Sunday, January 22, 2017

The Meteorological Good, Bad, and Ugly

In the wake of yesterday's Cottonwood Snowmageddon, we take a look back at the meteorological good, bad, and ugly.

The good is that with all the snow over the past 3 days, the total snow depth at Alta-Collins has eclipsed 100" (250 cm).


STEENBURGH WINTER IS HERE!!!

For the first time since the 2010/11 ski season, we have over 100 inches of snow at Alta-Collins before February 10.  Steenburgh winter is that magical period where we have such a snowpack before February 10, when the sun begins to have a much stronger influence on snow conditions in the Wasatch range.  Enjoy the deep snow and the low angle sun while they last.

Now, getting to the bad: Forecasts for Friday night.  From 4 PM Friday to 8 AM Saturday, Alta-Collins got 15", including 13" from 5 PM Friday to 5 AM Saturday with 0.55" of water.  

Such accumulations were much greater than forecast.  I did not put numbers on the snowfall for this blog, but expected only a modest refresh of a few inches.  The National Weather Service Little Cottonwood Forecast issued on Friday afternoon called for a 70% chance of 2-5" (with 0.15-0.35" of water) from 5 PM Friday to 5 AM Saturday and a 30% chance of 1-2" (0.05-0.14" of water).  As noted by commenters in the previous post, the "bros at the bird" beat us this time.  It's hard to say how much of the snow removal, traffic mess, and avalanche closures yesterday in the Cottonwoods were related to the underforecast, but I suspect it was a contributor.

BTW, the NCAR Ensemble handled the event pretty well.
NCAR Ensemble Forecast initialized 0000 UTC 20 January 2017

NCAR Ensemble Forecast initialized 0000 UTC 21 January 2017
Further evidence that the US needs a high-resolution ensemble ASAP.

The ugly?  There isn't one.  We have a 100" base on January 22nd, with snow in the forecast.  Avalanche conditions could be better, but after the past few down years, we shouldn't complain.  Just be careful out there.

Friday, January 20, 2017

Weak Eastward Penetration during Yesterday's Refresh

Snow totals in the upper Cottonwoods were generally in line with expectations with about 5 inches falling.  An exception was the Park City Ridgeline and Brighton area which came in with perhaps 3 inches according to the Utah Avalanche Center.

For much of the day yesterday, radar echoes were strongest over western portions of the central Wasatch, with echoes weaker along the Park City Ridgeline and in the Brighton area.  You can see this fairly well in the radar plot below.  I suspect this reflects the relatively weak flow associated with the trough yesterday.  Easy to diagnose in hindsight.  Much more difficult to anticipate in advance, mainly because such precipitation distributions are highly sensitive to many factors, including storm depth.


Overall, the NAM comes out looking good again.  No change in my view of the NCAR ensemble, which I think is extremely valuable, but for the most part, it overdid it yesterday.

We may see a few snow showers today, but accumulations will minimal.  Another modest refresh tonight.

For yesterday's refresh, Snowbird gets my hype-of-the-year award for the tweet below.


Thursday, January 19, 2017

Thursday Refresh

Due to my travels in Japan, I haven't skied in the Wasatch since last year!  Last calendar year that is (31 December 2016), so I have virtually no idea what the skiing has been like the past several days, although I've heard reports that a refresh is needed.

We'll get some snow today and tonight that should help.  Radar shows relatively light echoes covering much of northern Utah at 1559 UTC (0859 MST) this morning.


This is consistent with the light snow falling on campus and along much of the Wasatch Front this morning.  Similarly, snowfall rates in the mountains have so far been light as well.

I expect things to pick up in the mountains during the day today as the trough axis swings through.  Right now, this looks like a modest refresh, with 4–8 inches falling at upper elevations in the Cottonwoods through 5 AM tomorrow morning.

One thing that surprised me looking over the models today is how bullish the NCAR ensemble is.  By 5 AM tomorrow (20/12Z on the plot below), most members are putting out 0.6 to 1.15 inches of water at Alta.  Those members are much wetter than the NAM and inline with the GFS, which is usually too wet.

