Sunday, September 29, 2019

Concerns about Lightning Safety at Rice-Eccles Stadium

Readers of this blog know that I frequently discuss issues with lightning safety at outdoor stadiums.  Prior to yesterday's game, I discussed the difficulties of the game-day forecast and the potential for thunderstorms during the game (see prior post Difficult Game Day Forecast). 

I did not catch the start of the game as we were out with friends, but when I turned on the game at some point into the first half, I heard some discussion about their camera people leaving for safety.  Nevertheless, the game was going on.  I thought this was quite odd.  They returned to the field at some point and the game went on, with the Utes achieving a convincing victory.

This morning I decided to look into things.  Information on lightning strikes is available at  According to their site, there was a lightning strike 4.3 km (2.7 miles) from Rice-Eccles stadium at 2:49 UTC/8:49 PM MDT during the game. 

The NCAA Sports Medicine Handbook (available here) has an entire section on lightning safety.  To quote, "lightning is the most consistent and significant weather hazard that may affect intercollegiate athletics."  In it, they note that NOAA estimates that 62% of lightning strike fatalities occur during outdoor organized sport activities.  During this season's Utah-BYU game there was a delay due to a nearby lightning strike.  Nobody likes these delays, but they are important for participant safety.  Further, they are also important for spectator safety.  The American Meteorological Society Statement on Weather Safety at Venues and Public Gatherings (available here) provides some examples of lightning and other severe weather fatalities and injuries at outdoor events.

I don't know why last night's game was not stopped.  I do believe the U and whoever they have monitoring the weather should review their protocols. 

Saturday, September 28, 2019

Difficult Game Day Forecast

Tonight's game against Washington State is scheduled for 8 PM and I'm glad I don't have to give Kyle Whittingham the forecast as it is extremely challenging.

Below is the HRRR forecast valid 0300 UTC (9 PM MDT).  If this forecast validates, we'll be post-frontal at Rice-Eccles Stadium, with precipitation, in the form of showers and thunderstorms, just to the north. 

We are, however, unable to forecast the development and position of showers and thunderstorms with precision at such long lead times.  In a best case scenario, those showers and thunderstorms stay north of Rice Eccles and game conditions are fine.  In a worst case scenario, we see thunderstorms at the stadium, leading to delays and the like. 

It is worth noting that the Storm Prediction Center has us in marginal risk for organized severe storm episodes today and tonight. 

Here's their summary...see Northern UT to central WY below. 

Bottom Line: There are a range of possible weather outcomes for the game tonight do include the threat of thunderstorms that could be severe.  Much will depend on the position of the front and associated band of showers and precipitation. 

Friday, September 27, 2019

Epic Early Season Snow for Some

Source: NWS.  Downloaded 5:42 MDT Friday 27 September
A potentially historic early season snowstorm is expected to affect Montana over the next few days as an unusually deep early season trough develops over the western United States combines with easterly upslope flow and moisture of Pacific and Gulf of Mexico origins.  Call it a perfect storm if you must. 

The latest National Weather Service forecast calls for 24-30 inches at Marias Pass in Glacier National Park, but higher totals, exceeding 36 inches, in some mountain locations.

Downscaled SREF forecasts show mean snowfall totals for the period exceeding 36 inches in higher elevations of Glacier National Park where every SREF member is generating more than 24 inches of snow.  

The event will feature strong surface winds, which will make driving conditions even in the plains very difficult. 

Meanwhile, in the Wasatch, we're looking at a warm, south wind period Saturday with a cold frontal passage at some point over the weekend.  Phasing between the moisture and the cold air just never seems to come together right for this one.  Most of the latest SREF members generate less than an inch of snowfall for Alta Collins, although there are a few that go for more. 

It's not out of the realm of possibility we do better than that, but I would keep expectations low.  Low expectations are the key to a happy life.  It's too early for snow anyway.  

