Category Archives: Fronts

A Day and Night of Great Utah Storm Chasing

Finally, a decent storm!  Our storm chasing activities this month were somewhat curtailed by Mother Nature.  Storms were limited, but we made the most of the three major opportunities we had, including our efforts yesterday and today.

Yesterday, we set up camp in the South Jordan Train Station parking lot for the frontal passage and post-frontal precipitation.


We operated here in 2011 when it was in the boondocks.  Although development is spreading westward (note the building behind the DOW), we were able to use it effectively yesterday, although we may need to find an alternative for future deployments.

There was a great deal going on and I suspect we have a great dataset for both an MS thesis and at least one paper.  Here are a few snippets.

The photo below from U undergraduate Spencer Fielding is taken facing ESE shows the situation shortly after the surface cold front has passed and was located just south of Point of the Mountain.  A pronounced cloud rope was evident above the frontal interface with the cloud structure suggesting ascent in the strong southwesterly flow over and downstream of the front.  It was black as coal over the Cottonwoods, but we saw very little on radar during this period.  Instead, the precipitation was falling further downstream (relative to the flow aloft) in the area east of Mill Creek Canyon.  This appeared to be a classic situation of precipitation growth, transport, and fallout with the growth happening in the ascent region, but the particles needing sufficient time to grow big enough to fall out, which happened further downstream.  It was unclear if the mountains really mattered at all in this period.  Making all of this easy to see was the fact we seemed to be in a rain shadow east of the Oquirrh Mountains.


That Oquirrh rainshadow was a prominent feature for a couple of hours after frontal passage.  The image below is a vertical slice taken facing northwest toward the Oquirrh Mountains (Grey region marked with an "OM."  The color fill is Doppler velocity, which measures the speed of the flow toward (cool colors) or away (warm colors) from the radar.  The northwesterly flow is clearly evident.  Note how it descends into the Salt Lake Valley southeast of the Oquirrh Mountains.


That descending flow is quite consistent with surface observations at the time which showed WNW flow at most sites along and east of the Mountain View Corridor (highway 85) in the western Valley.  This was about the time of the frontal passage at our location.


An hour later, the front had pushed through Point of the Mountain.  There was a clear boundary over the southwest Salt Lake Valley between the downslope westerlies and the along-valley northerlies.  Curiously, the downslope flow was colder than the northerly flow to the east.  Downsloping air warms compressionally, so for that to occur, precipitation must have cooled the airmass.


The photo below was taken shortly after the MesoWest analysis above and was taken facing NNW.  Here you can see quite well the dark, low-level clouds that accompanied the front into the Salt Lake Valley.  To the left, however, one can see snow (fibrous clouds) that is spilling over into the lee of the Oquirrhs.  The sublimation of that snow presumably contributed to the cooling of the airmass.  Note also how the subsidence has eroded the low-level cloud away on the west side of the photo.


Looking west, you can see this spillover really well in the radar reflectivity.  Note in particluar the shallowing of the echoes toward the radar due to the downslope flow.  One might think of this as a "snow foot."



Eventually, the cold air deepened and the flow aloft veered (turned clockwise) to westerly and eventually westnorthwesterly and we got quite a treat as the area around Little Cottonwood Canyon finally lit up.  One example is below, which is a vertical slice taken facing east directly up Little Cottonwood Canyon.  The bright yellow/orange colors are ground clutter produced by the sloping canyon floor.  Above that clutter, there's not much happening in the lower canyon, but from mid canyon up, there are strong returns, indicating heavy snowfall.


The photo below was taken about 2 hours earlier when it was still light, but shows perhaps what was happening with little or no precipitation falling at the base of the mountains and in the lower canyon (v-shaped notch in background to left of radar dish), with heavy snow in the middle and upper canyon.  


At times, away from the mountains, we also saw some beautiful fall streaks.


Google it and look at the photos and you'll know what I mean.

