Thursday, April 30, 2015

Some New Toys

For those of you who like to get your weather geek on, I've begun to populate with some new products, specifically full vertical resolution model soundings from select locations in Utah and a few other sites of interest.  Here's an example from the NAM for 2100 UTC (3 PM MDT) this afternoon.

These soundings are really fantastic and should be helpful for forecasting.  For the NAM, they are available every hour, which will be great for periods of rapid change.  If you want to see a loop of the hourly soundings, click here.

I'm hoping to have these for the GFS as well, although they have lower vertical and temporal resolution and thus won't be as sharp or useful.

The old soundings will be deleted as I need to cut some CPU time from somewhere!

Wednesday, April 29, 2015

Don't Trust the GFS Precipitation in the Central Wasatch

This winter, NCEP upgraded the Global Forecast System (GFS), with improvements in both resolution and physics.  While it is likely improved forecasts of fronts, cyclones, and other major weather systems the precipitation forecasts over the central Wasatch are simply abysmal.

I noticed something was up immediately following the upgrade.  NCEP began to provide direct model output (known as BUFR) for the model grid point closest to Alta.  I began downloading and plotting this data [see (GFS at bottom) and] and noticed immediately that the GFS was generating far too much precipitation.

I then began downloading the native (13-km) resolution GFS grid from NCEP and examining the precipitation, which showed extreme amounts of precipitation over many mountain areas.  Here's an example from 23 January through 22 Feb (courtesy of a colleague).  During this period, the Alta-Collins and Alta-Top Cecret observing sites received a total of about 65 mm of precipitation (snow-water equivalent, grey and black lines)).  The ECMWF forecasts (blue) tracked this pretty well, but the GFS (maroon-I think!) produced nearly twice as much precipitation as was observed.  The NAM-4km (magenta) is even worse, but that's a story for another day.
On the plus side, NCEP smooths the GFS forecast to a 0.25ºx0.25º latitude–longitude grid before distributing it to NWS forecasters.  On this smoothed grid, the forecast looks better and closely tracks the ECMWF, falling just behind observed.
As we move into the spring, we're still seeing massive overprediction.  In the loop below you can see the problem.  Nearly nonstop precipitation over the central Wasatch and a few other mountain areas.  Note how these precipitation  bull's-eyes don't move and, like the Energizer Bunny, just keep going.

If we extract the time series for Alta (lower left), we see that the 84-hour forecast shown above (ending 6 PM Saturday) generates almost 1.5 inches of water equivalent.  If we look at the forecast through the middle of next week, Alta gets a whopping 4 inches of water equivalent.  Booyah!

We're entering into a spring pattern and we may see some rain or snow showers from time to time over the next few days, but barring the formation of an intense thunderstorm right over Alta, we're not going to see anything near 1.5 inches of water equivalent.

The bottom line is that the GFS precipitation forecasts are way overdoing precipitation in the central Wasatch, as well as some other mountain areas.  Let's hope this issue is addressed by NCEP in the future as the GFS would be even more valuable if it provide more reliable precipitation forecasts.

Monday, April 27, 2015

The Alta Low Snowfall Record Falls

My friends at UDOT tell me that Alta-Guard sits at 276.8" of snowfall for Nov–Apr.  The previous low was 314.5" in 1976/77.

With nothing but a flurry or two possible through the end of the month, the record is in the bag.  Congratulations, you survived the lowest snowfall season since WWII, although I still rate 1976/77 as a worse ski season (see Has There Ever Been a Snow Season Like This?).

Of course, the usual caveats apply (see Limitations of Long-Term Snowfall Records).

Sunday, April 26, 2015

Good Late Season Snow – With Dense Fog

There was some good upper-elevation snow to be had today, but visibility was about as nasty as it can get in the Wasatch.  I opted for a tour in the Supreme area and was glad I did as the trees were essential.  Conditions were quite good above about 9500 feet.

