Saturday, December 30, 2017

Not Your "Average" La Niña Pattern

Because of the presence of La Niña conditions in the tropical Pacific, La Niña is an easy target to blame for the wacky weather of the past month or two, but is it the real culprit?  Let's have a look.   

La Niña is a component of the natural see-saw of oceanic and atmospheric conditions in the tropical Pacific characterized by anomalously cold ocean temperatures in the central and eastern tropical Pacific.  These anomalously cold temperatures are evident in sea surface temperature anomalies from earlier this month.  Note in particular, the tongue of anomalously cold water extending along the equator from South America to the dateline. 

Source: Climate Prediction Center
If one looks at composites (or averages) of La Niña winters, one finds patterns similar to those illustrated in the bottom panel below with high pressure over the central Pacific near about 150ºW, cold conditions over northwest North America, wet conditions in the Pacific Northwest, and dry conditions across the southern tier of the United States.  

Another way to look at this is in terms of hight anomalies at upper levels.  Areas in warm colors below correspond to anomalous ridging, and areas in cool colors anomalous troughing.  At upper-levels, the primary circulation features are an anomalous trough near Hawaii, ridging in the North Pacific, and troughing over northwest North America. 

The pattern over the past month hasn't really looked like that.  Instead, we've had anomalous ridging along the west coast of North America. 

In addition, while it has been dry across the southwest US and most of the southern US, it has also been dry in the Pacific Northwest.  The only areas of anomalously high precipitation is in the northwest U.S. Rockies and adjoining plains and in lake-effect areas.  

Thus, this is a pattern that doesn't fit the average La Niña pattern all that well.  That isn't to say La Niña isn't playing some role.  It could be playing an important role, with our use of relationships based on averaging past events the real problem.  On the other hand, it is also possible that we need to look at what is happening from a broader, global perspective.  Some discussion of this topic is provided by the California Weather Blog.  This is an area of active research, and one that will probably get even more attention after this winter, which has generated some remarkable weather extremes across the United States.  

Friday, December 29, 2017

Wacky Waves, Winter Warmth, and Pollution Perspectives

Kory Davis sent me the great cloud photo below taken looking east yesterday at about 8:30 AM from Snowbasin.

Photo courtesy Kory Davis
This is a wonderful example of breaking waves produced by Kelvin-Helmholtz instability, named after physicists Lord Kelvin and Hermann von Helmholtz.  Such instabilities are produced by vertical wind shear, leading to breaking waves that are similar in appearance to waves breaking on a beach.  Such instabilities occur frequently in the atmosphere, and produce turbulence, but aren't always easily seen. 

Moving on to today's weather, there's much to discuss.  First, how about our afternoon temperatures.  At 2242 UTC (3:42 PM MST), many sites along the east bench are running in the 50s, including a 56 at the University of Utah.  Alta is 43, Mountain Dell 51, and Kimball Junction 50.  THIS IS NO WAY TO RUN A WINTER.  

If you look carefully, evidence of the valley inversion is apparent, with temepratures in the 40s in Kearns, Taylorsville, and along the Jordan River north of South Jordan.  Also apparent is the strong influence of the lake breeze, with the Salt Lake Airport sitting at only 44.

How about we give the PurpleAir network a little love today and use it to take a look at the distribution of pollution around the region.  Sometimes caution is needed in interpreting the data collected by these low-cost sensors, but they look to be quite useful today.  Note that the highest PM2.5 concentrations are in the downtown area.  Lower values are found on the east bench, the west bench, and in the southern Salt Lake Valley.  One can find predominantly clean air up I-80 to the east.


The ups-and-downs in air quality over the past few days have been astounding.  The DAQ sensor at Hawthorne Elementary has had periods each day with PM2.5 at levels ranging from unhealthy for sensitive groups to good.

