Escaping Smoke is Hard

During winter, the Salt Lake Valley often sees pollution episodes with elevated particulate matter due to the presence of low-elevation cold pools that persist due to the limited solar heating.  This enables particulate matter that forms from the emissions from automobiles, industry, home heating, and other sources build up within the cold pools, resulting in poor air quality.  This poor air quality is reflected in elevated levels of particulate matter with a diameter of 2.5 micrometers or less, or PM2.5.  Such events are known colloquially as inversions.

Because the PM2.5 is typically trapped in the cold pool, one can usually find clean air by moving to higher elevations.  The depth of the pollution varies from event to event and sometimes from hour to hour as the cold pool sloshes around in the valley.  At times, the benches may be in the clear air.  At other times, elevated PM2.5 can extend higher.  

An example, from December 15, 2025, is shown below.  Taken from about 6000 feet in the Avenues foothills, one can very clearly see the layer of pollution within the valley, but clear air at upper elevations.  

PM2.5 associated with wildfire smoke in the summer is more complicated for a number of reasons.  First, the heat and intense vertical motions associated with wildfires can inject wildfire smoke anywhere from the surface to more than 15 km.  Because the wind often varies in strength and direction with height, the transport of that wildfire smoke can also vary in direction.  As an example, note how smoke from the Grasshopper Fire in northern Oregon spread in two different directions yesterday morning. 

Source: College of DuPage

As a result, smoke can be found even at the highest elevations and PM2.5 concentrations do not necessarily decrease with height.  Sometimes they can even increase with height.  

I had planned to hike up Snowbird this morning, but thought this might not be a good idea after looking out the window and taking a look at the PM2.5 estimates from Purple Air.  Unlike in winter, elevated PM2.5 wasn't confined to the Salt Lake Valley.  One could find values > 35 ug/m3 (i.e., unhealthy for sensitive groups) even at some locations in the Snyderville Basin.  Although there are fewer upper-elevation sits, at 8:57 AM MDT the highest PM2.5 was actually near the mouth of Little Cottonwood (53-68 ug/m3), Red Pine Lodge at Canyons (83 ug/m3), and Silverfork in Big Cottonwood Canyon (61 ug/m3).   

Source: Purple Air

It's hard to say if the top of the tram would have been above the smoke. The 9 AM webcam suggested it was close, but smoke clearly enveloped the terrain to high elevations.   

Source Snowbird

I ended up going for a less strenuous walk in the Avenues foothills.  Although the PM was still elevated, my back likes having the kinks worked out. Visually inferring PM2.5 concentrations can be tricky for a number of reasons, but the view toward the Oquirrhs seemed to confirm that this morning concentrations may have been higher above the valley floor as one could see the base of the Oquirrhs (red arrow) but not the mid or upper elevations (blue arrow).  This doesn't mean the valley floor had low PM2.5 values, but just that they appeared to be lower than at mid elevations.  

The National Weather Service has a modeling system to predict wildfire emissions and smoke transport and concentrations known as HRRR smoke. Its analysis for 1300 UTC (0700 MDT) this morning actually showed higher concentrations in upper elevation areas (e.g., Stansbury, Oquirrh, and Wasatch Ranges) compared to the intervening valleys. Although concentrations were lower than observed, it at least had the right idea. 

The HRRR smoke forecasts suggests perhaps some improvement later today and tonight.  We will see if that pans out.  Smoke forecasts are tricky.  

Note that the situation this morning with lower PM2.5 in the valley cannot be generalized to all wildfire situations.  There are times when PM2.5 concentrations from smoke can be higher at lower elevations.  Escaping wildfire smoke requires more investigation (and maybe a bit of luck).  The HRRR smoke forecast can be helpful.  I like to access them at https://sites.gsl.noaa.gov/desi. Select "HRRR-CONUS" as the dataset, an appropriate runtime (those starting at 0, 6, 12, or 18Z will extend out 48 hours whereas the other go to 18 hours but are more recent), and then "surface smoke" from the surface drop down menu at top. 

Cooler, Smoky Air

It felt almost pleasant this morning as the the low temperature was 70 or possibly 69 at the Salt Lake City Airport.  70 is actually the average low at the airport, which just goes to show how insane summer has become around here that it feels cool and refreshing when we finally have an average low.  Average in this case is based on 1991-2020, so if we went back further, we're probably still running above the 20th century average.  

Countering the coolness was the smoke, which thickened overnight.  PM2.5 concentrations at Hawthorne Elementary climbed substantially after midnight and are now above 40 ug/m3, in the unhealthy for sensitive groups category.  

Source: MesoWest

Yesterday's MODIS imagery shows several major sources of smoke in the Great Basin and Pacific Northwest including fires on the eastern slopes of the Cascades and in northeast Washington that exploded over the weekend, sadly causing serious losses of homes and structures in Spokane.  There are also fires in Oregon and southwest Idaho that are major smoke producers, as well as the Widemouth Fire in central Utah.  

Given the weak trough that came through last night, which resulted in a shift of the flow to northwesterly, the source of the smoke in the Salt Lake Valley today is not the Widemouth Fire but sources to the northwest and west.  

Sadly there's still a lot of summer 2026 to go.

Preparing for Wildfire

Fire is on the minds of many of us who live in Utah and the western United States.  Conditions favorable for explosive wildfire spread are increasing in severity and frequency.   More and more I realize that my "instincts" about wildfire risk in my region, neighborhood, and home probably do not match reality. 

I encourage everyone to take a look at an article written by Tom Cova, a Professor in the University of Utah's School of Environment, Society, and Sustainability, that was published in today's Salt Lake Tribune.   

