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.