that these are cumulative statistics for all the stations indicated above, so there may be some variations by region.   

Time series of fractional SWE/P as a function of elevation with least-squares fit
trend line shown as solid if the trend is significant at the 95% confidence
interval and dashed if it is not (Source: Pierce et al. 2008, with figure
caption paraphrased from their text and Fig. 5).

My interpretation of Gillies et al. (2012) is that they found a similar influence of elevation in Utah.  In particular, they conclude that, "it was estimated that the proportion of winter (January-March) precipitation falling as snow has decreased by 9% statewide over a half century, with greater reductions occurring at lower elevations (< 2000 m)" (my emphasis).  

Attribution

What has caused these declines in low-elevation snowpack?  This is a challenge as the climate of the western United States is influenced not only by long-term climate trends, but also variations in Pacific sea surface temperatures and related large-scale atmospheric circulations (examples include El Nino and the Pacific Decadal Oscillation).  

Pierce et al. (2008) use climate model simulations to evaluate the relative roles of natural climate variability and climate change related to anthropogenic greenhouse gas and aerosol emissions.  Their results suggest that roughly half of the trend is due to anthropogenic warming.  

One study that suggests the anthropogenic influence may be smaller, at least in one region of the contiguous western United States, is Stoelinga et al. (2010).  They found that the spring snowpack in the Cascade Mountains declined 23% from 1930–2007 (a longer period than examined by Pierce et al. 2008, although the trend is not quite significant at the 95% level).  For the 1950–1997 period, they find a 48% decline, but argue that about 80% of this is due to natural variability.  

Summary

The detection and attribution of snowpack trends in the western United States is challenging.  It is my conclusion based on recent studies (including many not discussed above) that there have been long-term declines in snowpack over the lower elevations of the western United States, but that these declines decrease with elevation and trends at upper elevations appear to be negligible.  It is likely that anthropogenic warming contributes to the low-elevation snowpack decline, but that natural climate variability may also be playing a role, at least in some regions.  

Evaluating the contribution of anthropogenic climate change, natural climate variability, dust loading, and land-surface change is an area of ongoing research.  In addition to improving our understanding and prediction of past and future long-term snowpack trends, this is an important interdisciplinary research problem that is very relevant for improving seasonal, year-to-year, and decadal scale climate forecasts for water resource management.

Earth Day Scorcher

It was a spectacular day yesterday in Utah.  Temperatures were well above average, with the 80ºF high at the Salt Lake City International Airport.  The unofficial maximum temperature at the base of Alta was 64ºF.

Overnight temperatures at Alta were about 5ºF warmer than Friday night.  The window for decent corn will probably be earlier and shorter today than yesterday. 

At the Salt Lake City International Airport, today's record high is 83ºF.  We have a good chance of equaling or topping that as it should be a couple degrees warmer today.  In case you are wondering, the all-time record high for April is 89ºF.  

Keep the Skis Waxed

Not only because spring snow is slow, but also because the models are hinting at a possible return of winter late next week.

The Cottonwoods Are Utah's Powder Shangri-La

"Skiers will eventually find that the Brighton Basin, or the heads of the [Cottonwood] canyons within a short radius of this winter paradise, offer the best skiing to be found in the Wasatch Mountains."

- S. D. Green, U.S. Weather Bureau Meteorologist, 1935

Today we conclude our discussions of the snow climates of northern Utah by answering a question that I am frequently asked.  Is there a place in Utah with a better climate for deep-powder skiing than the Cottonwood Canyons?

No.  Or maybe better put given the uncertainties, probably not.  

The S. D. Green quote above pretty much sums up the situation.  Green was the original Wasatch Weather Weenie, a meteorologist and an avid backcountry skier.  Some of his photographs are part of the ski archives in the University of Utah Marriott Library.  He and the early ski pioneers recognized pretty quickly that there was something special in the upper Cottonwood Canyons.  

S. D. Green photo of skiers near the Wasatch Mountain Club lodge,
Brighton, UT.

We'll answer the question by looking at the available data.  Since extensive snowfall observations are not collected throughout the Wasatch, we'll rely on the PRISM digital precipitation analyses produced by Oregon State University and SNOTEL observations.  

The PRISM analyses suggest that the annual precipitation in the mountainous areas of northern Utah is greatest in two areas.  One is in the Cottonwood Canyons (southernmost red box), the other is on Ben Lomond Peak in the northern Wasatch (northernmost red box).  

Average annual precipitation (inches) in the mountainous regions
of northern Utah.  Source: PRISM Climate Group, Oregon State University 

This analysis is of course not perfect.  It doesn't fully resolve terrain effects and their are data gaps, but we can say that it is likely that the Cottonwoods and Ben Lomond Peak (and adjoining Willard Peak) are the wettest locations in northern Utah.  Further, while this is annual precipitation, most of the precipitation that falls over our mountains falls during the cool season.  At Alta, for example, 70% of the precipitation falls from November to April.  Therefore, the spatial pattern in the mountains above is strongly influenced by precipitation during the cool season.