I returned to Alta only to discover that I had actually been transported to the Cascades.  That's the Albion and Sunnyside lifts from the Albion Grill.  Visibility perhaps 150 meters.

The high humidity air made for some nice hoar frost on my son's hair.  Not sure if the vapor source was the atmosphere or his perspiration.  He is a teenager after all.

Saturday, April 25, 2015

The Last Dump of the Season

The skiing wasn't half bad this morning as the cream-on-crust skied pretty well despite the fact that it only added up to about 7 inches in the upper elevations and far less in the lower elevations.  It was nice to see white snow again, although the snirt was very quickly starting to appear in tracked areas by noon.

I've lived long enough in Utah to know that we can get some freshies nearly any time of year and I've seen some good dumps in May and even early June.  Even if that happens this year, the snow we get tonight will be, for all intents and purposes, the last dump of the season.  

I suspect that something in the 4-8 inch range is the most likely scenario for accumulations in upper Little Cottonwood through 9 am tomorrow, with a modest chance we'll do better than that.  With an early start, the high altitude (>9500-10000 ft) north aspect ski touring might be decent tomorrow as the snow should be right-side up and falling on what we got last night.  

Friday, April 24, 2015

Dr. Seuss on Western Hydroclimate

Snow remains in the forecast and hopefully we'll have some white stuff this weekend for Alta's closing day #2, to help Snowbird extend their season, and perhaps to give us an extra week or so of white corn in the backcountry.  However, I have something more entertaining to share than a discussion of the weekend forecast.

Jeff Lukas is a research scientist with the Western Water Assessment.  He recently presented a Dr. Seuss inspired summary of recent and future climate and water trends for Colorado that has gone viral amongst my colleagues.  It's worth a 5 minute look and does an excellent job summarizing current knowledge of our changing hydroclimate.

Although emphasizing Colorado and the recently completed report, Climate Change in Colorado, it is quite applicable to Utah.  

Thursday, April 23, 2015

The Winter That Wouldn't Start Now Won't End

I had what I thought was a great last ski day of the year last weekend.  With warm weather on tap for this week and a layer of dusty snow not buried far below the surface, I figured that we'd be skiing nothing but snirt by this weekend.

I'm now thinking that I will be doing some skiing this weekend.  The models are advertising the passage of two troughs, the first late Friday

and the second late Saturday.

These look to be fairly warm systems bringing mountain snow and valley rain through Saturday, with somewhat cooler air moving in for Sunday, although even then current forecasts suggest snow levels at or above bench level.  Given the warmth of the storm, expect heavy, near-Cascadian snow that may become somewhat drier on Sunday.

Nevertheless it's white.  Our downscaled ensemble forecast plumes show the two periods of heavier precipitation with a mean water equivalent after subtracting off the showery stuff today and early tomorrow of about 1.5 inches with a range of about .75 to 2 inches for the bulk of the ensemble members (some go for higher, but that's a lower probability outcome).

We'll have to see how all this comes together, but fresh snow for the weekend looks likely. 

Wednesday, April 22, 2015

How I Told Commuter Services to Stick It

"Good intentions score big, good actions score bigger"
–Mark Fischetti, Scientific American
Yesterday, in honor of Earth Day, I did something I've been dying to do for some time.

I told Commuter Services to stick it!

Actually, that's an over statement.  I called them up and politely cancelled my parking pass.  Although Earth Day provided some motivation, the cancellation was the result of several factors, not all environmental.  Depending on your situation, you might be able to do it too.  Skip to item 4 if you don't want the back story.

I've always been a multimode commuter.  For many years this involved biking in the summers and driving in the winters.  I was never a bus fan primarily because I found it so inconvenient, but circumstances have changed and the bus has become my primary mode of commuting.  Here's why.

1. The development of smart phones and bus-tracking apps.  I no longer waste time at the bus stop.  I get ready to go to work, fire up a bus tracking app (I use UTA tracker), and then do something productive until the bus is a couple blocks away.  During the winter, I don't stand outside freezing.

