Another New Product from the Utah Snow Ensemble

By popular demand, we now have a tabular output product from the Utah Snow Ensemble for Alta-Collins available at https://weather.utah.edu/text/ensgefsdslccforecast.html.  This should provide access to the most recently processed run and provides tables of total water equivalent precipitation, total snowfall, 6-h water equivalent precipitation, 6-h snowfall, snow-to-liquid ratio, and wet-bulb 0.5C height.  Times are local (mountain) time.  Cells are color filled by magnitude.  An example of the first two tables is below.

The row labeling for each table goes from the minimum value of the 82 ensemble members to the maximum value.  The P stand for "Percentile" so that P10 means the 10th percentile.  That means that 10% of the ensemble members are below that amount and 90% are above it.  

P50 thus is right in the middle, or what statistical types call the median.  50% of the members lie between P25 and P75 (this was corrected from a typo that said P50 incorrectly), or what statistical types call the interquartile range.  If you want to get an idea of what the range of the middle of the forecasts is, these are the rows to use.  

This is a lot to stick on a web page, so you may need to get used to zooming in and out and scrolling up and down.  I don't have time to program a proper web site so you get what you get and don't throw a fit.  At least that's what my mother told me.  

The table above is from last night's 0000 UTC forecast cycle.  We are finally starting to see some hope of a bit of storminess during the holiday period, with median snowfall at 22.4" by 5 PM Saturday 28 December.  That sets the over-under.  Are you betting over or under?

Do You Believe in Santa Claus?

These are desperate times in the natural snow department.  I supposed it could be worse.  Alta has had 105" of snow so far this year and has a base depth of 37", but these are meager numbers for mid December.  Snowpack water equivalent is the best measure of the robustness of the snowpack and we are well below median for that metric at most high-altitude observing sites in the Wasatch including 50% of median at Snowbird, 56% at Brighton, and 43% at Thaynes Canyon.  Somehow, Mill D North sits at 104% of median. Good for them, but that's still only 5.7" of water.  

The snowguns continue to blaze, but this is Utah, home of the so-called Greatest Snow on Earth.  Is there hope of a Christmas Miracle?

The truth is that the ensembles are not giving us much love. Last night's Utah Snow Ensemble came in with most members under 0.5" of water at Alta-Collins for tomorrow's storm.

A look towards Christmas shows that there are some members giving us the goods, but there are 82 members in the ensemble, and the reality is that half of them are producing 0.5" of water and 6" of snow or less.  That's the glass half empty perspective.  The glass half full perspective is that the half of them are producing more than 0.5" of water and 6" of snow.  If you are an eternal optimist, the wettest 8 members produce 1.33" or more of water and 17.4" or more of snow, which would do wonders.  

The odds are low, but this is the holiday season. Maybe we can get a Christmas Miracle.  Surely you believe in Santa Claus?

A Welcome Storm

The Alta snowstake cam suggests we're getting more action that may have been advertised by the models with perhaps 6-7 inches on the stake earlier this morning before the ski patrol wipe (kudos to my friends at OpenSnow for producing the Alta Snow Stake time laps so I could grab this image). 

And then another inch was added since then (not shown).  

The automated sensors are coming in a bit below that with about a 6" increase in the total snow depth and 6" on the interval board (ignore the spurious 34 reading at 08:00).  

Those numbers are a bit higher than advertised by the HRRR and GFS over the past few runs.  Below are the forecasts for Alta-Collins going back to 0000 UTC 12 December for snowall amounts through 9 AM this morning.  Only one of the HRRR runs went for 6" and the most recent was only an inch.  The GFS was a bit more optimistic overall, but never got above 4.5 inches.  