Next, we'll bore into the climatology of the Cottonwoods and Ben Lomond.  I'm going to use SNOTEL data, which isn't perfect, but should suffice.  At the Snowbird SNOTEL, the snowpack SWE peaks at about 44", whereas at the Ben Lomond SNOTEL, it peaks at about 40".  

Source: Colorado Basin River Forecast Center

The data periods at these two sites are, however, different (Ben Lomond goes back to 1979, but Snowbird only back to 1990), so this is a bit of an apples and oranges comparison.  In addition, the Ben Lomond site is located at only 8000 feet, compared to 9640 feet for Snowbird.  Nevertheless, the evidence suggests that Ben Lomond is a fairly snowy place, and, at a given elevation, possibly as snowy or even snowier than the Cottonwood Canyons.  I have taken groups of students up Ben Lomond during the spring and the snowpack near the SNOTEL site is usually quite impressive.  I call it "pound for pound the snowiest place in Utah", which means for its elevation, it is the champ.

But, Ben Lomond is not the undiscovered deep-powder Shangri-La for a few reasons.  First, it only reaches to 9700 feet.  The terrain around the Cottonwoods is higher, reaching over 11,000 feet in places), which helps make up for contrasts in precipitation at a given elevation and results in a colder climate that better preserves powder.  Second, the storms at Ben Lomond are typically monsters that occur during southwesterly flow.  This gives a big snowpack, but often creates high avalanche hazard and is not optimal for a high frequency of powder days.  Finally, I've skied on Ben Lomond many times and it is an extremely windy place on the ridge and above timberline.  Finding good powder in the alpine requires good fortune.  

Thus, while there is good skiing to be had throughout northern Utah's mountains, the closest thing to powder Shangri-La really is the Cottonwood Canyons.  As Don Henley sang for the Eagles in The Last Resort, "there are no more new frontiers, we have got to make it here."  

The Oquirrh/Stansbury Snow Climatology

Sharp Peak in Utah's Oquirrh Mountains

A few readers have contacted me asking about the snow climate in the Oquirrh and Stansbury Mountains west of Salt Lake City.  The Oquirrh Mountains flank the western Salt Lake Valley, reaching elevations of up to 10,620 feet, while the Stansbury Mountains are the next range to the west and reach 11,031 feet at the summit of Deseret Peak.

To my knowledge, there are no long-term, publicly available snowfall records from the upper elevations of either mountain range.  It is possible that Rio Tinto/Kennecott has such records as they have explored ski area development on their land in the Oquirrhs at various times over the past few decades, most recently a few years ago (e.g., here, here).  Others have proposed resorts in the Oquirrhs as well (e.g., here).  

SNOTEL observations from the Rocky Basin site at 8900 feet in the central Oquirrhs show that average snowpack SWE reaches a maximum of about 25 inches in early-mid April (blue line).

That's about 40% lower than the 42" average at the Snowbird SNOTEL (which is a bit higher at 9640 ft), but comparable to the 25" average at the Thaynes Canyon SNOTEL at 9200 feet at Park City Mountain Resort.  The only SNOTEL in the Stansbury Mountains, Mining Fork, has an average maximum of only 19", but is located at 8000 feet.  These observations suggest that the snowfall in the Oquirrh and Stansbury Mountains is lower than found in the Cottonwood Canyons, but comparable to that on the Wasatch Back near Park City.  

I have backcountry skied in the southern Oquirrhs and the Stansbury Mountains near Deseret Peak and it is my impression that the snowpack is perhaps 50–60% of that found at comparable elevations in the Cottonwood Canyons.  In addition, in some areas the upper elevations of these ranges are more open and wind affected.  

Ski touring in the Oquirrh Mountains
Photo Credits: Tyler Cruickshank

During January of last year, we found wind-jacked snow in and above the Twin Couloirs on Deseret Peak in the Stansbury Mountains, but nice settled powder at lower elevations.  

Given that they are narrow, I don't believe the contrast in snowfall between the west and east sides of these ranges is significant, but that's purely speculative.  The central and northern Oquirrh Mountains receive more lake effect, but the southern end is broader and higher, which might result in more snow in other storm types.  

To summarize, there is snow in the Oquirrh and Stansbury Mountains, but less than found in the Cottonwood Canyons.  Wind is a bigger issue in these ranges as well, although there are wind-shelted areas that can provide the goods when conditions are right. 

Backside Climatology

In an earlier post, we discussed some of the remarkable weather contrasts that existed between the Cottonwood Canyons and the Wasatch Back during the day on Sunday.  This included a pronounced cloud shadow that formed over the Wasatch Back and Park City Mountain Resort (PCMR), as well as the occasional spillover of precipitation across the Wasatch Crest.  Also, it was readily apparent that the snowpack at PCMR was substantially thinner than that found in the Cottonwood Canyons.

Dorothy, we're not in Little Cottonwood anymore.

Although the lack of snow at the base of PCMR on Sunday is partly related to elevation (the area pictured lies between 7000 and 8400 ft) and aspect (east to northeast), even on elevations and aspects similar to those at the resorts in Little and Big Cottonwood Canyon, the snowpack was markedly thinner.  This is confirmed by SNOTEL observations from Sunday at Snowbird in Little Cottonwood Canyon (31.5" SWE and 78" snow depth @ 9640 ft) and Thaynes Canyon in PCMR (14.5" SWE and 44" snow depth @ 9200 ft).

