Storm Update

The surface cold front passed the airport last night at a bit before 1 AM MDT as denoted by a 5F or so drop in temperature and a veering of the winds from southerly to WNW.  Look carefully in the top plot below for the abrupt drop in temperature and rise in dewpoint and humidity.  

Source: mesowest.utah.edu

And so the endless summer of 2024 came to an end.  Although we could be flirting with the upper 70s again next week, we'll call that fall or autumn, whichever you prefer.  

Showers and then widespread precipitation then moved into northern Utah with the precipitation in the early morning hours being more widespread than I anticipated yesterday.  That's a good thing as we have had little to no precipitation for over 3 weeks and this is beneficial precipitation to saturate soils and settle down the Yellow Lake Fire. 

As of 7 am, the snow line is sitting somewhere around 9600 feet.  Alta web cams show the white stuff covering the ground above 10000 ft. 

Source: https://www.alta.com/weather

But the High Rustler Cam shows the snow line just a bit off the top of the run, which is at 9800 feet.  

Source: https://www.alta.com/weather

And one can see a trace of snow on the snow stake at just over 9600 feet.

Source: https://www.alta.com/weather

I put snow line in italics above as I consider that the level at which snow is sticking and accumulating on the ground.  This is different than snow level, which is the level down to which snow is falling.  This time of year, they can be different given the warm ground.    

Radar and the models suggest we'll see precipitation for a bit more this morning, then more scattered showers with a possible break before the main trough moves in later today and tonight.  A look at the HRRR forecast for the wet-bulb zero level shows it falling gradually to about 8500 feet by 6 PM this afternoon and then dropping abruptly to 5500 feet from about 10 AM to 2 AM as the main trough approaches. Thus, we will probably see snow levels dropping to 8000 feet today before it drops to near bench level tonight.  

Much could happen tonight as the main trough approaches Utah.  Through 7 AM this morning, the HRRR had produced about 1.5" for Alta-Collins.  This is a bit ahead of reality (which is closer to 0.3") for that elevation, but might not be too bad at 10,000 feet.  It then adds another 9" or so through tomorrow night. Given the crap shoot nature of what is going to happen as the upper-level trough approaches and closes off to our south, I'm going to stick with my 8-16" forecast from yesterday for Alta-Collins.  

A Storm System with Lots of Moving Parts

Enjoy the last day of summer today.  Big changes are coming in the form of a storm system with a lot of moving parts.  

Let's have a look at some of the details as predicted by the GFS and HRRR.  For this 2100 UTC 16 Oct (3 PM MDT this afternoon), the GFS calls for dry southwesterly flow at 700-mb (lower left) and by extension, the surface.  I don't expect strong winds today, so let's just call it breezy.  

For the same time, the more detailed HRRR is producing some scattered showers over central Utah and the western Uintas.  This could produce some strong gusty winds for the Yellow Lake Fire. I am hoping for the best there during this windier pre-frontal period. 

Summer ends sometime tonight with the arrival of the surface cold front.  The latest HRRR has the cold front entering the northern Salt Lake Valley around 0500 UTC 17 Oct (11 PM MDT this evening).  Unlike some cold fronts which have an intense line of precipitation, this one looks to come in mainly dry initially with just some scattered showers. 

The GFS forecast valid 0600 UTC 17 Oct shows that this front and it's accompanying upper level trough are relatively weak and it is really the stronger trough over the Pacific Northwest that is going to bring the bulk of the precipitation to northern Utah.  

Indeed, by 0000 UTC 18 Oct (6 PM MDT Thursday), that trough has dug into Nevada with widespread precipitation over northern Nevada and northern Utah. 

All of this suggests some periods of precipitation on Thursday, becoming more widespread later in the day and into Thursday night.  

Let's have a look at the forecasts for upper Little Cottonwood.  The latest HRRR produces two periods of precipitation, one early Thursday morning just after the initial frontal passage, the other late Thursday and Thursday night with the second (and deeper) trough.  The wet-bulb zero level is very high initially (near 11,000 feet) and even after the initial frontal passage is between 9500 and 10000 feet.  The snow level is typically a bit below this, so some of the early precipitation could be rain even up to 10,000 feet.  By Thursday morning though, snow levels may be down to about 9000 feet or so.  It is during the approach of the second trough that temperatures and snow levels finally come down significantly Thursday night.   

In total, through 6 AM MDT Friday, the HRRR puts out a bit over an inch of water and 9 inches of snow for Alta-Collins (~9600 ft). 

The GFS covers a longer period so the plot below is a 7-day forecast.  In total, through Friday afternoon, it generates about 1.3" of water and 12" of snow for Alta-Collins.

Finally, we have the Utah Snow Ensemble.  The GEFS Mean through 00z 19 Oct (6 PM Friday) is around  1.75" of water and 18" of snow and the ENS Mean is around 1.5" of water and 15" of snow for Alta-Collins.