We eventually deployed to a site we could scan further north for the orographic and lake-effect precipitation that fell overnight.  I went home and tried to sleep, but it was largely a night spent looking at the radar and hoping we were getting great data, which we were.  You'll probably hear a lot of talk about the snow being lake effect, but in the mountains, most of it wasn't.  It was just good old mountain lifting doing the job until early this morning when we got a bit of lake effect.

We surveyed the lake-effect showers for most of the morning.  At one point, we decided to forget about doing anything except two rapid fire vertical slices taken through the lake-effect convection as it moved inland.  A still is below, but these slices were taken every 16 seconds.  I can't wait to process the data and see a video.


All of this brings to an end the OREO field phase.  Although storms were limited, we actually have some great data from three major storm cycles, and the students enjoyed a smorgasbord of precipitation and wind phenomenon.  

Special thanks to the National Science Foundation for sponsoring this visit and the Center for Severe Weather Research (CSWR) for making it happen.  The CSWR deserves extra special thanks for letting us keep the DOW a few extra days to catch this latest storm.

Yesterday’s Storm Chasing Bounty

Yesterday's frontal passage was a bummer for skiers, providing little in the way of the white stuff in the Cottonwood Canyons, but provided us with plenty of excitement thanks to the Doppler on Wheels.

We deployed that morning to a rattlesnake speedway in the Utah desert where a dark cloud rose from the desert floor and we headed straight into the storm.  If you have no idea what that means, watch the video below. 


More accurately, we set up along the side of the causeway to Stansbury Island, just north of the Tooele Valley.  When I drove out to meet the team, the surface front had already pushed into the northern Tooele Valley, with low level "fractus clouds" seen in the photo below at levels just abouve the ground, near its leading nose.  At this time, the front was quite shallow.  


The rattlesnake speedway in the Utah desert was actually the Stansbury Island Causeway, ideal for surveying the frontal structure over the Tooele Valley and precipitation processes over the Oquirrh Mountains.  We could also can over the Stansbury Mountains (background below). 


The shallow nature of the front was very apparent in the radar data we collected.  A Doppler radar is capable of measuring how fast scatterers in the atmosphere, in this case snow and rain, are moving toward or away from the radar.  This allows us to use radar scans, known as PPIs, which are oriented at a slight angle to the horizon, to infer changes in the wind across the area and in the vertical.  In the plot below, cool colors represent flow toward the radar, warm away, with the radar in the middle of the image.  There is a clear indication of flow from the northwest near the radar and south-southwest at ranges more removed from the radar site.  Since the radar scan is tilted at a slight angle to the horizon, this is an indication of strong vertical wind shear in the frontal zone not far above the Earth's surface.  


We can also configure the DOW to scan in vertical slices, known as RHIs.  The RHI below is oriented to the east and scans over the southern Great Salt Lake, eventually hitting the lower slope of the Oquirrh Mountains near Point of the Mountain where they rise above the south lake shore.  Doppler velocities in this image are primarily away from the radar, consistent with northwesterly flow, except near the ground just to the west of Point of the Mountain.  This reflects the splitting of flow around the north end of the Oquirrh Mountains, with the flow there having perhaps a slight NNE component, which results in a weak flow component toward the radar (green).  


The DOW is also a polarimetric radar, which means it sends out and receives radar energy in two planes, one horizontal and one vertical.  The shape of the raindrops or snowflakes can be inferred using this information.  One product we use to do this is known as "differential reflectivity," with reflectivity the amount of radar energy is scattered by back to the radar.  If the horizontal and vertical radar energy is similar, the differential reflectivity is zero, and the precipitation is likely circular.  If on the other hand, the horizontal radar energy is larger, then the precipitation is wider than it is high, and the differential reflectivity is positive.  Dendritic snow, those wonderful flakes with six arms that produce blower powder, often produces high differential reflectivity, because the flakes tend to fall "flat." 

The RHI below shows two layers of high differential reflectivity (indicated by yellows).  One is near the ground and reflects the melting layer in which snowflakes are sticking together and falling relatively flat.  The other is farther aloft and likely reflects a layer in which dendrites are growing and falling.  The temperatures in this layer were likely between -12ºC and -18ºC, which favors the formation of dendrites.  


