The Mt. Timpanogos Lightning Fatality

Friday was a remarkably active thunderstorm day in northern Utah, including a verified tornado in the Wasatch Mountains east of Fruit Heights.  

Sadly, a 19 year old man was killed by lightning on the summit of Mt. Timpanogos Friday morning.  Media reports suggest the victim was taking shelter during the storm in the shack on the summit, with authorities called around 6 AM.  Lightningmaps.org provides access to strike data for 24-hour periods.  The map below is for 0600 UTC 18 September to 0600 UTC 19 September (Midnight to Midnight MDT Friday) showing the high density of strikes in the Mt. Timpanogos area and surrounding region.  The prevailing flow was such that the storms on Friday were moving from southwest to northeast and you can see a strong clustering extending from southwest of Utah Lake over Mt. Timpanogos.  

Source: https://www.lightningmaps.org/

It's unclear precisely when the lightning strike occurred, but at the time of the reported call to authorities, a strong cell with maximum reflectivities > 50 dBZ was located just to the southwest of Mt. Timpanogos with lower reflectivities extending downstream of the summit.  Lightning can occur many miles away from the strongest cells, including as much as 25 miles from rainfall and beneath the anvil that spreads downstream from the storm, so this could be the cell that produced the lightning, although this requires confirmation with a better estimate of the time of the strike and analysis of the lightning data.  The earliest reported strike I could find on Mt. Timpanogos was at 1223 UTC (0623 MDT) and there was one later on the summit at 1415 UTC (0815 UTC).  These are a bit later than the reported call and produced by cells that developed later than the image below. Lightning detection is also not perfect so it is possible that the strike was missed.  More work is needed to piece together the timeline.  

I don't know anything about the circumstances leading up to Friday's fatality and as a long-time hiker, skier, and backpacker, I've certainly been in some less-than-desirable situations when it comes to thunderstorms (and other weather events).  In fact, I decided to become a meteorologist after experiencing a thunderstorm on a backpacking trip with my father.  I never wanted that to happen again!  

So let's spend some time on lightning safety review.  For those of us who travel in the backcountry, well away from substantial buildings, the pamphlet below provides useful guidance for lightning safety from the National Weather Service and National Outdoor Leadership School.  

Source: https://www.weather.gov/media/owlie/backcountry_lightning.pdf

It sounds like the victim and maybe others were taking shelter in the shack on Mt. Timpanogos.  Perhaps this was to get out of the rain or possibly hail that was reported in other storms.  Unfortunately a shelter like that does not provide lightning protection.  This is true not only of the shack on top of Mt. Timpanogos, but many beach, golf, picnic, and other shelters if they are not equipped with a lightning protection system. Such shelters are often designed for sun or rain protection, but often do not provide any lightning protection.   More substantial buildings like most homes in the United States are not equipped with a lightning protection system, but the wiring and plumbing provide some protection from the lightning electromagnetic pulse.  This is why it is best to move indoors into a house or substantial building when you hear thunder.  The biggest threat from lightning in such buildings is fire, but there's usually time to move to safety if that occurs.  Metal-topped vehicles like cars also provide protection, but soft-topped vehicles, convertibles, golf carts, and side-by-sides do not. Hence the National Weather Service recommendation below.

If you are interested in the details of what buildings and shelters do and do not provide lightning protection, see Use Caution with the Word 'Shelter' in Lightning Safety by Daile Zhang and coauthors.  

Northern Utah Mesocyclone

The NWS issued a tornado warning this afternoon for an area including portions of Weber, Davis, and Morgan Counties.

Of concern is one of the best developed mesocyclones (a rotating vortex about 10 km across) I've seen in northern Utah.  Radar reflectivity showed a classic "hook echo" associated with the storm as it impinged on the northern Wasatch near Farmington. 

NWS Doppler radars measure the radial velocity toward or away from the radar, so mesocyclones produce a very clear signature of rotation with a couplet of inbound (green and blue) and outbound (red) velocities colocated with the hook.  This is indicative of the small scale rotation associated with the storm. 

The mesoscale rotation does not necessarily mean that there is a tornado.  About 25% of mesocyclones produce tornadoes, but that's certainly enough to warrant a warning in this case.  

Let's hope this was just a well-developed mesocyclone that didn't produce a tornado.  

Latest CPC Seasonal Outlooks

CPC released their latest seasonal outlooks yesterday.  These are based on overlapping 3-month periods.  For November to January and December to February, all or nearly all of Utah is in "equal chances" for precipitation, which means there is no significant shift in the odds for below normal, near normal, or above normal compared to climatology.  