My view is that something in the 8-16" range for a storm total through Friday afternoon at elevations near and above 9500 feet is probably reasonable.  If the trough digs a bit further to the west, we may come in lower than that.  In addition, the trough is expected to close off to our south, so instead of getting a prolonged period of unstable northwesterly flow as sometimes occurs with mobile troughs, we will instead see a transition to northerly, then northeasterly, and possibly easterly flow from late Friday through Saturday.  The Wasatch could see a little snow during that period, but probably not a lot.  Accumulations at mid-elevations will be lower.

Thus, I'll call it a purgatory storm.  Somewhere between heaven (enough snow to ski) and hell (no snow to ski). If this forecast verifies, we could descend into a faceted hell shallow snowpack on shady aspects if we don't stack up more snow on top of it in the near future.  

An End to Endless Summer?

Believe it or not it is looking increasingly likely that we will see an end to endless summer later this week.  The models are advertising the movement of an upper-level trough over the western later this week.  We may first begin to feel it's effects Wednesday with an increase in southerly flow and a last warm day as the trough approaches and then cooler air moves in later in the week.  The GFS forecast for 1200 UTC 18 October has a bonafide cold front over northern Utah.   

It's a bit soon to lock in on that solution, so let's have a look at the Utah Snow Ensemble for Alta-Collins.  All of the ensemble members are dry through 0000 UTC 17 Oct (6 PM MDT Wednesday).  Then nearly all produce precipitation at some point during the 2.5 day period through 1200 UTC 19 Oct (6 AM MDT Saturday.  

Timing and amounts vary with the ECMWF ENS members generally being drier than the GEFS.  Through 1200 UTC 19 Oct (6 AM MDT) the former produces an average of about 0.75" water and 6" of snow at Alta Collins.  The latter is at about 1.6" of water and 16" of snow.  A look at the plan view plot for the Wasatch Front at that time shows a mean for the ENS and GEFS combined of 10.6" at Alta Collins and even some light amounts in the valley (<1").  

The spread is still large though, so we will see how this comes together in the coming days.  However, our long stretch of days with above average high temperatures, which extends back to September 22, looks to be over later next week and probably after Wednesday unless the trough and cold front are slow to arrive. 

Possible Snow?

I've been watching the new Utah Snow Ensemble looking for both bugs in the code and a pattern change.  I did find one bug (you probably didn't notice its effects) that has been corrected and now I'm seeing the possibility of a pattern change.  

As illustrated by the plume diagram for Alta-Collins below, the central Wasatch are locked into dry weather through 00000 UTC 14 October (6 PM MDT Sunday).  

Then, a weak trough drifts across northern Utah.  A few ensemble members bring some showers in either on the 14th, 15th, or 16th, but most keep us dry and it remains mild.  Not much to get excited about there.  

However, after 0000 UTC 17 October (6 PM MDT next Wednesday), we start to see the possibility of a shift.  The median wet-bulb temperatures drop to below 7000 feet and the many of the ensemble members produce precipitation and snow.  The forecasts though produce a wide range of precipitation and snow totals.  Additionally, while most of the ensembles cool us off (note how the median wet-bulb 0.5 levels drop), a few still keep us mild.

We will see how this plays out in the coming days.  

Anchoring Bias and Ensembles

Anchoring bias causes us to rely heavily on the first piece of information that we encounter.  I think of it a lot when I am interpreting model guidance, especially ensembles.   

Here's an example, which I draw from the new 82-member Utah Snow Ensemble.  One of the first products I often pull up is the plume for Alta-Collins.  I find it impossible not to immediately look at the snowiest ensemble member, which for last night's run is producing about 28.5" of snow from 3" of water.  

Like a drug, my heart rate doubles and I feel the adrenalin rush.

Then I pull up the four-panel total snowfall forecasts through 240 hours.  Again, my eye moves immediately to the lower right, which is the maximum snowfall.  Beautiful!  28.5"!

The problem is that such amounts are extreme outliers and highly improbable. What you don't see in a first glance at the plume is that more than 90% of the ensemble members aren't producing any snowfall.  That would probably be the best anchor for a forecast.  

Fight your primal urges when looking these forecasts, especially if your eye is drawn to the maximum.  

Crazy Heat in Phoenix

The wheels have come off in Phoenix or maybe better put burned off.  They have now set or tied daily records on what appears to be 8 consecutive days, set an all-time record of 117 for September and set an all-time record of 113 for October.  The murderer's row of daily records is:

Sep 24: 108
Sep 25: 113
Sep 26: 110
Sep 27: 113
Sep 28: 117
Sep 29: 113
Sep 30: 108
Oct 1: 113

The eight-day mean average high temperature of 111.8F obliterates the previous eight-day mean average high temperature record after September 1 of 108.6, which was set in the seven day period ending on 11 Sep 1979.  

The graph below shows the 8-day average mean temperature for 24 Sep - 10 Oct each year since records began in the Phoenix area, and illustrates just how anomalous the 8-day period has been.  

Min temperatures during this period have also set a record for the 8-day period.

Forecast highs for the airport are in the high 100s to 110 for the next several days.  What a stretch of fall misery.