Monday, September 27, 2021

The Week Ahead

Last weekend was spectacular in northern Utah with valley high temps in the 80s, blue skies, and fall colors.

Brief change is coming in the form of a cold front and upper-level trough that will move in tomorrow.  These features are, however, affecting our weather already by transporting smoke into the area, which was apparent on my ride into work this morning.  

Although the trough will bring cooler weather, right now, most of the model forecasts have the precip skipping northern Utah. Below are statistics based on the 87-hour period through 1800 UTC (1200 MDT) 30 September.  Mean precipitation and the probability of precipitation > 0.01" show a clear band of low values through most of western and northwestern Utah.  It appears the trough will produce precipitation mainly in the northwest and then east and southeast of northern Utah where it plays with some monsoon moisture.  

A small number of SREF members produce some light precipitation and snowfall amounts at Alta.  

Most likely we will see some fits and starts of precipitation, but nothing significant.  What a shame.  With the start of the water year later this week on October 1, we could use some rain to start raising soil moisture levels.  

Following the frontal passage, temperatures will rebound and smoke permitting next weekend looks like a good one.

Monday, September 20, 2021

About Last Night

The coldest airmass of the season so far is now in place over Utah.  A few low temperatures overnight reported to MesoWest are:

Alta-Mt. Baldy: 21˚F
Alta-Base: 28˚F
Cardiff-Big Cottonwood: 27˚F
Silver Creek Jct: 30˚F
U of U upper campus: 41˚F
U of U lower campus: 45˚F
KSLC: 47˚F (could be 46)

Sorry to disappoint those of you enamored with cold temps in Peter Sinks, but it only go down to 31˚F there.  

As an added bonus, unlike the post-frontal air in recent weeks, this one is quite clean.  PM2.5 concentrations are at or below 1 ug/m3 at the U, about as pristine as it gets.  

My ride in this morning was brisk.  Probably the first day since at least early June that I pulled on the uninsulated but full-fingered gloves.

Speaking of biking, U Bike Week starts on Thursday, Sep 23.  There are a number of activities planned including a free breakfast next Thursday, Sep 30.  See for details.  

I wish more people rode to campus and have been hoping that the emergence of e-bikes might break down barriers for some.  I used to ride my mountain bike to work, but switched to an e-bike a couple of years ago as it made it easier to ride up the hill home in my work clothes.  Try one if you haven't already.  It's a fun way to commute to the U.  

Sunday, September 19, 2021

Foliage and Trail Update

More colors this weekend than last in Big Cottonwood Canyon, but many trees remain green.

There's been some remarkable trail work between Mill D North and Dog Lake, presumably by the Uinta-Wasatch-Cache National Forest and possibly partners.  Hats off to them for this upgrade, which is a major improvement that should be far more durable than the old trail and is a better option for aging hikers like myself.  

Given the ongoing Avenues Foothills trail imbroglio, I am reminded that the world will be a better place with fewer roads, less development, and more and better trails.  

Friday, September 17, 2021

Keep an Eye to the Sky this Weekend

It's a somewhat surprisingly complicated weather forecast for the weekend, but not that atypical for September as we transition from the so-called monsoonal flow pattern to the cool-season westerly storm track. 

We'll begin with a look at the GFS forecast valid 0600 UTC 18 September (0000 MDT Saturday).  At this time, a deep upper-level trough (black contours) and frontal system are pushing onto the northwest coast.  Ahead of this system is a much weaker trough over California.  It doesn't look like much, but it contributes to a surge of monsoon moisture (color contours are precipitable water – a measure of the total atmospheric water vapor) into Utah (red arrow).  

As the deep upper level trough and cold front push into the Pacific Northwest, the weak trough slowly moves northeastward and the monsoon moisture streams into northern Utah.  By 1800 UTC 18 September (1200 MDT Saturday) the GFS is generating some scattered precipitation in northern Utah, mainly in a narrow region coincident with the monsoon surge. 

Thus, I would be aware of the threat of showers and thunderstorms tomorrow in northern Utah, especially from about noon through midnight.  These storms could be strong, as indicated by the NWS infographic below.  

Source: NWS, downloaded 8:35 AM MDT 17 Sep 2021

As indicated by that graphic, keep an eye to the sky and on forecasts and radar imagery if you have outdoor plans.  It's not possible to precisely predict the timing and distribution of these storms a day in advance.  

Your outdoor plans on Sunday should also consider the weather forecast as that is when the upper-level trough and the cold front arrive.  The timing of the surface front varies by a few hours amongst the various models.  The NAM brings it through the Salt Lake City International Airport after 6 PM MDT Sunday, whereas the GFS is much faster and brings it through just before 3 PM.  

The Euro also calls for frontal passage closer to 3 PM.  Based on this, I suspect the morning will be prefrontal, with the front coming through at some point in the afternoon.  Expect the temperatures to drop precipitously following the frontal passage and on Monday morning they will be around 20˚F at 11,000 feet.  