Source: https://www.cpc.ncep.noaa.gov/products/predictions/90day/

In other words, that period could go any way.  There's no useful skill.  

Later in the winter, during January to March and February to April, most of Utah is in the first level of "above normal" or, in the case of southern Utah, especially later in the winter, in the second level.  For southern Utah, this means there is an increase in the odds of above normal precipitation to 40-50%.  For northern Utah, that shift is much smaller and perhaps to around 35% or so for the Wasatch Range.  

Source: https://www.cpc.ncep.noaa.gov/products/predictions/90day/

In other words, something like a 36% chance of above normal, 33% of normal, and 31% of below normal.  

The long and short of this is that the far southwest has the highest likelihoods of above normal.  These odds decline and northern Utah is pretty much in a spot where there is not a lot of weighting of the dice.  The outlook this year largely reflects historical influences of El Nino and strong El Ninos, although predictions by climate forecast systems (numerical models that forecast several months in advance), and other tools are also used.

So, whether we see an above average snow season in the Wasatch this winter is still pretty much up in the air.  My advice is to just ignore all these predictions for the Wasatch and spend that time getting into shape and enjoying the outdoors.  

Status of weather.utah.edu

Some of you have asked about the status of weather.utah.edu as it has been down for off campus access now for at least 24 hours.  

I was told that "the campus default deny seems to be blocking weather.utah.edu from outside of campus."  I really have no idea what this means other than you can't get to it from off campus.  The people that help me with the web site told me they have contacted the powers that be, but it remains down.  

Those of you who are affiliated with the University of Utah, including my students, can access weather.utah.edu if you fire up the campus VPN from an off campus location.  That's the best I can do for now other than hoping we'll be back in business soon. 

I'll just say that the struggle at the U is real.  

An Out-on-the-Limb Maximum Temperature

Yesterday's high of 93°F at the Salt Lake International Airport was a record for the day but not really all that exceptional when you consider that the record high on the previous day (September 11) is 99°F and a couple days later (September 15) is 97°F.  Somehow September 13 has never gone out-on-the-limb for maximum temperature compared to the surrounding days.  Such are the realities of climate statistics. 

That said, yesterday's maximum temperature was out-on-the-limb in terms of what was forecast.  Several days ago, we posted that the forecast for tomorrow (Sunday) was uncertain.  Some models, like the GFS, were bringing in a cold front on Saturday, resulting in much cooler weather for Sunday.  Others, like the ECMWF AIFS brought the front in during the day on Sunday.  I'm not sure if any called for the cold front to come in Sunday evening.    

These days one can process the forecast from well over 100 ensemble forecasts to produce a range of possible forecast outcomes.  This is what the National Weather Service does for their National Blend of Models (NBM) forecast.  On Tuesday we posted about the enormous spread for the maximum temperature forecast for yesterday (see An Uncertain Sunday) with the middle 50% of high temperature forecasts lying between 68° and 84°F (see the tallest red box in the left-hand graphic below).  

That middle 50% is sometimes referred to as the interquartile range.  However, half the forecasts lie outside of that box.  The whiskers surrounding the box extend through the middle 80% of forecasts (or from what is called the 10th to 90th percentile).  In this case, the top whisker extended to 92°F.  That means that 90% of the forecasts called for a maximum temperature at or below 92°F.  

I went back to that NBM forecast distribution and it turns out it ended at 92°F.  In other words, not a single forecast member called for a maximum at or above 93°F.  The observed maximum temperature on Sunday was outside of the entire predicted ensemble range.  

Ideally that would not happen except in the most exceptional circumstances, but our ensembles are not perfect.  Forecasts do sometimes verify outside of the ensemble range and that's a sign that we still have work to do.  

Part of what made the forecast for Sunday challenging was the occurrence of something called anticyclonic wave breaking (AWB).  If you have been to the beach, you have experienced wave breaking.  As water waves approach the shore, they decelerate and steepen.  Eventually the waves steepen to a point where they overturn and break down into turbulence, or what scientists call wave breaking.

Breaking wave. Source: NOAA.

The waves you experience at the beach are gravity waves that occur at the interface between the high-density water and the lower-density atmosphere.  Gravity waves also occur in the atmosphere where gravity wave breaking is one important cause of aircraft turbulence. 

In AWB the waves that are breaking are not gravity waves but very large-scale atmospheric waves known as Rossby waves.  Rossby waves are named after Carl-Gustaf Rossby, who did pioneering work on the jet stream and the dynamics of upper-level waves in the middle 20th century.  He was even featured on the cover of Time Magazine in December 1956.

