Monday, February 19, 2018

Everybody Gets Some, Plus Tonight's Lake-Effect Intricacies

Finally, a great storm.  All elevations, all locations.  Some cherry picked storm totals so far based on reports to the NWS (time of measurement in parentheses):

Smithfield, Cache Valley (4 PM Sun): 12"
Ogden (5 PM Sun): 2"
West Valley City (6 AM Mon): 5.2"
Salt Lake Airport (5 AM Mon): 3.6"
Cedar Hills, Utah County (5 AM Mon): 9"
Powder Mountain (5 AM Mon): 12"
Brighton Crest (5 AM Mon): 16"
Alta-Collins (3 AM Mon): 14"
Spruces (4 AM Mon): 10"

Through 8 AM, Alta Collins is up to 18", blowing my 7-14" expected through 9 AM out of the water. 

With the KMTX radar up-and-down overnight (and currently down), getting a good handle on the action is more difficult than usual.  One thing that caught my attention is described in the tweet below, issued last night.  From 8-10, Alta-Collins recorded 6" of snow (3"/hr mean rate) and I saw some tweets of impressive accumulations near Alpine as well.  During this period, a very pronounced band extended across northern Utah County and far northwest Wasatch County, and stronger radar reflectivities also lingered over the upper Cottonwoods. 

One option when KMTX is down is to examine data from the Terminal Doppler Weather Radar (TDWR) operated near the east shore of the Great Salt Lake west of Farmington.  The primary purpose of this radar is to detect hazardous weather and wind shear over the airport.  It is blocked significantly by the surrounding topography, and provide little to know information about what is happening over the Wasatch and the broader region.  You can see, however, returns related to snow showers over portions of the Salt Lake Valley. 



Periods of snow look to continue today and tonight in the mountains and also in the valley.  The latest NAM forecast shows us transitioning into deeper northwesterly flow during the day today, with the flow transitioning to westerly overnight.


The situation for tonight is really interesting.  The latest forecasts show a pocket of remarkably cold air moving over northern Utah, with the latest NAM forcasting -22ºC over the Great Salt Lake at 1200 UTC.


In fact, the absolute minimum is an astounding -22.9ºC at 1500 UTC (8 AM).  That is a remarkably low 700-mb temperature.  It would not be a record for February (-25.9ºC is the all-time low), but we don't see too many days around here with temperatures that cold.

The Great Salt Lake is actually not that warm compared to climatology, but still, the average lake-surface temperature is almost 4ºC. 


As a result, the HRRR is fairly excited about a possible lake-effect event tonight.


Our statistical forecast system, based on the 6Z NAM, shows elevated lake-effect probabilities ovrenight tonight, peaking at 90% at 2 AM.   Note how the affected area shifts from the Cottonwoods to the northern Wasatch with the flow shift overnight. 


There are, however, three important issues to keep in mind.  The first is that wind directions beneath these upper-level troughs are tricky to forecast, so one can't count on that forecast precisely. 

The second is that we tend to fixate on lake effect, when we need to keep in mind that we could see post-frontal snow showers generated by other processes.

Finally, the third is the cold.  This is a remarkably cold airmass.  Temperatures at crest level are going to be so cold that instead of favoring dendrites, they will favor higher density crystals.  I've seen situations where this has put a damper on snow amounts in the past.  There aren't a lot of days at Alta that are this cold, but if you look at the snow-to-liquid ratios when the 650-hPa temperature is below -20ºC, there is a tendency for lower values. 


I'm thinking another 3-6" in the upper Cottonwoods through 6 PM this afternoon.  After that, I'm happy to sit in my ivory tower and watch this one play out.  Hopefully the radar will come up. 

Sunday, February 18, 2018

Six Questions to Answer When Forecasting

Exciting weather is happening and on the way, so it seems fitting to organize this post around the six questions to answer when forecasting

1. What has happened?

This "winter" has thus far been like 3.5 consecutive Novembers rather than a typical November, December, January, and February sequence.  The average temperature at the Salt Lake City International Airport for 1 December - 17 February was 37.8ºF.  That's just a shade lower than the November mean of about 40ºF.  As everyone knows, we're well below average for snowfall and snowpack, especially in the lowlands.  The photo below was taken this morning looking up City Creek Canyon and indeed it looks more like a scene you might see after a November snowstorm than one would expect in mid February. 

 
This context is important as I suspect most people are entirely unprepared for what is coming.  It will probably seem like the first storm of the year.

2. Why has it happened?

This is a good question and one that I can't answer satisfactorily.  The easy answer is that the warmth and snow drought reflects persistent high pressure and a storm track that has remained predominantly north of northern Utah.  Why that has been the case remains a subject of debate.