0Z 12 Dec HRRR: 1.1”

6Z 12 Dec HRRR: 6.0”

12Z 12 Dec HRRR: 3.3”

18Z 12 Dec HRRR: 2.4”

0Z 13 Dec HRRR: 1.0”

6Z 13 Dec HRRR: 1.0”

0Z 12 Dec GFS: 0.0”

6Z 12 Dec GFS: 3.7”

12Z 12 Dec GFS: 2.5”

18Z 12 Dec GFS: 3.4”

0Z 13 Dec GFS: 4.5”

6Z 13 Dec GFS: 4.5”

And a total of say 6-8" is also near the top of the snowiest members of the Utah Snow Ensemble over the runs since 0Z 12 Dec.  The snowiest member I could find from the last three runs of the ensemble was in the forecast below which produced about 7.5" for Alta Collins through 9 AM MST (16Z 13 Dec).  Even that was an outlier.  

Consider yourselves blessed.  Let's hope the snow keeps coming as we need all we can get.

The Future Is Uncertain

The physicist Niels Bohr once said that "prediction is very difficult, especially if it's about the future" and that is very evident when looking at the latest Utah Snow Ensemble (USE) plume for Alta-Collins.  The 82 members of the USE show remarkable spread for total water equivalent and snowfall over the next 10 days ranging from nearly zilch to about 2.5" water/38" of snow.  

The period features three storm systems and the 82 members can't seem to show much agreement in the intensity and timing of snowfall associated with each of them. 

We'll likely to be in a better place at the end of the 10-day period than we are now (it helps that the bar is set so low), but don't try to pin me down on details such as should I get a parking reservation at Alta for Friday, Saturday, or Sunday.  The first system moves through Thursday night and Friday, and the second Saturday night into Sunday, but I don't have any confidence concerning how much each of them will produce.

Your guess is as good as mine.

Views of a Highly Stratified Atmosphere

During the most recent inversion event, fog developed over the Great Salt Lake, eventually expanding to cover most of the lake and environs.  Below is a series of images from the NASA Terra satellite, which flys over roughly mid morning on December 4th, 6th, and 8th, the latter being yesterday (Sunday).  

It was on the 4th that fog first became evident, with a couple of regions of fog extending westward from near Antelope Island over the southern bay of the lake.

By the 6th, fog covered most of the lake.  

And finally, by yesterday (Sunday, 8 December), the lake was nearly entirely covered and spread into populated areas that included the northern Salt Lake Valley and portions of Davis County.  

Given the limited lowland snow cover in this instance, it is likely that the evaporation of water from the lake played an essential role in fog formation in this event, leading to fog development and persistence mainly over and near the lake.

I often tell students not to conflate the inversion with the pollution or the fog layer.  In part, this is because the stratified nature of the atmosphere during inversion events is such that neither the fog nor the pollution are perfectly colocated with the inversion.  Instead they are embedded in it or trapped beneath it.  The former was the case yesterday.

Temperatures in the morning sounding from the airport yesterday increased with height from the surface to 817 mb, or 6010 feet.  

Source: SPC

However, the fog was much shallower than that.  In fact, one was above it at about 5200 feet elevation in the Avenues foothills.

And at times the LDS office building and other tall buildings downtown rose above it.  

There was also evidence of pollution above the fog layer.  This might be inferred from the photos above but is better illustrated by one taken towards the sun and University of Utah.  In it you can see the fog, as well as complex layering of aerosols (i.e., pollution) in the atmosphere.

During inversion events, the atmosphere is often highly stable and "inverted" through a layer that is much deeper than either the fog or pollution layer.  Emissions and the transport of pollution by atmospheric flows and turbulence often occur within or beneath the inversion.  In these instances, the atmosphere is often quite stable (and often "inverted") even above the fog or pollution.  Variations in the timing and location of emissions, and the transport of them by atmospheric flows and turbulence, yield layers of "haze".  Natural emissions of water vapor from the lake are also trapped at low levels, resulting in fog formation over and near the lake, but not necessarily through the same depth as the pollution.  

In this most recent event, we ended up with a highly stratified atmosphere with fog and pollution layers.  

Thankfully, we had something to drive more mixing in the atmosphere yesteday and last night, and that was increasing flow and the decrease of temperatures aloft which were associated with an approaching trough.  Ultimately this mixed out the atmosphere, to everyone's great relief.  

Breathe easy.

Intricacies of this Inversion

We are currently mired in the first major inversion and air quality episode of the season, although I don't think it's been as bad as it could have.  