Although cold enough for snow in the higher elevations Sunday night, most of the model runs are favoring little to no snowfall at this time.  Below is the downscaled SREF for Alta-Collins as an example and of the 26 members, only 1 generates more than 2" of snow and the rest are 0.6" or less.  

  Still, I suspect the airmass, even in the valley, will definitely make it feel like fall on Monday.  

Thursday, September 16, 2021

Coronavirus/COVID Update

Sharing below this week's COVID-19 update by Dr. Michael Good, CEO of University of Utah Heath.

The good news is that COVID-19 cases on the University of Utah campus have been quite low, with an average rate so far of 9.1 per week.  We now have 83% of University employees and 70% of students fully vaccinated.  Another 6% of students are partially vaccinated.  Let's push these numbers higher and continue to mask.  I thank the students enrolled in my class for their consistent masking.  They have made the semester easier and less stressful for me and I suspect their fellow classmates.    

The bad news is that the coronavirus continues to rage through the unvaccinated population in Utah. Hospitalizations and ICU levels are now almost as high as they were during the peak of the pandemic last fall and winter.  Cases amongst children (1-14 years old) are much higher this fall than last.  In contrast, cases and hospitalizations amongst the vaccinated are much lower and the former has even begun to go down the past couple of weeks, perhaps due to adjustments in activities and behavior or greater masking amongst the vaccinated.  

I have blogged previously about the threats of digital misinformation to society (see Challenges of Science Communication in the 21st Century, posted just before coronavirus exploded in the U.S.).  We are seeing this playing out now.

Wednesday, September 15, 2021

Let's Not Lose Our Minds

Over the past several days, the extended range models have been suggesting that we will see a switch to cooler weather later this weekend or early next week.  The latest (0600 UTC) GFS forecast for 1200 UTC (0600 MDT) Monday morning is unlike anything we've seen in some time with a deep upper-level trough centered over the interior west and Utah.  

700-mb temperatures on Monday are as low as about -4˚C, certainly cold enough for snow down to about 7000 or 7500 feet. 

If you must ask, here's a peak at our downscaled NAEFS product.  Most members generate 0-3" of snow, but there are a few that are a bit more enthusiastic.  

A lot can happen in the next 5 days.  I wouldn't read too much into detailed scenarios like lake-effect potential at this time.  It's going to get colder, but let's see how forecasts evolve over the next few days before worrying about fine-scale details.  

Also, no need to panic about persistent weak layers from this storm.  Usually September snow melts out nearly everywhere if not everywhere and that will certainly be the case if we only get a couple of inches.  

Sunday, September 12, 2021

Smoke-Free Wasatch!

 Today was an absolute peach of a day with smokeless air, blue skies, and hints of fall.  

We've had few days like this over the past several weeks and I'm glad I was able to take advantage of it.  

I hiked up Gobblers Knob via Butler Fork and Mill A Basin.  If you're curious about the leaves, things are just starting to get going with a few trees and bushes changing as illustrated in the pictures above and below.  

The aspen groves of Mill A Basin and upper Big Cottonwood are still green, although if you look carefully, you can see one patch of yellow along the Park City ridgeline in the 2nd photo below.  

No snow yet, but it is acceptable to click on the photo below and start dreaming.  

The fall equinox is only ten days away.  

Thursday, September 9, 2021

The Winter Outlook

I'm not much of a fan of long-term outlooks for snowfall in northern Utah's mountains.  There is a tendency in the western United States for the southwest to be dry when the northwest is wet and vice versa, so northern Utah sits near the transition zone between these two regimes.  That tendency is sometimes referred to as a precipitation dipole, and it is related to the see-saw between El Niño and La Niña in the tropical Pacific Ocean, with the former favoring a wetter southwest and the latter a drier southwest, although such correlations aren't perfect.  

For this winter, it is looking increasingly likely that La Niña conditions will predominate.  This means cooler than average sea-surface temperatures in the tropical Pacific east of the dateline and warmer than average temperatures in the tropical Pacific west of the dateline.  This affects the location of tropical thunderstorms and related precipitation complexes and, in turn, the position of the jet stream in midlatitudes.

The National Weather Service Climate Prediction Center (CPC) currently expects La Niña to develop in the next couple of months, with a 70-80% chance of La Niña during this winter (Dec-Feb).  For more, see the diagnostic discussion they issued this morning.  

Frequently seen characteristics during La Nina winters include a colder northwest North America, a wetter US Pacific Northwest, and a drier southern United States, as depicted below.  

There is, however, variability from event to event, so I think of such patterns as a loading of the dice.  And, because northern Utah is in the transition zone between the wet northwest and dry southwest, there's not much useful correlation between La Niña (or El Niño) and snowfall at Alta.  This is illustrated by the graph below, which shows the 3-month accumulation of snowfall at Alta Guard compared to an index of La Niña/El Niño strength (more discussion of this at  

In my view, the future of seasonal forecasting lies with numerical modeling.  Such forecasts are, however, in their infancy and perhaps where numerical weather prediction was in the 1970s.  Seasonal forecasting requires the ability to simulate not just the atmosphere, but the oceans, the biosphere (plants and their response), and the cyrosphere (ice and snow coverage) and their interactions.  We are making great progress in these areas, but we need higher resolution modeling, better "coupling" between these Earth system components, and much larger ensembles.  