Source: DAQ
Yesterday in the Salt Lake Tribune, head of EPA region 8 noted that he believes that Utah's air quality can be improved, in part because of Colorado's success in Denver.  He notes, correctly, that part of the trick is figuring out the chemistry.  That's all fine and dandy, but here's my three New Year wishes that I would like to see happen now:

1. Upgrade and improve real-time air quality monitoring.  It is an embarrassment that there is only one "official" real-time monitor in Salt Lake County and that the data from this monitor is often more than an hour old.   There are large spatial and temporal variations in air quality that exist in our county (and other non-attainment counties in northern Utah) and citizens deserve better information about what they are breathing right now.   Alternatively, take greater advantage of lower-cost sensors, accounting for observational issues that arise from their design limitations.

2. More proactively work to improve air quality.  As a scientist, I always cringe at rallying cries for more research.  Yes, we do need more research, but the assumption that because we don't know everything, we know nothing is a bad one.  We know enough to move forward on new initiatives today. 

3. Show greater commitment and be results oriented.  I'm sorry, but I've lived here for over 20 years and I have heard the same song and dance for a long time.  If this state truly cared about the air pollution, we would have bent the curve years ago.  Near as I can tell, the primary difference between states that have made substantial air quality improvements (e.g., California) and those that haven't is political will and the desire to do what is difficult. 


After publishing this post, I realized I may have been overly harsh on points 2 and 3.  There have been long-term improvements in many air quality indicators, however, the worst PM2.5 events (98th percentile) have shown little trend over the past 10 years (see  Improvements basically flatlined after 2003.  The shift to tier 3 gas will hopefully help, but more can be done.  It's really a matter of whether or not you are satisfied with the status quo.  I am not.  

Thursday, December 28, 2017

Skiing, Inversions, Pollution, and All That

It's been an interesting break.  After hemming and hawing about whether or not to get out of Dodge, we decided that there was enough snow and decent enough air quality that we'd gut out Christmas week at home.  By and large, I think it was a wise decision.

The cross country skiing has been pretty decent and has provided a good cleansing of my aorta.  Back country skiing?  We got in a tour the day after Christmas.  I can't say the skiing was good, but it was an education for my son who had never been out on such a hair-trigger, high-hazard day before.  I made him suffer too, with a 2000 vertical foot climb through heavily brushed slopes to start the day.  It's important that youngsters recognize it's not all fun and games out there. 

We went up for a couple of high-speed laps at Alta this morning.  It was only my 2nd day of lift served this year.  Conditions were surprisingly good for carving.  I'm always amazed that when conditions here are about as bad as they can get, it's still far better than most of the good days I had when I lived in upstate NY.  That being said, it's incredible to think that Mineral Basin is still closed and probably will be through the 1st of the year, unless Snowbird has some real tricks up its sleeve.

Despite the poor off piste coverage, the coverage on the groomers was adequate and by 11, the corrals were full and there was stiff competition for Jerry of the Day honors.  After a few laps of human-breakaway-slalom, we decided to return home and get something done.

Which brings us to the weather.

Here's your pearl of wisdom for the day.  Don't count on a pattern change until you see the whites of its eyes.  Stop looking at the 10-day forecast and buy yourself a fat bike.  Alternatively, go to Jackson.

Now maybe you can provide me with some pearls of wisdom.  Is it just me or is this the weirdest inversion ever?  Remember all that fog, stratus, and pollution on Christmas Day and Boxing Day?  Well, the fog has dissipated, we just hit 44˚F at the airport, and the PM2.5 concentrations have been up and down like a yo-yo the past few days.

Source: DAQ
I'm sure there's still plenty of gunk behind the lake breeze where the cool air over the lake won't release the goods to the free atmosphere, but this situation is far better than I could have hoped a couple of days ago.  More time at the office is needed to decipher this riddle.  Comments on what's going on appreciated.

Wednesday, December 27, 2017

Dismal Pattern Continues

The large-scale pattern has not changed.  I repeat, the large-scale pattern has not changed.

Yes, we got a couple of miracle storms prior to Christmas, but on the hemispheric scale, we are still dealing with a highly perturbed, wavy, high-amplitude pattern. 