An interesting aspect of the storm was a near-complete lack of any enhancement of precipitation over the mountains.  It was a frontally forced event, rather than a mountain forced event.  In fact, it clearly precipitated more over the Tooele Valley than over the Oquirrh Mountains.  Only in the late stages of the event did the front finally decide to move over the Oquirrhs.  An example is the RHI of horizontal radar reflectivity below, taken looking east-south-east across the northern Oquirrhs.  The yellows are ground clutter from the radar energy bouncing off the Earth's surface, with the sloping area representing the western slope of the Oquirrh Mountains.  Above the ground clutter, the transition from light to dark blue reflects increasing precipitation toward the mountains, but this doesn't reflect an orographic effect, but is the frontal band as it moved into the Oquirrh Mountains.  


It's such a pity that the storm didn't produce much in the Wasatch.  However, thanks to the mobile capabilities of the DOW, we were able to go to the storm and get a wonderful dataset.  

Postfrontal Dustpocalypse!

A strong cold front raced across northwest Utah this morning, reaching Salt Lake City around noon bringing a blast of moderately strong pre- and post-frontal winds, the latter accompanied by blowing dust.

Dustpocalypse Now!
Observations collected every minute from the William Browning Building (WBB) on the University of Utah campus show a wind shift from SW to WNW from 1153 to 1155 MDT.  Winds continue to turn through NW at 1200 MDT.  From 1153 to 1200 MDT, temperatures fell 10.3ºF.  Pre-frontal wind gusts reached as high as 42 mph a couple hours ahead of the front and peaked at 49 mph at 1209 MDT, just behind the front.



Adding to the story was the post-frontal blowing dust.  At Wendover in far western Utah, the post-frontal visibility dropped to as low as 4 miles, likely due to blowing dust.  However, at the Salt Lake City International Airport, minimum visibilities reached 1 mile, suggesting that dust emissions from the area surrounding the Great Salt Lake and the west desert contributed.

The dust made the cold front very apparent as it entered the Salt Lake Valley (h/t to @UteWeather for tweeting the image below, taken facing from the U toward downtown Salt Lake City).  One can see the classic frontal "nose" to the left of the photo, with friction resulting in a slight forward tilt of the front with height in the lowest one or two hundred meters, above which the front slopes back over the cold air.


The post-frontal air was nasty.  PM2.5 concentrations spiked to 120 ug/m3 on campus immediately following frontal passage.


I guess if you're not going to have much snow, weather excitement like this is better than nothing.

Addendum @1235 MDT:

Shortly after writing this post, the PM2.5 at our mountain met lab topped out over 200 ug/m3 (note scale change from graph above). 


There's some uncertainty in these measurements, so perhaps we should be cautious about the absolute values.  That being said, the air was pretty nasty out there and remains so as I write this at 1235 MDT.

About Last Night

April showers bring May flowers.  The combination of abundant moisture and good large-scale forcing yielded a solid frontal precipitation band that swept through northern Utah last night.  Below is the KMTX radar image from about 0100 UTC (1900 MDT) yesterday evening.  Something for everyone.
Source: NCAR/RAL
About all we missed out on was severe thunderstorms.  There were some lightning strikes in the area, as indicated below, but I didn't see any strong wind or hail reports on the SPC web site this morning.  That's probably for the best.  It is only in the warped mind of a meteorologist that one is disappointed when severe weather doesn't materialize.

Source: lightningmaps.org
Rainfall reports reported to the National Weather Service show accumulations over .9 inches at several sites along the east bench.  The airport came in with 0.65 inches.  Those are good totals for a relatively brief storm.  

As of 7 am, Alta-Collins has observed precisely 1.00" of water and 7 inches of snow.  I suspect that the first tenth of an inch or so of water fell as rain as temperatures at that location (9662 ft) were in the 40s until 6 PM.  After that, cream on crust.  The snow depth is back up to the 125" US-unit psyche point.  Nice, but for those of you attending the March for Science this weekend, that's 317.5 cm.