Tuesday, September 24, 2019

Thinking Probabilistically

Meteorologist and MIT Professor Ed Lorenz is considered the father of chaos theory.  His "discovery" of chaos was accidental.  He was running a computer model on an early digital computer and decided do a rerun from the middle of a simulation.  The computer worked with 6-digit precision (e.g., numbers like .207689), but he had a printout based on 3-digit precision (e.g., .208).  When he input the 3-digit variables and did the rerun, he got dramatically different results, despite the fact that the difference between 6-digit and 3-digit precision is very small. 

This is the essence of chaos.  Small differences (sometimes very small) can evolve into differing outcomes.  It is one of the reasons why the atmosphere cannot be predicted precisely and why ensembles of computer model forecasts, which attempt to account for uncertainty that arises from chaos (and other sources) are so important for weather forecasting. 

Unfortunately, ensembles require computer time and, as a result, contemporary operational ensembles do not adequately resolve the western United States.  We provide products at that attempt to account for this shortcoming, albeit imperfectly.

The North American Ensemble Forecast System (NAEFS) is comprised of 42 forecasts produced by the U.S. National Weather Service and Environment and Climate Change Canada.  We take the NAEFS low resolution precipitation forecasts and we downscale them to 800 m grid spacing assuming seasonally varying climatological orographic precipitation gradients.  This gives us 42 high resolution precipitation forecasts from which we can derive statistics like mean, maximum, minimum and probabilities of exceedance for specific amounts.  An example is provided below. 

My colleagues here at the University of Utah, Court Strong and Lucas Bohne, are working on a better approach for the downscaling – one that would account for variations with the large-scale conditions.  I'm not sure if we'll be able to plug that in this winter, but it's something we hope to add soon.

Although the downscaling provides probabilistic water equivalent, we know you also want snowfall amount and perhaps even information on snow density.  Thus, we've developed algorithms for estimating snow level and snow-to-liquid ratio.  These are then applied to each ensemble member to provide high resolution snowfall forecasts, as illustrated below. 

Currently, we use the same algorithm across the western United States to estimate snow-to-liquid ratio.  We are currently working on regionally derived algorithms that we think will work a little better.  Those may be added incrementally over the next few months.

Another way to visualize these forecasts is using plume diagrams and violin plots, as presented below for Alta Collins.  On the left side are plume diagrams, graphs of the total accumulated water equivalent precipitation (top left) and snowfall (bottom left) produced by each ensemble member.  We identify members of the US (GEFS) and Canadian (CMCE) ensembles, their respective means, and the overall mean.  One can see, for example, that the Canadian ensemble is much wetter than the US ensemble. 

Also presented are violin plots of 6-h accumulated water equivalent precipitation (top right) and snowfall (bottom right).  These provide a statistical summary of the ensemble data.  The median for the total NAEFS ensemble is indicated by a horizontal red line, the middle 50% of the ensemble members by a vertical black bar, and the middle 90% of ensemble members by a vertical red line.  These take a while to get used to viewing, but provide a tremendous amount of information.  In the snowfall plot (bottom right) we also present the median snow-to-liquid ratio (dark grey line) and middle 50% of snow-to-liquid ratio estimates (grey fill area) predicted by our algorithm.  No grey shading prior to 06 UTC 29 September indicates that the precipitation falls as rain at this location.  After that, through 12 UTC 30 Sep, some ensemble members produce snow, whereas others produce rain.  After 12 UTC 30 Sep, all the members produce snow. 

I am a big fan of these ensembles as they provide estimates of the range of possibilities rather than a single outcome.  On the other hand, just like a single model forecast has capabilities and limitations, so does an ensemble.  Ensembles have biases and shortcomings as well.  We know, for example, that the US ensemble (GEFS) is underpredicts the full range of possible outcomes, especially at lead times less than 3 days.  Including the Canadian ensemble helps overcome this, but we're not sure yet to what degree or if it is overdone. 

