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.

Book Review: The Darkest White

 

Perhaps no modern figure has had a greater impact on a snow sport than Craig Kelly.  He was the first truly big legend in snowboarding, ushering the sport from its nascent fringe to mainstream, although he might object to the word "mainstream." Many people consider him the greatest snowboarder of all time. 

Kelly's life is the subject of Eric Blehm's book, The Darkest White, which was released earlier this year. It covers his transition from BMX to snowboarding and his ascent to world champion at a time when snowboard racing and freestyle competitions were not well structured.  Kelly excelled in all competitions.  It was the early days, but imagine if Mikaela Shiffrin, in addition to racing everything from slalom to downhill, was also throwing in some half pipe for the hell of it.  Kelly walked away from all of that to pursue freeriding and eventually train to become a certified mountain guide at a time when there was considerable prejudice against snowboarders. 

The book covers those aspects of Kelly's life, although I found that part of it a bit chronological and maybe too focused on competitions.  I suspect snowboarders will appreciate it more. 

The real page-turning part of the book is the description and diagnosis of the avalanche that sadly took his life and the lives of six other people in the Selkirk Mountains.  While working on his guideship skills, Kelly joined a group at Sellkirk Mountain Experience to work with the famed mountain guide Reudi Beglinger.  While ascending La Traviata in two groups, an avalanche was triggered catching 13 tourers.  Several were recovered successfully, including Ken Wylie, an apprentice guide, who was rescued after 35 minutes of full burial.  However, seven died including Kelly who was buried 9 feet deep and recovered after abount an hour. 

I was quite aware of the avalanche as well as another that season that killed seven others because I spent a week on a ski touring trip nearby with Golden Alpine Holidays.  It was on our collective minds during our tours and we spent a good deal of time skiing in low-angle terrain and not disturbing the monster in the basement.

It's difficult for me to say if the book (or all of the discussion you can find online) paint a fair and accurate assessment of what happened and why.  Hindsight is of course 20/20, but I spent a lot of time reflecting on the snowpack assessment and human decision-making in the account that Blehm presents.  There is a lot to be learned and I often reflected upon my own experiences and lapses of judgement, of which there are many.  I suspect Craig would appreciate everyone learning from what happened, especially if it results in safer backcountry experiences. 

To conclude, I share the photo below that I took of a memorial to Craig at Seki Onsen ski area in Japan in 2017.  I don't know what it says, but I was impressed to find it at a small (but very snowy) resort in the Myoko Kogen.

Déjà Vu All Over Again

I had a mild bike ride to the office this morning for November 11 and the forecast high for this afternoon is 65 for KSLC. 

Tomorrow will be different. 

However, it is a bit of a case of déjà vu all over again as it is a system that reminds me of the others we have had so far this fall with the strongest part of the trough and the so called "dynamics" moving to our south.  We get a frontal passage and some precipitation, but looking at the models, the system is just not put together great to give us a really big dump. 

Below is the GFS forecast valid 1200 UTC 12 November (6 AM MST Tuesday).  The strongest part of the 500-mb trough is basically over Northern Arizona and southern Utah and passing to our south.  The surface front is pretty much over Salt Lake City, but the 700-mb temperature contrast with it is somewhat diffuse and the precipitation with the front scattered and not all that organized.  

The GFS time height section shows very dry air ahead of the front and then a period of about 6-hours with deep moisture as the front moves through.  We do get into northwesterly post-frontal flow Tuesday night, but it is fairly dry at low levels (below 800-mb).  This is not the kind of forecast that causes my heart to flutter. 

The latest models are calling for a small storm at Alta.  Through late Tuesday the HRRR generates 0.25" of water and 3.5" of snow and the GFS  0.37" of water and 5" of snow.  The experimental RRFS has a remarkable amount of spread with one member producing essentially a trace and another up at 10".  

I'm not sure what to make of that, but I view that 10" as something close to the upper limit of what I'd expect under the best of circumstances, with a significant boost from lake-effect interactions with the terrain behind the trough.  

Finally, the Utah Snow Ensemble is generally in the under 12" range with means from the two ensemble systems at about 4" and 6".  Yes, I know there's more snow after this system in this forecast but I'm not going to address that here.  Whatever you do, don't be biased by the highest members! (See Anchoring Bias and Ensembles for why). 

