Category Archives: Dust

Storm Chasing Update

Prefrontal southerlies are cranking over the Salt Lake Valley, which is filled with dust as I write this just afternoon. 

We have a complicated day/night of storm chasing ahead of us.  The Doppler on Wheels is currently deployed near Daybreak where we hope that the skies are dusty enough to give us a nice picture of the cold-front penetration through the Salt Lake Valley.  We are also hoping that behind the front we will eventually get some precipitation for observing some of the interactions between the Oquirrhs and the Wasatch Range, as well as fine-scale precipitation structures in the central Wasatch. 

After this evening, we still haven't figured out what the heck we're gonna do.  The model are advertising the passage of a secondary trough during the late night hours.  At 1000 UTC (3 AM MST), the trough is pushing through the Bountiful area in the 1700 UTC initialized HRRR forecast. 

We may have some orographic snow showers and possibly some lake effect as well, but the devil is in the details.  Thus, we have some consternation about where to put the DOW.  It is a mobile platform, but we can do better science if we can operate in one area for an extended period.  We'll see what happens.  I've mentioned that this is a crapshoot enough already.

Postfrontal Dustpocalypse!

A strong cold front raced across northwest Utah this morning, reaching Salt Lake City around noon bringing a blast of moderately strong pre- and post-frontal winds, the latter accompanied by blowing dust.

Dustpocalypse Now!
Observations collected every minute from the William Browning Building (WBB) on the University of Utah campus show a wind shift from SW to WNW from 1153 to 1155 MDT.  Winds continue to turn through NW at 1200 MDT.  From 1153 to 1200 MDT, temperatures fell 10.3ºF.  Pre-frontal wind gusts reached as high as 42 mph a couple hours ahead of the front and peaked at 49 mph at 1209 MDT, just behind the front.

Adding to the story was the post-frontal blowing dust.  At Wendover in far western Utah, the post-frontal visibility dropped to as low as 4 miles, likely due to blowing dust.  However, at the Salt Lake City International Airport, minimum visibilities reached 1 mile, suggesting that dust emissions from the area surrounding the Great Salt Lake and the west desert contributed.

The dust made the cold front very apparent as it entered the Salt Lake Valley (h/t to @UteWeather for tweeting the image below, taken facing from the U toward downtown Salt Lake City).  One can see the classic frontal "nose" to the left of the photo, with friction resulting in a slight forward tilt of the front with height in the lowest one or two hundred meters, above which the front slopes back over the cold air.

The post-frontal air was nasty.  PM2.5 concentrations spiked to 120 ug/m3 on campus immediately following frontal passage.

I guess if you're not going to have much snow, weather excitement like this is better than nothing.

Addendum @1235 MDT:

Shortly after writing this post, the PM2.5 at our mountain met lab topped out over 200 ug/m3 (note scale change from graph above). 

There's some uncertainty in these measurements, so perhaps we should be cautious about the absolute values.  That being said, the air was pretty nasty out there and remains so as I write this at 1235 MDT.

Wind, Dust, Snow and All That…

I love spring storms, so I'm feeling like a kid in a candy shop today.

Winds picked up yesterday and gusted strongly overnight in advance of a developing trough and surface front over Nevada.  After midnight, peak gusts at upper-elevation locations in the northern, central, and southern Wasaatch are 82, 77, and 70 mph, respectively.  In the valleys, the Great Salt Lake Marina hit 60 mph and a sensor near the juncture of UT-201 and I-80 in the Salt Lake Valley hit 63 mph.

Winds as I write this have actually slackened just a bit.  Obs from the juncture of UT-201 and I-80 show two periods of strong winds overnight, on prior to midnight, the other from about 1:30-4:30 AM.

Source: MesoWest
With the development of the front and surface trough over Nevada today, as well as daytime surface heating, we will see strong winds today.  Given the prolonged nature of the event, dust is likely as well.  

The models are still calling for two fronts to move through northern Utah this weekend, the first late tonight or early Saturday morning:

the second late Saturday or early Saturday night:

 The NAM has backed off a bit in both instances for precipitation at Alta, especially the first front tomorrow morning, and ultimately produces a storm total by Sunday morning of 0.82" of water and 13.5" of snow.

The SREF continues to show a large spread from only 0.25" of water to over 2.5", with strong clustering based on model core (ARW or NMB - essentially, two different models are used for the SREF).  The really wet members all produce considerable precipitation early tonight, and thus get things started early.

So, here we sit, having looked at this storm for about a week, and still no guarantees!  Sunday still looks like the better ski day as snow piles up during the weekend, but how good it is will depend on whether or not we are in the upper-half of these forecasts.  Finding a smooth underlying surface or getting enough snow to fully bury the frozen coral reef will be the key to good skiing on Sunday.  One plus is that we are looking at cold temperatures Saturday night, with 700-mb tempreatures currently forecast to drop to about -14ºC, which should yield a right-side-up snowfall. 

What Is a Haboob?

Dan Pope of ABC 4 Utah shared a remarkable photo yesterday of a Haboob near Phoenix, Arizona.  The credit is ambiguous, so I share the full facebook post below to provide as much credit as possible.

