I Got Nothin'

Recent weather has left me with little to say.  Hot, dry, smoky July doldrums with some afternoon convection.  For all intents and purposes, the pattern doesn't change much over the next several days.  The ridge wobbles around, but there are no big changes.  The best we can hope for is a little thunderstorm activity to at least break up the monotony.  Perhaps you felt some drops and wind last night.  Sadly, I didn't.

The National Weather Service National Blend of Models forecast for the Salt Lake City International Airport shows the situation well.  Forecast highs near 100 each day.  Perhaps a slight cool down late next week when a trough passes to our north and the ridge weakens just a bit.  

Sad.  

Temperature Observations: It's Complicated

Yesterday's maximum at the Salt Lake City International Airport was 106°F, a record for the day and just a degree short of the all-time high (107°F).  The airport measurement site has been much maligned by some as being "jacked" (i.e., too high) or unrepresentative by some.  This is a subject that we've discussed previously in this blog (see What's Up @ KSLC and Records Falling).  What I have noticed in social media is that there is a tendency to assume that the airport site is wrong and the other sites are right.  There is also a tendency to assume that the differences in maximum temperature at the various sites are due to either calibration issues or local siting characteristics.  In reality, there are many many factors that affect maximum (and minimum) temperature readings.  

There are tens of thousands of weather sensors in the United States providing data to the internet.  The MesoWest project at the University of Utah (https://mesowest.utah.edu/), which began as a collaborative endeavor with the National Weather Service in the 1990s, provides access to these observations (there are other sites that do this as well).  On the MesoWest, you can plot the maximum temperatures over the past 24 hours, which I did this morning for the Salt Lake City area.  As you can see in the numbers below, there is incredible variability.  The official airport observing site (KSLC) has the highest maximum (106°F).  It is located just south of the airport on the left-hand side of the map.  Sites near it, however, range from 103 to 105, and just bit to the northeast you can find a 101 and 102 near or at the Rose Park Golf Course.  At the University of Utah, the range is even larger, with stations reporting maximum temperatures as low as 97 and as high as 105 (the 105 is covered).  

Some of these variations are real.  Urban areas involve rich tapestries of land use that affect local temperatures.  The Rose Park Golf Course, for example, is heavily irrigated.  Other areas are concrete and built up.  During one late afternoon last summer, analyses of late afternoon temperature produced by NOAA over Salt Lake City using field measurements and satellite data show the relative coolness of the Rose Park Golf Course compared to areas near the airport or to the east of I-15.  

Source: NOAA/CAPA

But there are other reasons why maximum temperatures vary from site to site.  The reality is that many different types of instruments and processing algorithms are used to measure temperature.  Instruments have varying response times (and accuracies) and differing averaging and reporting intervals are used. This is especially important for a maximum (or minimum) temperature, which can be the result of a relatively brief spike.  

As an example, below is a time series of yesterday's 1-min temperature observations from the University of Utah observing site at the mouth of Red Butte Canyon.  The maximum temperature at this site based on 1-minute averaged temperature observations provided at 1-minute intervals was 105°F.  There were two spikes between 1400 and 1600 MDT when 105°F was reached.

Let's suppose that this station instead reported data hourly.  There are some stations that report hourly averages.  The average temperature for the 1-hour period ending at 1500 MDT was 103.1°F and for the 1-hour period ending at 1600 MDT was 103.3°F.  In this case, MesoWest would have reported a high of 103°F instead of 105°F.

Alternatively, let's suppose that this station reports a 1-min average temperature every hour.  In this case, the 1-min observation at 1500 MDT was 103.3°F and at 1600 MDT was 102.1°F.  This too would have yielded a high of 103°F instead of 105°F.  

Many stations do not report a maximum (or minimum) temperature.  They simply provide temperature observations in discrete intervals.  In this case, the "maximum" temperature is actually the highest reported temperature.  The National Weather Service observing site at the airport, however, does report a maximum temperature every six hours.  This maximum could occur between observation times.   Older reports describing the characteristics of the measurement system (known as ASOS) suggest that these maxima and minima are based on 5-minute averages updated every minute.  If this is still the case, the maximum temperature reported at the airport is based on 5-minute average temperatures calculated every minute.  I am not sure if this is still the case.  

Ultimately, it is important to recognize when comparing maximum or minimum temperatures at observing sites that there is more happening than just instrument siting and calibration.