Source: https://content.time.com/time/covers/0,16641,19561217,00.html 

Rossby waves are the large-scale upper-level troughs and ridges that we observe at upper-levels (i.e., jet-stream level) in the atmosphere.  These waves are so big (1000 km or more across) that the rotation of the Earth strongly affects their behavior.  

Like waves at the beach, Rossby waves can also "steepen."  In contrast to water waves that steepen in the vertical, Rossby waves "steepen" in the north-south direction, which is what happened last week over the north Pacific. At 1200 UTC 10 September, there was a ridge over western Alaska and a trough over the northeast Pacific.  This ridge-trough system amplified over the next 24 hours, with the 500-mb height contours "steepening" and turning clockwise (anticyclonically) on the downstream side of the ridge.  Eventually, the 500-mb height contour "broke" in the sense that higher 500-mb heights were north of lower 500-mb heights.  This is consistent with the red contour crossing the purple meridian three times at the end of the plot sequence below.  Think of the ridge pushing over that meridian as an overturning wave with the surfer in the the tube or trough at lower elevations.  

AWB is associated with the amplification of a strong upper-level ridge, which is often related to the release of heat by condensation in precipitation systems just upstream of the ridge.  Although AWB events are often predicted reasonably well in terms of their occurrence, the details (i.e., the "turbulence") can be hard to get right.  Sunday was a hard forecast because the strength and position of the trough and cold front was challenging to anticipate.  The 93°F we hit on Sunday reflects the fact that the AWB produced a stronger trough than anticipated, delaying its progression across the Pacific Northwest.  

Surfers can be surprised by breaking wave behavior.  So can meteorologists.  

Reset of the Central Wasatch Snowpack

This is not a comprehensive sample, but it appears that all of the persistent snow patches that linger deep into summer and fall that I can typically spy from the top of Snowbird are gone.  

I hiked up the bird this morning and it was about as clear as it gets when I arrived on the summit.  I couldn't spy a single patch of snow below the American Fork Twins (this has been the case for at least a couple of weeks) or in upper Hogum Fork where snow tends to linger.  

Thus it appears that this summer we have had a full and complete reset of the central Wasatch snowpack, although perhaps one of you out there can tell us that there is a patch lingering somewhere.  Central Wasatch does not include Mt. Timpanogos where it is more likely that something remains around the Timpanogos Snowfield and rock glacier.  

Snowbird though is preparing to do something about this as I noticed they are beginning to deploy their snowguns.  

Meanwhile, Ski Utah is pinning all of their hopes on the Super El Niño.  One of my students gave me the sticker below from yesterday's Utah Avalanche Center Backcountry Benefit.

It remains hard to say if a vote for Super El Niño will yield change we can believe in.  I suspect a continuation of the incredibly poor status quo from last winter is unlikely, but whether a change in the majority climate party turns inflation (i.e., rising temperatures) around and results in improved job (i.e., powder) prospects for Wasatch citizens remains uncertain.  

Thank You NOAA

Yesterday I submitted a final report for the last grant I received from the National Oceanic and Atmospheric Administration (NOAA), which ended on July 31.  This brings to an end 28 years of continuous funding from that agency, so I'd like to give a big thank you to NOAA, the NOAA/National Weather Service, and NOAA Oceanic and Atmospheric Research for supporting the work of my graduate students and collaborators at the University of Utah and beyond.  

If my math is right, I've served as principal investigator (PI or lead scientist) or co-principal investigator (coPI or someone with shared leadership responsibilities) on 15 NOAA grants from 1998–2026.  In 1998, we began to receive annual grants from NOAA for weather support for the 2002 Olympic Winter Games, including the installation of weather stations at Olympic Venues and around the Great Salt Lake, the creation of what became MesoWest (led by John Horel), the development of numerical and statistical forecasting systems for northern Utah, and a major field campaign to understand winter storms in the Wasatch Range known as IPEX.  Impacts of this work for regional and Olympic forecasting were described in a series of articles in the February 2002 Bulletin of the American Meteorological Society, coinciding with the 2002 Olympics.  

After the Olympics, we received a series of grants over the next 15 years or so from the National Weather Service Collaborative Science Technology and Applied Research (CSTAR) program, which was designed to transfer basic and applied research into operations through collaborative research between academic groups and National Weather Service Forecasters.  This work enabled continued expansion of MesoWest (led by John Horel) but also efforts to advance understanding of the meteorology of the western United States including the Great Salt Lake effect and atmospheric rivers.  

