Tonight's Downslope Windstorm: Don't Be Complacent

I'm pretty blown away (pun intended) by the model forecasts for tonight and tomorrow (Sunday) morning.

The 0600 UTC NAM is generating strong easterly flow across the northern Wasatch Front with 700-mb (near crest-level) winds reaching 50 knots in places.  There is also a strong pressure gradient from Evanston to Ogden.  Coming overnight and in the early morning, these are ingredients for a strong downslope wind event.

The National Weather Service has issued a high-wind watch for the Cache Valley and northern Wasatch Front for sustained winds between 30 and 40 mph with gusts in excess of 60 and localized gusts in excess of 75 mph in some areas, especially Davis and Weber Counties.

Take the time today to secure loose outdoor items if you are in areas affected by these downslope winds and monitor forecasts at http://www.wrh.noaa.gov/slc/.

This Is the Biggest Storm Cycle of the Season

Here's one for you.  When did we have the biggest storm cycle of the ski season?  December?  January? February?

Nope.

The answer is right now, and it isn't even close. 

The chart below shows the running 5-day accumulation of water equivalent at the Snowbird SNOTEL station since October.  In terms of water equivalent, the 5-day accumulated total through today is 4.3 inches.  The next highest is December 25th with 2.8 inches.

All I can say is what an event, especially Tuesday and Tuesday night when we picked up 16 inches and about 2.25 inches of water at Alta-Collins.  Although snow was in the forecast, that was impressive.

And we got a bit more last night and will get a bit more today.  Note the inexact terms "a bit more."  I'm feeling beaten and battered by these closed lows and their whimsical precipitation patterns.

Winter Rises from the Dead

Source: Rankin-Bass

Just when you thought winter was over, it rises from the dead.

A whopping 16 inches at Alta-Collins in the last 24 hours has pushed the total snow depth up to 115 inches, the deepest since late March.  

That 16 inches contained 2.27 inches of water.  That's a water content of 14%.  Thick and creamy, but I bet anyone out this morning won't be complaining.

The pattern remains showery and unpredictable for the next few days, but don't wait.  The best time to get good turns this late in the spring is when it is snowing.  

It Really Wasn't a Very Good Snow Year

Although April 1st is commonly used as a key date for Utah snowpack, in the upper elevation north facing terrain in the Wasatch Range, the peak snowpack water equivalent isn't usually reached until late April.

It's late April now and the results are clear.

It really wasn't a very good snow year.

Oh, you probably had some good skiing.  There were a few good stretches and 2015/16 was certainly better than 2014/15, but if we're interested in snowpack at the end of the snow accumulation season, this year fell well short of par.

Let's take a look at the Snowbird SNOTEL.  Median (magenta line) and average (blue line) snowpack water equivalent both peak in late April at about 44 inches.

In contrast, this year (green line) we peaked around April Fools Day at about 34 inches and subsequently lost several inches of water equivalent due to an early ripening and melting of the snowpack.  April is the new May, at least this year.

What's that you say?  It snowed for Alta's closing day?  Perhaps, but that barely put a dent into the snowpack demise.  This week we may get a bit more snow, but we are about 15 inches below average, and that's a lot.  There won't be any making that up this year.

Be sure to take out your frustrations on all those people predicting a huge snow season because of the Super El Nino.

Fun and Emotions on Closing Day

As far as closing days at Alta go, today was Cascadian, and there's nothing wrong with a baker's dozen (13 inches) of creamy crud in late April.

I haven't skied at the resorts much this year, so we tried to make up for it by skiing the crud hard in the morning and enjoying the scenery, including this beautiful pileus cloud.

My son Erik made his college decision yesterday.  He was accepted to a number of great schools, but will be attending the U and skiing the Greatest Snow on Earth next year.   Today confirmed it was the right choice (as will the reasonable tuition bill and high quality eduction he'll get the next few years).  No, he's not majoring in meteorology!