3. What is happening?

Wow, what a windy night.  Strong south winds at all elevations.  In the past six hours (ending at 9:20 AM MST), several sites in the northern Wasatch and Bear River Range have gusted over 70 mph and ridgelines in the central Wasatch have seen gusts as high as 71 mph.  I can find many sites in the mid elevations reporting gusts over 50 mph.  Sherwood Hills (5658 ft) near Sardine Canyon guested to 64 mph.   If there was much powder left over from yesterday's feast, I suspect it's been blown to Jackson Hole.  Peak gust at the Salt Lake Airport so far is 43 mph.

With these strong winds, we are seeing some dust.  Concentrations were especially high in the western Salt Lake Valley, which I suspect is due to emissions from the area west of Utah Lake as we have seen in recent events



4. Why is it happening?

The answer here is an approaching frontal trough that at 1500 UTC (0800 MST) was sagging southward into northern Utah.  This has created a strong pressure gradient to its south, with strong gusts at all elevations. 


5. What will happen?
6. Why will it happen?

Loaded questions!  I'll answer them together as it is easier.  As I write this, the surface front just passed Hill Air Force Base.  The HRRR shows it progressing slowly southward today, with frontal passage in the northern Salt Lake Valley around 2000 UTC (1300 MST).  Thus, expect to see a wind shift early this afternoon, if not sooner in the valley. 

Periods of snow are likely in the northern Wastach today and will develop in the central Wasatch later this afternoon.  It's a bit of an oddball situation as the latest NAM shows the frontal band over far northern Utah through 2100 UTC, but then rather than bringing it through continuously, redevelops it to the south tonight. 



Tomorrow brings the post-frontal crapshoot beneath the upper-level trough where much depends on flow direction, moisture, and instability.  The NAM forecast below isn't too bullish on snow, but the 6 Z GFS is more enthusiastic and keeps us in wrap-around moisture (not shown). 


It's worth a look at the 12-km NAM-derived forecast for Alta.  Measureable precipitation begins around 5 PM and is strongest from about 6 PM to 11 PM with the frontal forcing.  Periods of snow continue through 6 PM tomorrow in the unstable post-frontal period.  Total water equivalent is 0.6" with 10" of snow by 9 AM tomorrow and 12" by 7 PM tomorrow. 


Take a peak also at the temperatures for Mt. Baldy.  Keep in mind these are for 11,000 feet, a bit above the lift-served terrain, but this provides some idea of how cold the airmass will be.  It goes sub zero by noon tomorrow and down to -8ºF by 9 AM Tuesday.  I don't think we've seen air this cold yet this year.

To summarize, this looks to be an all elevation storm and you should be prepared for winter conditions.  We haven't had a stress test like this in some time. Thankfully, it is the President's Day Holiday, which will hopefully help with tomorrow's commute.  For Alta-Collins, I lean toward 7-14" by 9 AM tomorrow.  I have a bit more heartburn for totals after that given the variability I'm seeing in the models.  Some snow is likely, but the range of possibilities is large.  Hug a ski patroller or avalanche forecaster when you see them.  They will have a tough job the next few days. 

Finally, this is a high-impact, rapidly evolving situation.  Keep an eye on official forecasts at http://www.weather.gov/slc/. 

Friday, February 16, 2018

Powder Fever, Real Winter, Olympic Dreams

Much to talk about today.  I'll split this post into two parts, Powder Fever/Real Winter and Olympic Dreams.

Powder Fever/Real Winter


Wow, what a morning.  Yesterday's storm blew away my expectations for both snowfall amount and water equivalent.  The Utah Avalanche Center reports 12-18" of new snow in the Cottonwoods and along the Park City Ridgeline.  This morning is bluebird.  It's hard to imagine a better ski day after all the pain and suffering of the winter to date.  I suspect many dawn patrollers were out this morning and that it will be powder panic in and around the Cottonwoods this morning.   In the backcountry, the buried weak layers still give me heartburn.  Check the avy report and don't let powder fever cloud your decision making.

The models have also been shifting to higher totals for the storm sometime late Sunday to Monday, as evinced by the downscaled NAEFS plume for Alta. 
Totals currently being spit out by the Euro and the NAM are a bit lower.  Let's see how it comes together and keep our fingers crossed. 

One thing is for sure.  It is going to feel like winter early next week.  REAL WINTER.  The current GFS is dropping 700-mb temperatures to -21ºC by 11 AM Tuesday morning.  My 20/20 rule tells me that anything above 20ºC or below -20ºC is exceptionally warm or cold, respectively, for these parts.  You wanted winter.  You're going to get it.