Strong Fronts: Timing is Everything

Spring is the time of strong cold-frontal passages, as defined based on large, rapid temperature changes, especially in the Intermountain West.  This is quite clear if we look at the frequency of strong cold-frontal passages across the western United States by month, which shows a peak in the late spring (May).  There is a similar peak in the Intermountain West, but it is more pronounced and reaches a maximum in June.
Source: Shafer and Steenburgh (2008)
There are two major reasons for the spring peak.  One is that it is still a synoptically active period, with frequent trough passages.  The second is that surface heating is also quite strong, which leads to daytime frontal intensification.  As a result, the frequency of strong frontal passages is highest in the late afternoon (~1800 local standard time) and shifts to later in the day as one moves from winter to summer.

Source: Shafer and Steenburgh (2008)
Intermountain fronts strengthen during the day because in our part of the world the pre-frontal environment to be cloud free or feature thin, high clouds, whereas the post-frontal environment typically features deeper, often precipitating clouds.  As a result, during the day, there is a contrast in surface heating across the front, with the pre-frontal environment heating faster than the post-frontal environment.  The direct effect of this heating contrast is to increase the cross-front temperature difference.  An indirect effect is that it produces a thermally driven from from colder to warmer air, which helps sharpen the temperature contrast.  In many events, post-frontal cooling from precipitation further augments these effects.

An example of this daytime frontal sharpening is provided by forecasts of today's frontal passage.  The GFS 700-mb (10,000 ft) temperature forecast for 1200 UTC (0600 MDT) this morning shows a frontal zone over Nevada with temperatures on the warm edge of the frontal zone around 4ºC.


By this 0000 UTC (1800 MDT) this afternoon, however, prefrontal temperatures have warmed to 6ºC and the front entering northern Utah has sharpened significant, with the isotherms (lines of constant temperature) packed much closer together.  This is an example of the frontal sharpening process.


Precipitation behind the front will likely play an important role in the frontal sharpening of this event.  Shifting to the high-resolution rapid refresh (HRRR) forecast for 2100 UTC (1500 MDT) this afternoon shows the expected precip just behind the well-defined frontal shift that is entering northwest Utah at this time.  Lovers of wind and dust will be pleased to see strong southwesterly flow ahead of the front.  Yup, another dust layer is likely for the mountain snowpack.


The HRRR has the front entering the northern Salt Lake Valley at 0200 UTC (2000 MDT) this evening, perhaps a little late for a truly colossal temperature change, but it still should be a significant drop.  Note that the HRRR calls for the surface front to outrun the precipitation with time, something we often (but not always) see.  Those of you hoping for a dump like last weekend will be disappointed to know that the models pretty consistently weaken the post-frontal precipitation band as it moves into our area.  This time, Mother Nature says, NO SOUP FOR YOU!


Curiously, there are parts of the world where daytime surface heating can weaken cold fronts.  Portions of Australia, for example, observer stronger cold-frontal passages at night than during the day.  These are areas where the climate favors dry conditions both ahead and behind the front.  Because the post-frontal airmass is shallow, daytime heating is more confined vertically behind the front, leading to more rapid heating in the cold air than the warm and frontal weakening.

Cold-Front Double Whammy Update

Morning has broken and the GOES-16 visible imagery is simply beautiful and shows clearly the two frontal bands that will influence our weather today and tonight, the first over northern Utah and the Salt Lake Valley right now (as if you needed a weatherman to tell you that) and the second moving across northern Nevada.  

Source: College of DuPage
Talk about well defined.  Really, you aren't going to see two more distinct frontal systems than that in the Intermountain West.

Radar imagery through 1406 UTC (0806 MDT) shows the frontal band progressing across northern Utah with a well defined back edge.  Perhaps I will be able to get in a bit of gardening today after all.


