Category Archives: Fronts

A Good Front Followed by Bad Orographics

A cold front moved across northern Utah last night bringing much needed snow from the valley floor to the highest peaks.  Powder panic brought gridlock to most (probably all) routes to the Cottonwoods, providing an all-too-frequent reminder that we are loving our canyons to death. 

Those who braved the traffic were rewarded with knee deep powder.  I would describe the skiing as good, not great.  A spongy layer of high-density snow to start would have helped reduce the bottom feeding, but this year, beggars can't be choosers.  

Snowfall produced by the front went largely as advertised by the models, at least in the upper Cottonwoods.  Alta-Collins had .59" of water and 7 inches of snow through 8 AM this morning.  This compares very well with the NAM forecast we discussed on Tuesday, which put out .64" of water and 8 inches of snow (see Frontal Snowfall Event on Tap for Late Tomorrow and Tomorrow Night).  The observed water totals are also near the middle of what was advertised by the University of Utah downscaled SREF ensembles.  The model wizards can be happy about this period.

I suspect that those hoping for a true storm ski day were a bit disappointed, however, with today's offering.  Light snow fell for much of the day, but since 8 am it added up to only .15" and 2 inches of snow at Alta Collins, which is probably a bit more than what we saw where we were ski touring in Big Cottonwood Canyon.  

Quite frankly, the orographic forcing today simply sucked, which we discussed as a possibility yesterday (see Probabilistic Snowfall Forecasting).  The morning sounding shows the situation quite well.  The atmosphere was quite moist, but also generally stable below 700 mb (10,000 ft), with a strong stable just above, which is associated with the front aloft.  The flow at low levels was northerly, but swung to southwesterly in the strong stable layer.  This is simply not a recipe for orographic enhancement.  

Source: NCAR/RAL
During the afternoon, the flow on Alta-Mt. Baldy slowly shifted to northwesterly to westerly, but remained weak and with high atmospheric stability, wasn't generating much at upper elevations.  

Instead, the radar loop below shows the development of the strongest echoes along the east bench and within the lower canyons through 0028 UTC (5:28 PM MST).  

Driving down Big Cottonwood late this afternoon, it was clearly snowing harder in the very bottom of the canyon and along the east bench than it was in the upper canyon.  Note that you can also see this effect in the Oquirrh Mountains where the radar returns, especially later in the loop are stronger on the lower and mid elevation western slopes.  

The devil is in the details.  

What Happens to Fronts over the Western US

It's a question that fascinated me 30 years ago and continues to fascinate me today.

What happens to fronts over the western US?

The short answer is a lot, and it varies from case to case.

Let's begin with a loop of the front making landfall later today and tonight.  This loop presents sea level pressure in black contours, 850 mb (1500-m) temperature in red contours, and either radar or 3-hour forecast accumulated precipitation in color fill.  In this loop, note how the front appears to decelerate as it pushes inland across Oregon and northern California and ultimately takes on an "S" shape with the inflection near the trough that develops downstream of the Sierra Nevada. 

These and other effects are more apparent if we look at individual times.  At 1800 UTC 17 January, the system has the appearance of a classic occluded cyclone with a relatively smooth and continuous cold front extending from just off the Washington coast into the subtropics. 

Eighteen hours later, the front is pushing into Oregon and northern California (I've given up analyzing the fronts north of this location as they have become difficult to track).  The front at this time is beginning its inland transformation and is decelerating over southern Oregon and northern California.  At the same time, there is a temperature contrast just ahead of the front over California and the eastern Pacific (circled with a thin light blue line).  This is an important feature as a new front is beginning to form at the leading edge of this zone of temperature contrast.   

By 1800 UTC 19 January, the front and the temperature contours have taken on an "S" shape with the inflection over northern Nevada.  This S-shape is the result of several processes, including the deceleration of the front over northern California and Nevada, and the development of a new front ahead of the old Pacific cold front over the eastern Pacific.  Note in particular that the precipitation band accompanying the old Pacific front is well behind the leading edge of cold air at this time, consistent with a new front forming ahead of it. 

By 1800 UTC 19 January, the front has a strongly distorted into an S-shape.  One can quibble with the precise position of the cold front in my analysis over southern California and Baja California, but the S-shape is very clear in the temperature contours.  Again, note the separation between the cold front and the model precipitation. 

The processes responsible for this evolution are complex and not well captured by simple models of frontal evolution.  Where to position a front in an analysis is always a subject for debate, but the development of the S-shaped appearance is common as cold fronts make landfall in this part of the world. 

If you are wondering what all this means for snow, tough luck.  I've already spent too much time on this post and need to get some work done!  Maybe tomorrow.  Maybe. 

A Remarkable Forecast of a Beast of a Storm

Intense frontal cyclones don't come much prettier than the nor'easter rampaging up the east coast today.  Satellite imagery and radar for 1200 UTC shows the beautifully wrapped up system with the low center just a shade ESE of Virginia Beach.  Precipitation was heaviest near or just offshore (radar imagery well offshore is nonexistent). 

If we slap on the RAP 925-mb analysis (roughly 750 m above sea level and a good level for seeing the frontal structure) we see the classic structure of an intense frontal cyclone predicted by yesterday's NAM (see prior post) with the cold and occluded/warm fronts oriented at right angles, weakening of the temperature contrast associated with the cold front near the occluded/warm fronts (known as the frontal fracture), the frontal temperature contrast associated with the occluded front maximizing near and west of the low center, and the occluded front extending through the low center as a back-bent occlusion.  One can also see a near cutoff pocket of warm air near the low center (a.k.a. the warm-core seclusion) as cold air encircles the system.  The area in purple shows an intense low-level jet with winds in excess of 40 m/s (80 knots, light purple) wrapping cyclonically from the west to south of the low center, culminating in a maximum in excess of 45 m/s (90 knots, dark purple) known as the poisonous tail of the back-bent occlusion.  In this case, the wind maximum likely represents a sting jet, a local wind maximum near the tip of the comma cloud head that is produced by the descent of strong winds from aloft.  For more on this subject, see What is a Sting Jet?

The 30 hour NAM forecasts that we presented yesterday were quite remarkable and I've reproduced them below for comparison with the RAP analyses above.  The frontal structure, low center position, and low center intensity are very well captured.  The NAM forecast central pressure of 961 mb is a bit overdone compared to the RAP analysis 967 mb, but analysts at the National Weather Service Weather Prediction center put it at 960 mb, so the NAM forecast is certainly within the uncertainty. 

Such a forecast is a remarkable scientific achievement.  We shouldn't take it for granted.  Through the mid 1980s, operational numerical modeling systems frequently failed to predict intense frontal cyclone development of this type.  Scientific papers describe errors in central pressure forecasts of as large as 55 mb.  You read that right.  55 mb.  Basically, a complete and total failure to predict the cyclone development. 

Today, it's almost impossible to believe numerical forecasts could be that bad, but they were, and many on the high seas paid with their lives.   It is only through advances in understanding, observing systems, computing infrastructure, and numerical modeling techniques that we knew a storm like the one above was coming (in fact, many days in advance).  Surely there will be some issues with details of the local forecasts that require further research and model improvements, but forecasts of intense frontal cyclones have come a long long ways. 

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.

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.

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.