Heat Wave Update

Northern Utah has so far been sheltered from the southwest heat wave, but that will be changing this week.  

All-time records set over the weekend include 120F in Las Vegas (old record 117F) and 124F in Palm Springs (old record 123).  Bishop, Barstow-Daggett, and Desert Rock tied their all-time highs.  

The Salt Lake City International Airport has been "blissfully cool" so far this July with temperatures maxing out in the 80s from July 1-4 and then in the low-to-mid 90s from July 5-7.  Minimum temperatures have been in the 50s and 60s.  

Temperatures, however, will be climbing during the first part of the work week and are forecast to reach the triple digits from Wednesday through Sunday.

Those forecast highs are close to, but not above records (104 for Tuesday, Wednesday, and Thursday; 105 for Friday; 107 for Saturday; 104 for Sunday), but they are close enough that one can't rule out a record being tied or falling.  

For kicks and giggles, let's look at the forecasts for Saturday, although Friday also looks blisteringly hot.  Saturday's record high just happens to be 107, the highest on record for the SaltLake city Airport (note that 107 has been reached five times).  The GFS forecast 700-mb (about 10,000 ft) temperatures for the airport at 3 PM and 6 PM on Saturday are 20.2C and 20.5C, respectively.  The highest observed by weather balloon in northern Utah is 20.2C, so this is "rarified air."  The middle 50% of forecasts from the National Blend of Models for Saturday fall between 104 and 106 degrees.  The lowest 10% are 102F or lower.  However, the upper 10% are 108 or higher, so it is not out of the realm of possibility that we crack a record. 

Take it easy and check on vulnerable friends and neighbors during this period.  

Hell Is Coming

Enjoy the "cool" weather this week because I just looked at the forecast and hell is coming.  

One of my rules of thumb is the 20/20 rule for temperature.  Anything below -20C or above 20C at 700 mb (about 10,000 feet) is exceptional for northern Utah.  

The latest GFS forecast for 0000 UTC 12 July (6 PM MDT Thursday) have us in the 20–22C range.

And the latest NBM forecast for the Salt Lake International Airport shows a gradual warming trend after tomorrow well into the triple digits by next Thursday with highs into the triple digits.  Of the forecasts used for the NBM, the median high is 104. The upper 25% are 108 or greater.  

Source: https://apps.gsl.noaa.gov/nbmviewer

The NWS is already messaging the potential for dangerous, record-breaking heat.  

Source: NWS, obtained 3 July 2024

A cruel reminder that July is a four-letter word.

Cool Change

With high temperatures this past weekend of 100 and 101 at the Salt Lake City International Airport, today's "cool change" is greatly appreciated.  As I write this at about 3:10 PM, the temperature at the airport is only 81 and it is looking likely that we won't even eclipse 90 today.  

Those long in the tooth might remember that Cool Change was a 70's hit for the Little River Band.

The Little River Band was from Australia, where the Cool Change is an actual meteorological phenomenon that produces dramatic temperature falls from oppressive summertime heat in southeast Australia.  

The current drop in temperature is perhaps not as large or dramatic as the Australian Cool Change, but we'll take it.  I addition, the extended forecast is actually a good one for early July, with persistent ridging upstream along the Pacific coast (e.g., GFS forecast valid 0000 UTC 7 July below) putting northern Utah in northwesterly flow.  

That's much better than the ridge being parked over us or over the four corners.  As a result, forecast highs for the airport from the National Weather Service are very pleasant this "work" week:

Tuesday: 83
Wednesday: 88
Thursday: 84
Friday: 89

Thursday is the Fourth of July, so this week is broken up for many by the holiday.  84 would be wonderful.  If you are wondering, the lowest maximum temperature observed on the Fourth of July in Salt Lake City was...wait for it...62 in 1902.  I thought that would be as much of a black swan outlier as you'll find in early July (the next lowest maximum on the Fourth of July is 72 in 1993 and 1912), but on the previous day (July 3, 1902) the high was only 58 with 0.45" of rain! 

Someone needs to go back and see if that really happened. 

June Is the New July

I woke up this morning thinking it must be the end of July.  Heat.  Thunderstorms. Basic yuckiness.  Graphs from the National Weather Service show only 6 days this month with a maximum temperature below "normal" and only 4 days with a minimum temperature below normal.  

