This Digging Trough Is a Tease

The loop below shows a beautiful cold front and upper-level trough migrating through the long-wave ridge over the eastern Pacific with a path that would seemingly take it to the Wasatch Mountains.  A powder dump is coming right?

1200 UTC 10 Dec – 2100 UTC 11 Dec 2011 IR and 500-mb
geopotential height analysis

Wrong.  This is a digging trough that it is amplifying and moving strongly southward along the Pacific coast.  It then slides across northern Mexico and the southwest U.S.  The frontal precipitation moving into the Pacific Northwest today stalls and decays just upstream of Utah, leaving us with just a few scraps.

GFS 500-mb geopotential height and 6-h accumulated precipitation analyses
valid 0000 UTC 12 Dec – 1800 UTC 14 Dec 2012

This is a fairly common track for storms during the cool season.  These troughs are a tease for Utah, but the southwest needs to get their share from time to time.

Sepia Sky

It's a real bummer given that it has been snowing on and off for a few days, but there's already a sepia tint to the lower atmosphere over the Salt Lake Valley.  Pity.

Cottonwoods Magic Kicks In

When you are a meteorologist, there are bound to be some days when you take your lumps.  Today is one of those days.  All clues pointed to a fairly weak storm today, but the magic of the Cottonwoods kicked in and we're now beyond the 2-4" I expected.  However, I'm not going to complain and neither should you.

The 1502–2112 UTC (0802–1412 MST) radar loop below shows the highly localized nature of the snow, which has been confined primarily to the Salt Lake Valley and Wasatch Mountains to the east.  

Lake effect?  Except perhaps near the end of the loop above, I don't think the lake has played much of a role so far.  The flow at the top of the Snowbird tram has been westerly, which isn't favorable for carrying lake-effect precipitation into the Cottonwoods.

In addition, usually lake-effect precipitation is characterized by cellular radar echoes and convection.  I didn't see much around the Salt Lake Valley for much of the morning and early afternoon to indicate there was any lake-effect convection.  Perhaps the lake will eventually kick in, but thusfar, I don't think it's played a significant role.

Instead, there was local precipitation enhancement within a cloud band that extended from southeast Oregon, across northwest northeast Nevada, and then eastward across northern Utah.

Source: NCAR/RAL

Incredibly, the area around and upstream of the Cottonwoods was the only place it was snowing along that cloud band.

Why?  I have ideas, but none of them are very compelling or easy to explain in a few sentences.  Maybe you have better ones.  I'm just going to say it was the Cottonwoods magic and not spoil a good thing.

The new snow has piqued my interest, but I'm currently laid up with a bad back and unable to ski.  Transporting my son from Snowbird this afternoon was serious torture.  The only turns I got involved following the red snake back down the canyon!

Get some for me tomorrow.  

Right Flow, Wrong Location

For a few days each winter, we settle into a pattern with northwesterly flow, but with the storm track just a bit too far to the north and east.  When this happens, the Tetons and the Park Range (i.e., Steamboat) tend to get more snow than the Cottonwoods.

That will probably be the case over the next four days.  The northwesterly storm track will extend from Washington into Colorado, leaving a "pearl necklace" of precipitation maxima over the Cascades, northern Idaho Mountains, Tetons, and Park Range.  The Cottonwoods are just south of the major action.

Source: NCEP

Right now it looks like we will see some snow showers over the weekend, but major accumulations will require the lake to get going and keep going.  The lake is always fickle, so it's a possibility, but right now I don't see a major lake contribution.  Thus, we'll probably see 2–4 inches, just enough to cover up the rocks.

A Sea of "Smaze," Fog, and Stratus

It's deep.  You could just smell the fresh air when you got within about 50 vertical feet of the ridge of the Avenues Foothills this evening.  Most of the stuff over the Salt Lake Valley appeared to be thick "smaze" (smog and haze), with some low clouds in places, but a fairly solid deck of fog or stratus was evident over the Great Salt Lake.  The cold pool was much stronger there.  Note the lower temperatures (~40ºF) compared to over the Salt Lake Valley (43–48ºF).

Source: MesoWest

The burn off of the fog at the Salt Lake Airport showed up real well in ceilometer observations.  A ceilometer is a vertically pointing laser and receiver.  Energy scattered back to the laser can be used to identify clouds, haze, etc.  The image below shows a time series of vertical profiles from the ceilometer from 1253–1753 MST today with time increasing to the right (the y-axis is height above ground level in meters).  The fog shows up as the red and maroon colors.  It dissipates around 1420 MST, with the lingering lower returns produced by the "smaze." 

Source: MesoWest Ceilometer Network

Haze, Smaze, and Fog

A foggy morning in the Avenues
Photo taken 7:20 AM MST 6 Dec 2012

The weather has now sunk temporarily into deep depression as fog formed over the Salt Lake Valley overnight.  The seeds for the fog were sown Tuesday night and Wednesday when rain fell from mid-level clouds into a shallow cold pool (a.k.a., inversion) that was present over the Salt Lake Valley.  This jacked up the relative humidity near the valley floor and led to wide spread haze within the cold pool yesterday.  The haze was very apparent late yesterday afternoon and was topped in places by shallow stratocumulus or "scud" clouds.

Looking southeast toward the Wasatch Mountains from the Avenues
at 5:15 PM MST 5 Dec 2012 (click to enlarge)

Usually I bristle at the use of the word haze during cold-pool events.  Usually, the poor low level visibility is produced by pollution, which should be called smog.  Haze forms only at higher relative humidities when condensation can occur on microscopic, water-soluable particles that are floating around in the atmosphere.  These particles can be natural, but they can also be produced by human activity.  One could use the term "smaze" to describe haze that forms in these polluted environments, as was the case yesterday.  

The sounding from yesterday afternoon tells the story pretty well.

Source: NOAA/NWS/SPC

The surface temperature and dewpoint were 47ºF and 45ºF, respectively, which equates to a relative humidity of 93%.  This is high enough for haze particles to form, but not high enough for those particles to grow into larger cloud droplets.  Thus, clouds formed only in a few places at the top of the cold pool where there was just enough turbulence, lift, or moisture for the atmosphere to reach 100% relative humidity.  

Above the cold pool, the relative humidity was lower and the air less polluted.  The visibility in this layer, which extended to cloud base, was outstanding.  

Overnight, fog and low clouds formed over the valley.  These can be seen from the photo from the top of Snowbird, which is framed well by ice on the lens.  

Source: Snowbird

Don't worry, the inversion is weak.  The fog will probably lift and dissipate today, and we should be further stirring things up the next couple of days as cooler air moves in.