2. Campus construction.  Have you driven to campus lately?  It's an unmitigated disaster!  New buildings going up everywhere.  Parking lots torn up left and right.  My bus stop is actually a couple of minutes closer to my office than any of the available lots and, if the close ones are full, it's five minutes closer.  The bottom line here is that items 1 and 2 make the bus is almost as fast as driving, although I have a short commute and a fairly reliable bus line.

3. The Northwest Parking Garage.  This is the 350 stall monstrosity that they are constructing on 100 South just west of the Sutton Building.

The U talks a pretty good game about sustainability, but we have a long ways to go until we are committed to a less car-intensive future.  My disgust with this project has made it even easier not to drive to campus.  Further, if I want to park in the garage when it's done, I have to upgrade to a $870 T permit.  Nada.  Not going to happen.  I guess that's one way to financially incentivize the use of mass transit, but it's a pity they couldn't do it without building the garage.

4. The availability of day passes.  Nearly all of us need to drive to campus from time to time.  Encouraging more mixed-mode commuting is one way to reduce the demand for parking and traffic on campus and Commuter Services now has a day parking pass option on its web site!  I don't know when this appeared and it's very hard to find.  In fact, it doesn't appear explicitly under the "Parking on Campus" drop down menu on their main page, on the Parking Permits FAQ page, on the Faculty/Staff Permits Page, or on the Student Permits Page.   Basically, if you don't know about this option, you have no hope of finding it.  Call me paranoid, but one has to wonder if they really want to promote it.

In any event, here's what you need to do.  Go to their online parking portal and click on "purchase permits."  Login and keep boring into the pages.  Eventually you'll get to a page entitled "select permit and permit agreement" and lo-and-behold, there they are.  Half and full day rates.  Sweet!

I haven't actually tried to day park in this way yet, so I don't know how well it will work in practice, but financially this is a great option for a mixed-mode commuter like me.  An A-Pass for a faculty member costs $414 a year.  At the $10/day A-lot rate, that's 41 days of parking a year.  I don't come anywhere near that right now.  Alternatively, the daily U-lot rate is only $5 and that's probably what I'm going to do while I experiment with this daily parking approach.  Of course, an annual U-Pass is only $115 (23 days of parking) and maybe that would be easier for convenience, although available U spaces are usually far from my office and the days I drive I often need to get on and off campus quickly.

Bottom Line
If you are or are considering mixed mode commuting with only infrequent parking on campus, temporary full and half day passes are available on the commuter services web site.  You may be able to axe your parking pass.

Unfortunately, it's not immediately obvious how to do this on the Commuter Services web site and the need to login to a web page and buy a permit each time seems overly cumbersome.  What is ultimately needed is either a smart-phone app that one can just tap once or twice to buy the permit upon arriving on campus OR a system in which you sign up and their automated license plate scanning system simply dings you everytime it scans your plate.  

For students, I'm not sure what's available, but think you are constrained to the $115 U permit, so your savings going this route might be minimal unless you drive very infrequently.

Tuesday, April 21, 2015

Smoke from Russian Fires

A somewhat smoky/dusty sunrise of the Salt Lake Valley this morning
Several days ago, wildfires set by Russian farmers in preparation of spring planting raged out of control, killing 23 and leaving more than 5,000 homeless.  It was a terrible catastrophe.

Smoke from these fires, along with perhaps some Asian dust, has since spread across the Pacific.

Source: NASA
And is currently draped across portions of the western U.S.  Below is an image from MODIS from yesterday.  The smoke is easiest to see against the dark background of Pacific Northwest fires west of the Cascade Crest.  

Here's an image showing the estimated aerosol optical depth.   Warmer colors indicate higher aerosol concentrations.  

Evidence of the smoke can also be seen in air quality observations, which show a gradual upward trend in PM2.5 over the past two days.