He really made my day when he woke up and asked me if I wanted to ski with him today.  Such days are priceless.

It was very apparent today that the student has become the teacher.  The kid can really outcharge me now.  Old age hasn't helped, but really, he's just better than me.

I missed the Frank World Classic last weekend, but those with better knees than me provided some entertainment today.  I suspect the afternoon was even more entertaining.

I kept the skis on the snow.  Final run down High Boy.

The day did not end there, however.  My father always wanted to ski and golf on the same day, but we never got around to it.  Erik and I made up for that today.  

No grass was harmed by golf carts in the making of these memories.

Snirty Dancing

Dust is now obscuring South Oquirrhs #utwx pic.twitter.com/BNh9VMzyhM

— Jim Steenburgh (@ProfessorPowder) April 22, 2016

Earth Day Cynicism, Storm Optimism, and Other Tidbits

The sun has risen on a day that I awake feeling strangely conflicted by an emotional mix of Earth Day cynicism, meteorological optimism, and scientific excitement.  A few reasons why in rapid-fire mode:

Earth Day: So What?  

Sorry to be a cynic, but I don't think Earth Day has accomplished much.  It should be retired.  I'm not against what it stands for in any way, shape, or form, but we spend too much time discussing simple things that we can do to reduce our carbon footprint.  Youngsters out there – you can think much bigger and do what my generation couldn't.

Passing of Prince

Although I never was a big fan, I came of age when Prince truly was a mega star and we tip the hat to him today for Purple Rain.  It's always great to see hydrometeors referenced in album and movie titles.  

Wind and Dust?

We should see some south winds today and possibly some blowing dust around the state.  The frontal timing isn't quite right for a really big blow in the valleys as ideally you want the strongest flow to be phased with the peak in afternoon temperatures.  Instead, the maximum low-level flow, at least in the 0600 UTC NAM forecast, comes in later this evening and overnight as illustrated by the time-height section below.  

Nevertheless, the predicted flow for today is sufficient to support the NWS forecast of 25-35 mph south winds with gusts in excess of 45 for many of the valleys and basins of western Utah.

Closing Weekend Dumpage

Forecasts for mountain precipitation during and following the frontal passage have been fairly erratic, but we are going to get some of the white stuff.   The 0600 UTC NAM generates nearly an inch of water and 9 inches of snow at Alta from Saturday morning to Sunday morning.  Cream on crust seems likely for closing day at Alta.  If we get an inch of water or more, the skiing might even be decent.  Temperatures are such that most of this snow will be high density, but in late April, beggars can't be choosers.  

Is Our Nearly Snirt Free Spring Coming to an End?

As far as dust storms go, this past cool season has been pretty quiet.  We've had a few minor events, but nothing significant, and the Wasatch snow cover remains fairly white and relatively snirt free (snirt = part snow part dirt, the dirt meaning dust).

The lack of dust so late into the spring is fairly unusual.  Estimates of total dust flux at the Salt Lake City international airport show a pronounced peak in April.

Source: Steenburgh et al. (2012)

What's been missing this year are strong south wind events that can tap into dust emission sources in southwest Utah.  We simply haven't had them, but that's about to change.

With the approach of an upper-level trough, we will see the development of south winds tomorrow.  The 1200 UTC NAM forecast valid at 0000 UTC 23 April (6 PM MDT Friday Afternoon) shows the upper-level trough over northern California with 30–40 knot SSW 700 mb (10,000 ft) winds and 15–25 knot SSW surface winds over western Utah.

The situation is not quite ideal for a strong wind-driven dust event as the surface trough is not especially deep and the surface front is still over eastern Nevada (ideally we want the front moving into northern Utah late in the afternoon), but it is the first event we've had this spring with the potential to stir things up.  We'll have to see tomorrow if the winds are strong enough and if the land-surface is ready for dust emissions.  I prefer my corn white, so I'm hoping that's not the case.