A Haboob is a dust storm generated typically in arid regions by the outflow from a thunderstorm, convective cloud, or precipitation system.  Within these clouds and precipitation systems, cooling by precipitation produces a downdraft or downdrafts, a cold pool at the surface, and strong winds.  The leading edge of the cold pool and strong winds is known as a gust front and, in areas where this leads to dust emissions from the surface, typically demarcates the leading edge of the Haboob.  I've taken considerable artistic license to crudely sketch this out in the photo below.

Haboobs occur in arid regions around the world.  Areas where the land surface has been disturbed, enabling or enhancing the potential for dust emissions, are vulnerable to Haboob development.  Many iconic photos from the Dust Bowl are Haboobs with dust emissions in that area strongly related to poor agricultural practices combined with long-term drought.  Even today in Arizona and much of the American Southwest, land-surface disturbance is an aggravating factor in Haboob frequency and intensity.

Another Blow Is Coming

It was an eventful day yesterday with a power outage on campus in the morning, storms in the late afternoon (northern Wasatch Front) and evening (Salt Lake Valley), and more power outages overnight.  The Salt Lake Tribune reports that 85,000 people lost power overnight after a "cascade of outages" struck numerous areas of the Wasatch Front an Tooele.  If high winds were indeed the cause, chalk this one up to cheapness.  Bury those electrical lines and we wouldn't be as vulnerable to these events.

Turning from commentary to weather, I mentioned in Wednesday's post that the cold front would tease us Thursday afternoon and then retreat back to the north and the west and that's exactly what has happened.  Check out how the frontal precipitation band pushes into northern Utah, doesn't like what it finds (insert your favorite colorful reason why here), and then decides to tuck tail and move back to the north and west.

You don't see that every day and it is a result of the digging trough along the Pacific coast, which has resulted in a backing (counterclockwise turning) and intensification of the flow over the western interior.  So far, that flow intensification hasn't been felt on the ground in most of the Salt Lake Valley.  There are, however, hints that things are going to change as the day progresses.  Note, for example, the 40 knot flow at Stockton Bar and 30 knot flow at Dugway in the MesoWest plot below.

Those are areas where the cold pool from last night's storm has mixed out.  That cold pool is currently preventing the strong flow from aloft from mixing to the surface in the Salt Lake Valley and some other lowland locations.  Indeed, the morning sounding shows a deep cold pool, but it is topped by strong flow, suggesting that we'll see a rapid increase in wind speed once that cold pool has been eroded.

And, the NAM forecast for 0000 UTC (6 PM MDT) this afternoon shows a band of 700-mb flow of 40-50 knots extending from southwest Utah over the Salt Lake Valley.  Another blow is coming.

So, if you are looking at the limp flags this morning and are wondering if the high-wind warning issued by the NWS for western Utah and the Salt Lake Valley is going to verify, have no fear.

Soon those winds will be here and they may be bringing dust with them.

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.

Desperate Times for Meteorologists

The weather of late has been great for recreation, but pretty damn boring if you are a meteorologist.

Today's "excitement" involves a trough passage across northern Utah.  It's pretty much cloud free, so we need to dig deep to notice the change.

The NAM forecast for this afternoon shows the trough axis at all levels just past Salt Lake City with westerly or northwesterly flow in its wake.  

The impact on temperatures is quite pronounced as we've been nearly flatlined late last night and for much of the day today.  This is an indication that the transport of cold air into northern Utah, or what meteorologists call cold advection, is nearly perfectly balancing the rise in temperature from solar heating.

Source: MesoWest
Although this is a pretty wimpy trough from a moisture perspective, it is bringing in some dust or smoke.  I was on Grandeur Peak this morning and the visibility at low levels was fairly poor over the northwest Salt Lake Valley and the Great Salt Lake.

That's our weather excitement for the weekend.  Yes, things are boring, but hopefully Mother Nature is storing it up for November.

Yesterday’s Air Quality Was Briefly Bad, But Pioneer Day Was Worse

If you were out and about yesterday morning just after the frontal passage, you may have gotten a mouth or eye full of dust.  Visibility was low, the sky grey, and the mountains partially or even fully obscured at times.

Looking east at the dust obscured Wasatch Range from the University of Utah at 10:25 AM 27 July 2015
Observations from our mountain meteorology observatory on the east side of campus near the mouth of Red Butte Canyon show the pronounced spike in PM2.5 that occurred during the dusty period, with a maximum of around 45 ug/m3.

Now here's what's interesting.  If you thought yesterday was bad, Pioneer Day was worse if you were in an area that was affected by the fireworks.  The PM2.5 trace below is from the DAQ sensor at Hawthorne Elementary, just south of Liberty Park.  You can see the bump in PM2.5 concentration yesterday morning to a maximum of around 18 ug/m3 (the DAQ observations are hourly averages, which results in a lower peak than seen in the higher frequency data from out mountain meteorology lab), but if you go back to the evening of the 24th (Pioneer Day), the peak is much higher, reaching 47 ug/m3.  That peak was likely produced by the Liberty Park fireworks.  
Source: DAQ