Mother of All Cold Pools

The Peter Sinks are limestone sink holes located in the Bear River Range east of Logan.  There's not much too them.  They are just shallow depressions at high (~8000 ft) elevation.

Photo: John Horel

They are famous for their incredibly low minimum temperatures and strong cold pools.  The lowest temperature ever recorded in Utah (and second lowest ever recorded in the contiguous US), -69.3F, was measured in the Peter Sinks on 1 Feb 1985.

Last night, it got down to -31ºF in Peter Sinks.  Yeah, that's not that cold compared to the all time record, but it is 46ºF colder than the minimum temperature observed on the rim of the sink less than a mile and 300 vertical feet away!

Minimum temperatures since midnight MST 4 December 2012

 Check out the difference in the temperature trace at the two stations.  Temperatures remain relatively steady overnight on the rim, fluctuating between about 15 and 25ºF, whereas on the floor of the sink, they drop like a rock shortly after sunrise and hover near -30ºF most of the night.  Note the change in scale between the two graphs.  

Indeed, it pays to know where to pitch your tent on a clear calm night in the Bear River Range.  

Got Chapstick?

Adding to the incredible mountain weather today is the remarkably dry air on the mountaintops.  On Mount Baldy (11,000 ft), the dewpoint and relative humidity dropped to as low as -44ºF and 4%, respectively.

Now that's what I call Chapstick weather!

Great Mountain Microclimates Today!

This morning's observations suggests that there are some remarkable inversions in the canyons and mountain valleys of the Wasatch today.  For example, at 11 am it was 6ºF at the base of Snowbird, but 25ºF at the top of the tram.

Click to enlarge

Especially interesting are the observations near the base of Alta.  It was 26ºF and 29ºF at the two sites on the south facing aspect to the north of the resort, but only 12ºF at the site near the base of the resort (note: the plotted position of this observation is off by perhaps 300 feet - it should be further southwest in a spot that is typically shaded this time of year).

It appears that some very strong cold pools formed in the canyons overnight, and these cold pools are persisting in those areas that are still topographically shaded and haven't yet seen the sun.  The 10:45 am web cam shot of Alta shows this well.  This camera is sited to the west (left) of the two warm observations in the image above.  They are all in the sun.  Areas the canyon near the north facing slopes are in the shade or have only recently seen some sun.  The 12ºF observation is taken near the lower right corner of the image.

Perhaps someone can comment on the veracity of these observations, but I have seen situations like this previously when it is clear and very dry near the winter solstice when the sun angle is quite low.  Often you can find all sorts of cold pockets in areas where it is shaded and the air is calm.  A screaming descent of Snowbird today must be interesting.

A Strengthening Inversion with an Uncertain Future

Weather balloons launched from the Salt Lake City Airport show the strengthening and descent of the inversion over the past four mornings.

Loop of 0500 MST soundings from 31 Dec 2012 and 1, 2, and
3 Jan 2013.

During the period, the 700 mb (near mountain-top level) temperatures increased from about -15ºC to -5ºC, whereas the surface temperatures decreased from about -8ºC to -12ºC.  This produces a 9.4ºC temperature inversion this morning between 814 and 720 mb (6280 and 9400 ft).  Yup, we're pretty much screwed.

About the only positive thing in this instance is that the base of the inversion is at 814 mb (6280 ft).  This means that the pollution can mix through about a 2000 ft deep layer this afternoon before it hits the capping inversion.  That will keep pollution levels lower than if the inversion were closer to the valley floor.

I've been quite concerned about this event given the model forecasts for the development of a very stagnant pattern over the western US, but the latest GFS forecast has changed dramatically from yesterday.  Rather than closing the approaching trof off the California coast and ultimately setting up a Rex block (see previous post) it is bringing into the western US as an open wave.  This change can be seen by comparing the forecast from yesterday's 0600 UTC GFS forecast (top image below) to today's 0600 UTC GFS (bottom image below) for 1800 UTC (1100 MST) Sunday.

Let's hope something close to the more recent run verifies, as it would break up the inversion on Sunday and give us some snow for the mountains.  I'll be more than happy to wipe egg of my face for a crappy long-range forecast if that happens.

Anatomy of Split Flow

Over the next several days, the jet stream will be taking a vacation from the Intermountain West, leaving us high and dry and, for those who live in the Salt Lake Valley, mired in pollution.

During this period, pronounced split flow will develop over the western United States.  Split flow features two jet streams, one to the north called the polar jet and one to the south called the subtropical jet, as can be seen in the GFS forecast for 1200 UTC (0500 MST) 8 January.  

0600 UTC 2 Jan 2012 GFS 250-mb (jet stream) wind vector, wind speed (contours),
and 6-h accumulated precipitation forecast valid 1200 UTC 8 Jan 2012

The split flow reflects a large-scale pattern known as a Rex Block, which features an upper-level ridge "over" an upper-level trough.  Note in the image above how Utah is sandwiched between an upper-level ridge to our north and an upper-level trough to our south.  If this forecast verifies, it will be a quick flight for those traveling from Denver to Salt Lake on the Jan 8th as the jet-stream level flow is completely reversed and easterly over western Colorado and Utah.  

The Stratus Sandwich

It was worth a trip to the mountains today just to check out the various cloud layers.  At low levels, we had a deep cold pool entrenched over the Salt Lake Valley that was topped by stratus clouds that tongued their way up Little Cottonwood Canyon.

Looking west down Little Cottonwood Canyon @ 2:37 PM MST
1 Jan 2013.  Click to enlarge.

Above the cold pool, there was a layer with strong northerly flow and an altostratus cloud deck at mountaintop level.  The altostratus was widespread in the morning, forming a sort of "stratus sandwich," but became more broken during the afternoon.  During this period, the clouds lingered over the west-east oriented ridges, with clearing where the crest-level flow was sinking into Little Cottonwood Canyon, as depicted above.

The deep valley cold pool is actually a good thing.  During bad inversion events, the inversion is located very near the valley floor and all our pollution is confined to a very shallow layer.  Today, however, the inversion was located about 3000 feet above the valley floor, which is why there are stratus clouds tonguing so far up Little Cottonwood.  That means our pollution is being distributed through a deeper layer.  Unfortunately, the inversion will be lower and the air pollution worse later this week.

Also, there's not much hope for snow in the 7-day forecast.  Time to start appeasing the gods...