One key advancement during this period was improved understanding of the inland penetration of atmospheric rivers over the western United States.  This work was led by one of my graduate students, Jon Rutz, who also served as an intern at the National Weather Service while in graduate school.  His paper, Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the western United States, published in 2014, has been cited more than 700 times and amongst other things illustrated key pathways for atmospheric river penetration into the western US interior.  It also illustrated the more "fickle" relationship between atmospheric rivers and precipitation over the interior compared to the coastal ranges, Cascade Mountains, and Sierra Nevada upstream.  This work is important not only for weather prediction, but also for understanding and projecting future changes in climate and snow hydrology over western mountain ranges.  

About ten years ago, as the CSTAR program was reaching an end, Justin Minder at SUNY Albany contacted me about a new opportunity at NOAA for winter weather funding.  This led to three more grants, the most recent ending on July 31, exploring ways to improve snowfall prediction not only over the western US, but across the continental United States.  This was a dream project for me.  Our most recent publication based on this work explores the use of machine learning to improve snow-to-liquid ratio prediction.

Readers of this blog have benefited from this work as many of the forecast techniques we've developed have been tested and evaluated on https://weather.utah.edu/.  The Utah Snow Ensemble, for example, could not have been developed without the capabilities developed with this NOAA funding.  We are working on upgrades to weather.utah.edu that will more prominently feature the many snow guidance products that we've developed over the years.  Below is a preview of what the new site will look like.

Our most recent proposal to NOAA to continue our work was recommended for funding but was ultimately unfunded due to cuts during the DOGE operations.  While disappointing, I've had an unusually productive run of NOAA funding and am grateful for 28 years of solid support.  I am also grateful for talented students and collaborators who have worked with us on these projects and made the journey fun and exciting.  All of my graduate students have benefited from NOAA funding and are now contributing to the success and growth of the weather enterprise.  Thanks again NOAA.  

An Uncertain Sunday

Forecast uncertainty doesn't always grow continuously with forecast lead time.  Sometimes there is an issue with the intensity and timing of a weather system that can cause uncertainty to increase temporarily and then decrease at longer lead times.

A good example is the forecast for northern Utah this weekend.   The models all bring in a strong cold front, but there are variations in the timing and intensity.

The GFS forecast valid 1200 UTC 13 September (0600 MDT Sunday) shows one scenario with the cold front through northern Utah, a deep trough at 500 mb centered over the central Idaho Mountains, and cold post-frontal flow over northern Utah.  700-mb temperatures in far northwest Utah are below -2°C.  

The ECMWF AIFS has different ideas.  The front is much slower and for the same time valid above (0600 MDT Sunday), the front is just getting to Salt Lake City and the upper-level trough is centered over northeast Oregon.  The 700-mb temperatures in northwest Utah are around +6°C, 8°C warmer than the GFS.  

Put these and a gazillion other forecasts into the National Weather Service blend of models and you get a lot of forecast spread for temperatures on Sunday.  Note how the spread of the box-and whisker plot of forecast maximum temperatures (red box-and-whisker with annotated temperatures) gets really large on Sunday.  The middle 50% of the forecasts like within the colored red box which has a range from 68° to 84°F!  

The middle 80% of the forecasts lie between the ends of the whiskers (the white lines that extend from the red box).  The numbers aren't annotated, but the range is between 64°F and 93°F.  And to think your TV meteorologist will have to pick one number to put in their forecast for Sunday afternoon!

Max temperatures on Sunday are as much of a crap shoot as you can imagine and a prime reason not to look at only one model.  This is a classic case of what meteorologists call anticyclonic Rossby wave breaking over the north Pacific and western North America, which introduces uncertainty in the strength and movement of the upper-level trough as it amplifies along the Pacific coast.  I'd check the forecasts in a couple of days if you are planning a weekend backpack to high elevations.  

Flies in the Labor Day Ointment

Weather for the start of the Labor Day weekend today (Friday) looks pretty good.  The large-scale pattern hasn't really changed dramatically for some time.  There's a trough centered over the Pacific Northwest coast and a ridge over the central US.  These features have been around for some time, shifting in strength and position.  Right now, the southwesterly to southerly flow between these features is relatively dry, so shower and thunderstorm activity is suppressed or limited.  Today should be quite nice across most of Utah other than some gusty winds, as indicated by the ECMWF HRES forecast for 0000 UTC 5 September (1800 MDT Friday).  