It is what it is, another modest system that will add a bit more to our November snowpack.  I'm thinking 5-10" for Alta Collins. Although it is clear that the ensembles are saying the range of possible outcomes for this event is pretty big, there isn't much here to get me thinking about more than a foot.  That said, none of these models are particularly good at dealing with the post-frontal northwesterly orographic snowfall enhancement in the Cottonwoods.  Then again, they also aren't advertising a great environment for that.  

Colorado and (gasp!) Texas Running Away With It

 Looking for snow?  Forget the Collins glacier.  Go to Alta or maybe Texas.  

Over the last four days, Colorado, New Mexico, far western Oklahoma, and the upper northwest corner of Teas were absolutely pounded.  This includes the high planes.  Texline, TX recorded 24" of snow.  Boise City, OK, not Idaho, recorded 26".  

The big winner was a site 12.9 miles ENE of Fort Garland with 53.3".  The NWS does not provide specific locations for privacy reasons, but that looks to be a site in the low pass through the Sangre de Cristo between the San Luis Valley and the I-25 Corridor.  Some big numbers as well in the high plains of Colorado and the mountains of New Mexico.  A look at Angel Fire (40") this morning.

https://www.angelfireresort.com/weather/

I've always wanted to ski there just for the name.  One of the best in the business. 

The fattest snowpacks in the Utahrado region are now in the San Juans and Sangre De Cristo Mountains. The big winner (blueish dot) is Beartown at 11,600 feet which is sitting at 7.7 inches.  This is the equivalent of their median snowpack on December 20th, so they are running about 6 weeks ahead of median. 

Source: NRCS

Hayden Pass in the Sangre De Cristos now sits at 5.8 inches, the equivalent of their median on December 28 and way above anything on record, although observations at this site start only in 2008.  

There are, however, other sites at record levels for this date in the Sangre de Cristos, Pikes Peak, and Buffalo Peaks.  These are historically dry areas, so an extreme event like this is really exceptional. 

It's a weak La Nina year and this has guided seasonal forecasts.  Here's one for Nov-Jan from the Climate Prediction Center that now looks on track to bust for at least parts of southern Colorado and northern New Mexico.  

This one event has produced about 2/3 of the average Nov-Jan precipitation showing how an extreme weather event can strongly contribute to seasonal precipitation and snowfall.  This is a characteristic of the cool-season snow climate of some regions of the western United States that is often overlooked when seasonal forecasts are being issued.  It introduces an element of randomness to the year-to-year variability in western precipitation that can limit the reliability of seasonal forecasts based on long-term means and similar analyses.  There is a good paper by Lute and Abatzoglou (2014) showing that 20-38% of the annual snowfall water equivalent and about 2/3 of the year-to-year variability in that metric can be attributed to the top ten decile (10% largest) snowfall events in portions of the western United States.  Basically, a handful of big events, sometimes just one or two, make or break the season.

Congratulations to the early season snowfall winners.  

The RRFS Snow Ensemble

We are excited to share that the RRFS Snow Ensemble is now available on https://weather.utah.edu and has replaced the old SREF product.

The RRFS Ensemble is a 6-member ensemble that is under development for future operational use by the National Weather Service.  Based on the FV3 dynamical core (i.e., the software that solves the atmospheric equations of motion), it is run at 3-km grid spacing and provides forecasts out to 60 hours.  The RRFS Ensemble is projected to go operational in 2024, although there have been a number of issues and challenges identified during testing that may affect that, in particular related to the forecasting of convective storms in the midwest.  The RRFS Ensemble has not been carefully evaluated over the western US, so one of the reasons we are producing this product is to evaluate its fidelity for orographic precipitation.  

We are also interested in testing our techniques for snow prediction.  Thus, what we call the RRFS-Snow Ensemble is basically an ensemble in which we plug in new techniques to predict snow-to-liquid ratio (SLR) and snow amount.  A summary of the data and methods and graphics is provided below. 

Precipitation Downscaling

Although we downscale the Utah Snow Ensemble from the lower resolution global ensemble grids, the RRFS is providing forecasts at 3-km grid spacing. Thus, we are currently doing no precipitation downscaling and just using the raw model grids.  There might be some advantage to downscaling the RRFS eventually, but for now, we're not doing it.  