Sunday, April 19, 2015

Closing Day (#1)

It was the first closing day of the season at Alta today.  Although the freshies are gone, the turns were quite good and you could carve a hell of a trench for much of the afternoon.

As usual, costumes and revelry prevailed for the day.

Special thanks to this dude for throwing in some old-school air and for keeping his clothes on.

Snirty snow is already making an appearance in wind scoured and sun exposed areas.  A brown spring is in our future.

There was something in the air throughout the day obscuring the views.  I was wondering if it was smoke from the Siberian fires.  Perhaps one of you out there can take a look and comment.

Now is a good time to say thanks to all the avy pros, patrollers, snowmakers, groomers, and lifties for their exceptional efforts this difficult snow year.

Friday, April 17, 2015

Dusty Snow: The Monster in the Basement

"A very strong cold front arrived on April 15th with near hurricane velocity winds.  The storm knocked down trees, signs and power poles all over Utah and filled the air with choking dust.  In the next week, 42 inches of snow fell on top of the chocolate brown layer of dust.  Backcountry recreationists had their last fling with some of the best powder of the season on mostly stable snow.  Then, finally, spring arrived."
Sound familiar?  That quote is from the Utah Avalanche Center 2001-2002 annual report.  The very strong cold front on April 15th was produced by the 2002 Tax Day Storm, which like the storm we experienced earlier this week, was a huge dust producer.

AVHRR satellite imagery at 2046 UTC 15 April 2002.  Dusty areas not obscured by clouds identified with warm colors in inset at upper left.  Source: West and Steenburgh (2010).
The snow that year was remarkably brown once we were well into the melt season.  We are guaranteed a repeat of that this year, at least in those areas where a snowpack remained prior to this week's storm.

One of our readers, Scot Chipman, sent me the photo below of the snowpack at Snowbird (Volkl thanks Scot for his careful product placement!).  26 inches of beautiful white snow on top of a layer of dust deposited on the snowpack by wind and falling snow during Tuesday afternoon and evening.

As that white snow melts, it will percolate through the snowpack and eventually the dusty layer (a.k.a. snirt - part snow part dirt) will be sitting on top.  Here's how things looked in May last season.

Yes, that was snirty, but this year will be far worse.  I wonder how tolerable late-season tours are going to be once the snirty monster in the basement is sitting on top.

A few people have asked me if all this dust is due to the drought.  Drought may be an exacerbating factor, but the primary factor in dust storms in Utah is strong flow over a disturbed land surface.  Cryptobiotic crusts (pictured below), desert pavement, and other natural surfaces in the desert west are very resistant to erosion.  However, as can be seen by the trails below, they are also easily disturbed.

Source: Secrets of the Greatest Snow on Earth
Thus, we have had significant dust deposition even in relatively snowy/wet seasons.  The photo below was taken at Snowbird in June 2011, at the end of a remarkably snowy/wet season.  Plenty of dust.

Further, the processes influencing dust emissions from playas (i.e., dry lake beds) are very complex and not as straightforward as the surface being wet or dry (see Reynolds et al. 2007 for an example).

The good news is that the snirt is temporarily buried under the new snow in most areas.  Instead of ski it if it's white, try ski it while it's white.

Thursday, April 16, 2015

Storm Aftermath

As Mr. Rodgers liked to say, it's a beautiful day in the neighborhood!  It feels so good to have a taste of winter, even if it is in April.  Snow totals in the mountains are crazy big in Little Cottonwood. Snowbird has a 45 inch storm total, Alta-Collins 32 inches based on the automated sensor.  As discussed in the previous post, Snowbird Dumpage, the biggest snow totals appear to be in mid Little Cottonwood and  drop off as one moves through Twin Lakes Pass into upper Big Cottonwood and then even more dramatically as one moves further east to Park City.  As meteorologist and backcountry skier S. D. Green said in 1935, "Skiers will eventually find that the…heads of the [Cottonwood] canyons…offer the best skiing to be found in the Wasatch Mountains."  That dude knew his weather.