Note, for example, the dynamic tropopause (jet-stream level) analysis from 0000 UTC 27 Dec (1700 MST Tuesday/Yesterday).  Deep trough over the eastern US.  Deep trough off the coast of Asia.  High amplitude ridge over the eastern Atlantic. 

You want snow, go to the lee of the Great Lakes or western Japan.  Both have been getting pummeled. 

And, there's no end in sight for the dry weather.  Forecasts below are from the 0000 UTC 27 December initialized ECMWF and GFS models through the end of the holiday period (0000 UTC 3 January/1700 MST 2 January).  Storm track to our north.  Dry southwest. 

There are a few members of the NAEFS that drag the moisture down south enough to give us a little more action around later in the 7-day forecast period.  Thus there is a little hope, but I'm keeping my expectations low.  Plumes below are for Alta-Collins and show a small number of ensemble members giving us some action on or after the 30th of December.  Most generate no more than 4 inches of snow.  Sad!

Is this La Niña?  I'm not ready to endorse that viewpoint.  Yes, there are aspects of this pattern that are consistent with La Niña, but there are other aspects that are not.  Mechanisms affecting the large-scale circulation are multifaceted and complex.  Maybe we'll deal with this in a future post. 

Monday, December 25, 2017

Christmas Brings Gravity Waves and Unexpectedly Poor Air Quality

Christmas turned out to be a fascinating weather day.  The Christmas Eve warm front and precipitation left a shallow cold pool over the Salt Lake Valley with widespread fog and dense haze in the morning.  I noticed some strong gravity waves from my vantage point in the upper avenues, one of which is pictured below.  Note the bulge in the depth of the fog, similar to the crest of a wave on the water. 

Although the fog burned off in many areas, a shallow lens of haze remained over much of the valley during the afternoon. 

My thinking during the day was that this was probably predominantly natural haze as PM2.5 concentrations were quite low on Christmas Eve morning.  However, the PM2.5 observations from Hawthorne Elementary show we reached levels unhealthy for sensitive groups Christmas evening.  In fact, the increase from Christmas Eve morning is astounding.  Normally one sees an increase in PM2.5 during a developing and persistent inversion of about 10 ug/m3 per day.  What happened from Christmas Eve to Christmas was more than double that.

Source: DAQ
Clearly, the DAQ was caught with their pants down on this one as their web site, updated at 1:22 PM on Christmas Afternoon, had us in "unrestricted action." 

So, why was the PM2.5 so high?  I'm not sure why, but there are a few possibilities.  One is that in this case the inversion didn't develop aloft and descend slowly as is often the case when a ridge builds in.  Instead, it developed as the warm front warmed temperatures aloft and precipitation cooled the near-surface airmass, resulting in a shallow cold pool right from the get go.  Thus, any emissions were confined to a shallow layer.  Indeed, the sounding collected in the late afternoon on Christmas shows a very shallow inversion based right at the surface, with a well mixed layer aloft.  Little wonder the upper benches were so clear, with a different story in the valley.  

I'm not entirely satisfied with that as an explanation, however, since the buildup in pollution seemed remarkably rapid.  I wonder if the holiday "sparked" an increase in yule log burning, and that this, along with the shallow nature of the cold pool, resulted in the rapid increase in PM2.5.  Having more wood fires with such a shallow cold pool would be a recipe for a rapid pollution rise.  Being in unrestricted action, perhaps many more people than usual decided to go for the holiday fire.  

A third possible factor is that the high humidity is causing somewhat inflated values, although the sampler used for the time series above I thought was somewhat immune to this effect (some samplers are not as water droplets inflate values).

Regardless, it's unfortunate that pollution along the valley floor was so high during this event.  

Sunday, December 24, 2017

Much Needed Tailwind for Santa This Year

With a full-latitude, high-amplitude ridge parked over and just north of Alaska, Santa will have a much needed tailwind after taking off from the North Pole for Utah.  Just check out the strong northerly jet stream (green contours) that will rocket him southward from the North Pole and across much of western Canada tonight! 

