Wasatch Weather Weenies

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 snowf...

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  https://attheu.utah.edu/facultystaff/bike-unity/ for details....

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.  

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, m...

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 vacc...

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 thi...

 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.  

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 str...

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....

Tonight is the opener for the University of Utah football team.  Masks aren't required, but clear bags are.  Plan accordingly! Have you bought your clear bag yet? As a reminder, Rice Eccles Stadium only allows clear bags! Make sure you're ready for tomorrow's game with one of your own. Buy one here: https://t.co/iwvOOuwtuP pic.twitter.com/RV0XzSTrLi — Utah Red Zone (@UtahRedZone) September 1, 2021 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 Septembe...