Normal for the National Weather Service is currently based on the 30-year average for 1991-2020, but warming of the climate system and expansion of the Wasatch Front urban area was already having an influence during that period.  Thus, I thought I'd take a look back at how this June so far compares with the Julys we used to have around here.  

So far, this June (based on data from the 1st to the 26th) has an average temperature at KSLC of 77.3°F.  It is evident from the graph below that this is much warmer than the typical range of temperatures observed prior to about 2000.  In fact, it is the 2nd warmest such period on record, behind only 2021.  

Source: https://xmacis.rcc-acis.org/

I wondered how that compared to the Julys we had around here back in the day, so I pulled those numbers.  It turns out that so far this June is comparable to the average July temperature during the 21st century (based on data from 1901–2000) of 77.05°F.  It is also comparable to the 30-year averages for Julys from 1941–1970 (77.0°F), 1951–1980 (77.5°F), and 1961–1990 (77.8°F).  

You know, the good old days.  

Since there are always questions about the Salt Lake Airport observations, I'll add that the Bountiful Bench cooperative site has also experienced it's 2nd warmest June 1–26 on record this year (tied with 1988), behind only 2021.  Records there only go back to 1975, so comparisons of with the climate of the 20th century are not possible for that site.  

Great Salt Lake Seasonal Maximum

Data collected from the USGS at Salt Air suggests that the elevation of the south arm of the Great Salt Lake reached its seasonal maximum in May at about 4195 feet and is now declining.  

Source: https://waterdata.usgs.gov/

Data for this location extend back to 1847, so we can put the 4195 foot elevation into historical context.  There are two prior periods where lake levels dropped below 4195 feet.  The first was from about 1934–1946 when the lake episodically went below 4195 feet.  The second was from about 1959–1971 when the lake dropped to just below 4192 feet and was below 4195 feet for a several year stretch with episodic drops below that level in surrounding years.  

https://waterdata.usgs.gov/

The north arm remains lower than the south arm and is currently at an elevation of about 4193 feet.  

A real question moving forward from here is will our string of high snowfall winters continue?  Let's hope so.

What an Airmass!

Mother Nature delivered the cold air as promised and we have a wonderful mid-June airmass in place over northern Utah this morning.  

Temperatures reported within the 1 hour ending at 1324 UTC (0724 MDT) show 40s in the Salt Lake Valley with 20s and a few 20s in the mountain valleys on the Wasatch Back.  

A few minimum temperatures (so far) that caught my eye:  Natural History Museum of Utah 36, Alta-Guard 25, Top of Snowbird Tram 19, and I80/Silver Creek Junction 24.  

The aiport reports minimum temperatures every six hours and at 6 AM their minimum was 45. There's a chance it could go a degree below that.  If you are wondering, the record low for today is 37 (set in 1928), so that's safe.  

It could be a long time until we see temperatures this low again.  Enjoy.  

From Frying Pan to Freezer

Wednesday: 99.

Thursday: 100.

Had enough?  

I have some good news. The forecast high for today is only 95F. Hooray!

But it get's better.  We will see a gradual cooling trend through early next week.  First, the ridge that has tortured us this week moves downstream and a broad upper-level trough gradually moves overthe western US. By 0000 UTC 16 June (6 PM MDT Saturday), northern Utah is in southwesterly flow aloft and in the leading edge of cooler air.  

That cooler air gradually bleeds in until Monday when an even stronger trough and surge of bonafide cold air push into the state.  

The NWS forecast nicely shows the high temperatures for the airport dropping each day from yesterday's 100 to today's 95, Saturday's 89, Sunday's 83, Monday's 79, and Tuesdays 70. 

The trend is our friend. If you extrapolate that forward, winter will be here later this month!

Sadly that's not the case, but I am looking forward to the cooler air.  Tuesday might also prove to be the coldest day we see around here for a long time.

Triple Digit Heat?

There has been a lot of talk about hitting 100 today or tomorrow.  As I type this, the National Weather Service Forecast National Blend of Models, which forecasts a distribution of max temperatures based on forecasts from many modeling systems, is producing a median forecast of 99°F for the airport today.  The middle 50% of forecasts, also known and the interquartile range, is between 98°F and 100°F.  