Olympic Dreams

For me, the Winter Olympics have kicked into high gear now that the alpine skiing events are underway.  The compressed schedule might not be what the athletes want, but as far as I'm concerned, it's great for me.  Having the two tech event runs bracketed around a speed event means skiing dominates the broadcast, which makes me happy, although NBC still managed to skip Wendy Holdener's first slalom run yesterday, which was the fastest in the field.  I bet Austrian TV is smart enough not to skip the run by the skier ranked 4th in World Cup slalom points.  The DVR has also proven it's worth as I've been recording cross-country overnight and getting up early to watch before heading into work.

The schedule for tonight and the weekend is enticing.  Super-G 7PM MST tonight, Women's 4x5 km relay 2:30 AM MST Saturday, Men's GS 6:15 PM MST Saturday, and Men's 4x10 km relay 11:15 PM Saturday.  Throw in some freestyle skiing, and there's much to look forward to.  The main weather concern for these events would be wind, which hasn't been as bad in recent days, but still has shown its ugly face at times.  Hopefully all will go off without weather having a significant impact.

A few comments on the cross country.  I had the good fortune of attending both the Men's 4x10 km and Women's 4x5 km  during the Nagano Games in 1998.  The Men's 4x10 km is the Superbowl of Nordic skiing and at the time, the Norwegian and Italian ski teams had an incredible rivalry, that is well documented in Bud Greenspan's excellent Olympic documentaries.  The Italians upset Norway on their home snow during the Lilliehammer 1994 Olympics, with Silvio Fauner nipping Bjorn Daelie, the greatest male cross-country skier of all time, at the line by 0.4 seconds. 

In Nagano, the Norwegians changed strategy, putting Bjorn Daelie in the second or third leg (I forget which).  Below, Daelie is chasing down a competitor from the Italian team.


Instead they put Thomas Alsgaard, a better sprinter, in the anchor leg and he was able to take the win for Norway at the line by 0.2 seconds.  What a race!

An equally exciting race followed in 2002 in Salt Lake, with Alsgaard once again sprinting to victory.

I think the Norwegian team this year is overwhelming, but these team sprints are often closer than expected and always worth watching. 

Which brings us to the women's 4x5 km relay.  In Nagano 20 years ago, the US team finished in 15th place, just ahead of last place Canada.  I attended the race with high mucky mucks from the US Ski Team and Salt Lake Olympic Committee, who talked at length about how to get better.  It was great to be a fly on the wall for that one. 

Fast forward to 2018 and the US women's team is one of the best in the world and Jessie Diggins has been knocking on the door for Olympic medals.   The US women have never had an Olympic cross country medal and the men haven't had one since 1976.  The 4x5 is in play, as well as the team sprint.  Let us hope that this is the year. 

Thursday, February 15, 2018

Ah, the Blessed Steenburgh Effect

Sidelined with a broken bone in my hand this week, you reap the benefits.

Alta Collins has recorded 0.49" of water and 8" of snow through 7 am this morning (the 18" snow interval depth is spurious in the data below, so it's unclear if we may have ticked up or down from the 8" at 6 am).

While not a big storm it's pushing toward my arbitrary "deep powder" threshold of ten inches.  It's also pushing the upper end of predictions.

The radar imagery shows we'll add more to that total, especially in the next hour or two.


Nice to see it snowing in the lowlands as well.  Campus was covered in a thin blanket of white at sunrise this morning.


Make a few turns for me.

Tuesday, February 13, 2018

The Dribs and Drabs Will Continue Until Morale Improves


In times like these, one learns to appreciate the smaller dumps in life.  My usual definition of a deep-powder day is a 24-hour snowfall of at least 10 inches, but we've only had one of those at Alta since December 3rd!

On the other hand, recent dribs and drabs have certainly helped the skiing some, even as we continue to lose ground to climatology for snowfall amount and snowpack water equivalent.  The 6" of quick snow Saturday afternoon and 7.5" yesterday did create some smiles.  Maybe 6" is the new deep powder day.

You'll be hearing some talk of a pattern shift probably in the coming days, and indeed there are some changes afoot.  The GFS forecast valid 5 AM MST next Tuesday, for example, has a trough over the northwest U.S. and a ridge over the east, something we haven't seen a lot of this winter.



Similarly, the ECMWF model has a trough in the west (with some differing details) as do most (but not all) GEFS ensemble members.