That tailwind is a godsend because the reindeer will need to be fresh when they arrive in northern Utah.  With clouds and precipitation along much of the Wasatch Front and in the Wasatch Mountains, they will have a difficult job, even with Rudolph leading the team.  Mountain snow will reduce visibility and, with temperatures climbing, Santa may encounter a rain/snow mix or even a bit of rain in the lower elevations, depending on the timing of his deliveries.

Rumor has it that Santa is considering hanging around and skiing freshies tomorrow at Alta.  Apparently he was spotted yesterday. 

It's rare for Santa to leave the North Pole at this critical time of year, but given the rarity of powder this year, he told the elves to buckle down and cover for him for a couple of hours as he needed a fix to lift the spirit before Christmas.

Happy Holidays from the Wasatch Weather Weenies!

Saturday, December 23, 2017

Water Equivalent Is More Important Than Snowfall Amount

Most ski reports and skiers tend to focus on snowfall amount as the measure of storm size, but for many applications, it is the water equivalent of the snowfall (i.e., how much water it contains) that is the most important measure.  After all, the water equivalent determines how much weight (and stress) you are adding to the snowpack, which is important for avalanches, and how much water you are adding to the snowpack for water resource purposes.  

Really, Utah skiers should also be focusing on water equivalent right now instead of snowfall amount.  We need base, and base is a function of how much water is in the snowpack.  Six inches of 15% water content snow goes a lot farther as six inches of 5% for building up base since it contains three times as much water mass.  

As I write this post at 7:15 AM, I'm encouraged by the current storm so far.  Storm-total water equivalents in the Wasatch so far include 0.9" at Ben Lomond Peak (through 6 am), 0.8" at Ben Lomond Trail (6 am), 0.74" at Snowbasin-Boardwalk (7 am), 0.6" at Lookout Peak (6 am), and 0.38" at Alta-Collins.  Those aren't big numbers, but they are welcome and they will help a great deal.  Plus, as can be inferred from the radar image below, snow showers look to continue.  

Source: NCAR/RAL
Indeed, the latest HRRR keeps us in the moisture plume for most of the morning, as evident in the forecast valid 1800 UTC (11 AM).  

Things should taper off in the afternoon, but let's hope we squeeze as much out of this as possible.  It will not get us near an average snowpack, but it's going to help with the ski conditions quite a bit at the resorts.  It should allow North Fork Park to get open for cross country skiing.  I think we're in some sort of ski purgatory right now for backcountry skiing, with a thin, weak snowpack in some areas and not really enough to ski elsewhere, but this gets us closer.  

Friday, December 22, 2017

The Next Storm Could Surprise

Based solely on the large-scale pattern setting up for tonight and Saturday, one might not be very enthused about our snow chances.  There's a huge, high-amplitude ridge centered over Alaska and a ridge upstream of Utah with a short-wave trough moving across Montana.  Show this and nothing else to 10 meteorologists and ask them what will happen in Utah and they probably would say not much.

However, the Devil is in the details, and those details suggest this storm could surprise.  The pattern is highly unusual because of the size of the ridge over Alaska, and we end up in a situation in which two airmasses of dramatically different origin are coming together, one from the subtropics, the other from the high latitudes.  This leads to a strong mid-level front and plume of moisture that extends inland across Utah.

By itself, that probably wouldn't do much, but we have something going for us, the Wasatch Range.  The GFS forecast above shows that the terrain effects are very important, with strong modulation of precipitation as the moisture plume runs over the various topographic features of Oregon, Idaho, Utah, and Colorado.

So, what do the models spit out?  I'll focus on the period through 0600 UTC 24 December (11 PM MST Saturday), although the bulk of the precipitation is expected to fall tonight and during the first half of the day tomorrow (the end of the precip is a bit ill defined with some models calling for it to last longer than others).

For Alta, the 0Z Euro puts out about 0.4 inches of water, the 6Z GFS is just over an inch, and the 6Z NAM 0.8".  The downscaled SREF plumes below show a pretty wide spread, enough to give serious heartburn to any meteorologist, with a range from 0.1" up to 1.75" by 0600 UTC 24 December.