We're spending quite a bit of time on these products this year, with one of my graduate students, Mike Wessler, leading the effort.  Expect to see better products coming in the future.

Monday, September 23, 2019

A Tricky Case of Downstream Development

You may have heard some rumors of potential mountain snow in the extended-range forecast.

From a forecast perspective, it's a tricky case of downstream development in which a high-amplitude ridge develops over the North Pacific, which leads a deep trough digging into the western United States. 

Below is the dynamic tropopause (jet-stream) level forecast from the GFS valid at 0600 UTC (0000 MDT) Sunday.  Note the high amplitude ridge over the Gulf of Alaska and the deep trough along the Pacific coast of the continental U.S.

Such a trough will bring some cold air to the western U.S., but the details of the downstream development, such as the strength and position of the trough, embedded disturbances within it, and interactions with Pacific moisture, are difficult to anticipate with precision at this time. 

For example, we could look at the 42 forecasts produced by the North American Ensemble Forecast System (NAEFS), which we downscale and apply some algorithms to for estimating snowfall at specific points in the western U.S.  One of the 42 forecasts generates over 20 inches of snow at Alta-Collins this coming weekend (see lower left-hand panel).  A total of five generate 8 or more inches of snow. 

Exciting, but 36 members produce no snow at all as although a number of members produce precipitation, most don't bring the cold air in fast enough to lower snow levels to Alta-Collins level over the weekend.

I call these "Gimli" forecasts after the dwarf warrior in Lord of the Rings, whose quote below is one of my favorites. 

Basically, mountain snow this weekend is a possibility, but significant accumulations by Sunday afternoon are a low probability possibility based on current forecasts.  Keep an eye on forecasts, but recognize there are a number of possible outcomes depending on how things come together. 

Friday, September 20, 2019

No Expansion of Campus Parking

Today is the #climatestrike.  While I won't be participating, I am supportive of efforts to transform our energy economy to one that is cleaner and ultimately carbon free.

Along these lines, I did notice that KSL ran a piece yesterday about University of Utah students who have started an online petition calling for more student parking on campus.

In the 24 years that I have been a professor at the U, I have seen the remarkable growth of the Wasatch Front and campus and how this has resulted in increased gridlock around campus.

When I arrived here in 1995, there was no Huntsman Cancer Institute or Hospital.  There was no Primary Children's Outpatient Services building or attached parking garage.  There was no Sorenson Molecular Biotechnology (USTAR) building or Warnock Engineering Building. There was no Student Life Center, Garff Building, or Eccles Business Building.  I could go on.

I don't have long-term statistics to look at parking trends, but the University has destroyed a great deal of open space where the golf course used to be for parking.  It (and Primary Children's) has erected parking garages.  In the case of garages, the cost per space is $24,000.  Apparently the U added almost 500 student parking spaces last year.

According to the Kem C. Gardner Policy Institute, Utah's population is expected to increase from around 3 million in 2015 to 4 million in 2032 and 5.8 million in 2065.  The Salt Lake Tribune reports that more than a half dozen new high rises are currently planned for Salt Lake's urban core.  Have you seen the transformation of Sugarhouse in recent years?  Salt Lake is transforming from Small Lake City to Tall Lake City.

I love my car and the freedom that it brings, but do we really want a future where even more people are driving to campus?

Let us continue to invest in campus housing, transit, and ride sharing.  Let's incentivize multimodal transportation options that encourage people who drive 5 days a week to perhaps drive 2 or 3 days a week instead.

I recognize that transit doesn't work for everyone.  I need to drive to campus from time to time too.  But we need solutions that don't involve adding more parking spaces and having more people drive to campus.

Wednesday, September 18, 2019

Intermountain Frontogenesis

Frontogenesis is meteorological jargon for the birth or strengthening of a front.  Intermountain frontogenesis thus refers to the birth or strengthening of a front over the Intermountain West, as will happen today.