Snow-to-Liquid Ratio (SLR)

Snow-to-liquid ratio (SLR) is based on a new random forest algorithm developed using SLR observations from more than 900 Community Collaborative Rain, Hail & Snow Network (CoCoRaHS) observing sites.  Thank you to all the volunteer observers and the CoCoRaHS team!  In particular we are using a subset of CoCoRaHS observing sites at which the observers are taking manual cores of the snowfall data, which we hope will reduce issues related to precipitation undercatch, which is a problem with the water equivalent measured by many gauges.  The random forest is trained using data from across the contiguous United States and in testing has performed better than existing operational techniques in both the western and eastern United States.  

Snow Level

Identifying snow level in the west or precipitation type in the east is a bit of a thorny issue.  The so-called "wet-bulb" technique that we use in the western United States works fairly well when the temperature decrease with height is close to what meteorologists call a wet-adiabatic lapse rate.  It doesn't work well if the atmosphere is stable and/or has a warm nose above freezing aloft.  

As a result, we decided to use a more physics-based approach to identify if snow is occurring.  At each model grid point, we calculate the melting energy in the model soundings.  This is the amount of energy available to melt snow in the sounding.  The technique is based on Bourgouin (2000), although we use wet-bulb temperature rather than dry-bulb temperature to calculate melting energy (special thanks to Kevin Birk of the National Weather Service for providing some of the initial code for this work).  Currently we are applying our random forest SLR without adjustment if the melting energy is ≤ 2 J/kg and assuming the precipitation is all rain if the melting energy is ≥ 9 J/kg.  If the melting energy is between those values, we reduce the SLR between the random forest value and 0 based on linear interpolation between the two thresholds.  

Those thresholds are based on published values, but admittedly, the data is not comprehensive.  It may require some modification over time.  However, they do give results similar to the wet-bulb method when the lapse rate is near moist adiabatic and can deal with more complicated temperature profiles.

Note that we are not attempting here to diagnose freezing rain or sleet.  The melting energy approach we are using will basically give us a SLR of 0 in those instances.  So, our plots only show forecasts of accumulated snow.  

We are working with another group to possibly incorporate a machine learning technique for precipitation type in the future, but it may be a while before we get to that.  

Four-Panel Plots

We provide loops of four-panel plots of the following variables for several regions, including over the central and eastern US: 

• Total precipitation (water equivalent) since the beginning of the forecast period
• Total snow since the beginning of the forecast period
• 24-h precipitation (water equivalent)
• 24-h snow
• 6-h precipitation (water equivalent)
• 6-h snow
• 1-h precipitation (water equivalent)
• 1-h snow
• Wet-bulb 0.5°C height above ground level (based on the lowest wet-bulb 0.5°C level)
• SLR

SLR and snowfall are calculated in 1-h intervals, with the resulting 1-h accumulations summed to provide accumulations over longer periods.  Thus, the 24- and total snowfall should not be confused with the change in snow depth on the ground over long time periods, which would be affected by settlement.  

Each four panel plot includes the the RRFS control forecast at upper left, the ensemble mean at upper right, the ensemble minimum at lower left, and the ensemble maximum at lower right.  Below is an example of the total snowfall through 60-h over the Wasatch Front.

For some of the caveats of interpreting these plots, see my blog post on The Utah Snow Ensemble. 

Plume Plots

We are also providing forecast plumes and violin plots for many locations, including several in the northeast US, to provide more information about SLR uncertainty.  These are identical to those for the Utah Snow Ensemble, so refer to The Utah Snow Ensemble blog post for information on interpreting these plots.  The RRFS ensemble, however, only has 6 members, so there are not a lot of forecasts and the violins are going to be based in part on interpolation fro sparse data.  Some groups use time-lagging (i.e, using older forecasts) to increase the ensemble members (but also decreasing the forecast period), but we're not bothering with that for now. Perhaps at some point we will change the lower left panel from wet-bulb 0.5 level to melting energy, but for now we're keeping it consistent with the Utah Snow Ensemble.  The wet-bulb 0.5°C level is based on the lowest level in the sounding, so it will not tell you where the top of a warm nose is and may not be a useful variable in situations where there is a warm nose aloft.  

Caveats and Disclaimers

This is an experimental product.  In fact, it is an experimental SLR product post-processing experimental ensemble modeling system!  Feedback is helpful to us as we are trying to find ways to better forecast snow and its characteristics and squeeze everything we can out of the operational model suite.  Tell us what works and what doesn't.  

The RRFS ensemble is based on data and products from the National Centers for Environmental Prediction (NCEP), University of Utah, and other groups.  These groups do not accept any liability whatsoever for any error or omission in the data and their availability, or for any loss or damage arising from their use. 

This blog post may be updated as needed.