Monday, January 27, 2020

Sneaker Storms and Winter Perspectives

I haven't been blogging much about the forecast of late and maybe that's a good thing.  I keep looking at the models and thinking the next storm is sort of ho hum, and then it overproduces, resulting in a so-called sneaker storm.

Last night provided yet another example.  SREF forecasts initialized at 0900 UTC 25 January (2 AM MST Saturday) produced anywhere from 3-12 inches with last night storm with a mean of about 8. 


Thusfar, we're up to a 12 inch total, at the top of the ensemble range, and it's still snowing. 


I could dig further, but who wants to jinx this run?  Let it keep coming.

Shifting gears, it's now January 27th, which puts us nearly 2/3 of the way through meteorological winter (December-February) and the snow accumulation season.  In the Salt Lake Valley, it's been a relatively mild meteorological winter with a mean temperature at the Salt Lake City Airport through Jan 26 of 34.6˚F, the 16th warmest on record (i.e., since 1875). 

The coldest temperature observed so far this winter at the airport is a relatively balmy 20˚F, which was the minimum on three days in December and two in January.  That's pretty pathetic.  We've never gone so deep into meteorological winter without a colder temperature.

Source: NOAA Regional Climate Centers
Note, however, that we did get below 20 on two days in October, but that's not winter.  It's fall.  Remember, on October 30 we had the coldest day on record in October with a low of 14˚F.  We haven't been within 6˚F of that this winter!  

Finally, a quick comment on the state of the snowpack.  It's been a good snow season, but the snowpack is not as fat as you might think.  The Snowbird SNOTEL sites at 148% of median, but as of yesterday elsewhere in the Salt Lake and Bountiful area mountains Brighton is 113%, Thaynes Canyon 111%, Mill-D North 114%, Parleys Summit 112%, Lookout Peak 125%, Louis Meadow 110%, Hardscrabble 105%, Parrish Creek 111%, Farmington 105%, and Farmington Lower 112%.  To the south, Timp Divide and Cascade Mountain are below median (94% and 89%, respectively).  

Last night's snow will help a bit, but if I was a water manager, I'd keep doing my snow dances.  

Sunday, January 26, 2020

Reading the Clouds

Yesterday (Saturday) provided some great cloud displays that help illustrate important atmospheric circulations.

We'll start with the most "benign" —thermally forced cumulus clouds that formed on the mountains immediately north of Little Cottonwood Canyon yesterday morning.  These are a common site in Little Cottonwood and are the result of upslope flows that form over the canyon sidewalls.  It is likely that such flows form earliest in the morning on the south facing sidewall this time of year.


In contrast, nocturnal downslope flows can linger later in the morning on the north facing sidewall.  The upslope flow helps to form cumulus clouds when the temperature and moisture profile is right.

I also noticed some evidence of vertical wind shear and associated turbulence in the clouds near the Pfeifferhorn that morning, but nothing as dramatic or beautiful as what Luke Stone saw over Mount Timpanogos.

 As he notes, such clouds provide an example of Kelvin-Helmholtz waves, which occur where strong wind shear overwhelms the atmospheric stability, leading to overturning of the atmosphere.  If a cloud is present, the resulting pattern can look like water waves breaking near the beach.

Unfortunately, upper-air soundings are not collected mid-day, but if we look at the evening (5 PM) sounding from the Salt Lake City airport, we see that there was a layer (horizontal green line) at about 630 mb that is near saturation and in which the winds are increasing with height.  Mt. Timponogos is just a bit lower than this at about 660 mb.

Sounding source: SPC
Alternatively, the clouds could have been produced by shear right at the base of the inversion that was near crest level at about 690 mb.  Given the difference in time and distance, either of these are plausible. 

Finally, Allan Gardner posted the video below on Twitter.  This is an example of a hydraulic jump in which flow is moving rapidly downslope, becomes "supercritical", and then rises rapidly in the hydraulic jump.  

 The process is very similar to what can happen during flow over a rock in a stream.

Source: Aokomoriuta via Wikipedia Commons
As one might gather from the photo and video above, the region near and downstream of the jump can be quite turbulent. 

It's great when clouds provide guidance about the flow in the atmosphere, but Kelvin-Helmholtz instabilities and turbulence associated with downslope flows and hydraulic jumps can occur in the clear air.  They represent important hazards for aviation or recreationists such as parasailers.  In particular, both hazards and produce severe turbulence, often in areas that are otherwise free of clouds.  Just one of the reasons why I always wear my seatbelt on an airplane.