Oh, and in case you are wondering, some cream on crust is looking likely for the mountains on Saturday.

Sizzling Start to 2016

Data released yesterday by the National Centers for Environmental Information shows that 2016 got off to a sizzling start, with global temperature anomalies for January through March blowing away anything previously seen in the instrumented record.

In terms of raw numbers January through March of 2016 was 2.1ºF warmer than the 20th century average and a whopping 0.5ºF warmer than the same period in 2015.

With El Nino weakening and the and an increasing chance of La Nina developing later this calendar year, I have been wondering if 2016 would be warmer than 2015, but the numbers for January through March are so far off the scale that it's looking likely 2016 will set another new record.

We've climbed another step on the global warming staircase.

Where Are the Snowiest Regions on Earth?

A remarkable global snow climatology has recently been developed by scientists from the Space Science and Engineering Center (SSEC) at the University of Wisconsin–Madison (see Kulie et al. 2016; access to full article may be paywalled unless you or your institution are subscribers to the American Meteorological Society Journals).  

Producing such a climatology is a major challenge.  Reliable snow observations are fairly limited over land, especially in the high latitudes, and nearly non existent over the oceans.  Thus, Kulie et al. (2016) take advantage of the cloud profiling radar on NASA's CloudSat satellite to develop their climatology.  The CloudSat profiling radar has a wavelength of only 3.2 mm, much smaller than the 3 to 10 cm wavelength commonly used for traditional weather radars.  This makes the radar more sensitive to smaller cloud droplets and ice crystals.  

The Kulie et al. climatology incorporates CloudSat observations from June 2006 through December 2010.  Because of their global focus, these observations are binned into 1 degree latitude–longitude boxes (about 100 by 100 km at midlatitudes).  Thus, the climatology will not pick up on smaller scale snowfall microclimates such as those found in North America.  However, the results are still quite fascinating.

The figures below show the ratio of CloudSat observations with snow (an approximate measure for snowfall frequency) and the estimated liquid equivalent of snowfall.  Values are highest in the mid and high latitudes, especially along the Southern Hemisphere storm track and in North Atlantic and Barents Sea.  So much snow falling on water.  What a waste!

Top: Ratio (%) of CloudSat observations with snow. Bottom: Mean annual liquid equivalent of snowfall (mm/year).  Both in 1x1 degree grid boxes.  Source: Kulie et al. (2016).

Even at this very coarse resolution, the coastal mountains of northwest North America stand out, as do the southern Andes.  If you blow the image up, you can also find high values over high mountain Asia, especially in the western Himalayas and Karakoram, which experience a greater fraction of precipitation produced by disturbances in the midlatitudes westerlies.  Southeastern Greenland also stands out as a snowfall hotspot.  

Unfortunately, this grid spacing does not allow us to examine the small-scale characteristics of snowfall in many areas and largely smooths out the effects of narrow topographic features such as those found in the western U.S.  Looking forward to seeing what the can strain out on regional scales.

Outlier Mode over North America

The atmosphere is well into outlier mode over North America at present, contributing to high-impact weather over portions of the United States.

The situation over the past couple of days is highlighted below.  A slow moving closed-low over the U.S. southwest is sandwiched between ridges along the west coast and over the eastern U.S.

High-impact weather associated with this pattern includes a major snowstorm along Colorado's Front Range on April 16 and 17.  

Snowfall totals reached over 4 feet in the foothills west of Denver.

Source: NWS Denver/Boulder

The big winner in preliminary reports appears to be Pinecliffe in the foothills southeast of Boulder where a trained spotter reports 51.3 inches with a water content of 3.55 inches.

This morning, flooding is affecting the Greater Houston Area where as of this morning as much as 15 inches of rain has fallen in the past 24 hours.

Source: Harris County Flood Warning System

As of 5:20 AM, the Houston International Airport set a monthly record for daily rainfall with 8.85" (the old record was 8.16"), with more rain since falling.  The flooding event underway at present is quite serious.  Our best to everyone in the Houston area.  