Forecasts from the NAM for the 700-mb level (crest level or about 10,000 ft above sea level) for 1200 UTC (0600 MDT) this morning show a deep trough with cooler air over the Pacific coast.  Ahead of this trough, the flow is southwesterly and temperatures warmer.  Over Nevada, the focus of this talk, the 700-mb temperatures are around 6ÂșC in southern and central Nevada and 0˚C in extreme northwest Nevada.  There is a slight wind shift across Nevada with the flow weakly confluent, or merging.  

By 1800 UTC (1200 MDT), we see the first signs of frontal strengthening.  Note how the isotherms, or lines of constant temperature, have come closer together over northwest Nevada, indicating a strengthening gradient in temperature.  The wind shift across this feature has also strengthened. 

Through the afternoon, the front continues to strengthen.  Aiding this strengthening is a contrast in surface heating from ahead of the front, where skies are clear, to behind the front where skies are cloudy.  Note how temperatures in eastern Nevada have increased to 8˚C.  Additionally the wind shift along the front has strengthened.  

During the same period, low-level pressure troughing, indicated below by the 850 mb (about 5000 ft above sea level) height analysis, strengthens over central Nevada, Idaho, and Montana along the developing front.  

This is a classic case of Intermountain Frontogenesis, or frontal development over the Intermountain West, which is common in the fall and the spring.  Several factors contribute to frequent frontal development in this region including flow interactions with the Sierra Nevada, downslope warming in the lee of the Sierra Nevada, and the contrast in cloud cover across the front, which allows for daytime strengthening due to more intense heating of the prefrontal airmass.  In some instances, precipitation also cools the postfrontal airmass, although in this case the forecast precipitation (indicated by green and blue shading above) is too far behind the front to have a strong influence.  

Salt Lake City will be prefrontal today, so enjoy the wonderful fall weather.  The front is a slow mover once it gets near northern Utah, so timing frontal passage may be difficult for Salt Lake City.  It happen overnight or as late as tomorrow afternoon.  The front may meander back and forth across some locations.  The high-res NAM, for example, stalls the front over northern Utah tonight and places it over Salt Lake County at 1800 UTC (1200 MDT) tomorrow. 

The GFS solution similarly puts the front over Salt Lake County at 1800 UTC (1200 MDT) tomorrow.  

Much will depend on the exact placement of the front.  Keep an eye on forecasts.  Note, however, that the frontal passage itself appears to be dry.  Precipitation is well behind the front, with chances of showers not increasing until Thursday night (if not later).  Amounts for Thursday night and Friday are a bit of a crap shoot, with the ensembles putting out everything from near nothing to nearly a half of an inch at the Salt Lake City Airport.  

If you are feeling wonkish about these events and are willing to take a deep dive, a detailed analysis of a similar event, including diagnosis of key processes, is available from West and Steenburgh (2010).  

Monday, September 16, 2019

Buckle Up!

Changeable weather is on tap for the work week.

For today, increasing south winds as a strong cold front approaches from the west.  The situation at 1800 MDT this afternoon shows the strong pre-frontal southwesterly flow, which reaches 50 knots at 700 mb (crest level).

The NWS has issued wind advisories, high wind warnings, and/or red flag warnings for much of the state.  Salt Lake City has both a wind advisory and wind flag warning.  Today is a good day to batten down the lawn furniture and avoid playing with matches.  Winds will likely be strong this evening as well.

There's a little monsoon moisture to play with, so there's also a slight chance of thunderstorms.

Tonight the front comes through.  It will likely bring a few showers and possibly some thunderstorms.  By 0600 MDT (1200 UTC) tomorrow morning, we're post frontal and 700-mb temperatures are down to 0˚C, which is about 10˚C cooler than where they are this morning.

They eventually bottom out at around -2˚C by about noon tomorrow.  The bottom line is that it is going to feel like fall tomorrow.  We will likely only reach the high 60s in the valley, which is average for early October.