The article emphasizes the importance of being ready to evacuate given the fact that "fast-moving fires compress every step of an evacuation."  As Eisenhower said, "Plans are nothing, planning is everything."  This is especially true during extreme wildfire spread.  

Natural Air Conditioning

Residents of the northern Wasatch Front and northern Salt Lake Valley were treated to some natural air conditioning thanks to a complex of thunderstorms that moved across the area yesterday evening.  

Source: ral.ucar.edu

Some of the thunderstorms were severe and produced wind gusts of 60 mph in Sugarhouse, 64 mph in Tremonton, and 72 mph at the I215/I80 exchange.  

There was also beneficial rain and cooling. Temperatures at the Salt Lake City International Airport dropped to 67°F, the lowest temperature observed there since July 4, and I was able to get temperatures in my house below 70°F for the first time since we returned from Austria.  

We were also treated to a beautiful double rainbow as the storm moved off to the east.  

Unfortunately, as has been forecast for a few days, the heat is expected to return in spades by the weekend, with the National Weather Service issuing an Extreme Heat Watch for SaturdayAfternoon through Sunday Evening with temperatures up to 105°F and lows in the low to mid 70s for the Salt Lake Valley and other lowland areas of northern Utah.  

The cool was brief but fun while it lasted. 

All-Time Record Tied

Yesterday (Monday) morning's minimum temperature at the Salt Lake City Airport was an incredibly warm 85°F.  Daily records though are based on the minimum and maximum temperatures over the period from midnight to midnight local standard time and last night temperatures dropped to 82°F by midnight local standard time (1 AM MDT), so that stands as the minimum temperature for the day. 

Still 82°F ties for the highest minimum temperature at the Salt Lake City International Airport.  Other days that only cooled to 82°F are 23 July 2023 and 13 July 2021.  

The top 20 highest minimum temperatures in the Salt Lake City area are below.  17 of them are in the 2000s, which likely reflects multiple factors including climate change, urban growth, and local conditions at the observing sites.

Source: https://xmacis.rcc-acis.org/

All occurred though between July 3rd and 31st.  More evidence of the misery of July.

"Hotter than Hell"

I heard a Catholic priest say in downtown Salt Lake City this weekend that the weather is "Hotter than Hell."  Who am I to argue with a man of the cloth?

That said, let's take a quick look at the data and forecast as we approach the end of July.

The month actually started out with four days with minimum temperatures between 62 and 66 at the Salt Lake City International Airport.  And believe it or not on July 22nd, we actually had a maximum of only 87°F.  Other than thoat, it's been hot.  Since July 4th, every day but one has had a minimum temperature at or above 70°F and there have been nine days with a maximum temperature at or above 100°F, including the all-time record high of 109°F on July 12th.  

For the first 25 days of the month, the mean temperature at the Salt Lake City International Airport was 85.3°F.  This is not a record, but it is 4th all-time in the Salt Lake City area and 4.2°F higher than any equivalent period in the 20th century.  If you are wondering, the 20th century record is 81.1°F in 1985.  

The period also rates as the 4th warmest at the Bountiful Bench observing site with a mean temperature of 81.4°F, which is 3.1°F higher than any equivalent period in the 20th century.  There the 20th century record is 78.3°F in 1988.  

July is a month with less weather variability in Utah than we see in winter. On average, as shown in the mean 500-mb height analysis below, there is a mean 500-mb ridge to our east, with the axis centered over Colorado and Wyoming.  This results in the mid-latitude storm track being to our north.  Sometimes a midlatitude system can sneak in here, but it happens less frequently than in winter.  Instead, our weather is controlled more by variations in the strength and position of the ridge, which is a component of what is known as the North American Monsoon System. 

Source: https://psl.noaa.gov/cgi-bin/data/composites/printpage.pl

The forecast for the evolution of that ridge over the next several days does look hell like.  Below is the GFS forecast valid this afternoon [0000 UTC 28 January (1800 MST Tuesday)].  The 500-mb ridge (upper left panel) is centered to our east in northeast New Mexico.  Over northern Utah, the 700-mb temperatures (lower left panel), which are roughly at 10,000 feet or crest level, are around 18°C, which is quite high, but not record setting. 

That ridge though is expected to move slowly westward over the next few days and by 0000 UTC 2 August (1800 MDT Saturday) it is forecast to be parked over Utah.  The 500-mb heights are above 6000 meters, which is a value that typically draws the attention of meteorologists.  Forecast 700-mb temperature are exceptionally high and over 22°C in northern Utah.  In fact, at 6 PM on Saturday they are forecast to be 21.6°C, which would be more than 1°C higher than observed in the upper-air sounding record from the airport. 

The ridge in the ECMWF HRES is also parked over Utah, but not quite as strong.  That said, the 700-mb temperatures are also over 20°C in northern Utah and 22°C in portions of southern and central Utah. 

So, there is no rest for the wicked controlling the weather this week.  Lows at the airport will remain in the mid 70s or higher.  Maximum temperatures will be near or above 100.  If we're lucky, we'll get a thunderstorm, although the likelihood of that will be declining later in the week as the ridge moves overhead.  Temperatures will climb later in the week.  It's too soon to say if we might make a run for 105+, perhaps on Saturday, but it's in the cards. 

Hotter than Hell indeed.

Source: https://chatgpt.com/

Addendum: 3:50 PM MDT

The overnight low at the airport was an incredible 85°F.  The all-time record is 82°F.  That is a daily record, however, which means from midnight to midnight standard time, so for the 85°F to hold for an all-time record, we need to stay above that until 1 PM tonight.