In northern Utah, impacts of this high-amplitude pattern are comparatively benign.  A bit of mountain snow late last week to make for decent spring skiing and some modest easterly downslope winds late yesterday and overnight producing peak gusts of 47 mph in Farmington and 44 mph at the University of Utah.  

The Front Range Snow Climate Is Weird

It's mid April, winter is over, and of course Colorado's Front Range is getting pounded with snow.

The 24-hour snow totals ending this morning show a swath of 30+ inches in the foothills west of Denver.  Add another 10 inches or so to that for the 2-day storm totals in that area.

In the mountains, Loveland Pass is reporting a 48 hour snow total of 32 inches, Winter Park 24 inches, and Arapahoe Basin 25 inches (the latter is a 3-day total, but probably most fell in the last 48).

This late-season snowfall is not unusual for the Front Range.  For sites near and east of the Continental Divide, March and April are the snowiest months climatologically.  For example, at Nederland west of Boulder (period of record 1970-1988), the monthly mean snowfall in March and April is 23.9 and 24.3 inches, respectively, compared with 13.3 and 13.1 in January and February.  At Berthoud Pass (period of record 1950–1985, the monthly mean snowfall in March and April is 57.9 and 54.6 inches, compared with 49.8 and 42.4 inches in January and February.  

Those high averages averages in March and April are propped up by intermittent but large upslope snowstorms in easterly flow.  Some of the largest snowfall events in these areas have occurred in March and April.

What can I say?  The Front Range snow climate is weird.  They should try to shift that snow to winter where it can be better utilized.

Yesterday's Impressive Dumpage

I had a Maalox moment around noon yesterday having issued a fairly confident forecast for 2-5" of upper-elevation snow only to see things explode in the southwesterly flow and produce some remarkable precipitation rates in upper Little Cottonwood (see prior post Big Water Amounts).

The Alta-Collins precipitation trace below tells the story.  For the 1-hour periods ending at 11 AM and 12 PM MST, water equivalents were 0.36 and 0.39 inches, respectively.  The 0.75 inches of water equivalent in that period was over half of the 1.4" that fell yesterday.

I don't have a careful analysis to back this up, but during the cool season (Oct-Mar), I water equivalent precipitation rates of 0.3" per hour are about as high as they get at Alta.  I usually associate such precipitation rates with warm atmospheric-river events.  To get two hours > 0.3"/hr is quite impressive, although there is greater potential for larger water equivalent rates in the warmer storms of April.

The warmth, however, was a blessing and a curse.  Temperatures at Alta-Collins (9600 ft) did not fall to 32ºF until noon yesterday.  Thus, a significant portion of the water equivalent fell as rain at low to mid elevations and what fell at upper elevations Wednesday and Thursday morning was quite dense.  The .75" of water equivalent at Alta-Collins from 10 AM to 12 PM yesterday was accompanied by only 4" on the interval board, for a water content of about 18%.

At Alta-Collins, yesterdays snowfall total was 8" by 6 PM, with another inch overnight, bringing the 24-hour tally to 9" and the storm-total since Wednesday to 10".  Given the warmth of the early part of the storm, a huge increase in snowfall can be found from about 7500 to 10000 feet.

Perhaps some good turns will be found at upper elevations since the latter portions of the storm were colder and the snow is stacked right side up.

Big Water Amounts

For the last 2-3 hours, precipitation rates in Little Cottonwood Canyon have been exceptionally high.  During the cool season, it is quite unusual for snow water equivalents to exceed 0.3" an hour, but Alta-Collins reported 0.36" for the hour ending at 11 AM MDT and 0.39" for the hour ending at 12 PM MDT (ignore the 34" snow interval ob which is bad).

These numbers are consistent with those measured at the Snowbird SNOTEL station, so water equivalent rates of .3" or more per hour seem likely.