However, lurking in that forecast is Hurricane Marie off the coast of Baja (Hurricane Karina is also visible in the plot farther to the west).  Marie is expected to move northwestward over the next few days, and therein likes the forecast problem.  Although Marie will weaken, her moisture is going to move into the southwesterly flow ahead of the upper level trough, leading to another surge of moisture into Utah.  The ECMWF HRES forecast for 2100 UTC 6 September (1500 MDT Sunday) shows this well.  Marie (yellow L) has drifted ever so slightly northwestward and with high relative humidity air and strong vapor transport extending to her northeast and into Utah (red arrow).  

Right now Saturday looks good, but the specter or showers and thunderstorms will be on the increase on Sunday as this moisture surge moves into Utah beginning in the St. George area Sunday Morning but then expanding northward.

Although we developed the Utah Snow Ensemble for powder, it does include downscale precipitation forecasts.  Below is the forecast of 6-h accumulated precipitation for Sunday afternoon [1800 UTC 6 Sep to 0000 UTC 7 Sep (1200-1800 MDT Sunday)].  The upper left is the ECMWF HRES showing a band of precipitation extending across central Utah from St. George to the Uintas (upper left), although there is also some precipitation in other areas.  The ensemble mean also keys in on that area (upper right), again with some members producing precipitation in other areas.  

We'll see where this surge ends up along with its associated precipitation, thunder, and potentially flash floods.  Best to check forecasts carefully if you will be recreating on Sunday and Monday.  

Summer 2026 Is in the Books

Meteorological summer (June, July, and August) is now in the books and as usual it is great to have it in the rear view mirror.  

Summer was a scorcher by long-term standards at the Salt Lake City International Airport.  The average temperature was 80.2°F, the 7th highest all time (the record is 81.4°F set in 2022).  A look the long term trends in summer temperatures shows we are no-longer operating in the climate that prevailed during the 20th century.  The 10 hottest summers on record were all in the 21st century when the trend is clearly upward.  

Mean Average Temperature June-August at the Salt Lake City International Airport.  Source: https://xmacis.rcc-acis.org/.

We also set an all time maximum temperature record of 109°F on July 12th, tied the all time high minimum temperature record of 82°F on July 27 (set previously in 2021 and 2023), and saw 20 days with a maximum temperature of 100°F or higher (the record for summer is 27 in 2022 and for the year is 34 also in 2022).  

Precipitation at the airport provides a good example of the devil being in the details.  For the whole summer, the airport recorded 2.63" of precipitation, which is above average (2.02"), but June was well below average (.17" vs. .95), July just a shade above average (.62" vs. .49"), and no measurable rain fell the first 12 days of August.  Thus the summer featured a precipitation deficit through about mid August, when we got into a wetter pattern.  Percent of normal precipitation over northern Utah shows many areas above 300% of normal, although there are a few bands below average.  Many of our storms in the latter half of August moved from southwest to northeast across the region, and this is quite apparent in the map below.  

For me, the summer of 2026 was one in which I fully experienced the ugly face of global warming.  I was in Europe until July 8, experiencing the early end of snow cover in the Alps and the June heat wave.  Then I came home for our hot summer and the 109°F all-time high at the airport.  

I miss the climate of the 20th century.  I suspect I'll miss it more in the future. 

Thunderstruck!

It sure can get exciting around here when monsoon (and tropical storm remnants) can interact with a mid-latitude trough.  This morning in the Salt Lake Valley has been incredible with round after round of heavy rain and thunder.  These are not those high-based "virga storms" that we sometimes get around here, but the real deal with pretty good downpours and loud, crashing thunder.  Love it.

We discussed the ingredients for today's rain in the post from a couple of days ago entitled An Unusual Moisture Pathway.  Those ingredients included the remnants of tropical storm Iselle and a front that was expected to develop over central Nevada.  Indeed these ingredients came together ahead of a deepening trough along the Pacific Northwest coast.  The front is especially apparent at 700 mb (lower left panel below) where there is temperature contrast and wind shift between southern Utah and northern Nevada and Utah.  The large-scale wind pattern is one that meteorologists call confluent and in this case the confluence and temperature contrast are setup to favor precipitation development along the front.  

MRMS precipitation estimates from 0600–1800 UTC (0000–1200 MDT) shows a band of accumulated precipitation aligned along the front with embedded bands of heavier precipitation associated with thunderstorms that moved along the front and from southwest to northeast across northern Utah.  The switch from green to yellow shading occurs at 0.5".  A good fraction fo the Salt Lake Valley saw at least that much.  There are a few areas above 1.0" including portions of the Avenues. 