The SREF plumes for Alta-Collins below summarize the precip chances quite well.  24 of the 26 SREF members produce less 0.2 inches of water equivalent or less (upper left panel) and 25 of the 26 members produce an inch or less of snow (bottom left).  There are two members, however, that produce over 0.5" of water equivalent.

Thus, precipitation for this event will probably be limited unless you get hit with one of the stronger showers or thunderstorms.  A skiff of snow is also possible in the upper elevations, although accumulations of more than an inch are unlikely.

Looking farther out, another front approaches us late Wednesday or Wednesday night.  Timing of the frontal passage is a bit dicey right now as it is stalling as it approaches.

Keep an eye on forecasts for this one and buckle up for the work week!

Friday, September 13, 2019

How Good Is the New GFS?

With the cool season approaching, we now turn our attention to a critical question for the prediction of deep powder days in the western continguous United States (CONUS):

How good is the new GFS?

The GFS was upgraded in June and the changes were substantial.  The so-called "dynamical core" of the model, the guts of a numerical modeling system that are responsible for solving the equations that govern atmospheric motions, is all new and based on the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) Finite-Volume Cubed-Sphere dynamical core known as the FV3.

The equations that govern atmospheric motions are well known and based on physical principles such as the conservation of mass, conservation of momentum, ideal gas law, etc.  Solving these equations efficiently for a sphere (i.e., Earth) on a massively parallel computer, however, isn't straightforward.  The old dynamical core of the GFS was dated, not optimal for modern computer infrastructure, and problematic as model grid spacings continued to decrease.  Thus, a change was needed.

Numerical modeling systems also need "physics" to account for things like radiation, cloud, land-surface, and other processes that affect the atmosphere.  These are more difficult since we either don't know or can't define the physical equations that govern these processes, or they occur on scales that are smaller than can be simulated directly on current computers.  Cloud processes provide a good example.  We simply cannot simulate directly the formation of every cloud droplet, rain drop, drizzle drop, and ice particle in a cloud.  Shortcuts must be made.  This is called parameterization.

The new GFS is based primarily on the old GFS parameterizations, except for replacement of the old cloud parameterization with a new one developed at GFDL and a few tweaks to the land surface and ozone/water vapor photochemistry parameterizations.

Perhaps the above is TMI, but it establishes that the new GFS is substantially different than the old, so knowing whether or not this has led to changes in bias and skill is important for predicting deep powder days over the western CONUS.

Fortunately we have the answer, thanks to the efforts of Marcel Caron, a graduate student in my research group who recently completed his M.S.

Prior to the GFS upgrade, the National Weather Service went back and ran a developmental version of the new GFS (known as GFSv15.0) for previous years.  This is sometimes called reforecasting.  It allows for a comparison with the old GFS (known as GFSv14, without a .0 just to make things confusing).  Ultimately, the operational version of the new GFS that was implemented in June was based on GFSv15.1, but for the water equivalent of precipitation presented here, results are expected to be similar.

Further, just to make things interesting, we'll take a look at how the European Center for Medium Range Weather Forecasting (ECMWF) global model (HRES) does as well.

We will focus on the 2017-18 cool season for which we have forecasts or reforecasts from all three modeling systems.  Note that the HRES underwent an upgrade this summer as well, so it is at a slight disadvantage relative to GFS since we are evaluating the previous operational version rather than the current one.  The 2017-18 cool season was drier than average across much of the west, which is unfortunately, but you do what you can.  We will compare model performance at SNOTEL stations, which are located primarily in the mountains, and at ASOS stations, which are located primarily in the valleys.