Radar imagery over and upstream of Little Cottonwood showed frequent initiation of radar echoes in central and Utah County extending downtream over the Wasatch Range.  These echoes have gotten more cellular and intense in the last few frames.

At low elevation, all of this wonderful water has gone to waste.  The UDOT camera at the White Pine Parking lot (~7500 ft) shows just a hint of snow sticking here and there.  Nearly all rain here so far.

At high elevations, however, it is piling up fast.  Looks like 4-5" on the Snowbird Snow Stake near the top of Gad II (9900 ft).

Source: Snowbird

Looks like my outlook for modest accumulations today at upper elevations (see previous post) could be going down in flames.

Frontal Update, Part Deux

It's not quite the equivalent of Groundhog Day The Movie this morning, but it's close.

As was the case yesterday, a frontal boundary and precipitation band are flirting with northern Utah.  The main difference is that they are a bit further north this morning than yesterday.

The large-scale analysis from 1400 UTC shows a pronounced 850 mb (~1500 m/5000 ft) wind shift extending across central Nevada and northern Utah, including the Great Salt Lake, with a band of precipitation just to the north.

MesoWest surface observations similarly show the wind shift extending through Dugway Proving Ground, across the Great Salt Lake, to north Ogden.

Although we are south of the front this morning, temperatures are pretty similar to yesterday morning and in the mid 50s.  If you are outside and not paying attention to which way the wind is blowing, you will hardly notice a difference.

The 1200 UTC initialize HRRR model  calls for the front and precipitation to push into the Salt Lake Valley this morning with showers and possibly a thunderstorm today.  I think this forecast is a bit fast on the precipitation as it has the band over us by 8 AM this morning, but it will get here eventually.

What's the scoop on mountain snow?  Well, yesterday afternoon and overnight the Alta-Collins precip gauge picked up 0.33" of water, but the snow depth sensor maxed at only 1".  Temperatures ranged from 38ºF yesterday afternoon to 33ºF over night.  Ick.  Temperatures and snow levels will lower some with the frontal passage today, but I'll stick with my forecast from yesterday which called for a total of 3-6" of upper elevation snow from yesterday through this afternoon.  That means 2-5" today.

Over the past couple of days I've seen some big snow numbers forecast for storm totals through Friday.  I'm less enthused.  We never get into a good period of moist northwesterly flow and the frontal passage for today is a relatively warm event, leading to less snow per unit of water.  The NAM calls for a storm total today and tomorrow of about 0.6" of water, which our algorithms convert to about 6" of snow (10% water content overall, although it will be higher water content to start and lower later).  The NCAR ensemble has most members between 0.35 and 0.75" of water, although there is one with only about 0.08" and another with closer to an inch.

In addition, this is April.  Climatologically, the enhancement of precipitation by the mountains this time of year is quite a bit lower than it is during mid winter.  In January, the average precipitation at Alta is more than five times what it is in Salt Lake City, but in April, it's only about 2.5 times that.  Typically frontal and convective storms of the type we are experiencing today produce less enhancement of precipitation in the mountains, so jacking up the model totals by a lot, as one might do in winter, doesn't make sense in this pattern.

Thus, for a big dump, we will need the front or the post-frontal environment to be bigger producers than advertised presently by the NAM and NCAR ensemble.  While I can't rule that out, it's simply not a good bet and expecting more modest totals of 2-5 today and perhaps a couple inches more in snow showers tonight and tomorrow makes the most sense.

Frontal Update

In Monday's post, I mentioned that on or about Tax Day we often get some interesting weather in northern Utah.  Well, change is here.  It's not too exciting by spring standards, but it is something to talk about.  

This morning a surface cold front pushed through the Salt Lake Valley bringing a few showers.  As I write this, at about 11:45 AM MDT, the front sits right at Point of the Mountain.  North winds with temperatures in the 50s predominate the Salt Lake Valley, whereas in the Utah Valley, one finds south winds and temperatures in the 60s.  Perhaps the weather down there is nice, but BYU still sucks.

