Category Archives: Dust

Six Questions to Answer When Forecasting

Exciting weather is happening and on the way, so it seems fitting to organize this post around the six questions to answer when forecasting

1. What has happened?

This "winter" has thus far been like 3.5 consecutive Novembers rather than a typical November, December, January, and February sequence.  The average temperature at the Salt Lake City International Airport for 1 December - 17 February was 37.8ºF.  That's just a shade lower than the November mean of about 40ºF.  As everyone knows, we're well below average for snowfall and snowpack, especially in the lowlands.  The photo below was taken this morning looking up City Creek Canyon and indeed it looks more like a scene you might see after a November snowstorm than one would expect in mid February. 

 
This context is important as I suspect most people are entirely unprepared for what is coming.  It will probably seem like the first storm of the year.

2. Why has it happened?

This is a good question and one that I can't answer satisfactorily.  The easy answer is that the warmth and snow drought reflects persistent high pressure and a storm track that has remained predominantly north of northern Utah.  Why that has been the case remains a subject of debate.

3. What is happening?

Wow, what a windy night.  Strong south winds at all elevations.  In the past six hours (ending at 9:20 AM MST), several sites in the northern Wasatch and Bear River Range have gusted over 70 mph and ridgelines in the central Wasatch have seen gusts as high as 71 mph.  I can find many sites in the mid elevations reporting gusts over 50 mph.  Sherwood Hills (5658 ft) near Sardine Canyon guested to 64 mph.   If there was much powder left over from yesterday's feast, I suspect it's been blown to Jackson Hole.  Peak gust at the Salt Lake Airport so far is 43 mph.

With these strong winds, we are seeing some dust.  Concentrations were especially high in the western Salt Lake Valley, which I suspect is due to emissions from the area west of Utah Lake as we have seen in recent events



4. Why is it happening?

The answer here is an approaching frontal trough that at 1500 UTC (0800 MST) was sagging southward into northern Utah.  This has created a strong pressure gradient to its south, with strong gusts at all elevations. 


5. What will happen?
6. Why will it happen?

Loaded questions!  I'll answer them together as it is easier.  As I write this, the surface front just passed Hill Air Force Base.  The HRRR shows it progressing slowly southward today, with frontal passage in the northern Salt Lake Valley around 2000 UTC (1300 MST).  Thus, expect to see a wind shift early this afternoon, if not sooner in the valley. 

Periods of snow are likely in the northern Wastach today and will develop in the central Wasatch later this afternoon.  It's a bit of an oddball situation as the latest NAM shows the frontal band over far northern Utah through 2100 UTC, but then rather than bringing it through continuously, redevelops it to the south tonight. 



Tomorrow brings the post-frontal crapshoot beneath the upper-level trough where much depends on flow direction, moisture, and instability.  The NAM forecast below isn't too bullish on snow, but the 6 Z GFS is more enthusiastic and keeps us in wrap-around moisture (not shown). 


It's worth a look at the 12-km NAM-derived forecast for Alta.  Measureable precipitation begins around 5 PM and is strongest from about 6 PM to 11 PM with the frontal forcing.  Periods of snow continue through 6 PM tomorrow in the unstable post-frontal period.  Total water equivalent is 0.6" with 10" of snow by 9 AM tomorrow and 12" by 7 PM tomorrow. 


Take a peak also at the temperatures for Mt. Baldy.  Keep in mind these are for 11,000 feet, a bit above the lift-served terrain, but this provides some idea of how cold the airmass will be.  It goes sub zero by noon tomorrow and down to -8ºF by 9 AM Tuesday.  I don't think we've seen air this cold yet this year.

To summarize, this looks to be an all elevation storm and you should be prepared for winter conditions.  We haven't had a stress test like this in some time. Thankfully, it is the President's Day Holiday, which will hopefully help with tomorrow's commute.  For Alta-Collins, I lean toward 7-14" by 9 AM tomorrow.  I have a bit more heartburn for totals after that given the variability I'm seeing in the models.  Some snow is likely, but the range of possibilities is large.  Hug a ski patroller or avalanche forecaster when you see them.  They will have a tough job the next few days. 