First, the valley is mostly snow free.  This likely leads to a bit more energy input into the atmosphere (due to less sunlight being reflected back to space and increased ground heat flux) and perhaps a bit of a decrease in particulate matter generated by photochemical processes (although this is not my area so I'm speculating).  

In the case of the former, the soundings released at the Salt Lake International Airport at 0000 UTC each afternoon are too late to capture the structure of the convective boundary layer that develops each afternoon.  The convective boundary layer is the part of the atmosphere in which surface heating by the sun leads to vigorous vertical motions (the updrafts are sometimes called thermals) and mixing.  The depth of the afternoon convective boundary layer varies from meters to kilometers, with the former happening during the strongest inversions and the latter on hot days in the summer.

HRRR model forecasts have suggested that the afternoon convective boundary layers during this afternoon have been about 50 mb or 500 meters deep.  An example is below. 

That's deeper than we would see with snow on the ground and during our strongest inversion events, which might feature a boundary layer that's only a couple hundred meters deep, leading to a bit more dilution of this event.  

For those in the northeast Salt Lake Valley, this event has also featured some enhanced easterly flow.  This is a result of the structure of the "Rex Block" pattern over the western United States.  A Rex block features an upper-level high pressure system to the north and an upper-level low pressure system to the south.  Salt Lake City has been wedged between these two features, contributing to broad easterly to northeasterly flow in the northeastern part of the valley each morning, including this morning.  

Source: Mesowest

Such a flow pattern is not entirely unusual on clear mornings, but it seems like it may be a bit deeper and more effective at diluting the pollution each night.  PM2.5 estimates from the PurpleAir network this morning, for example, show considerable variability along the northeast bench, but several stations with low values west (and downstream in the morning) of Emigration Canyon and in the Olympus Cove Area.  In the case of the former, low values extend farther west to at least State Street.  In contrast, PM2.5 estimates are uniformly high in the western valley.  

Source: purpleair.com (screenshot at 0708 MST 4 Dec 2024)

The nocturnal "flushing" of the air pollution during this event is well illustrated by the measurements from Tracy Aviary in Liberty Park.  There is a clear see saw of PM2.5 concentrations, which drop each night and then increase each morning at about 9 AM.  

Similar but even more pronounced behavior is observed at our Mountain Meteorology Lab near the mouth of Red Butte Canyon on the University of Utah campus where we also have colocated meteorological observations.  Yesterday, for example, PM2.5 concentrations increased rapidly at about 10 AM.  Prior to that, the winds were ENE and coming down canyon.  At 10 AM, they became calm and then shortly thereafter shifted to SW, bringing in the gunk.  Then, at 5 PM, the PM2.5 began to drop.  This corresponded with the return of the ENE down valley flow.  That ENE downvalley flow persisted all night long, with PM2.5 levels dropping eventually to 5-10 ug/m2.

These trends cannot be explained by local emissions or photochemistry.  They are related to meteorology and the diurnal variations of winds in the northeastern Salt Lake Valley. 

Book Review: The Starting Zone

If you are looking for a good gift idea this holiday season, one that will expand the snow knowledge of your friends and family (or maybe yourself), look no farther than Karl Birkeland's recently published "book" The Starting Zone, available from avalanche.pressbooks.pub ($49.95).

I put book in quotes because The Starting Zone is an e-book available from the Friends of the Colorado Avalanche Information Center.  The Starting Zone is a living document, with tons of figures and videos and more references than you can shake a stick at (these are footnotes, so they don't clutter up the reading).  Everything is hyperlinked, allowing you to watch videos or access supporting materials with the click of the mouse or a tap of the finger (I read it on my tablet).  

I am not an avalanche professional or researcher, so my perspective is of a scientist in a complimentary field (meteorology) who has been backcountry skiing for more than 30 years but has little formal avalanche training.  I loved that the book was written in plain English with many opportunities to dig deeper into subject matter.  This appeals to my practical and sciency sides.