On Monday, a friend in the avalanche business told me that he was hoping we would obliterate the all-time low snow season at Alta-Guard.  We have suffered mightily, and it would be a shame if the statistics didn't bear that out when people look back a couple of decades from now.

If my records are right, it appears that Alta-Guard has reported 27 inches in this storm, bringing their snowfall since November 1 to 258 inches.  That's still 56.5 inches behind the all-time Nov–Apr low of 314.5 inches, set in 1976-77.  We still have a couple of weeks to go and as we learned the past two days, snow can pile up fast.  Nevertheless, the record is probably safe, although I'm not sure it will be the obliteration my friend was hoping for.  I'm guessing he's more than happy to enjoy a couple of powder days instead.

With 700-mb temperatures rising to about -5C, low clouds around, and an April sun, this powder is not going to survive for very long.  Get on it, but be careful.

Wednesday, April 15, 2015

Snowbird Dumpage

This afternoon, some great micrometeorological magic has been blessing Snowbird, which is now reporting 36" of new snow.  Snowfall amounts appear to decrease as one moves further east.  Automated sensors at Alta-Collins show 24" and near the base of Supreme 17".  I've also heard reports of lesser amounts from ski tourers at Solitude, and of lesser amounts at the top of the Tram compared to lower on the mountain at Snowbird.  

I'm always hesitant to jump to conclusions based on automated sensors, so this discussion should be read with caution.  Nevertheless, the radar seems to support the view of a decline in snowfall from Little Cottonwood to Big Cottonwood to Park City.  Although this often occurs in NW flow, it's appears to be especially pronounced this afternoon.  To show this, I'll show a radar loop in which the high terrain around Little Cottonwood and the Park City ridge line are identifed with red lines.  

In the loop below, which begins at noon, note how the intensity and frequency of radar echoes is greatest around and northwest of Little Cottonwood Canyon.  Both the intensity and frequency decline as one moves northeastward into upper Big Cottonwood, and then even further as one goes across the Park City ridgeline and finally to Park City and the Snyderville Basin.  

One caveat to this analysis is that the radar is partially blocked along the radial that covers Big Cottonwood, so there is some tendency for less intense and frequent echoes there due solely to that reason.  Nevertheless, there does seem to be some signal here.  

With regards to the decline in snowfall near the top of the Tram, if legit, I suspect it reflects the shallow nature of the storm and considerable snow growth at mid elevations.  

Those of you skiing today should add some comments on what you observed.  

Black Tuesday Becomes White Wednesday

Tuesday Afternoon to Wednesday Morning: Change You Can Believe In!
What a storm!  Massive temperature fall, high winds, blowing dust, and now heavy snow.  It's a meteorological dream!  What do we call it?  The Black Tuesday/White Wednesday Storm?  The Tax Day II Storm?  Make suggestions.  Naming rights will be auctioned off later today.

Let's look at this storm piece by piece.


We'll concentrate here on the University of Utah because we have a long record of 5-minute temperatures allowing for comparison with earlier storms.  At the University of Utah, yesterday's maximum temperature was 71ºF, achieved just before frontal passage, which occurred between 2:45 and 2:50 PM MDT. The temperature changes accompanying the frontal passage include 12.4F in 5 min, 17.8F in 10 min, 21.1F in 1 h, and 29F in 2 h.  The total temperature change from 71F at 2:30 to 32F at 8:05 was 39F.

The mother of all Salt Lake City frontal passages occurred during the 2002 Tax Day Storm.  This was a beast of an event and perhaps the closest analog to yesterday in terms of the strength of the front.  During the Tax Day Storm, the maximum temperature was 73ºF, also achieved just before frontal passage, which occurred between 3:20 and 3:25 PM MDT.  The temperature changes accompanying the frontal passage include 6.1F in 5 min, 11.2F in 10 min, 28.0F in 1 h, and 31.4F in 2h.  The total temperature change from 71.5F at 3:20 to 32F at 11:30 was 39.5 F.   