Source: https://hwp-viz.gsd.esrl.noaa.gov/wave1d/?location=KSLC

One way to interpret that is that it is more likely than not we'll be at 99 or a bit lower, but the odds of 100 or a bit higher are roughly 25% (I'd need access to higher precision data to provide a more detailed estimate).

Tomorrow, the numbers edge up slightly, with 100°F being the median forecast, so we'll call it 50/50 for tomorrow. 

Source: https://hwp-viz.gsd.esrl.noaa.gov/wave1d/?location=KSLC

Is this unusual?  Yes.  Since record keeping began in 1874, there have been 10 days with a high temperature of 100 or more on or prior to June 13.  This is based on observations taken in downtown Salt Lake City until about 1928 when the site was moved to the airport.  

The table above lists the 40 hottest days on record prior to June 13. One can see the impacts of a few early June heat waves, the earliest being in 1918, which has 3 of those days.  26 of the days have been since 2000.  

Please note that the National Weather Service has issued a heat advisory for much of the Wasatch Front and West Desert due to the high temperatures.  

Summer heat is sadly here.  There is some hope that tomorrow will be the worst of it (see plots above).  Let's hope that's the case.  

What's Up with the Snowfall Measurements at Alta Guard?

I was recently asked about the large difference in snowfall this past season reported by Alta Guard and Alta Ski Area and thought I would take this opportunity to talk about why they might differ and some of the challenges of observing new snow amounts.  

The Utah Avalanche Center provides a long record of snowfall at Alta Guard at https://utahavalanchecenter.org/alta-monthly-snowfall.  For this past season (November through April), their spreadsheet reports only 432 inches.

In contrast, for the same period, Alta Ski Area reported 606.5" (see https://www.alta.com/weather..note that I've subtracted 21.5" that fell in October from their seasonal total through April).  I am often asked how can there be such a large difference since they are "right across the street from each other." 

Actually, the observing sites are not right across the street from each other.  The Alta-Guard observing site is located just on the north side of SR-210 just down canyon for Our Lady of the Snows and just above the highway at an elevation of about 8660 feet.  Alta typically uses observations collected by their snow-safety at Alta-Collins, which is located in Collins Gulch at an elevation of 9662 feet.  

Map source: CalTopo

Thus, Alta-Collins is 1000 feet higher than Alta Guard.  Mean annual snowfall in the Wasatch Range increases by about 100 inches per 1000 feet, so we might expect these to differ by roughly that amount, but the differential this past season was 174.5 inches, so that doesn't fully explain the difference.

However, it turns out that there is another snowfall observing site at Alta, and that is the volunteer National Weather Service Cooperative observer.  It is my understanding that these observations are collected at the Alta Town Offices that are just up canyon from Alta Guard, as indicated below. 

For the same November-April reporting period, that site reported 498.7" if snow (this data can be obtained from https://xmacis.rcc-acis.org/ or the National Centers for Environmental Information).  There was one missing day in that record (Dec 20), but the ski area did not report snow on that day, so we will assume the seasonal total is complete.  

The difference between this site and Alta Collins is pretty close to what we would expect with that 100" per 1000 feet guideline, but 66.7" higher than reported by Alta Guard only about 500 feet away.  How can we explain this difference?

Here I can only speculate.  First, I do not currently have access to the UDOT observations from Alta-Guard, so I cannot rule out the possibility that there is some missing data that is not being considered or the possibility that there was an error transcribing to to the Utah Avalanche Center site.  I suspect this is not the case, but I cannot rule it out.  

Second, it could simply be that the characteristics at the Alta-Guard site favor snow densification, so that that snow depth from a given water equivalent would be lower. Such characteristics would include, for example, greater wind and sun exposure (and this is consistent with my understanding of the site characteristics).  I do not have access to the water-equivalent observations of snowfall observations from Alta-Guard, but the Alta-Coop site reported 50.68" of water-equivalent precipitation from October 1 through April 30.  It is possible that some of this fell as rain (mainly in October), but that is very close to the 49.8" of maximum snow water equivalent observed by the Atwater SNOTEL site that is just up the hill from the two sites.  Thus, for the sake of argument, I am going to assume that about 50" of water fell as snow this past season at the two sites.  