Karl is an excellent writer and does a great job of simplifying complicated subject matter.  I found the chapter on Dry Snow Metamorphism to be especially insightful.  I finally understand the differences between near surface facets and surface hoar and can now better explain the physical processes that distinguish metamorphism that leads to facets vs. rounds.  Karl utilizes laboratory experiments to illustrate many concepts, with extensive use of videos and images from micro-CT scans.  This also appealed to my sciency side, but also my bias toward visual learning.  I much prefer to be shown something rather than be told something or forced to infer from equations. 

There is one oddity with The Starting Zone.  It is still in the runout zone.  Sections I (Laying the Foundation) and II (The Mountain Snowpack) are finished and available except for one section (Spatial Variability at Multiple Scales).  The final section (Avalanche Release, Mitigation, and Forecasting) will be released by Fall 2025, although all three sections of the book are included in the purchase price.  Think of this as an opportunity to advance your eduction as you creep out on the slab but before you feel the collapse, hear the whumpf, and are carried down the hill by the snowy torrent.  Perhaps what you learn from Sections I and II will keep you from creeping out on that slab in the first place. Not everyone in that situation gets a second chance.

Will It Ever Snow Again?

December arrives with the dreaded "Rex Block" firmly in charge over the western United States. 

A Rex Block is a large scale pattern characterized by high pressure "over" low pressure as is evident in the GFS forecast valid for 0000 UTC 3 December (5 PM MST Wednesday) with an upper-level ridge centered over Oregon and a weak trough off the coast of Baja California.  

Rex blocks are named for meteorologist Daniel Rex who published an early article on blocking in 1950. 

Given the stability of this pattern, all members of the Utah Snow Ensemble are snow free at Alta-Collins through 0600 UTC 6 December.  After that, you can find a few members that try to bring a storm or two to Alta, but most keep us dry.   

It's easier to forecast the onset of a block than its demise, so perhaps the odds will ever be in our favor as we approach 10 December, but for now anticipate dry conditions, valley inversions, and faceting snow on shady aspects.  

Different Storm Pathways

What will certainly be the last storm of November and possibly the last storm for some time is now in the books.  

As a recap, I thought we would do a comparison of the total storm depth change at Deer Valley Ontario (9100 feet) and Alta-Collins (9662 feet) as they illustrate two different pathways for increasing the total snow depth about 15 inches.  I've take the liberty of stretching the Alta-Collins plot from MesoWest so that the y-axis scale increments are comparable in scale to those at Deer Valley Ontario.

Most of the snow at Deer Valley Ontario fell from about 0000–1400 MST Thursday (red shading).  This was during the pre-frontal storm stage discussed in the prior post (see Can You Help Explain the Overnight Snows).  

The Deer Valley Ski Patrol was caught mocking the Alta Ski Patrol on their snow-stake web cam.

Of course, Alta is Mother Nature's favorite son, and with and following the passage of the front late yesterday afternoon and last night (blue shading), she decided to give Little Cottonwood the goods.  

Two different pathways to 15ish inches.  In the end, everyone is happy. 

Can You Help Explain the Overnight Snows

Sometimes those with investment portfolios and trophy homes get the last laugh and that was the case last night with Deer Valley the big winner in the central Wasatch.  Below is a look at the Ontario Snow Stake Web Cam at 8:05 AM showing a solid 8".

Source: https://www.deervalley.com/explore-the-mountain/webcams

Meanwhile, on the other side of the Wasatch, at a ski area also frequented by people with investment portfolios and trophy homes but better known for deep powder, pickings were much slimmer.  

Source: alta.com

This is a pattern that does sometimes bless the Deer Valley side, although I confess I don't exactly know why, in part because of poor radar coverage, poor radar estimates, and limited observational data.  

The issues with radar coverage are apparent in the plot below, which shows the accumulated precipitation estimated from the National Weather Service Radar (KMTX) for the 6-hour period ending at 1400 UTC (7 AM MST).  The radar thinks the heaviest precipitation is in upper Big Cottonwood, in Brighton Basin, rather than to the east in the Deer Valley Ontario area. 