Before it retires in another week or so, we can also have a look at the NCAR ensemble, with a range from about 0.5 to 1.4 inches through 0000 UTC 24 December (5 PM Saturday).

Ultimately, much will depend on the location of the plume and the evolution of the flow.  The Euro and low-end of the ensembles say keep your expectations tempered, but even 0.4" of water would make this the 2nd biggest event since the 3 December storm that put down a season-saving 1.08" of water at Alta.  It's going to help.  Some of the ensemble members are putting out amounts much higher than anything we've seen in a while.  Those are not high probability possibilities, but they could come up with a lucky role of the dice.  Thus, let's see what happens and hope this storm surprises.

Skinny skiers should be happy as a little will go a long ways at Mountain Dell and North Fork Park and the precipitation should be all snow at both locations.

Thursday, December 21, 2017

About Yesterday's Terrible Air Quality

It's a 2-for-1 day at the Wasatch Weather Weenies.  If you want to read about the overnight snowfall, proceed to the previous post.  If you want to read about dust, air quality, pollution, and other apocalyptic aspects of the storm, stay here.

Yesterday morning, an extremely nasty plume of dust penetrated into the Salt Lake Valley, pushing air quality to unhealthy levels.

Although Salt Lakers are aware of poor air quality during inversion events, yesterday we dealt with an entirely different beast.  During inversion events, emissions from the combustion of fossil fuels and wood, as well as a few other sources, build up within the Salt Lake Valley.  Winds are light and the strong atmospheric stability prevents the mixing of pollution vertically.  In contrast, yesterday the poor air quality occurred during strong, southerly, prefrontal winds.

The graphs below show what happened at Neil Armstrong Academy in the Northwest Salt Lake Valley.  From Tuesday morning (19 December) through just after midnight Wednesday (20 December), PM2.5 concentrations fluctuated from about 0 to 19, with the peak around midnight.  These are values that indicated good to moderate air quality with just a little bit of pollution. 

Source: MesoWest
Then, after midnight, strong southwest winds developed, scouring the valley clean.  Those winds persisted overnight and into the morning.  However, beginning after about 6 AM, PM2.5 concentrations began to climb, eventually spiking to over 200 ug/m3, which is well into unhealthy territory.  That spike occurred just ahead of the surface cold front.  PM2.5 concentrations then dropped from 203 to 10 ug/m3 in 10 minutes as the front went through and brought in cleaner air. 

Another perspective is provided by a laser ceilometer at the University of Utah.  This is a device consists of a laser that points vertically through the atmosphere.  The signal returned back to the device can be used to infer pollution concentrations and the base of clouds. 

Below is a time-height section from about 1500 MST on Tuesday through 1500 MST Wednesday.  There is some moderate pollution evident on Tuesday afternoon and evening, but the airmass becomes relatively clean overnight.  The dust plume appears shortly after 6 AM and through about 10 AM extends to about 750 meters (2500 feet) above ground level. 

Image Source: MesoWest
As the front approaches, the depth of the dust increases.  I suspect this is due to the convergence of surface winds and the lifting of air near the front lofting the dust plume to deeper heights.  If one looks carefully, you can see evidence of the dust aloft event after the clean air behind the front has moved in at low levels.

So, what the hell is going on and where is all of this dust coming from.  We had a similar event on December 3rd, but the dust wasn't as think.  One of our graduate students, Derek Malia, pointed out to me that the dust plume was very evident in satellite imagery that day.  Sure enough, you can see it in the image below, originating in the south Cedar Valley west of Utah Lake.

Source: NASA
Yesterday, similar story.  I didn't have access to as sharp of an image as on 3 December, but one could still see the plume originating over the south Cedar Valley. 

Image Source: NASA
And, one can see it as a pink streak in the dust product from GOES-16.  Note that the plume continues downstream over northeast Utah, in that area likely above the shallow post-frontal airmass. 