Thursday, January 23, 2020

Perspectives on Linkages Between Arctic Warming and Severe Winter Weather

A hot climate topic in recent years has been the possible linkage between a warming arctic and midlatitude weather extremes, including severe winter weather.  An example of the types of headlines you've probably seen before is provided below from The Guardian

Source: The Guardian
This is a subject that has caused me a good deal of indigestion and that I've largely avoided in this blog.  This is because most of the evidence for this linkage has been based on observational studies, whereas model based studies have obtained differing results.  

A new review article is now out in Nature Climate Change discussing the differing perspectives obtained form observational and modeling studies.  The paper is led by Judah Cohen and involbes a large number of authors including HongPing Gu and Simon Wang of Utah State University and the Utah Climate Center.  

Source: Nature Climate Change
Sadly, the paper is paywalled, but the National Science Foundation Public Affairs Office has issued a plain-language summary that describes the issues at play.  It is available at https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=299896&org=NSF&from=news, although I've included a screenshot below if you want to take a quick look (click to blow up).

Source: NSF
To summarize the issues at play, the Arctic has been warming twice as fast as the global average, a trend that has long been anticipated and is known as Arctic Amplification.  The causes of this amplification are multifaceted and complex.  Feedbacks related to a loss of Arctic ice are most commonly cited by popular media, but as summarized below, there are others.  

Source: Cohen et al. (2020)
In turn, it has been hypothesized that Arctic Amplification may be affecting midlatitude weather.  For example, the decline of Arctic sea ice in the fall can intensity Arctic storm systems, which could potentially lead to weakening of the polar vortex.  A weaker jet stream might also be expected due to the weakened temperature gradient between the arctic and the lower latitudes.  These are simple generalizations and ultimately linkages between the Arctic and the midlatitude depend on several factors and are sensitive to regional changes in sea-ice loss and other subtleties.  

What one would hope is that observational and modeling evidence would like up and agree, but at present, they do not.  Observational studies reveal relationships and suggest linkages.  The period that can be investigated observationally is, however, relatively brief given the amount of variability in the atmosphere.  Modeling studies allow us to simulate much longer periods of time and conduct experiments in which we can hold one thing constant but change another, but are limited by model imperfections.  

At present, observational studies have identified some relationships between Arctic Amplification and midlatitude severe winter weather.  Recent modeling studies suggest, however, that the atmospheric response to recent Arctic sea ice trends is negligible.  As noted by Coehen et al. (2020), "A number of large ensemble modeling studies have come to the same conclusion — that is, there is little modeling evidence of an atmospheric response to the pan-Arctic sea ice trend."  

This is how science works.  Hypotheses are put forth and tested, often using multiple approaches.  Discrepancies are then investigated, methods refined, and hypotheses revised and retested.

I don't know how this process will conclude.  It is possible that we will learn that recent trends reflected variability and it was coincidental that there appeared to be a linkage between arctic warming and midlatitude severe winter weather.  It is possible that deficiencies in the modeling system or experimental designs will be identified.  Finally, it is possible that we're just not looking at the problem properly and that new theory and better theory will be developed.  Eventually we will figure this all out.  

Wednesday, January 22, 2020

Japan's "Nonsoon" Season

Western Japan has one of the most remarkable and reliable snow climates on Earth, but as the saying goes, climate is what you expect and weather is what you get.

During the East Asian Winter Monsoon, which over Japan features quasi-persistent northwesterly to northerly flow between the Siberian High and Aleutian Low, frequent cold-air outbreaks drive sea-effect storms near the Sea of Japan coasts of Honshu and Hokkaido Islands.  Mean annual snowfall exceeds 500 inches in some mountain areas, much of it falling from December to February.  

This year, however, the winter monsoon has been more of a winter "nonsoon."  Snow depth observations as of today, 22 January, show many sites with under 50 cm of snow and only four over 100 cm (about 40 inches).  

Snow depth observations in cm.  Source: http://www.data.jma.go.jp/obd/stats/data/mdrr/snc_rct/index_snc.html.
This is unusual as nearly all sites are well below average, especially on Honshu where many sites are below 40% of average.  One can find a few sites in northeast Honshu that are above 100%, but these are not in the heavy snow region near the Sea of Japan and thus snowpack in those areas is still scant.  For instance, Sannohe in Aomori Prefecture is at 142% of average, but that's with only 27 cm on the ground.  