By and large, these two events are very similar in terms of the temperature change.  Yesterday's temperature fall appears more rapid than that in the Tax Day Storm, but that might simply be an artifact of the 5-minute sampling and sensor lag.  The front could pass at any point in that 5-minute period and it is possible to miss the rapid nature of the temperature fall.  During the Tax Day Storm, for example, one of our students measured a 12.6F temperature fall in 10 seconds with a hand-held weather station.  We conclude that both events featured a remarkable temperature drop.  However, the total fall during yesterday's storm occurred in a shorter period, enabling a more dramatic transition from extreme dust to snowfall in just 3.5 hours.


The winds during yesterday's storm were impressive in both strength and coverage.  Going into the storm, I suspected we would see some valley gusts to around 60, but the event was even stronger with widespread gusts > 60 mph and some > 80 mph.  Peak gusts at valley and canyon locations include:

Parleys Canyon (5126 ft): 92 (during fropa)
Eureka (6584 ft): 87 (prefrontal)
Saratoga Springs: 87 (postfrontal)
English Village (Dugway): 82 (during fropa)
I-80 MP1: 81 (post frontal)
GSL Marina: 81 (likely during fropa, but no temp obs and wind direction squirrelly)
Lehi: 81 (postfrontal)
Baccus: 75 (postfrontal)
BCC S-Curve: 73 (prefrontal)
Wendover Port: 72 (postfrontal)
Highland: 71 (postfrontal)
Spaghetti Bowl: 66 (postfrontal)
Salt Lake International Airport: 64 (postfrontal)

Although the southerly pre-frontal flow was quite strong and produced an impressive dust storm, the strongest winds at most locations occurred shortly after the frontal passage.  A notable exception is the S-Curve site in Big Cottonwood.  That's deep within the lower canyon and it is quite remarkable that it got a gust to 73.  


Modis imagery from yesterday shows significant dust being transported to the Wasatch Front from several sites in southwestern Utah.  For the Salt Lake Valley, key sources include the Sevier Lake Bed, Sevier Desert to the east, and the Escalante Desert west of Cedar City.  Further to the west, one can see dust behind the cold front over southern Nevada.  

Source: NASA
PM2.5 levels reached hazardous levels during and around the time of frontal passage in the afternoon.  At Hawthorne Elementary, the hourly average PM2.5 reached 279.8 ug/m3.  I have never seen anything that high in the Salt Lake Valley during our dreaded inversion events.  

Dust events that reduce visibility at the Salt Lake Airport to 10 km or less occur, on average, 4.3 times per year, although there are large variations from year to year (Steenburgh et al. 2012).  From 1930-2010, 2.6% of these events featured visibilities of 1/2-1 km and 2.0% feature visibilities < 1/2 km.  Yesterday we were in the former category at the airport.  I don't have access to a long record of hourly PM2.5, but Hahnenberger and Nicoll (2012) note that during the dust event of 19 April 2008, PM2.5 levels at Hawthorne Elementary reached 191 ug/m3.  For hourly PM2.5, yesterday was probably amongst the nastiest events.  For daily PM2.5, Hahnenberger and Nicoll (2012) report a maximum of 49.9 at Hawthorne (and 55.7 at Lindon) on March 30, 2010.  We didn't reach those yesterday due to the short-lived nature of the event.  


Snowfall yesterday evening was associated with the frontal passage.  Overnight, snowfall was generated by several mechanisms, with some contributions from lake-effect processes.  Our Ute Weather Center reports that we are closing in on 7 inches on campus.  The Alta Collins stake hit 18 inches as of 8 am.  Not much to say here except HOORAY!  For those of you up there, get some faceshots for me.  Ski touring?  Keep in mind that the Utah Avalanche Center has issued an avalanche warning.  Keep powder fever in check.  

