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.  

A BIG MAY STORM

This is a blog post that deserves all caps.  The models continue to advertise a major May storm cycle that will bring substantial snowfall to the mountains of northern Utah. 

As I write this on Sunday morning, the south winds are blowing and temperatures in the Salt Lake Valley are in the 60s.  

But everything is about to change.  A look at the latest observations shows northwesterly flow and colder air to our west.  Along I-80 at at Knolls it's 43°F.  

The front will likely push through the Salt Lake Valley from about 9–11 AM this morning.  Precipitation will also spread across the area.  The coldest of the cold air arrives tonight.  The GFS shows unstable, northwesterly flow tomorrow morning with 700-mb tempeatures around -8°C.  In January, that would probably mean wet snow on the valley floor.  In May, it's probably down to near bench level.  Don't be surprised to see flakes late today or tomorrow.  

For the mountains, the models are putting out some big numbers.  The 6Z HRRR forecasts 19" of snow for Alta through 0600 UTC (0000 MDT) Tuesday.  

The GFS is locked in with similar numbers through that time.  Additionally, it keeps us in a cold pattern through Wednesday with a series of systems moving through and pushing the storm totals to about 3" of water and 40" of snow by Wednesday.  

These are serious May numbers.  I'll add that the forecast above also shows northwest winds on Monday on Mt. Baldy of over 40 mph.  Real winter, with real-winter avalanche conditions, and the possibility of a rapid change to wet-snow avalanches conditions should the sun appear.  

Let's be careful out there.

Good Skiing Early Next Week?

Forecasts for late Sunday and early next week are, how shall I put it, interesting.  Both the GFS (below) and the Euro are bringing a deep upper-level trough and strong cold front into northern Utah on Sunday afternoon.  

Then, in the wake of that system, there's a prolonged period of moist, northwesterly flow with a couple of weak disturbances thrown in to help juice things up.  Below is the forecast for 1500 UTC (9 AM MDT) Monday showing heavy orographic snow showers over the Wasatch.  

The forecast 700-mb temperatures at that time are also about -7°C, which could drop snow levels to near the benches.  

For upper Little Cottonwood, the latest GFS is going off and produces 3" of water and 35" of snow for Alta-Collins from around 11 AM Sunday through noon Tuesday.  

That is, however, an upper-end forecast, topped by only a few members of the downscaled NAEFS.  the NAEFS mean is closer to 2" of water and a bit over 20" of snow.  There's also a good deal of spread, indicating a wide range of possible outcomes. 

 

That said, it looks like cold air is coming and with it the potential for significant mountain snow.  We'll see if things come together for this early May storm.