One thing that I noticed in the overnight model runs is how colossally bad the 0600 UTC 3-km NAM forecasts were for this morning.  The end of the loop above shows a very wide and broad frontal precipitation band over the Salt Lake Valley and surrounding area, but the 3-km NAM has nothing of the sort.


In fact, that forecast is so bad that I thought there might be an error in my retrieval and processing software.  Thus, I surfed around and found another site that serves up the 3-km NAM, Pivotal Weather.  They have 3-hour accumulated precipitation plots (above is 1-hour), but it's still enough to confirm a massive forecast bust.


Basically, the 3-km NAM was completely clueless.  Resolution is worthless and even harmful if you can't get the basics of the large-scale flow, and it clearly didn't in this case.  The 12-km NAM wasn't much better.

Getting back to reality, the Alta-Collins site began to record precipitation at just before 6 AM and has observed 0.18" of water and an inch of west snow through 8 am.  It's a nice start, and they should see snow with the frontal band for the next 2-4 hours.  After that, how about scattered snowshowers and thunderstorms.  Yup, that nice clear gap this morning between the fronts could allow for good destabilization this afternoon and with strong flow and and approaching second front, we could see some stronger convection.  In fact, the Storm Prediction Center has us in a marginal risk category for severe thunderstorms.


Then we have the front later today and tonight, and I'm hoping for a powder day tomorrow.  The NCAR ensemble is optimistic.  One member is a bit under an inch of total water, but most lie in the 1-2 inch range, which would probably be enough for decent turns in many areas, especially if we end up in the upper end of that range.


I'm now jazzed enough about the two fronts that I'll go for a total of 1–1.75" of water and 10–20" of snow for upper Little Cottonwood from 6 am this morning through tomorrow morning.  The NWS numbers are a bit higher than that.  I'm a little reluctant to go so high because the latest forecast flow directions never quite come around to the coveted northwest direction and I'm unsure about precipitation during the hit-and-miss convection between fronts.

One thing is for sure, I'll be enjoying the weather the next 24 hours or so.  

Wind, Dust, Snow and All That…

I love spring storms, so I'm feeling like a kid in a candy shop today.

Winds picked up yesterday and gusted strongly overnight in advance of a developing trough and surface front over Nevada.  After midnight, peak gusts at upper-elevation locations in the northern, central, and southern Wasaatch are 82, 77, and 70 mph, respectively.  In the valleys, the Great Salt Lake Marina hit 60 mph and a sensor near the juncture of UT-201 and I-80 in the Salt Lake Valley hit 63 mph.

Winds as I write this have actually slackened just a bit.  Obs from the juncture of UT-201 and I-80 show two periods of strong winds overnight, on prior to midnight, the other from about 1:30-4:30 AM.

Source: MesoWest
With the development of the front and surface trough over Nevada today, as well as daytime surface heating, we will see strong winds today.  Given the prolonged nature of the event, dust is likely as well.  

The models are still calling for two fronts to move through northern Utah this weekend, the first late tonight or early Saturday morning:


the second late Saturday or early Saturday night:


 The NAM has backed off a bit in both instances for precipitation at Alta, especially the first front tomorrow morning, and ultimately produces a storm total by Sunday morning of 0.82" of water and 13.5" of snow.

The SREF continues to show a large spread from only 0.25" of water to over 2.5", with strong clustering based on model core (ARW or NMB - essentially, two different models are used for the SREF).  The really wet members all produce considerable precipitation early tonight, and thus get things started early.

So, here we sit, having looked at this storm for about a week, and still no guarantees!  Sunday still looks like the better ski day as snow piles up during the weekend, but how good it is will depend on whether or not we are in the upper-half of these forecasts.  Finding a smooth underlying surface or getting enough snow to fully bury the frozen coral reef will be the key to good skiing on Sunday.  One plus is that we are looking at cold temperatures Saturday night, with 700-mb tempreatures currently forecast to drop to about -14ºC, which should yield a right-side-up snowfall. 

Cold Front Double Whammy?

The latest model forecasts are advertising what I'll call a "cold front double whammy" for the weekend with two pronounced frontal passages.