Tuesday, April 14, 2015

The Black Storm of Utah

A remarkable dust storm has been raging over the Salt Lake Valley now for a few hours.  As is often the case, the dust became very concentrated along the cold front, resulting in what I'll call Black Storm conditions.  The Black Storm is a name commonly used for intense dust storms over China.

Evidence for the extreme nature of the dust is provided by visibilities at the Salt Lake City International Airport, which dropped to a half mile immediately following frontal passage.  In addition, observations at the University of Utah show a nearly instantaneous temperature fall from 70ºF to 52ºF with a concurrent PM2.5 spike to over 210 ug/m3.  For reference, the clean-air act standard is 35 ug/m3 (24-hour average).

I've been sheltering inside, but I'm sure the frontal passage was positively miserable to experience outside.

Storm Coming Together Nicely

I love days like this.  A quick update.

As anticipated, the new front has formed over eastern Nevada and Northwest Utah and is now draped across the Salt Flats and northern Great Salt Lake.

This is clearly a case of discrete propagation.  Despite the NW wind, it is currently 63ºF at Wendover.  That's much warmer than what was immediately behind the front 24 hours ago.

Remarkable dust plumes are being driven by the strong southerlies ahead of the front.  These plumes can be clearly seen in visible satellite imagery (not to mention by looking out the window). Major dust sources below are outlined in red boxes and include the West Desert in and around Dugway Proving Ground, the Tule Valley (I think there's a small dry lake bed there), and the Sevier Lake Bed.   There are probably others.

Visibility in the Salt Lake Valley is dropping like a rock.  We are seeing PM2.5 concentrations on our Trax mounted sensor in excess of 20 ug/m3.  Air quality is clearly deteriorating.  I wonder if they will need to close US-6 north of the Sevier Lake Bed where the plume is highly concentrated.

Corrections/Addendums @ 12:55 PM

Peak PM2.5 measured by the Trax-mounted sensor was actually 41 ug/m3.  Ick!  Careful look at Satellite imagery shows dust also being emitted from Sevier Desert east of the lake bed.

Typically the dust becomes highly concentrated at the cold front.  It will be haboob-like when it comes through.  You won't want to be outside during the frontal passage.

The Bottom Falls Out This Afternoon!

I hope you brought a jacket to work or school today as you are going to need it later this afternoon.

Yesterday we discussed the anticipated discrete propagation of a cold front across the Intermountain West (see Excitement At Last!).  Everything is coming together this morning for this to occur and for a strong cold front to move through the Salt Lake Valley later today.

We begin with a trick question.  Where is the front in the image below?

That band of precipitation in central Nevada really gets my attention, as does the shift in the wind from southerly at Elko to Northwesterly at Winnemucca.  Compared to balmy Salt Lake City, it's another world right now in Winnemucca (when isn't it — ha ha) with a temperature of 36ºF and a NW wind of 20 knots.

However, let me add the corresponding short-range HRRR forecast of 700-mb temperature and wind to that plot.  When I do this, you can see that indeed there is a trough associated with that band of precipitation over central Nevada, but also that cold air aloft is already advecting into Northwest Utah.

When it comes to cold fronts over the Intermountain West, you can't think of them as features that simply move continuously across the landscape.  Often times, a new front forms ahead of the approaching Pacific cold front.  This process is happening this morning.  The Pacific cold front is still over central Nevada, but a new front is beginning to form over Northwest Utah and eastern Nevada near the leading edge of the cold air at 700-mb and along a surface trough that extends northeastward from the Sierra Nevada across the Great Salt Lake Basin.  Below are the HRRR forecasts for 1500 and 1800 UTC showing this process, the latter with the observed satellite imagery.

Based on observations, I think the HRRR is developing the trough a bit faster than observed, but I expect to see a healthy trough and new front emerging over northwest Utah and eastern Nevada over the next couple of hours.