Source: https://mrms.nssl.noaa.gov/

In part, this reflects differential orographic blocking of the radar, which results in weaker returns (all else being equal) in the Deer Valley area.  It could also reflect overshooting by the beam if the growth of snow crystals in this situation is shallow.  Finally, the correlation between radar reflectivity and snowfall rate (including water equivalent rate) is much lower than it is for rain, so there are times when radar estimates are simply out to lunch.  In any event, the National Weather Service radar is not all that helpful for understanding what is happening in these events.  

There is also a complete lack of upper-air observations near Deer Valley, so we have to make due with the sounding from the Salt Lake City International Airport.  This morning's sounding shows southerly winds at low levels and westerly flow at 700 mb, roughly 10,000 feet. At issue is whether or not the flow in the Heber Valley in such a pattern is lifted and produces local, shallow snowfall enhancement on the Deer Valley ridgeline.  Some have speculated this is the case, but the hypothesis has not been carefully evaluated. 

Source: SPC

Perhaps a conflicting piece of evidence in this case is that the flow direction on Mount Baldy was not southerly overnight but southwesterly.  

If it was southerly or southeasterly, it would fit this hypothesis a bit better.  Of course, there's always the possibility that wind direction is affected by local conditions and the overall flow in that area is actually ascending out of the Heber Valley.  

I have another hypothesis, although it might not be as compelling as the flow direction one.  As shown in the sounding above, the crest level flow in this case was westerly and we had near saturated conditions through a deep layer, with strong flow in the upper troposphere.  On the other hand, the low level atmosphere in the Salt Lake Valley was dry with a relatively high cloud base.  A look at radar echoes for this period showed that they were not evident right over the immediate western face of the Wasatch, but somewhat downstream. 

My hypothesis is that in this event, we are seeing a situation where there is weak orograhic lift over the western Wasatch, but it is is deep, resulting in ice crystal generation aloft. Those ice crystals are carried downstream and fall out preferentially downstream of the Wasatch Crest over Deer Valley.  

There are examples of this happening over other ranges.  The best example I can think of is a case examined by Geerts et al. (2015) in the Range of Wyoming.  They flew through the storm in an aircraft with upward and downward pointing cloud radars (the dashed line below is the aircraft flight level).  These radars don't scan, but instead collect a continuous curtain of radar data above and below the flight track, allowing the detailed vertical structure o fthe storm to be observed.  The top panels are two different flight flight tracks during the storm.  In both cases, there are no low echoes upstream of the mountain and on the windward (left) side of the crest, reflectivities are highest aloft.  They calculated the streamlines of ice crystals and showed that those generated in this windward area aloft were carried downstream and fell out on the lee side of the mountain. 

Source: Geerts et al. (2015)

That paper is a favorit of mine because it shows how you sometimes need to think beyond where the mountains are forcing rising motion. You also need to think about transport and fallout.  This is particularly important when there is crystal generation aloft, possibly well above the crest.  

Anyway, that's my story and I'm sticking to it.  Perhaps you have other ideas and can help explain the overnight distribution of snowfall.

The Atmosphere Is a Complicated Place

Introductory meteorology textbooks depict a world of cold front, warm fronts, and occluded fronts. The cold front separates two airmasses, one the colder "polar" airmass, the other the warmer "tropical airmass."  The cold front is a long-lived feature in that intrudes into the warmer airmass, lifting it and producing a band of heavy precipitation.  Chance are you have seen conceptual models of this type.

If only the world were so simple.

The reality is that the atmosphere is a dynamic, complicated place in which troughs, fronts, and other atmospheric features are constantly evolving.  You can't put a line on a map in one area and expect it to move continuously, without evolution into another.  The developing storm for tonight and tomorrow is a prime example. There are a lot of moving parts, to take this discussion for what it's worth: A summary of a complicated atmosphere.

The GFS forecast valid at 1200 UTC 26 November (0500 MST Tuesday) is below.  I've identified some of the primary large-scale features of concern for the forecast.  The first is an atmospheric river (AR), characterized by an elongated filament of high integrated vapor transport (IVT) above 250 kg/m/s, that extends from the eastern Pacific across southern California and southern Utah.  The second is an upper-level trough at 500 mb (dashed line upper left) and 700 mb (dashed line upper right).  There is another also a developing trough downstream of the Sierra Nevada, evident at 700 mb, which I've identified with a solid line at lower left. 