Why this contrast?  To begin, it's helpful to first take a detailed look at the topography of the Wasatch Mountains.  Although the Wasatch Mountains are a fairly narrow range that run roughly north-south, the central Wasatch Mountains (large box below), which encompass Big Cottonwood Canyon (BCC), Little Cottonwood Canyon (LCC), and the Park City Ridgeline, is unique in two important ways.  First, the central Wasatch are broader than the rest of the Wasatch Mountains, such as the area around Snowbasin (small box).  Second, although the Park City ridgeline forms the hydrologic divide, the highest terrain is located to the west along the three west-east oriented ridges that rise above Big Cottonwood Canyon and Little Cottonwood Canyon.  Collectively, this forms an "island of high topography" that is the broadest and most substantial in the Wasatch Mountains (a couple peaks reach higher to the south, but they lack the width of the central Wasatch).  Thus, the highest terrain is actually upstream of the Park City ridgeline during many storm periods.

Although there can be storm periods during which the flow is southerly, southeasterly, or easterly and the Park City resorts benefit from temporarily being on the windward side of the Wasatch Mountains, during most storm periods, they are on the leeward side.  Thus, the mean annual snowfall is about 33–40% lower at PCMR than found at comparable elevations in Little Cottonwood Canyon.

Estimated mean annual snowfall as a function of elevation in Little
Cottonwood Canyon (LCC, blue) and Park City Mountain Resort (red).
Derived with clever wizardry using PRISM precipitation analyses
and data available from the Western Region Climate Center.    

This is consistent with snowpack SWE observations from the Snowbird and Thaynes Canyon SNOTEL stations, respectively, which peak at an average of about 42" and 25", respectively (a difference of about 40%).

The height and breadth of the central Wasatch topography contributes to this contrast.  Where the Wasatch are narrower, such as around Snowbasin, more precipitation generated on the windward side of the mountains spills over into the lee.  This is one reason why the average snowfall and snow depth at Snowbasin is greater than found in the Park City area.  

Making all this even more remarkable is the snowfall contrast between Alta and Park City occurs over a distance of less than 15 km.  

A Good Rain

A weak baroclinic trough is moving through this morning, but it has produced a nice period of steady rain over northern Utah.  Note how the radar coverage filled in nicely as the system approached Salt Lake City, giving us a good soaker.

With fairly continuous radar coverage, one can see some of the areas where the radar is partially blocked by the topography, leading to lower radar reflectivities (see red lines) farther from the radar.

Unfortunately, this is one of the challenges when operating a radar in complex terrain.

Backside Meteorology

The Wasatch Weather Weenies don't spend much time on the Wasatch Back during winter.  We like being in the Cottonwood Canyons where there's a greater variety of storms and a deeper snowpack.      However, thanks to some free lift tickets from a friend, we ran laps yesterday at Park City Mountain Resort (PCMR) where we got a great look at some of the leeward effects of the Wasatch Mountains.

As shown by the 1800 UTC (1200 MDT) 15 April 2012 NAM analysis, yesterday featured large-scale northwesterly flow at crest level and through most of the troposphere.

This led to the development of low clouds over the western (windward) slopes of the Wasatch Mountains, including the Cottonwood Canyons (in lower left-hand corner of red box below).  In contrast, a pronounced cloud shadow was found downstream of the Wasatch Crest (right side of red box).

Early in the morning, this led to beautiful sunny skies at  PCMR, with low clouds hanging over the Wasatch Crest to the west.  

Later in the morning, perhaps due to surface heating, the contrast was less abrupt, but still evident from the top of Jupiter bowl near the Wasatch Crest.  Looking southeastward across upper Big Cottonwood Canyon, it was mostly cloudy.

In contrast, looking eastward toward Park City, it was partly sunny with a pronounced cloud-free hole over the lowlands between the Wasatch Mountains and the Uinta Mountains further east.

In the early afternoon there was a nice example of what we call precipitation spillover.  Looking northward, shallow cumulus clouds were well developed and producing snow over the Wasatch Mountains.

The snow was subsequently carried eastward by the prevailing flow and "spilled over" into the lowlands around Park City.

This provides a visual example of what happens in more substantive Wasatch Mountain snowstorms.  Precipitation is generated as air is forced upwards over the windward side of the Wasatch Mountains.  However, some of the precipitation is carried downstream by the prevailing flow and spills into the lee. Precipitation typically decreases as one moves downstream of the mountain crest (there are some exceptions depending on storm characteristics and terrain size and shape).  Contributing to this decrease in precipitation is the warming and drying of the air as it sinks over the lee slopes, which results in evaporation and sublimation.  For example, in the images above, the clouds never penetrate eastward over the lowlands, despite the prevailing northwesterly flow because they are comprised of small droplets that evaporate quickly.  Only the larger snowflakes and ice crystals generated within the clouds  survive long enough to fall out on the lee slopes.

In a future post, we'll take a look at how these processes affect the average snowfall and snowpack across the Wasatch Mountains.