Penn State E-wall
However, the overall pattern is one that remains high amplitude.  Note, for example, the strong ridging over the eastern Pacific and the north Atlantic in the GFS forecast above.  Given the characteristics of this flow pattern over the eastern Pacific and western North America, I'm still not enthused about this pattern opening up the spigot from now through the President's weekend.

Instead, dribs and drabs are likely.  As shown in the NAEFS plume below for Alta, the next round of dribs and drabs looks to be late Wednesday through Thursday AM.  After that, there's a break and then a great range in the timing of possible dribs and drabs Saturday night through Monday.  As usual, there's a couple of more excited ensemble members, so my usual line of keep expectations low and hope for the best applies. 

There is one non-scientific reason for you to be optimistic.  I took a surprisingly hard fall skate skiing on Saturday and learned yesterday that I fractured a bone in my hand.  They tell me I can continue to ski with a splint, but this is likely to slow me down a bit more than usual.  Thus, there may be a partial Steenburgh Effect that increases the likelihood of a deep powder day, although perhaps not as much as when I'm out of town.  This effect, if it exists, will only last 6 weeks, so be ready.  

Monday, February 12, 2018

PyeongChang 2018: Cold, Wind, and Lessons for Salt Lake 2026/2030

You can always count on the Winter Olympics to serve up some good weather stories.  So far in PyeongChang, it's cold and wind.

Let's talk about the cold first.  The average temperature in PyeongChang during the Olympic Period is 22.1ºF, making its climate easily the coldest to host the Winter Olympics since Lilliehammer in 1994.  Note that the numbers below for sites other than PyeongChang are from Wikipedia and for the entire month of February.

PyeongChang 2018: -5.5ºC (22.1ºF)
Sochi 2014: 6.0ºC (42.8ºF)
Vancouver 2010: 5.5ºC (41.9ºF)
Turin 2006: 6.4ºC (43.5ºF)
Salt Lake City 2002: 3.6ºC (38.5ºF)
Nagano 1998: 0.0ºC (32.0ºF)
Lilliehammer 1994: -5.3ºC (22.5ºF)

Some of these sites have outdoor venues that are higher (and colder).  However, if you are a reporter returning to the host city each night, you are frequently experiencing a warmer climate.  There's no escape from the cold in PyeongChang.  Even in the Gangneung Coastal Cluster, near sea level, the average temperature is -0.5ºC, colder than all but Lilliehammer.

Thus, for people covering the games, even average temperatures are a big adjustment.  In addition, over the past two weeks, mean temperatures inferred from the maximum and minimum temperatures reported at Daegwallyeong in the Mountain cluster (the site used for the climatological PyeongChang discussed above temperature above) were below average except on Feb 9 and 10.
In addition, it has also been windy, resulting in low wind chills and affecting many of the outdoor competitions.  The Men's Downhill and Ladies Giant Slalom were postponed (I learned during my Olympic service that the word cancelled shall not be used under any circumstances) and watching events the past few days it was clear that wind was playing a role.

None of this is unexpected from a meteorological perspective.  The Korean Peninsula lies along the east coast of Asia, the largest continent.  During winter, dominant high pressure frequently drives cold, flow over the region.  The pre-Olympic Weather Report summarizing the climate of the region, issued in April 2017 by the Pyeongchang Organizing Committee, provides a summary of the main weather patterns that may affect the Games schedule, and explicitly states that:
Cold and dry air flow from the northwest...is the most dominant weather pattern during the Olympic and Paralympic Periods. The Siberian High brings cold and dry weather to the Korean Peninsula. When the air mass games strength, high winds and low windchill temperatures are the most influential factors in particular during the Olympic Period.
Thus, nobody should be surprised by this weather.  Climate is what you expect, and weather is what you get.

Which brings us to Salt Lake City's likely bid for the 2026 or 2030 games.  Should those games be awarded, it would be a mistake to assume either that: (1) the weather in 2026 or 2030 will be similar to that in 2002, or (2) we will have above average temperatures because of global warming.

If you remember back to the 2002 Games, Mother Nature blew out the air pollution just prior to the start of the Olympics and for the most part during the Games, the weather was great.  In fact, Pat Bagley featured it in one of his cartoons.



However, what happens in 2026 or 2030 is going to be very dependent on the whims of the Mother Nature.  February in Salt Lake is a fickle month and a lot can happen that can affect transportation, athlete and spectator safety, logistics, and competitions.  Persistent inversions with fog and air pollution.  Downslope windstorms.  Arctic intrusions with extreme cold.  Heavy snowfall in valleys or mountains.  Strong winds in the valleys or mountains.  Lightning.

Climatologically, Salt Lake has a great climate for the Olympics, but that doesn't mean the weather will necessarily be easy on us if we host the Olympics again.