Source; https://mrms.nssl.noaa.gov/qvs/product_viewer/

Those numbers may be a bit high compared to gauges, but a neighbor reported 0.69" and our lower University of Utah measurement stations are at 0.57" and 0.60".  

It's not over yet.  Showers and thunderstorms remain possible through at least tonight.  Some of those storms could be severe (e.g., wind gusts up to 70 mph, hail > 1 inch, heavy rain, lightning) as indicated by the National Weather Service Storm Prediction Center and Salt Lake City Forecast office.  

Source: NWS.  Downloaded ~1245 MDT 29 August 2026

Snow!

I'll just put these here for some high stoke, low probability snow possibilities.  There are some ensemble members producing snow in northern Utah late next week and weekend.    

OK, truth be told it's three ensemble members out of 82.  

I told you it was a low probability possibility.

An Unusual Moisture Pathway

The remnants of Tropical Storm Iselle are taking an unusual pathway into the western US interior and will be visiting northern Utah this weekend.  

Yesterday morning (1200 UTC 26 August), Iselle's remnants were to the west of the Baja Peninsula.  The color contours below are precipitable water in millimeters with warmer colors indicating higher values.  Precipitable water is a measure of the total water vapor in the atmosphere expressed as the depth of water if all of that water vapor were to condense and fall out.  Iselle's remnants were associated with a maximum in precipitable water above 65 mm.  

Iselle's remnants moved northwestward yesterday and last night.  As this happened, they began to interact with the southwest flow ahead of an upper-level trough off the Pacific Coast, resulting in a plume of high precipitable water movign northeastward toward California. 

That plume spreads eastward and ultimately serves as the locus for precipitation development over Nevada tomorrow afternoon (0000 UTC 29 August).  A relatively strong front also forms in that area during the day tomorrow due in part to daytime surface heating in southern Nevada, which is more cloud free, and northern Nevada which is cloudier and experiencing cooling beneath the precipitation.  This leads to a 700-mb temperature difference across Nevada of almost 10°C at 700 mb.  

That system then slides into northern Utah Friday night and Saturday.  At 1800 UTC 29 August (1200 MDT Saturday), the front is just to the south and east of the Salt Lake Valley with the GFS producing scattered precipitation (probably showers and thunderstorms) over northern Utah.  

A look at the ECMWF AIFS forecast shows it is much more bullish on precipitation Saturday morning.  The plot below (upper right) is a 6-hour accumulation whereas the one above for the GFS is a 3-hour accumulation, but still, the precipitation is clearly heavier and more widespread.  

Neither of these models is able to resolve the details of convective storms, so for now I have a wait and see attitude about how this will turn out.  Saturday though does look cooler and we can all be thankful for that.

What Hast the Thunderstorms Brought?

The last few days have brought regular afternoon and evening thunderstorms to the Salt Lake Valley.  It's worth taking a closer look at the drivers for the storms and who has gotten the rain.  

On average, the 500-mb heights in August are characterized by  a ridge that is centered roughly over west Texas and a trough over or just to the west of the Pacific coast.  This pattern yields mean easterly flow at 500-mb over central Mexico which turns clockwise (cyclonically in meteorology speak) near Baja California before moving into Utah.  

Source: climatereanalyzer.org

The pattern above is characteristic of the so-called North American Monsoon.  This monsoonal pattern has been amplified the past several days.  As an example, below are the 500-mb height anomalies (departures from average) for today.  Positive height anomalies over the western US and Mexico are strongest over west Texas and the Mexican border area to the west, right where the 500-mb ridge is on average.  In contrast, negative height anomalies are strongest just off the Pacific Coast.  

Source: climatereanalyzer.org

Thus, the current pattern is one in which the primary monsoon circulation features, the ridge over west Texas and the trough off the Pacific coast, are stronger than average.  

This pattern has brought us enough monsoon moisture and perhaps some large-scale lift from the Pacific trough to allow surface heating to generate scattered to widespread thunderstorms each afternoon and evening.  These storms have generally moved from southwest to northeast across northern Utah.  This can be seen in the radar-estimated precipitation for the past 24-hours, which covers last nights storms.  Precipitation was heaviest in a band oriented roughly from southwest to northeast near the southern shore of the Great Salt Lake and into the Bountiful area Mountains.  A second band of heavier precipitation extends over what I'll jokingly call the "Herriman rain forest" in the southwest Salt Lake Valley.   

https://mrms.nssl.noaa.gov/qvs/product_viewer/

If we look over a 3-day (72-hour) period, we see that everyone in the valley has seen some rain, but the big winner is the southern Oquirrhs and the Herriman rain forest, with one area near Herriman seeing over 1.25".  

https://mrms.nssl.noaa.gov/qvs/product_viewer/

There's a lot of variability in the valley.  One can find some areas around Cottonwood Heights that may have seen < 0.2", while other areas in the central and western valley are over 0.5".  The maximum near Herriman is over 1.5", but if you move a bit to the east toward I-15 there is a band with < 0.5".  Then there's another streak of heavier precipitation over Draper.  