So, we have a good idea that this dust is coming from the south Cedar Valley.  Google Earth shows that the dust emissions could be coming from agricultural fields in that area, which are perhaps exceptionally dry for this time of year due to the drought conditions.  Another possibility is emissions from a fire-affected area.  I haven't had a chance to dig into the past fire data to examine if this is a viable hypothesis. 

Hopefully, this is an issue that will remedy itself with precipitation.  If not, perhaps it wouldn't take much to reduce emissions from that area. 

Radar Struggles with Low Density Snow

The total snowfall through 7 am this morning on the Alta-Collins stake sits at 6".  With a snow water content of 0.36", that makes for a water content of 6%.  Perhaps not quite cold smoke, but close to it. 

Alta continues to see some light snow.  This stuff is barely discernible in the radar loop where all one sees are some stronger returns over the high terrain that are ground clutter and some very light returns in a couple of bands that move from east to west across the Wasatch Range, consistent with precipitation in the "wrap around" part of the storm. 

It's very difficult to determine precipitation rates at Alta and in other parts of the Wasatch Range during such conditions using radar.  There are a couple of reasons for this.  First, the precipitation is quite shallow and often the radar beam partially or totally overshoots the precipitation. 

Source: Secrets of the Greatest Snow on Earth
Second, snow scatters a lot less radar energy back to the radar compared to rain.  This is especially true for low density dendritic snow.  As a result, returns are limited and the correlation between the radar reflectivity and snowfall rate can be poor.

Ski Tourers: Note that the Supreme Area at Alta is now closed to uphill traffic.

Wednesday, December 20, 2017

Lessons in Cold-Frontal Precipitation

If you have studied introductory meteorological text books, you've probably seen schematics of frontal precipitation based on the one below, which is from a seminar paper published in 1922 that synthesized existing knowledge on frontal cyclone evolution into a coherent depiction known as the Norwegian Cyclone Model due to it's development in Bergen, Norway.

Source: Bjerknes and Solberg (1922)
The cold front is depicted on the left, with a narrow band of precipitation forming where the leading edge of colder air is intruding into the warm airmass.  This band of precipitation is known today as a narrow cold-frontal rainband (or alternatively, snowband if cold enough).  

Although, conceptually simple, not all cold fronts behave in this manner.  Sometimes a wide cold-frontal rainband exists upstream of the surface front, as depicted below. 
Source: Matejka et al. (1980)
In some instances, the wide cold-frontal rainband exists in isolation, with no narrow cold-frontal rainband present.

Which brings us to today's frontal passage.  At 1400 UTC (7 AM MST) there was a clear separation between the surface trough, which was draped across central Nevada and northwest Utah and a wide cold-frontal rainband (really a snowband) that was further upstream and northwest.  

The 1300 UTC initialized HRRR forecast valid 1900 UTC (1200 MST) very clearly shows the surface cold front, as defined by the wind shift, pushing into the Salt Lake Valley, well in advance of the trailing precipitation band.  

So, this is an instance where we will expect a mainly dry frontal passage, with precipitation moving in later (let's hope things fill in better than in the HRRR forecast above!).  

The visualization below depicts the structure of a similar front on the 27th of November when the surface front was moving across the Great Salt Lake Desert and into the Skull, Tooele, and Salt Lake Valleys.  The cold air behind the surface front was quite shallow and during this period the surface front was dry.  Deeper cold air was further upstream and accompanied by a wide cold-frontal rainband (rainband not shown).  I suspect todays frontal passage will be similar in structure.  

What controls the precipitation characteristics of cold fronts in our part of the world is not fully understood.  The topography appears in some instances to form a new surface trough ahead of the approaching Pacific cold front, and this trough sometimes becomes a new cold front ahead of the precipitation system.  On the other hand, we also see events that feature bonafide narrow cold-frontal rainbands.  These differences can, however, often be anticipated by the HRRR.