To begin, let's look at the ratio of total seasonal precipitation produced by each modeling system compared to that observed, which we call the bias ratio.  A value greater than one indicates that the model produces too much total precipitation and a value less than one indicates that the model produces too little.  All three models, for day 1 (12-36 hour) and day 3 (60-84 hour) forecasts, feature bias ratios that are well above 1 at most ASOS stations, with HRES being the wettest of the three.
Bias ratios at ASOS stations.  From Caron and Steenburgh (2019, submitted).
At SNOTEL stations, bias ratios are near or just below 1 for all three models with the HRES being the driest.  This probably means that the models are a bit drier than observed since precipitation gauges often suffer from undercatch so that they underreport the actual precipitation.

Bias ratios at SNOTEL stations.  From Caron and Steenburgh (2019, submitted).
Overall, these results are consistent with the resolution of the models.  They have effective grid spacings of 10-15 km and thus do not adequately resolve the topography, leading to excessive lowland and slightly underdone mountain precipitation.  Although you may have the desire to examine the biases at individual stations, resist this as the significance at a single site is probably low, especially at ASOS stations where the sample size is small.

Total accumulated precipitation, however, doesn't tell you anything about individual events, defined here based on 24-hour accumulations.  A model could have a bias ratio near 1, but excessively produce smaller precipitation events and underproduce larger ones.  So, it is worth taking a look at the frequency bias, or the ratio of how many events a model produces within a size range compared to observed.  Values greater than one indicates the model produces too many events and less than one too few, although we use a range between .85 and 1.2 to indicated "neutral" bias given the precipitation measurement uncertainties.  Results are, however, fairly consistent with what one might expect based on bias ratio.  At ASOS stations, all three models produce too many events for event sizes up to 7.62 mm (0.25", larger events not presented due to the small sample size), with the HRES producing the largest bias.  At SNOTEL stations, all three models produce near-neutral or marginally dry frequency biases, with the HRES producing the largest underprediction of event frequencies for larger events (>25.4 mm or 1").  

Event frequency bias at (a) ASOS and (b) SNOTEL stations.  From Caron and Steenburgh (2019, submitted).
Biases of the type above are valuable since they can help you adjust model forecasts, but ultimately, one also needs to know how well do model forecasts correspond to reality.  A model could produce the same frequency of large precipitation events as observed, but on the wrong days.  Such a model would have a great bias, but really poor skill.  

A common metric for examining model skill is equitable threat score.  I won't go into the details of how this is calculated, except to say that higher values are better and that a perfect model would produce an equitable threat score of 1.  Below are equitable threat scores for all three models (color and forecast day same as above figures) at ASOS (left) and SNOTEL (right) stations.  Whiskers indicate 95% confidence intervals if you are into that sort of thing.  At Day 1 (dashed lines) the difference between GFSv14 and GFSv15.0 is nearly indistinguishable in most event thresholds.  At Day 3, the GFSv15.0 is producing improvement for most event sizes.  The HRES produces more skillful forecasts in general, although the GFSv15.0 closes the gap by day 3, especially for larger events at SNOTEL stations where the HRES has an underprediction issue.  

Equitable threat scores at ASOS (left) and SNOTEL (right) stations.  From Caron and Steenburgh (2019, submitted).
One can, however, account for bias, which is done below.  Without going into details, when this is done, the HRES shows superior performance relative to the two GFS versions, and the GFSv15.0 shows improvement over GFSv14 on Day 3 (Day 1 improvement is much smaller).  

Equitable threat scores based on event percentile ranking at ASOS (left) and SNOTEL (right) stations.  From Caron and Steenburgh (2019, submitted).
Overall, the results above indicate that the new GFS should perform about as well or slightly better than the old over the western United States, with relatively similar biases.  It does not appear to perform as well as the previous operational version of the HRES.  This is especially true if one performs some bias correction.  Nevertheless, the differences are not huge, so the new GFS warrants consideration in the forecast process and is going to outperform the HRES in some events.  Knowledge of the general lowland and mountain biases described here should be helpful, but be aware that those biases do and will vary on a station by station basis, which could be important for point specific forecasts.

One quick caveat.  The post above is based on work that is currently in review. 