The 0600 UTC initialized NAM shows precipitation associated with the first front over northern Utah at 1200 UTC (0600 MDT) Saturday morning.  


And then the precipitation associated with the second front over northern Utah merely 18 hours later at 0600 UTC (0000 MDT) Sunday. 


Time-height sections show each of these features quite well.  Keep in mind that by convention, time increases to the left in these time-height sections.  Thus, you can see the pre-frontal southerly flow and low-level dry air that will predominate through late tomorrow on the right side of the diagram.  Tomorrow could be quite windy with blowing dust and perhaps some elevated convection and a slight chance of a thunderstorm.  The bottom falls out with the frontal passage.  Note how the freezing level (blue line) drops, bottoming out at just about 850 mb (5300 ft) early Saturday morning, bringing prospects of snow down to bench level.    

Then there is a rapid rebound in advance of the next front, with winds becoming southerly again and the freezing level rising.  Then, the second front ushers in even colder air Sunday night, with snow levels dropping to the valley floor.

We spoke a few days ago about the medium-range forecasts and the need to treat those with some caution.  We are close enough now that it looks like we will get some action this weekend, but far enough out that specifics regarding timing and accumulations are still hazy.  The first front comes in late enough that even if it drops a decent amount of water, it's probably going to create cream-on-crust conditions rather than true powder skiing.  The second system, assuming it comes in as advertised above, could yield some pretty good late-season skiing on Sunday, since right now it will be adding to Saturday's totals, may be a bigger storm, and will be right side up and colder.

Much depends on how much snow the two fronts put down.  I've largely given up on using the GFS, which has an overforecast problem and has produced about 2000 inches of snow this season at Alta.  If you want to have hopes consistently dashed, go with the GFS.  The 12-km NAM has generally been more reliable (although it was upgraded in March and we don't have a good sample on the new version yet) and generates 0.5" of water and about 5" of snow at Alta with the front on Saturday, and then goes crazy Saturday night and early Sunday, tacking on an additional 1.23" of water and 19" of snow by 9 AM Sunday.  What a treat that would be.  

The downscaled SREF shows quite a bit of variation in both the timing of the front (earlier in the ARW members) and the amount of precipitation produced, especially by the first front (more in the ARW members).  Typically members based on the ARW dynamic core are wetter, and that's the case for the weekend.  The spread here does sober up my excitement as there are quite a few members producing less than an inch of water, which is probably the minimum required for decent powder skiing given the current spring snowpack.  


So, I'm guardedly optimistic for a good day on Sunday.  Saturday looks to be a day of transition, with good skiing more likely late than early and the need for the first front to come through big to get the spring snow buried.  

A Night of Extreme Change

The "Steenburgh Effect" was in full play this weekend as I was out of town skiing in Jackson Hole and of course missed out blogging about the best frontal passage we've had in months.  Nevertheless, I saw what was coming and we ended up getting off the mountain at 1 PM yesterday, and racing home via Evanston, experiencing the full force of the pre-frontal southerlies, but arriving home just ahead of the post-frontal snowmaggedon.

I took a quick gander through the wind reports over northern Utah and it appears that nobody escaped the full force of the winds.  Gusts over 70 mph were reported at all elevations and in many areas that you might think of as being "sheltered."  Here are a few selected peak gusts, all of which were recorded in the pre-frontal environment.

Mountains
Arrowhead Summit (Sundance): SSW 110 mph (0010 UTC)
Windy Peak (Uinta Mts): SW 97 mph (0100 UTC)
Mt. Baldy (Central Wasatch): SSW 96 mph (0000 UTC)
Big Mountain Pass: SW 96 mph (2220 UTC)
Ogden Peak: S 89 mph (1530 UTC)

Mid-Elevation Hills and Canyons
Eureka (6584 ft): W 95 mph (2240 UTC)
Vernon Hill (5761 ft): SE 90 mph (2230 UTC)
Deer Creek Dam (5429 ft): SSW 83 mph (0110 UTC)
Mayflower Summit/US-40 (6929 ft): S 81 mph (0120 UTC)
S-Turns Big Cottonwood (6235 ft): W 78 mph (2230 UTC)