Below is the total precipitation produced by the High Resolution Rapid Refresh (HRRR) through this time (0500 MST Tuesday) early tomorrow morning just to highlight the higher precipitation amounts tonight in the central mountains.  From Provo north, precipitation is heaviest around Provo Peak, Cascade Ridge, and Mount Timpanogos. By and large, this reflects the position of the strongest IVT accompanying the AR over southern Utah.  

Precipitation over the Salt Lake Valley though is fairly limited.  During this period, drier air fills the valley at low levels and causes precipation sublimation or evaporation.  Eventually we get some precipitation, but it will largely be a "cloud storm" or "virga storm" tonight. 

By 0300 UTC 27 Nov (8 PM MST Tuesday), the AR has "penetrated" across the Rockies and into the central US.  In this case, don't think of the leading edge of the AR as a material surface.  The IVT across the southern Great Plans was already close to AR level earlier and the strengthening IVT in that part of the world led to IVT values ≥ 250 kg/m/s rapidly extending all the way to eastern Missouri.

Meanwhile over the Great Basin the trough downstream of the Sierra has acquired frontal characteristics.  This occurred ahead of the approaching 700-mb trough, as depicted in the lower left-hand panel below.  This is an example of discrete frontal propagation in which a new front forms ahead of the approach 700 mb trough, as often happens over the Great Basin.

With this front moving through, precipitation over the Salt Lake Valley becomes more widespread, as indicated by the 6-h accumulated precipitation forecast valid 0300 UTC 27 Nov (8 PM MDT Tuesday).  

By 1500 UTC 27 November (8 AM MST Wednesday) we are well behind the front and 700mb trough, which have merged into one feature that extends from California into the southern Great Plains.  At this time, mountain precipitation would be associated with unstable, postfrontal, northwesterly flow (a bit being produced by the GFS is in the red circle). 

So, there's a lot going on.  True AR conditions remain to the south of the Wasatch during this period, although we will get some mountain snow on the fringes of it.  Then we have the frontal passage late Tuesday and Tuesday evening, and the post-frontal period Tuesday night into Wednesday.

Let's look at some totals from the models. The HRRR is generating 1.79" of water and 19.3" of snow.  The first part of the storm is relatively warm, with the wet-bulb zero level reaching about 7500 feet  early Tuesday morning (call it a 6500-7000 foot snow level give or take at that time) before it falls late Tuesday into Wednesday with the frontal passage.  Snow through mid day Tuesday looks to be relatively high density (snow-to-liquid ratios between 8 and 11 to 1 at Alta Collins), after which we transition slowly into lower density snow. 

The GFS (not shown) is one of the drier models, putting out only 0.84" of water and 11" of snow.  

Below is a plan-view plot from the Utah Snow Ensemble for the total accumulated snowfall through 1200 UTC 28 Nov (5 AM MDT Thursday, although most of this falls through Wed evening).  The mean of the 82-member ensemble is at upper right, minimum lower left, and maximum lower right.  The mean for Alta-Collins is about 16", with a minimum of 8" and a maximum of 27". 

The large contrast between the low-end models and the high end models at this stage is a bit ulcer inducing.  The HRRR at 19.3" is nearly double the GFS at 11".  I'm inclined to be cautious in a situation like this and lean toward a storm total of 12–24" at Alta Collins.  That means this will likely come in as the biggest storm of the season so far. It will help a lot, but probably not be truly transformative.  A best case scenario would be for the AR to shift a bit northward and for the post-frontal period late Tuesday night and Wednesday to be highly productive.  

Catskills and Poconos for the Win

Looking for powder?  Head east to the Catskills and the Poconos.  Some decent 24-hour totals out there, including 16.5" near Delhi. 

It's a bit of a strange pattern, resulting from the downstream development that we've talked about in prior posts and which also led to the bomb cyclone in the eastern Pacific.  In the northeast, that eventually led to a deep closed low over the mid Alantic states. 