If that was the case, the mean water content of snow for the season at Alta-Guard was 11.6%, whereas at the Alta-Coop site it was 10.0%.  Such spatial differences in snow water content are not unusual in storms and skiers are well aware that there can be highly localized variations in snow conditions depending on wind and sun exposure.  In small storms, such a contrast is small and likely not to attract much attention.  In a storm that produces 1" of water, 11.6% water content yields 8.6" of snow, whereas 10% yields 10" of snow.  But over a season, it adds up to a more noticeable contrast.  

There are other factors that could be playing a role in the difference between Alta-Guard and the Alta-Coop site, including the frequency of sampling, which I have no information about. The higher frequency that you sample, or simply taking measurements as soon as the snow stops during a storm, yields a greater snow depth than if you do it a low frequency at specific times.  These are the realities of snowfall measurement.  

But there's more, and this is important if you are comparing seasons or looking at trends.  The measurement techniques and site used by UDOT (and the US Forest Service in the more distant past) have changed many times over the years.  Given the large spatial variability of snowfall and the sensitivity to measurement frequency and practices, this makes seasonal comparisons and trends problematic.  

The bottom line here is that one should expect there to be some differences in snowfall reported by Alta Guard and Alta ski area simply due to the elevation difference between their two sites.  One might also expect differences depending on local conditions at the two locations, or measurement practices.  The difference in snowfall between Alta Guard and the Alta Coop site illustrates this well.  I don't consider any of these observations to be "wrong."  The reality is that a single number for new snow depth is going to depend on exactly where you measure it.  As I like to say, "all observations are bad, but some are useful."

The Emerald Foothills

The Avenues foothills (and perhaps other areas long the benches, seem remarkably lush and green for early June.  Yesterday evening I did my first hike since returning from Scotland and was blown away by the vegetation.  

It's not unusual for the foothills to green up in the spring and for there to be wildflowers.  Often by the start of June, things are browning up, but the emerald foothills are hanging on this year.  Additionally, the coverage and height of the grasses and other vegetation in many areas seems exceptional, in some areas reaching or exceeding the height of my treking poles, which were set to 120 cm (48 inches).  

These are purely anecdotal observations.  I'd be curious to hear other perspectives.  One thing is for sure, things will be browning and crisping up more rapidly in the coming days as we move into hotter drier weather. 

Summer Begins

Technically meteorological spring ended and summer began on June 1st, but it will really feel like it is over in a couple of days.

Today, we will blissfully enjoy the the cool airmass in the wake of the weak front that came through yesterday afternoon.  The forecast high for the airport is a wonderful 77°F.

However, by Thursday, a high amplitude ridge is in full control over the southwest US and we are likely flirting with the low 90s for highs.  

If you are keeping score at home, that would be our first 90 of the year.  The highest temperature at the airport so far was 87 on May 28th.  

And here's the 6–10 day temperature outlook from the Climate Prediction Center.

Source: NOAA/CPC

Not good.  Hot weather this time of year just extends the agony of summer heat, which is a virtual lock around here in July.  

Was May Really Below "Normal"?

After posting on May 13th about the warmth over the previous 12 months globally and at the Salt Lake City International Airport (see It's Been Warm), there were a few comments concerning there being some below average days recently and May being colder than normal.

Was it?

Certainly not globally, but I'm going to assume those comments were specific to the Salt Lake City International Airport.  Let's have a look at the numbers.

For May 2024, the average temperature at the airport was 58.8°F.  This makes it the coolest May since 2019 (58.0°F) and the 5th coolest since 2000.  

However, whether or not 58.8°F was below normal depends on what you mean by normal.

The National Weather Service climate normals are based on 30-year averages that are updated every 10 years (and there are other national forecast services that do this).  So, when you see "normal" on the news today, it is based on an average from 1991–2020.  For May, that's 60.5°F, making this May 1.7°F colder than normal.  

But lets look at how the May 30-year climate "normals" have changed over time.  For brevity, I'll start with 1941-1970.  

1941-1970: 58.4°F

1951-1980: 58.6°F

1961–1990: 58.8°F

1971-2000: 59.2°F

1981-2010: 59.9°F

1991-2020: 60.5°F

So, May 2024 was above normal relative to the 1941-1970 and 1951-1980 climate normals, at normal for 1961-1990, and below normal relative to the 1971-2000, 1981-2010, and 1991-2020 climate normals.  It was also above normal relative to the 20th century average temperature (58.6°F).  