The frontal precipitation band is just to our north, resulting in more frequent shower activity along the northern Wasatch Front, with the Salt Lake Valley getting the occasional brush by shower.  

For today, I'd expect some frontal "wobbling" and more precipitation to the north, but still a chance of showers and thunderstorms for the Salt Lake Valley, especially this afternoon.  I remain perplexed as to how things will play out tonight and tomorrow, but am less enthused about the prospects of a deep powder day (or perhaps better put, deep crud day) on Friday than I was a couple of days ago.  There are a couple of reasons for this.  One is that the system has come in several degrees warmer than advertised earlier in the week.  The other is, despite some organization along the front, a prolonged period of heavy mountain precipitation isn't currently being advertised for the central Wasatch.  If we can get some organization over the Cottonwoods, we could be in business, but it's more likely that we'll see periods of snow (rain to as high as 9000 feet today in the central Wasatch if and when it precipitates) through tomorrow afternoon that won't add up to more than 3-6 inches in the upper elevations.  In situations like this, I like to have low expectations and hope for a surprise.  

Precipitation amounts will likely be greater to the north today, but with the high snow levels, much of that will fall as rain or extremely heavy snow.  Temperatures at Snowbasin Middle Bowl (7400 ft), despite the periods of precipitation this morning, are currently lingering near 40ºF. 

Tidbits

Two quick tidbits today as we wait for some meteorological action on Thursday.

New Book
University of Utah Emeritus Professor of Geology and Geophysics Bill Parry has a new book out entitled Geology of Utah's Mountains, Peaks, and Plateaus.

Although I haven't read it yet, I've skimmed it and it looks like a good one for Wasatch Weather Weenies readers.  You can pick up electronic, paperback, or hard cover versions here.

Seminar
Stanford Associate Professor of Earth System Science Noah Diffenbaugh will be giving the Global Change and Sustainability Center seminar next Tuesday, April 19, in 210 ASB on the University of Utah campus.  The seminar seeks to better understand how global warming affects extreme climate events.

Why Utah Meteorologists Love Tax Day

If you are a meteorologist in Utah, there are few days on the calendar as fun as tax day.

It seems like something exciting happens every year on or about April 15, and there are good reasons for it.  April 15 lies at the beginning of the Intermountain cyclone season, which spans from April to early June and frequently features the development of strong troughs or low pressure systems over the Great Basin downstream of the Sierra Nevada.  Such systems frequently bring strong winds and blowing dust.  April 15 also lies in the heart of the wettest month climatologically in Salt Lake, with April averaging a whopping 2.10 inches of precipitation, good for big storms.  Finally, April 15 is in the middle of the transition from winter to summer and we can get some extremely large temperature swings in April and everything from deluges to dumps.  If you don't like the weather in April, wait 5 minutes.

Perhaps the biggest and baddest example of extreme weather on April 15 is the 2002 Tax Day storm, which produced the second lowest sea level pressure observed in Utah since observational records began in 2002 and the strongest cold-front passage observed in recent decades.  With frontal passage in Salt Lake City, temperatures fell 13ºF in 10 seconds and 34ºF in 2 hours.  Blowing pre-frontal dust produced by the storm forced the closure of US-6 and left a chocolatey-brown coat of dust on the mountain snowpack for the remainder of the snowmelt season.

The storm advertised for later this week isn't as exciting, but will probably make things more meteorologically interesting.  As predicted by this morning's 1200 UTC NAM, a surface cyclone and frontal precipitation band are predicted to develop over the Great Basin by Wednesday afternoon (0000 UTC 14 April).

The surface trough and frontal band linger to our northwest for some time, but the NAM eventually brings them into the Salt Lake Valley in earnest on Thursday morning.

Yes, I know that's 14 April, not Tax Day, but that's close enough for Rock 'n' Roll.