Which brings us to the forecast.  I'm sticking with a 3-6" storm total at Alta through noon tomorrow (Thursday), most with the wide cold-frontal rainband (or more correctly, snowband) trailing the surface front, with a few snow showers from the wrap around late tonight and tomorrow.  The snow will be of the low-density variety, which is a shame, as we really need a pasting with high-density snow right now.  

If Alta gets more than 8 inches, consider it a Christmas miracle.  If they get less than 2 inches, you had better take some time to reflect on your behavior this past year, because you are clearly on Santa's naughty list.

Tuesday, December 19, 2017

Keep Expectations Low

I've been thinking of hanging it up for the season.  Not skiing, but forecasting.  It's starting to get depressing.

I needed to remind myself this morning what a decent storm looks like in the Wasatch.  Certainly over an inch of water in 24 hours.  Or, better yet, a good pasting and base builder, which would give more than two inches of water in 24 hours (maybe 3 at the upper end).  What I wouldn't give for that.

We do have a storm on the horizon.  The GFS forecasts a cold front pushing through Utah tomorrow at 5 PM MST.  The cold front is really a thing of beauty, wonderfully continuous in terms of the wind shift, temperature gradient, and precipitation band from NW Wyoming all the way to the White Mountains.  That's a rarity.

We look to get some mountain and valley snowfall with the front.  Following frontal passage, a strong surface cyclone forms in the four corners area.  Could be some nasty weather for the Canyonlands area.  The Wasatch, however, are on the edge of it, the so-called "wrap around" area.

A best case scenario for us would be for the front to be a good producer and the wrap around to be close enough to give us some additional snow.

Most members of the SREF ensemble, however, give us 0.1-0.3" of water at Alta Collins with the frontal passage tomorrow afternoon or evening, and little thereafter.  There are five members (out of 26) that go for either a more productive front and/or wrap round period, with an inch of water produce by one member.  One out of 26 ain't that bad right?

So, best to expect this to be a relatively modest event.  The snow will likely be of the low-density variety, so my take is 3–6 inches for Alta Collins with the frontal passage.  More will require a more productive front or wrap-around period than currently being advertised.  At least it's white.

Monday, December 18, 2017

Predictability Limits Don't Change Because of the Holidays

Christmas is still a full week away, but that isn't preventing people from making specific forecasts for the big day. 

This is a common issue when a major day or holiday approaches.  The desire to provide specifics far in advance.  10-day forecasts for trick-or-treating weather on halloween.  Guarantees of a white Christmas, etc.  Unfortunately, the existence of a holiday does not magically clear up the meteorological crystal ball or alter the fundamental chaotic nature of the atmosphere. 

I saw one forecast this weekend for a brutally cold Christmas day with a high of 18ºF.  Presumably this was based on forecasts like the one below from yesterday morning's GFS which did indeed have some exceptionally cold air over northern Utah with 700-mb temperatures below -20ºC.

However, last night's GFS paints a totally different picture with a ridge over the west and 700-mb temperatures of -4ºC over northern Utah.  For the ski areas, that's a swing in temperature of about 17ºC (30ºF)!

Cold air outbreaks on a continental scale can often be anticipated may days in advance, but the specifics for one particular area are more difficult to nail down.  Northern Utah is difficult because we tend to be on the western edge of the cold air.  The shift in the model forecasts above reflects the uncertainty in the forecast more than a model trend.  It doesn't mean we're going to miss the cold air (or be warm).  It simply means that we have a range of possibilities due to the chaotic nature of the flows that generate these cold-air outbreaks. 

We don't produce a 700-mb temperature thumbnail plot from the GEFS, but below is one for the "1000-500-mb thickness" (dashed lines, some in red), which is proportional to temperature in the lower atmosphere.  These are valid for 11 PM Christmas Eve, when Santa will be delivering presents to your home.  Some of these runs would be quite cold (e.g., lower left panel and 2nd panel from right in center row), but others would be more seasonable or perhaps even warm (e.g., first and second panels from left in center row). 