Even Cleveland got in on the action for a bit. 

Meanwhile, looking to our west, the upper elevation site (7617 ft) at the Mount Shasta Ski Park had a pretty good run the past couple of days for snowfall, but appears to be either in or just below the melting layer now.  Observations from that site show total snow depth increasing from 15 to 63 inches in about 24 hours,but temperatures also steadily increasing through the period. 

Currently it is 35F, so I suspect they are seeing either rain, slush, or wet snow.  Quite a recipe for a deep, upside down snowpack and rain-on-snow avalanches.  Such a waste.

It's About to Hit the Fan

Satellite imagery for the north Pacific Basin this morning is simply incredible.  I could teach an entire class based on it.  One can see all of the features discussed in the previous post, How to Break the Jet Stream, the amplifying ridge over the Bering Sea, the amplifying trough over the east Pacific, the explosively deepening cyclone off the northwest coast, and the developing atmospheric river to the south of the low center.  

Source: College of DuPage

The National Weather Service Ocean Prediction Center surface analysis for 0600 UTC 19 November (11 PM MST Monday) showed the nascent cyclone upstream of the California coast.  At that time it was what we call an "open wave" cyclone with a warm front, cold front, and intervening warm sector, with a central pressure of just under 1004 mb.  

Source: https://ocean.weather.gov/unified_analysis.php

However, in the satellite imager above, you can see the development of a clear comma-cloud signature overnight, an indication of rapid deepening.  The GFS forecast called for the low center to deepen from what we'll call 1003 mb at the time above to an unbelievable 941 mb by 0000 UTC 20 November (5 PM MST Tuesday).  I'm sitting here right now wondering if I've done something wrong.  That is a drop of 62 mb in 18 hours.  The weather.utah.edu products don't include the central pressure of cyclones, so I'll use the GFS forecast from Tropical tidbits for 0000 UTC 20 November (5 PM MST Tuesday) to illustrate this incredible bomb cyclone. 

Source: TropicalTidbits.com

We're fortunate that storm is a bit offshore as it means the worst of the winds will be a maritime issue (but still a threat that will alter shipping routes).  However, the atmospheric river accompanying the system has its sights set on northern California and it appears it will be a long-lived AR event as we discussed in the prior post.  Below is the forecast for 1800 UTC 20 Nov (11 AM MST Wednesday).  The lower-left hand panel shows the magnitude of the integrated vapor transport (IVT) as colorfill and IVT vectors.  IVT is a measure of the amount of water vapor passing over a square meter of the earth's surface every second.  High values, indicative of an atmospheric river, extend from the eastern Pacific into northern California.  

Going out another 24 hours, there isn't much change in location, although the intensity is higher.  

Below is the Utah Snow Ensemble Forecast for Mt. Shasta Ski Park in the southern Cascades of northern California.  There is strong agreement in the ensembles for substantial precipitation at this location with the lowest amounts for this system (i.e., through 0000 UTC 24 November) of about 5" and the highest around 11" (see upper-left diagram).  I've used a red line to indicate an important transition point in the storm.  Prior to that time, the wet-bulb 0.5°C level is below the site elevation and the precipitation falls as snow.  In fact, there is a very tight clustering of the snowfall amounts through about 0900 UTC 21 November near about 25 inches.  Through that time, the wet-bulb 0.5C level in all the ensemble members rises (lower left panel) and the snow-to-liquid ratio falls (lower right) so this will be some high density, upside down snow. 

Around 0900 UTC 21 November, the wet-bulb 0.5°C level begins to rise above station elevation in somemembers and eventually it rises above station elevation inall members.  The net result is that all members call for rain in the latter part of this storm period.  Precipitation in the upper-left panel keeps increasing, but snowfall is flatlines and the snow-to-liquid ratio goes to zero.

The saddest five words in the English language are "the snow turned into rain." That looks to happen in this case.

Of course the more serious issues may involve flooding.  There is a flood watch issued for much of northwest California, including the northern and central Sacramento Valley, mountains of southwest Shasta County, and areas to the west. Let's hope the precip numbers for this event come in lower than advertised. 

