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."