As things stand now, the event looks like a modest spring storm for northern Utah, certainly not of the caliber of the 2002 Tax Day Storm and many other strong spring events in Salt Lake City.  If the NAM were to verify, we would, however, see some significant changes in weather including some bench snow on Thursday, and perhaps some decent powder skiing in the mountains on Friday.

However, I've done this gig long enough to know not to read too much into the small-scale details of an Intermountain front event that is still about 3 days away.  There's a lot of sensitivity of these events to the track, speed, and structure of the primary upper-level trough and its interaction with the Sierra Nevada.

Thus, don't bet the farm on this morning's NAM forecast.  A lot could change the next couple of days.   Let's see how things look on Wednesday.  

Downslope Winds

Strong easterly winds (photo taking looking north) this morning in the Avenues foothills

Easterly downslopes winds are creating blustery conditions along the east bench and on the University of Utah campus this morning.  Observations at about 8 AM showed strong esterly flow from Parley's canyon north with gusts to 31 in the Avenues, 35 on campus, and 39 at the mouth of Parleys (annotated numbers below are current wind gusts in mph).  Winds are stronger along the east bench than in the canyons and most upper-elevation bench locations. 

There is also clear evidence of a rotor near the Salt Lake City International Airport.  Note how the easterlies extend to downtown, but the flow at the airport is westerly, as depicted below.

Source: Whiteman (2000)

Rotors sometimes feature sub-rotors (smaller-scale circulation centers embedded in the larger-scale rotor), strong wind shear, and turbulence.  I've often wondered if the airport is just far enough west that planes on take-off and approach avoid these features or if its a bumpy ride on days like this.

The cause of these downslope winds is easterly to southeasterly 700-mb (10,000 ft) flow being driven by a closed low off the coast of California and an elongated upper level ridge extending northwestward from Texas to the Pac NW.

Peak gusts so far include 47 mph on the Bountiful Bench, 45 mph in Farmington, and 43 mph at the University of Utah.  These winds should slacken in the next few hours.

The City Creek Canyon Antiwind

Thermally driven flows are produced by spatial variations in heating and cooling associated with topographic and land-surface contrasts.  Examples include sea and land breezes, up- and down-valley flows, and up- and down-slope flows.

During the day, one typically expects up valley and up slope flows in areas of complex terrain, whereas at night and in the morning, one expects down valley and down slope flows.

Schematics of thermally driven flows from Whiteman (2000)

Within narrow valleys, the up- and down-valley flows usually dominate the slope flows once the former are fully developed, such as is found in the late afternoon or early morning.

However, those generalizations apply to situations where the local forcings dominate and the larger-scale flows have little influence.  One can find many examples where the flows in canyons and along slopes deviate from what one would expect from local forcings.  One such example is lower City Creek Canyon.

If you frequently run or bike in the lower portion of City Creek Canyon in the afternoon, one typically finds what meteorologists call an antiwind.  An antiwind is a wind that blows opposite what one would expect from local thermal forcing.  In City Creek Canyon in the afternoon, one expects up-valley flow, which would be associated with SSW flow in the lower canyon and SW flow in the upper canyon above the elbow about 1 mile past the gatehouse (blue arrows below).  

Instead, there is often a N or NW wind in the lower canyon (indicated by the red arrow above).  One can also frequently find a N or NW wind in the Avenues, which is downslope.  Indeed that was the case yesterday in the late afternoon as I entered City Creek Canyon on my bike from 11th Avenue.  Note also the north wind at the upper Avenues observing site in the figure above.

Such winds are of course completely demoralizing for a bicycling meteorologist.  My expectation is up-valley flow in the afternoon.  To have a down-valley headwind when I'd like an up-valley tailwind is simply unacceptable.