Percent of average snow depth.  Snow depth observations in cm.  Source: http://www.data.jma.go.jp/obd/stats/data/mdrr/snc_rct/index_snc.html
Near Niseko Resorts on Hokkaido, Kutchan currently has a 78 cm snow depth, which is only 61% of its 128 cm average on this date.  Kutchan is near the base of Niseko and snow depths are probably greater on the mountain, but with records going back to 1944, it does illustrate the unusual low-tide conditions.  

Perhaps not surprisingly, the deepest measured snowpack, indicated by the reddish-orange square in the top image above, is Sukayu Onsen in Aomori Prefecture on the north end of Honshu.  Sukayu Onsen has a mean annual snowfall of 694 inches, so a bad year there is still better than a good year most anywhere else.  There, in the Hakkōdo Mountains, the snow depth is 191 cm, but average is 248 cm.  

One reason for the "nonsoon" so far this winter is that the Siberian High and Aleutian Low are weaker than average.  Below is an analysis of the sea level pressure "anomaly" (or departure from average) for the one month period ending January 15.  Negative anomalies over much of northern Eurasia are consistent with a weak Siberian High and positive anomalies over much of the north Pacific are consistent with a weak Aleutian Low.  


One might ask why those pressure systems are weaker than average.  Ha ha.  I don't know the answer to this and I don't have the time to investigate.  You get what you pay for with the Wasatch Weather Weenies blog.  If you want me to investigate further, buy me a ticket to Hokkaido.

Tuesday, January 21, 2020

Thursday's Frontiers of Science Evening Lecture

This Thursday Evening's Frontiers of Science Lecture is one you don't want to miss.  Dr. Craig Clements, Professor and Director of the Fire Weather Research Laboratory at San José State University will be speaking on Advances in Observing the Wildfire Environment: Understanding the Role of Fire-Atmosphere Interactions
The talk is scheduled for 6 PM in 220 ASB (Aline W. Skaggs Biology Building).  Free parking is available in the Chemistry Lots.  Complete information is available at https://science.utah.edu/news/frontiers-of-science/.

Craig is a University of Utah alum and the "Indiana Jones" of wildfires.  You've probably heard of tornado storm chasing.  Well, Craig is an extreme wildfire chaser, using scanning Doppler lidars and other instruments to better understand how fire-atmosphere interactions yield extreme fire behavior. 


Craig is a great and engaging speaker.  This is definitely a lecture you don't want to miss. 

Monday, January 20, 2020

Greed Is Good

One of the "worst" days I've had this year.  Photo: Erik Steenburgh.
It's been a pretty good ski season thus far and, thinking like Gordon Gecko, I think our motto going forward should be "Greed Is Good."  

NRCS SNOTEL data as of 18 January shows that most northern Utah observing sites are at or above median.  In the Wasatch Range, only Timpanogos Divide (93% of median) and Cascade Mountain (82%) are below median.  

Source:NRCS
The Snowbird SNOTEL has both the highest water equivalent (23.8") and percentage above median  (143%) in the Wasatch Range.  A look at this water year's trace (blue line below) compared to water years since 2011 shows we are running well above all water years except 2019 and 2011 (year denotes the last year of the water year — i.e., 2019 = 2018/19).  Further, we were ahead of 2019 during much of December and are only a little behind, so the difference is negligible.  

Source: CBRFC
The bottom line is that this year is pretty good by recent standards.  We've discussed how the 2010s were pretty "meh" for snowfall compared to the 1980s and 1990s in previous posts (see Your Parents Had More Powder Than You).  A look at the the traces above shows that the 2010s were characterized by either really big snow years or really bad snow years.  There's no middle ground and no seasons that end up near median maximum SWE, which is 43 inches. 

How the rest of the year ends up is anyone's guess, but we can look forward to the week ahead and see what is likely.  We have a system passing to the south tonight and tomorrow and then another system passing to the north Tuesday night and Wednesday.  While not a direct hit, the Wasatch should see some periods of snow tonight through Wednesday morning.  Through 11 AM Wednesday (22/18Z in the plumes below), the SREF generates a mean of about 8 inches of snow at Alta-Collins, with most members between about 3 and 12 inches.  


After that, it's a bit of a dirty ridge scenario for Thursday, but accumulations, if it snows, will probably be light.  Thus, a reset this week depends on coming in on the high end of projections through Wednesday morning.  The most likely scenario is 4-8 inches in upper Little Cottonwood this week.  Let's hope we do better than that and get a real reset.  Greed is good.