I've never been a fan of the word "normal" as it is often used in meteorology.  In a stationary climate, "normal" is not a single number since it is normal for the weather to vary.  In a changing climate, or what climate scientists might call a non-stationary climate, what is normal is difficult to define and very dependent on the period used.  

A better synopsis of this May is that it was near average for temperature relative to the temperatures observed in Salt Lake City during the 20th century.  It was slightly below average based on temperatures observed in recent decades.  

This will become an even bigger issue in the future, especially for phenomena that are less well sampled, like extreme precipitation events.  

Back from Scotland

It's been a long blog break while my wife and I were visiting Scotland.  A visit to the northernmost part of the United Kingdom might not be on your bucket list, but it didn't disappoint.  The Scottish Highlands contain some ancient mountains, lots of lakes (lochs), and spectacular coastlines.  I'll give a quick summary.

We started in Edinburgh, which was quintessentially Scottish with weathered buildings overlooked by Edinburgh Castle.  

Edinburgh is technically in the lowlands, but you can =get in a short climb above the city by ascending Arthur's Seat in Holyrood Park.  

Leaving Edinburgh, we got a glimpse of the lower slopes of the "original" Ben Lomond, which rises above Loch Lomond. 

Mother Nature blessed us with clear skies on the day we had booked an excursion out to Staffa Island.  We enjoyed the coastal scenery and puffins.

One of the reasons I like to go to places like Scotland is cool weather, clouds, and rain.  We get enough of the sun in Utah.  I like to travel to places where the rain gear and winter hats come out for a day or two.  Lightweight winter gloves were even on for a while on this hike. 

Call them hills if you want, but the ancient, wind-swept, treeless landscapes of the Scottish Highlands make for some spectacular scenery, even in the rain.  And the single-track, two-way roads can be a lot of fun to drive. 

Scotland has a rich climbing history.  Below is a monument to Norman Collie and John Mackenzie, pioneering mountain guides on the Isle of Skye and beyond.  

We paid a visit to Cairngorm Mountain Resort, one of Scotland's ski areas.  Seemed hard core not because of steepness, but exposure to the elements.  This is wind swept terrain.  Respect for the dedication.    

We did a hike up to one of the coires (glacial cirques) near the ski area.  Beautiful spot.  

If you are wondering, haggis was eaten and scotch was drunk.  

It's Been Warm

Global temperatures have been remarkably warm over the past 12 months or so.  April numbers are not quite out yet from the National Centers for Environmental Information, so the analysis below is based on the 12-month period ending in March.  Average temperatures were 1.26°C (2.27°F) above the 20th century mean, easily the warmest such period on record.  

Source: NCEI

Locally, the airport also saw its warmest April–March on record with an average temperature of 56.8°F.

Source: https://xmacis.rcc-acis.org/

It was a pretty good ski season, but a lot depended on elevation.  At the Salt Lake City Airport, total snowfall from November to April was only 28.3", 14th lowest since 1876 (lowest on record is 14.3" in 1934).  Mean November to April temperatures were 42.4°F, 4th highest on record (highest on record is 43.6°F in...you guessed it...1934).  It was a dream winter in many respects meteorologically.  Not a lot of snow to shovel in the valley and a lot to ski in the mountains.  

The global warmth is related to global warming, but El Nino probably contributed some to the extreme warmth of the past 12 months.  El Nino is now weakening and we may be transitioning to La Nina conditions by fall. That will probably result in a drop in global temperatures from the insane peak of the past 12 months.  However, these are small fluctuations on top of the long-term trend due to global warming, which will continue for at least the next 2 to 3 decades.  After that, much depends on ongoing and future greenhouse gas emissions.  

May Snow in Context

Yesterday, two skiers died tragically in an avalanche somewhere near Big Willow Cirque on Lone Peak (details still forthcoming).  This after a storm cycle in which Alta Ski Area reported 42" of snow and even the benches received substantial accumulations in some areas.  How unusual is such May snow?

This is a hard question to answer concretely mainly because the available records at bench and upper elevations are spotty, inconsistent, and limited.  For bench level, I will fall back to my usual standby, the Bountiful Bench cooperative observer site, which is at an elevation of 4950 feet.  This is a bit lower than perhaps one would like, but it has continuous records back to 1975 with no missing data.