This is the nature of these hit-and-miss thunderstorms.  I wish we had got a bit more at my place, but am glad we got something.  

ECMWF AIFS Forecasts Now Available

Forecasts from the ECMWF Artificial Intelligence/Integrated Forecast System (AIFS) are now available on weather.utah.edu.  Look for ECMWF AIFS in the left-hand nav bar.  An example forecast is below.  Based on the data available, most of our favored four-panel variables are available, but due to limitations in the data provided, there is no forecast of outgoing longwave radiation (used for a forecast infrared satellite image) in the upper-right panel and the precipitation is six hourly, which is a longer accumulation period than many of our products.  

As illustrated in the link at the top of this post, AIFS forecast skill at 500 mb is roughly the equivalent of the ECMWF IFS (called HRES on weather.utah.edu) through about day four. At longer lead times, it exhibits somewhat better skill.  The AIFS forecasts after 9 days are about as good as the IFS at 8.5 days, so they are basically giving you an extra half day of skill at longer lead times.  That's not enough to abandon looking at the IFS, GFS, or ensembles, but it certainly makes sense to make the AIFS an important edition to your forecast workflow.  

The AIFS forecasts are produced at ~31 km grid spacing and provided on at 0.25° grid.  The latter is similar to what we use for the GFS-0p25deg and ECMWF HRES products on weather.utah.edu.  However, the AIFS forecasts are smoother, especially for precipitation, which is a reflection of the training dataset and techniques.  

In the long run, I am interested in adding the AIFS ensemble to the Utah Snow Ensemble.  That is a 51 member ensemble run at ~31 km grid spacing and available at 0.25 deg.  We have put in for computer time and engineering support on the new University of Utah Redtail supercomputer, but a lot has to fall into place for this to happen.  It's unclear if I have enough bandwidth and compute power to make this happen otherwise, although one never knows.  

Musings on AI and Higher Education

Classes start at the University of Utah this coming Monday, August 24.  Campus is getting busier and energy levels are rising.  The start of a new school year is coming!

I was on sabbatical last academic year and although I did teach for a full semester at the University of Innsbruck, it feels like higher ed has changed dramatically since the last time I taught at the U in spring 2025.  One reason for this is the rapid advancement of AI, including weather forecasting models and generative AI assistants like ChatGPT.

My approach to teaching has been one that has brought new tech into the classroom.  The weather analysis and forecasting classes I teach today use tools and techniques that did not exist when I started as a professor and in some cases even a couple of years ago.  

As we go into the new semester, I've been thinking a lot about how to utilize AI in the classroom but also how to ensure that students are using AI tools responsibly.  The lowest-hanging fruit is bringing AI forecast modeling systems, like the ECMWF AIFS or Google WeatherNext into my lectures and weather discussions and using them alongside traditional numerical weather prediction model forecasts.

ECMWF AIFS forecast.  Source: pivotalweather.com

I'm not sure yet if I'll have time to develop my own graphics (my preference since I can tailor things for the western US and Utah) or add these systems to products like the Utah Snow Ensemble, but we will at least be using graphics from other sites.  

I am more conflicted about generative AI and related tools that may have some potential to accelerate learning but can also be used in ways that prevent students from the hard work and deep thinking needed to develop the creativity and critical thinking skills.

Students need an advanced education that includes the foundational knowledge, expertise, judgement, and vision to extract maximum benefit from AI.  There are no shortcuts to the top. A scientist needs a well trained mind just as an athlete needs a well trained body.  

As an educator, I want to ensure that AI is a powerful assistant for learning and not a training shortcut that leads to cognitive atrophy.  If I can borrow from a recent article by Mya Poe and Dylan Dryer, our students should be charting the path of scientific discovery, not machines.  

Summer Skiing Is Dead

Following up on the post from August 8th (The Death of Summer Skiing), as reported over the weekend by weathertoski, the last operating ski area in the Alps, Passo Stelvio in Italy, closed on Saturday, August 15th.  There are no summer skiing options left in the Alps.  