What is the cause of this antiwind?  On days with weaker large-scale flow, such as yesterday, it is likely that the larger-scale thermally driven circulation associated with the Great Salt Lake and Playa (i.e., the Lake Breeze) and the larger, broader Salt Lake Valley (up-valley breeze) overwhelms the up-valley flow in City Creek Canyon.  This flow pushes across the Ensign Peak ridgeline and then descends into the lower Canyon.  In some situations, there is a large-scale northwesterly flow, such as found behind a cold front, that further enhances or drives the antiwind.

Source: Whiteman (2000)

Of course, there are days when there are strong up-valley winds in City Creek Canyon.  These may be days when these larger-scale northwesterly flows are weak or, better yet, there is a large-scale southerly wind pushing up the canyon.  You want a City Creek PR?  These are the days to go for it.

One thing I have noticed over the years is that the flow in upper City Creek Canyon often is weaker or even up valley.  I suspect this is related to several factors, including the fact that the ridge on the north and west side of the canyon is much higher in the upper canyon than the lower, which often blocks the larger-scale northwesterlies.  Aspects of the terrain geometry both in the canyon and around it also play a role.  

In case you are wondering, yes there still is some snow on the road in the very upper Canyon within about a quarter mile of the top.  For early April, that's nothing.  April is the new May once again. 

How Does PM2.5 Pollution Vary with Elevation in the Salt Lake Valley?

Although the wintertime PM2.5 period is now past us, a number of you have asked how much the air quality varies with elevation in the Salt Lake Valley.  I'll answer this question like a scientist.  It depends.

First, let me note that the best situation for knowing the air quality before you recreate or go outside is to have high-quality real-time monitoring at many locations across the Salt Lake Valley.  The Division of Air Quality does not provide this.  We get data in real time (usually an hourly average with a bit of a delay) from Hawthorne Elementary and that is it.  The air quality can, of course, be considerably different in other areas of the valley.

The University of Utah has recently installed sensors at a few locations as well as on TRAX trains, which helped during this past winter.  There is also the grassroots purpleair.org network.  Overall, these sensors helped last winter, but the coverage remains incomplete and we still still need to vet the purple air sensors, which seem to overestimate PM2.5 concentrations.  There's still more work to be done until we have a good idea about the PM2.5 concentrations in your neighborhood.

With that in mind, let's talk about what we do know about the air quality dependence on elevation?  In January and February 2011, several University of Utah and DAQ scientist collected PM2.5 measurements at several locations in the Greater Avenues area during several poor air-quality episodes.  Twenty-four average PM2.5 concentrations from these locations are plotted below and show that in most episodes there was a trend toward lower concentrations.

Note, however, that there are variations in the size (and even sign) of the decrease.  In other words, it depends.  In the 4 Feb event, for example, PM2.5 was higher at 1500 m than it was at 1300 m.  In addition, these are 24-hour averages and so they are more representative of the long-term PM2.5 exposure than what you might get when you go outside to recreate at a particular time of day.  In the morning, one sometimes gets clear air near the mouths of the canyons and on the upper benches due to local slope and valley flows transporting air from higher elevations, whereas in the afternoon, those locations are in the gunk.  This leads to a lower 24-hour average, but if you are recreating in the afternoon, you are breathing lousy air.

In addition, there is a tremendous amount of variability in the characteristics of air pollution from event to event.  Much depends on the strength and height of the capping stable layer or inversion and the local meteorology.  For example, there are situations when the pollution is trapped in a shallow layer and the air is much cleaner on the benches than along the valley floor.

On the other hand, there are times when even the upper-elevation benches are enveloped in smog.

Source: KSL

My personal rules of thumb are if I'm above the smog, I recreate without concern.  If I'm in the smog, then I generally assume that the PM2.5 levels on the bench are probably near what they are to those measured on the valley floor and make decisions accordingly.  Every now and then I will identify a situation where the bench PM2.5 levels are not as bad as on the valley floor and I will opt to recreate. Nearly always in these situations I've seen a very recent PM2.5 observation from the U or TRAX line to support this conclusion.