That being said, we can take a look at what these modeling systems are producing for this winter (December-February).  Forecasts of precipitation anomalies (departures from average) from several modeling systems are below.  One can see the influence of La Niña in these forecasts with most systems producing a drier southwest and southern United States and a wet Pacific Northwest.  One outlier is the GEM_NEMO modeling system, which is keeping the entire west coast dry.  

Source: CPC,

These models do not resolve the fine-scale terrain effects of the western United States.  This is just one of many shortcomings.  Some call for northern Utah to be wetter than average (e.g., NCEP_CFSv2).  If you are looking for hope, none have the region around the central Wasatch below average, although I don't put much stock in that.  

We can also look at the temperature anomalies, and here we see most models are calling for a warmer than average winter across most of the continental US, although there is one exception, the GEM_NEMO again which goes for a colder than average western US.  

Source: CPC,

Putting all of this together, I'm concerned about this winter extending the drought in the southwest US.  Although not a guarantee, the dice seem to be loaded for below average precipitation and above average temperatures.  It's tough to say what will happen in the northern Wasatch given that we're in the transition zone. Ski it if it's white.  

Tuesday, September 7, 2021

Smoky Forecasts

It's been a long, hot summer with frequent periods of poor visibility and air quality from fires in California, Oregon, and other regions.  

A lot of people have asked me about smoke forecasts, so here's a quick primer.

The primary tool that I use for smoke forecasts is the High Resolution Rapid Refresh (HRRR) forecast model run by the National Weather Service. Sometimes referred to as HRRR-smoke, to my knowledge, this is the only current modeling system that predicts smoke transport across the contiguous United States.  This requires estimates of smoke emissions from fires and techniques to transport and remove smoke aerosols within the HRRR modeling framework.  I believe it also has the capacity to simulate the effects of smoke aerosols on cloud and precipitation processes, which is sometimes referred to as aerosol-aware microphysics.  

It's important to recognize that the injection, transport, mixing, and removal of smoke aerosols varies vertically.  Below is an example of a shallow smoke layer over Little Cottonwood Canyon this past Saturday.  A variety or processes must be accurately simulated to account for these effects.  

The HRRR is run hourly to 18 hours in the future and every six hours (at 0, 6, 12, and 18 UTC) to 48 hours.  I tend to favor the latter runs with their longer forecast periods.  I don't know of many sites that provide HRRR-smoke forecasts, but will discuss two here.  The first are the free forecast graphics from NOAA that are available at  A number of products are available including near-surface smoke, 1000 ft above ground level (AGL) smoke, 6000 ft AGL smoke, and vertically integrated smoke.  I typically use the near-surface smoke for anticipating ground-level air quality and the vertically integrated smoke for anticipating impacts on sunsets and other effects.  

For example, in the forecasts below from this morning's 1200 UTC initialized HRRR, the near-surface smoke concentrations along the trough forecast to extend from Oklahoma to the Great Lakes are relatively low, but the vertically integrated concentrations are much higher.  In that region, air quality impacts will be small, but one would expect a deep orange or red sunset due to the existence of smoke aloft.  

Source: NOAA/GSL

Source: NOAA/GSL

The HRRR-smoke graphics are conus scale and don't provide a lot of detail.  If you want more regional or local graphics, you could subscribe to or

Source: OpenSummit

One thing to keep in mind is that these are model forecasts so the George Box quote "All models are wrong, but some are useful" applies.  Adjust your plans as needed and consider a higher error tolerance if you are someone who is more sensitive to pollution.  I typically use the HRRR-smoke forecasts for planning, but adjust outdoor pursuits based on air quality observations and visual evidence.   I haven't been living in a cave this summer, but I shifted the timing of my activities and shortened their duration depending on the conditions.  

Thursday, September 2, 2021

Stadium Safety

Tonight is the opener for the University of Utah football team.  Masks aren't required, but clear bags are.  Plan accordingly!

The Storm Prediction Center has us in the marginal risk category, meaning that isolated severe thunderstorms are possible.

Source: SPC

If I were attending the game, I would keep this in mind.  Things could work out just fine, but if there is a thunderstorm in the vicinity, it is best to take appropriate measures, which likely means moving to a hard-topped vehicle such as your car.  Stadiums are generally not safe from lightning, even in covered open-air areas.  I always get a pit in my stomach when a thunderstorm is near Rice-Eccles stadium, most recently in September 2019 when there was lightning near the stadium, the camera people left due to lightning concerns, but the game went on (for more see  

Hopefully the storms say away and the Utes win convincingly.