Thus, I'm curious to see how this one pans out in the Cottonwoods.  My 4–8 leans toward the NAM, which has been a reliable performer and was initialized more recently.  The 1200 UTC NAM, for example, puts out 5" of snow and 0.41" of water at Alta Collins.  Something verifying closer to the wetter NCAR ensemble members, would, however, be more satisfying.

Wednesday, January 18, 2017

The Inversion Tightens the Noose

Salt Lake Valley residents awoke to a shallower but dirtier layer of pollution this morning
In my previous post issued on Monday (Why Deep Cold Pools Are "Good"), I discussed how deep valley cold pools and pollution layers are actually "good" compared to shallow ones.  When the inversion (a layer in which temperature increases with height) is elevated, pollution is distributed through a deeper layer, resulting in lower pollution concentrations.

Over the past two days, however, the inversion has lowered, tightening the noose on the Salt Lake Valley.  You can clearly see this in the soundings from Monday morning (top) and this morning (bottom).  In the Monday morning sounding, the inversion was about 1 km above the valley floor, and the atmospheric temperature profile allowed for mixing through that 1-km deep layer.  In contrast, this morning the inversion is based pretty much on the valley floor.  



Source: SPC
As a result, pollution concentrations at low elevations have increased.  Note in particular how they went into overdrive yesterday.  Yes, there are a few ups and downs, but for the most part the valley floor (Hawthorne Elementary) is observed much higher PM2.5 concentrations from yesterday afternoon through this morning than observed the prior days.  
Source: DAQ
There is some good news.  An approaching storm should lead to improving air quality tomorrow.

Monday, January 16, 2017

Why Deep Cold Pools Are "Good"

I arrived back in Salt Lake City yesterday afternoon to somewhat murky skies.  After 2 weeks in Japan, my first activity wasn't work related.  Instead, I simply lied on the couch, ate junk food, drank beer, and watched football like a good American.  It's good to be home!

Today, however, I'm at the office and taking a closer look at those murky skies.  The sounding from this morning shows a very deep cold pool over the Salt Lake Valley, with a capping inversion based at around 775 mb, which is at an altitude of 2264 m (7428 feet).


Consistent with the deep cold pool and more elevated inversion, the Cliff Cam at Snowbird shows smog and stratus extending ominously up Little Cottonwood Canyon.

Source: Snowbird
Although people often become alarmed about deep pollution (a.k.a. inversion) events, for residents at lower elevations along the valley floor, they typically feature lower PM2.5 concentrations than shallow events because pollution is distributed through a deeper layer.  Indeed, while the air quality isn't good, we're only in the moderate category and levels for the event so far maxed out at "only" 30 ug/m3 about two days ago.
Source: Utah Division of Air Quality
PM2.5 levels would be much higher if the inversion was closer to the valley floor and the cold pool was shallow, which results in pollution accumulating in a much smaller volume of air.

So, the bottom line is that when you see stratus and pollution tonguing up to Snowbird, that's probably "good" for valley residents.  On the other hand, it could mean you being exposed to some pollution at mid elevations in the canyons.

Sunday, January 15, 2017

Honshu's Climate Transition

I'm now in Narita airport awaiting my flight to return to the states.  My travels from Myokokogen to Tokyo today provided an excellent example of the remarkable climate transition that exists across the Japanese island of Honshu.

This was the scene last night when I went to bed.  Just like Alta, it doesn't need a reason to snow in Myokokogen.  It needs a reason to stop.


Did it stop overnight?  Nada.  Another 50 cm (20 inches) at the lodge and in town and 70 cm (28 inches) on the upper mountain.  Crazy.

My hat goes off to Michael and Tamami at the Myoko Mountain Lodge for going well beyond the call of duty to get me out of Dodge.  When I woke up at 6 AM, Michael was already out blowing out the walks and cars and Tamami was calling the railroads.  No service to the nearby station, so Michael drove me off the mountain and got me on the train.  Kudos as well to the remarkable Japanese snow removal systems for keeping the roads passable.


I suspect that the climate transition across the Cascades and Sierras is more abrupt than that of Honshu, but nevertheless, the contrast from the Snowpacalypse in Myokokogen to the Tokyo area was staggering. In Tokyo, on the western side of Honshu, it was a beautiful day with the only snow in sight on Mt. Fuji.