Source: Penn State E-wall
Thus, endorsing a specific, individual forecast makes little sense at this stage.  On the other hand, it is important for meteorologists to discuss the range of possibilities for big holidays when travel volumes are likely to be high. In that regard, mentioning that there could be a cold surge into the area seems appropriate, if it is also noted that it is still unclear if that surge will push into the Great Basin or remain east of the divide and over the northern Plains. 

Sunday, December 17, 2017

Accessing ECMWF Forecasts

The European Center for Medium Range Weather Forecasting (ECMWF) produces the leading medium range forecast model and ensemble prediction system in the world.  However, their products are generally paywalled and I am unable to access them freely for real-time applications (Note: They do generously provide their products for research applications that are not real time, which we greatly appreciated). 

As a result, weather weenies and hobbyists who want to examine the ECMWF forecasts have typically had to pay for a subscription.  However, is now providing access to ECMWF forecast graphics without such a subscription.  Below, for examples, is the 5 day forecast valid 000 UTC Friday 22 December. 

I don't know how long this jailbreak will last, so enjoy while you can. 

Friday, December 15, 2017

Storms that Stretch Like Taffy

Over the past several weeks, we've had a parade of storms that, to steal a line from Howie at Alta, "stretch like taffy."

This is a symptom of the large-scale patter that has dominated our weather during the period.  Upper-level troughs either move through or over the ridge over the eastern Pacific and western North America into an area characterized by strong deformation.  Deformation is a word that describes flow patterns that tend to "stretch" and change the shape of weather features.  A good analogy is what you do with silly putty.  Push it down on something like comics in the newspaper, pick it up, and stretch it.  The figures on the silly putty get stretched out and deformed.  In other words, deformation.

An example is provided by the forecast for the next couple of days.  The loop below depicts the forecast evolution of the dynamic tropopause, basically a map of the upper-level pattern.  Troughs are blue, ridges are red, and the wind vectors depict the jet-stream level flow.  Such a map shows deformation very well.

One can see two pieces of taffy in this loop.  The first is early in the loop over Mexico and the central US.  That is the trough that brushed by us on Wednesday and Wednesday night.  Note how it gets stretched out int eh first part of the loop before a piece breaks off over Mexico and gets entrained back into the large-scale flow.

The second is the trough that is associated with our storm tomorrow.  Note how it too gets stretched out as it moves across the eastern Pacific and the western U.S. before it eventually forms a "treble clef" like structure over the southwest U.S. (nod to musical friends).

As I mentioned, just about anything slipping through the net of the persistent ridging the past several weeks has experienced this.  Ultimately, the amount of precipitation the mountains gets depends on the gory details.  Troughs have been infrequent, but when the come through, some have been productive, others non-productive.

Which brings us to the Saturday storm.  If you want an optimistic solution, look no farther than the 6Z NAM (Note: I"m writing this at 6 AM, so you are stuck with the early model runs for this post).  It holds the trough together in a way that the central Wasatch get some frontal precipitation tomorrow morning with some additional post-frontal snow showers through tomorrow evening.

Total precipitation through Saturday evening is .32" of water and about 4.5" of snow at Alta Collins.  Not much, but this season, we'd be happy to get it.

The GFS solution is only slightly different, with a less productive frontal period.  Maybe in this case we're looking at 2-4" at Alta.

True depression is provided by the 0Z ECMWF forecast which is the driest of all (not shown) with less than 0.1" of water at Alta.  The total precipitation forecast through 12Z Sunday from the ECMWF shows that the front basically dies before it gets to the central Wasatch and says NO SOUP FOR YOU!

Our downscaled SREF product has most members producing between 0.2" and 0.7" of water at Alta Collins, but this might need to be tempered by the fact that the downscaling is based on average mountain enhancement, and this is largely a frontal event with weak flow, so it may be overdoing the mountain effects.

So much will depend on exactly how and where the taffy stretches.  Keeping expectations low and hoping for the best seems to be the key to happiness this season.  A total for Saturday Morning through Saturday evening of 2-4" represents the most likely scenario for Alta Collins, but I would not be surprised if we got a little less or a little more.  More than 8 inches is quite unlikely.  Ho Hum.