They suggest this may be the first time there's no (lift served) summer skiing operations in the Alps.  It's likely that applies to the Northern Hemisphere.  Mt. Gassan (Japan) appears to have closed in early July.  Timberline on Mt. Hood appears to have closed on July 19.   I believe late operating resorts in Norway are also closed.  

2026.  The year that summer skiing died. 

The Afternoon Deluge

Monsoon thunderstorms developed over the Salt Lake Valley this afternoon and brought heavy rainfall to the northeast corner of the Salt Lake Valley, especially the University of Utah. 

The event started with the development of a cell over or just downstream of the Salt Lake International Airport.  This one moved off over North Salt Lake.  Shortly therafter, two cells formed just to the south of the track of the first cell and pounded the Avenues and Avenues Foothills.  

Then things got interesting as a cell exploded west of Murray and moved northeastward to the University of Utah.  This was the beginning of a series of training storms that pounded the area around the U before things settled down.  The full sequence is below.  I'm hoping the video will upload properly as I'm working with some limitations currently.  

Storm total Radar precipitation estimates through 2200 UTC (1600 MDT), which does not quite capture the entire storm period so expect these to go up a bit show more than an inch of rain in a SW to NE oriented band from just north of Taylorsville to the foothills above the University of Utah.  

Source: https://mrms.nssl.noaa.gov/qvs/product_viewer/

Maximum accumulations of > 2" were centered on Dry Creek Canyon and Mt. Van Cott, pretty much over and just north of the burn scar above the U hospital.  

Source: https://mrms.nssl.noaa.gov/qvs/product_viewer/

Some transport of falling rain can occur below beam elevation, so exactly where this rain fell on very small scales is hard to say, but this does appear to have been a significant stress test for flash flooding in the burn scar area.  

As I type this, I don't have access to estimates through the end of the storm or information on the burn scar.  Hopefully it held up as that would be a significant victory.  

El Niño Is a Lock But Snow for Wasatch Less Certain

The National Weather Service Climate Prediction Center (CPC) issued their monthly El Niño/Southern Oscillation (ENSO) Diagnostic Discussion today and El Niño is looking like a lock for this winter.  They give a 90% chance of a very strong El Nino during Northern Hemisphere fall and winter.  

By most definitions, a strong El Niño is already in place over the Tropical Pacific Ocean.  Sea surface temperatures more than 1°C above average already extend westward along the equator to nearly 180° with areas more than 3°C above average near the coast of South America.  Anomalously warm sea surface temperatures also extend poleward to the west of North and South America.  

ENSO strength probabilities from CPC are near or above 90% for September-November through November to January.   The odds of a strong event decline later in the winter, although the odds of El Niño conditions persisting remain high.  

El Niño is a tropical climate phenomenon with impacts even in the mid and high latitudes as it affects jet stream and storm track patterns.  During Northern Hemisphere winter (December – February) over North America, El Niño can be associated with anomalous warmth of Alaska, western Canada, and the northwest United States and wetter conditions in the southern US and northern Mexico.  Linkages to the mid and high latitudes are weaker in summer.  

Source: https://www.climate.gov/news-features/featured-images/global-impacts-el-ni%C3%B1o-and-la-ni%C3%B1a

For the Wasatch, the relationship between El Niño and snowfall is relatively weak.  It is popular for people to look at past events for analogs, but there isn't a true analog to this event and strong events are relatively rare during the historical data period, so the sample size is small.  Additionally, El Niño is an important factor, but there are other phenomenon that can influence the weather in any given winter, including the possibility that what happened in a previous strong El Niño year more strongly reflects randomness than a real signal.  The CPC outlook (now almost a month old so maybe there will be an update) still puts northern Utah in the "equal chances" category for December to February precipitation, meaning they don't consider there to be better skill than chance for what will happen this winter in our neck of the woods. 

Source: CPC

They are, however, giving better odds for above average temperatures, although that's not all that surprising given recent trends. 

Source: CPC

My view is that the usefulness and reliability of these seasonal outlooks for anticipating what the ski season will be like is limited in the Wasatch.  In addition to the limited correlation in our part of the world, how the snow comes and how the temperatures fluctuate matter.  A warm winter can yield a good snow year at low elevations if the storm characteristics are right.  A year with above average snowfall due to big snows in November and December is very different from one with big snows in March and April.  

As such, I'm comfortable saying that I can't confidently predict how this coming season is going to play out in the Wasatch.  It's probably better to spend your time getting in shape for ski season than worrying about what the season will bring.