First lets look at snowfall during May. This station has observed 1" or more of snow in 20 out of the 50 Mays on record, with a mximum of 27.9" in May 1975.  Impressive!  Second is 1983 with 15.6", followed by this May with 10.2", although it should be noted that this May is not over yet.   

Source: https://xmacis.rcc-acis.org/

For this location, this May was the snowiest since 1983, although it should be noted that this May is not over yet.  

If one looks at the largest 3-day May accumulations, they have all occurred in 1975, 1983, or 2024 (note that some of these overlap).  

https://xmacis.rcc-acis.org/

Going down to the largest 2-day May accumulations, events in 1991 and 1979 also sneak in there, with this May coming in at #5.  

https://xmacis.rcc-acis.org/

And finally for a single day, the 5.7" that fell on May 6 of this year and 4.5" on May 5th are good for #5 and #7 since 1975.  

So, at least at this location, I would describe this latest May storm cycle as unusual but not unprecedented.  One can find snowier 1-3 day periods in at least two other periods, although it has been over 40 years since that happened. 

The Bountiful Bench site reported 10.2", but in Public Information Statements issued by the National Weather Service, I saw as much as 11.5" reported from Bountiful at 5117 ft at 7 AM on Tuesday and 12" at the City Creek Canyon Water Plant at 5340 ft at 4 PM Monday.  I also saw a report of 15" at 5600 feet in Emigration.  It's harder to place these in context, but I suspect the general view that it's unusual, hasn't happened recently, but is not unprecedented probably holds, but that's admittedly a conjecture.  

At Alta, the cooperative observer by the municipal offices has recorded 40.9" of snow so far this month.  Their records are not as complete, but there are 12 prior Mays that top that.  The 18.3" that fell this May 6th was the 3rd highest for a single day in May (topped by May 9, 1986 with 20.5" and May 11, 2000 with 20") and the 30.1" that fell on May 6 and 7 was also good for 3rd highest.  For a five day period, which one can find big cycles in May 1986 and 1993 that were actually quite a bit bigger than this May.  The maximum 5-day accumulations in those months were 67.5" and 66.0", respectively, compared to 50.1" this May.  

https://xmacis.rcc-acis.org/

Given that there's spottier records for Alta, it's a bit harder to convert that into a frequency or return period.  My take is what has happened this May at upper elevations is certainly not unprecedented and I'm not sure I would call it unusual like the bench snowfall. If we had more continuous records we might find a storm like this week's happening every 5-10 years at this elevation in May.  Back to 1971, even with substantial missing data, the Alta Coop site has observed at least 40" of snow in May 13 times, which is roughly once every 4 years, but the return period for more intense, shorter period snowfall would probably be longer.  

That's enough handwaving for today. 

Winter Continues

 It didn't take long yesterday after the front went through for precipitation to change to snow at my place (5,000 ft elevation).  The photo below was taken at 1:52 PM in the afternoon when lots of big aggregates (combinations of two or more ice crystals) were falling.

Aggregates are common in warm storms because ice is "stickier" at temperatures near 0°C.  

By evening the grass was covered, and by morning it looked like proper winter.  Fortunately, the roads are warm and driving to the office this morning wasn't too difficult despite the snow covered roads.  

As I write this, the radar looks quite active, so I anticipate we will see snow continue along the east bench for at least a couple more hours before the valley precipitation becomes more scattered.  

Meanwhile in the mountains, Alta is closed to uphill skiing. The automatic snow interval sensor at Alta-Collins has been a bit erratic for this storm period, but if I blink and throw out what I think is questionable data, it looks like they got 8" overnight Saturday and yesterday and then another 11 inches overnight through 6 AM this morning. So, we'll call it 19".  Total water is 1.6".  I suspect if you are ski touring this morning, you're getting the best of the best when it comes to May skiing.  

The models call for this initial cold trough to move downstream today with a weak ridge moving in this afternoon (emphasis on weak).  That will probably bring a lull to the action in the afternoon with rising temperatures in the valley likely resulting in the snow turning to rain at some point today, even on the benches by mid afternoon.  A wildcard is the possibility of a thunderstorm, which might push the snow levels down temporarily again.  

Then another short-wave trough comes in tonight and lowers snow levels again to near bench level, with another trough dropping down into northern Utah Tuesday night.  From 9 AM this morning to 5 AM Wednesday morning, the GFS puts out another 20" for Alta. 

Winter continues.