This new front will likely remain to the north and west of the Salt Lake Valley through about 2 PM when it will push into the Salt Lake Valley.  Expect strong south winds ahead of it and northwest winds behind it, with some dust.  Although there's a possibility of showers and thunderstorms popping up along it, the bulk of the precipitation will come in a couple of hours later.

That's when things get even more interesting.  The NAM is advertising an extended period of precipitation through about 0300 UTC (9 PM MDT) along the Wasatch Front with temperatures and snow levels falling.  The entire event will be snow in the mountains and start as rain in the valleys, but snow levels will likely be down to the benches by 6 PM or so.

So, we're likely going from a high temperature in the upper 60s today at about noon to snow on the benches by 6 PM.

Snow showers are possible at all elevations overnight and early tomorrow as a very cold airmass moves over us and we get a secondary trough passage from the north.  The lake temperature right now is about 11.7ºC and there's a pretty good chance that we will see some lake effect overnight.  Where, when, and how much is a bit of a crap shoot.

For the Cottonwoods, I think something in the 4-8 inch range is probably reasonable for through about 9 PM tonight.  After that, much will depend on the intensity, frequency, and location of the post-frontal snow showers.  By and large, I think the odds of a "deep powder day" (10 inches or more) by tomorrow morning are better than 50/50 and high enough that I'd advocate for getting up and checking the snow report in the morning.  It is mid April after all and we've had few powder days this year.  For a storm total, the NWS forecast of 8-16 inches seems pretty reasonable as the most likely range of possibilities.  I'd be disappointed if the storm total was less than 8 inches.  I wouldn't be shocked if we did better than 16, although I don't think this is a high probability outcome.

Addendum @ 10:45 AM

It just hit 71ºF at the airport!  So much for my high 60s.  Gonna be an even bigger drop to tomorrow.

Monday, April 13, 2015

Excitement At Last!

Tuesday and Wednesday look to be the most meteorologically exciting period we've had around here in some time as a strong cold front develops over the Intermountain West.  There's something for everyone including strong winds, blowing dust, mountain snow, and then, for Tuesday night and Wednesday morning, the chance of mountain and valley snow depending on how everything comes together. 

The forecast for 1800 UTC (1200 MDT) today shows a Pacific cold front making landfall along the Pacific Northwest coast.  Ahead of the landfalling cold front, however, temperatures increase fairly dramatically from the Pacific coast to the Intermountain West (circled).

As the Pacific cold front pushes into the interior, a new cold front forms ahead of it from that temperature gradient that currently exists from the Pacific coast to the Intermountain West.  By the time we get to 2100 UTC (1500 MDT) tomorrow, the front pushing through Utah is essentially a new front, with the Pacific airmass well behind it over Nevada and central and western Idaho.  

Meteorologists refer to this as discrete propagation and it is a common occurrence over the Intermountain west, especially in the spring.  See if you can pick up on it in the loop below.

As a result, tomorrow will likely bring a two part system.  We will have a cold-frontal passage in the early afternoon.  This frontal passage could bring some showers, but will otherwise probably be dry with the possibility of blowing dust (both ahead of and behind the front depending on how things come together.  The bulk of the precipitation will lag the surface front and come in two or three hours later.  

Precipitation forecasts for this system have been all over the place (see There's Always a Storm in the Extended) and they continue to give me heartburn.  The latest NAM is calling for 0.8" of water and 16 inches of snow for a storm total at Alta through Wednesday evening.

A few inches in the mountains late tomorrow afternoon and evening looks pretty likely, but Tuesday night and early Wednesday are the really hard part of this forecast.  The latest runs call for moist, northwesterly flow and very cold air for mid April, with the potential for the lake to get going as well.  Right now things look good for mountain snow, with the potential of some action in the valley too, but we are right on the edge of the action and a slight shift could change that.  We'll take a closer look tomorrow.