Salt Lake Valley
SR-201/I-80: WSW 82 (2310 UTC)
Olympus Cove (4960 ft): SW 73 mph (0030 UTC)
Kennecott Tailings Magna (4440 ft): SW 72 mph (0000 UTC)
Draper (5052 ft): W 68 mph (2140 UTC)
University of Utah: S 64 mph (2315 UTC)

The driver of these strong winds was a strong cold front that developed over the Great Basin and moved across northern Utah yesterday afternoon.  The intense surface trough accompanying the front featured strong pre- and post-frontal pressure gradients.


Although the strongest gusts were generally observed ahead of the front, the winds behind the cold front were nothing to laugh about either.  Here at the University of Utah, the peak gust of 64 mph occurred about 90 minutes ahead of the frontal wind shift.  Although there was a temporary drop in the wind speeds following the frontal passage, they picked up again, with a prolonged period of W-NW flow and gusts above 30 knots, as the frontal precipitation band came through.


The lag between the surface based front, as indicated by the pressure trough, and the precipitation band was clearly evident in the analysis for 0200 UTC below.


The combination of heavy snow and strong W-NW flow made for a quick coating of white at our place in the upper Avenues (about 5000 feet) by about 8 PM MST.  It's always hard to take night-time pictures of snow, especially with an old cell phone, but the combination of snow and wind made for about the nastiest weather I've seen in the valley this winter.


I suspect this morning provided a rude awakening for many as temperatures on campus are around 24ºF with NW winds gusting from 15-20 mph over the past couple of hours.  Throw in icy sidewalks and you have conditions best described as "sporting."  After a high on campus yesterday of 59ºF, it is safe to say that it was a night of extreme change.

Also this morning in the post-frontal environment we have some beautiful orographic clouds draped over the mountains, such as these over the Oquirrh Mountains as I walked to the bus at about 7 am.


Thanks to the post-frontal convection, graupel has been falling at times on campus.


Enjoy this taste of winter.

Meteorological Winter Going Out with a Bang

After abandoning us for a stretch in February, it is clear that Meteorological Winter (the months of December, January, and February) is going to go out with a bang as snow impacts the morning commute across much of the Salt Lake Valley and the northern Wasatch Front today.

The large-scale setup for this event is worth a peak as it features many of the characteristics that we often see when an upper-level trough swings across the Great Basin.  At 2000 UTC (1700 MST) yesterday, the system was "vertically stacked" along the Pacific Northwest coast with the 500-mb low center (as indicated by the 500-mb height contours in black) located nearly over the sea level pressure low (as indicated by color contours).  To the south, a pronounced lee trough existed downstream of the Sierra Nevada, from which a surface trough extended into western Utah.


As the upper-level trough slid southward and eastward overnight, the Great Basin trough extended northeastward across northern Utah and become the locus for surface development and cold-front formation, with precipitation immediately upstream becoming more widespread.



It is that area of precipitation that is now bringing snow to the Salt Lake Valley and northern Wasatch Front, and will eventually impact Utah County.  The latest radar loop shows scattered precipitation over Utah County and far southern Salt Lake County, but otherwise fairly continuous snowfall from roughly 6200 South to Ogden.


Really, this is not a worst-case scenario because temperatures are modest and road crews have been out, but it's still bad.  Utah Commuterlink traffic flows show a pretty horrendous picture with low speeds (0-30 mph) along many of the main highways in Salt Lake, Davis, and Weber Counties.  Note how Utah County remains "in the green."  Weather, like politics, is local, and they've escaped the snow impacts so far.


Things are looking good for a few inches of snow to pile up on the campus grass this morning.  The system has already been a good producer in the mountains (6 inches at Alta-Collins, perhaps 9 inches at Snowbasin-Boardwalk).  Looks like free refills until the front pushes through later today.