How to Break the Jet Stream

A major transition in the structure of the jet stream will occur over the next few days, resulting in the development of an omega block over the Norh Pacific Basin and high-impact weather for northern California and the Northwest United States.

The plot below is a combined sea level pressure (black contours) and dynamic tropopause (jet-stream level) forecast for 0000 UTC 18 November (5 PM MST Sunday). I have identified the jet stream over the North Pacific Basin and North America with a black line.  The forecast shows a deep low pressure system over the Sea of Okhutsk (at the tip of the L1 arrow).  Downstream of it, there is strong southerly flow at the surface.  L1 and the strong southerly flow ahead of it don't look at that disruptive, but combined with warming due to condensation in the precipitation system accompanying them (not shown), they serve as the proverbial straw that break's the camel's back. 

By 0900 UTC 19 November (2 AM MST Tuesday), L1 has weakened, but the ridge downstream of it has amplified substantially.  Concurrently, the surface high pressure system, H1, has also amplified.  This is an example of the mutual amplification of an upper-level ridge and surface high pressure system.  However, that's not all that is going on.  The trough downstream of that ridge is also amplifying, and another low pressure system, L2, is starting to develop over the eastern Pacific.  

By 0000 UTC 20 November (5 PM MST Tuesday), the jet stream pattern over the Pacific Basin is highly amplified (call it wavy if you want) and L2 has turned into a monster as it and its accompanying upper-level trough mutually amplify.  L2 is what we call an explosively deepening cyclone, or bomb because its sea level pressure drops so fast.  Before numerical weather prediction and our ability to anticipate explosively deepening cyclones, such a storm would have been a shipwrecker, coming out of nowhere to  produce dangerous winds and seas that are a marine nightmare.  Today, we know it is coming, but it will still be a beast. 

Finally, by 1800 UTC 20 November (11 AM MST Wednesday), the omega pattern is fully developed with the high-latitude ridge and mid-latitude troughs forming a clear "omega" pattern covering the North Pacific Basin. 

This "breaking" of the jet stream results from the progressive amplification of a series of upper-level ridges and troughs (and accompanying surface high and low pressure systems) through a process called downstream development.  The end result is a high-amplitude flow pattern and in many cases high-impact weather.  

In this case, the high impact weather is in the form of L2, an explosively deepening cyclone, and it's accompanying atmospheric river, which looks to bring heavy rainfall to portions of northern California and the Pacific Northwest. Looking at the GFS forecast for 1800 UTC 20 November (11 AM MST Wednesday) shows L1 off the coast of Washington and British Columbia (upper right panel in the figure below), but the accompanying atmospheric river, identified by the high integrated vapor transport (IVT) values in the lower right plot below, is aimed straight at northern California.  This is a flow configuration that can produce heavy rainfall in the coastal mountains and southern Cascades.

The IVT associated with this system is high, but not exceptionally strong.  In the forecast above it tops out between 750 and 1000 kg/m/s, whereas extreme values can reach over 1500 kg/m/s.  However, due to the high-amplitude nature of the flow, this is a slow moving system, so the AR will be pointed at that area for a long time.  The Center for Western Weather and Water Extremes (C3WE) has created an AR scale to rate the intensity of atmospheric rivers and their impacts that considers both the strength and persistence of high IVT.  On a scale of 1-5, this one rates a 4 on the coast of northern California based on the control run of the Global Ensemble Forecast System (GEFS), another model used for weather prediction.

By now you are probably wondering what all this means for Utah.  It probably means a bit of a break after the weak system that moves through on Monday.  After that system, we look to be to the south of the AR, in an area that is relatively dry.  As a result, the Utah Snow Ensemble plume for Alta-Collins shows a bit of snow around 0000 UTC 19 November (5 PM MST Monday), and then 3 days of what looks to be dry weather.  After that, there is a wide range of possibilities.  

To summarize, the next few days will provide an example of how Mother Nature can break the jet stream and produce a high-amplitude flow pattern, explosive cyclogenesis, and high impact weather.  I'd say buckle up, but Utah is not in the crosshairs, at least for the next few days.