Finally, this is a high-impact, rapidly evolving situation.  Keep an eye on official forecasts at http://www.weather.gov/slc/. 

About Yesterday’s Terrible Air Quality

It's a 2-for-1 day at the Wasatch Weather Weenies.  If you want to read about the overnight snowfall, proceed to the previous post.  If you want to read about dust, air quality, pollution, and other apocalyptic aspects of the storm, stay here.

Yesterday morning, an extremely nasty plume of dust penetrated into the Salt Lake Valley, pushing air quality to unhealthy levels.

Although Salt Lakers are aware of poor air quality during inversion events, yesterday we dealt with an entirely different beast.  During inversion events, emissions from the combustion of fossil fuels and wood, as well as a few other sources, build up within the Salt Lake Valley.  Winds are light and the strong atmospheric stability prevents the mixing of pollution vertically.  In contrast, yesterday the poor air quality occurred during strong, southerly, prefrontal winds.

The graphs below show what happened at Neil Armstrong Academy in the Northwest Salt Lake Valley.  From Tuesday morning (19 December) through just after midnight Wednesday (20 December), PM2.5 concentrations fluctuated from about 0 to 19, with the peak around midnight.  These are values that indicated good to moderate air quality with just a little bit of pollution. 

Source: MesoWest
Then, after midnight, strong southwest winds developed, scouring the valley clean.  Those winds persisted overnight and into the morning.  However, beginning after about 6 AM, PM2.5 concentrations began to climb, eventually spiking to over 200 ug/m3, which is well into unhealthy territory.  That spike occurred just ahead of the surface cold front.  PM2.5 concentrations then dropped from 203 to 10 ug/m3 in 10 minutes as the front went through and brought in cleaner air. 

Another perspective is provided by a laser ceilometer at the University of Utah.  This is a device consists of a laser that points vertically through the atmosphere.  The signal returned back to the device can be used to infer pollution concentrations and the base of clouds. 

Below is a time-height section from about 1500 MST on Tuesday through 1500 MST Wednesday.  There is some moderate pollution evident on Tuesday afternoon and evening, but the airmass becomes relatively clean overnight.  The dust plume appears shortly after 6 AM and through about 10 AM extends to about 750 meters (2500 feet) above ground level. 

Image Source: MesoWest
As the front approaches, the depth of the dust increases.  I suspect this is due to the convergence of surface winds and the lifting of air near the front lofting the dust plume to deeper heights.  If one looks carefully, you can see evidence of the dust aloft event after the clean air behind the front has moved in at low levels.

So, what the hell is going on and where is all of this dust coming from.  We had a similar event on December 3rd, but the dust wasn't as think.  One of our graduate students, Derek Malia, pointed out to me that the dust plume was very evident in satellite imagery that day.  Sure enough, you can see it in the image below, originating in the south Cedar Valley west of Utah Lake.

Source: NASA
Yesterday, similar story.  I didn't have access to as sharp of an image as on 3 December, but one could still see the plume originating over the south Cedar Valley. 

Image Source: NASA
And, one can see it as a pink streak in the dust product from GOES-16.  Note that the plume continues downstream over northeast Utah, in that area likely above the shallow post-frontal airmass. 


So, we have a good idea that this dust is coming from the south Cedar Valley.  Google Earth shows that the dust emissions could be coming from agricultural fields in that area, which are perhaps exceptionally dry for this time of year due to the drought conditions.  Another possibility is emissions from a fire-affected area.  I haven't had a chance to dig into the past fire data to examine if this is a viable hypothesis. 


Hopefully, this is an issue that will remedy itself with precipitation.  If not, perhaps it wouldn't take much to reduce emissions from that area. 

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.