Wednesday, September 11, 2019

September Lake Effect and Mountain Snows

University of Utah students may be wondering if they should run over to the Merrill Engineering Building and learn how to build an ark as it's been a wet one with some intermittent heavy downpours this morning.

Radar imagery shows a number of srong cells developing over or near the Great Salt Lake and moving downstream.  Northern Salt Lake County and Davis County have seen the most intense cells.

Seven-day mean lake temperatures show that the lake was quite warm prior to the passage of last night's trough with a mean temperature of 23.3˚C.

That's no surprise given the remarkable warmth observed earlier this month.  Why the convection isn't organizing a bit more downstream of the lake is a bit less clear, but it could reflect the general instability and moisture of the airmass and the relatively weak flow.  Given that we really don't understand what controls the mode of lake effect, I won't speculate further.

Although the heaviest stuff is falling in the mountains near campus, the central Wasatch are getting something and it is snowing down to about 9500ish feet, as suggested by Alta's camera in Albion Basin.

A little higher up, things are looking wintery.

The flow direction is such that I don't think the lake is contributing to the snowfall in upper Little Cottonwood, but my take is that it is affecting the intensity of the convection in northern Salt Lake and Davis Counties.  In any event, there is white stuff up high. 

Tuesday, September 10, 2019

Don't Obsess over the Outlook

Seasonal outlooks have little useful skill for skiing.  By popular demand, here's mine for the 2019/2020 season.  Basically the same as last years, with a couple of minor changes.  

I am completely comfortable telling people that we have no idea how the season is going to turn out.  Oh sure, you can find some people selling you what you want to hear, and they might get it right.  I could tell you, for example, that there's a 2/3 chance that this winter will give us near-average or above average snowfall at Alta.  Alternatively I could say there's a 2/3 chance that this winter will give us near-average or below-average snowfall at Alta. 

Pick whichever one makes you feel happy.  Don't obsess over the outlook. 

Monday, September 9, 2019

Signs of Fall (and Winter)

The times they are a changin', with the transition from summer to winter now clearly underway.

We're finally starting to get a tease from the mid-latitude storm track here in Utah.  The loop below shows yesterday's trough passage, as well as a deeper trough expected to affect our weather this week.

Yesterday's trough, and associated showers and thunderstorms, yielded a high for the day of only 79˚F.  We haven't had a day with a high that low since June 22nd, so it was well deserved.

The trough anticipated this week will be eventful in a few ways.  First, in advance of the trough, we will see increasing temperatures and southerly flow on Tuesday, which will likely complicate efforts to contain wildfires in central Utah.

The trough itself and associated surface cold front and precipitation band is currently forecast to push into northern Utah Tuesday night. Although it is coming in at night, with some monsoon moisture and instability to play with, it could bring in some thunderstorms in addition to rain.

Behind the trough, 700-hPa (about 10,000 ft above sea level) temperatures are currently forecast to drop to as low as 0–1˚C on Wednesday, sufficiently cold for snow at upper elevations if precipitation occurs.

Critical for the snowfall forecast in this case is how much precipitation we get once temperatures have dropped to below about 1˚C.  Currently, our downscaled SREF guidance has 5 out of 26 members generating > 1 inch of snow, with a maximum of 5 inches, at Alta Collins near 9500 feet.

Other members produce nothing, due either to slightly higher temperatures or a lack of precipitation.  Overall, if one looks like 15 to 25% of SREF members produce snow in the upper elevations of the Wasatch and the Uintas.

Thus, do not be surprised to see some white on the higher peaks by late Wednesday, although accumulations are likely to be limited.  Nevertheless, keep an eye on forecasts if you are planning high elevation adventures such as a backpack trip this week.

Moving across the pond, winter made an appearance in the Alps over the weekend as a short-wave trough dug from the North Atlantic into central Europe, bringing with it cold air to lower snow levels to mid elevations.

Some of the more impressive scenes were in northeast Italy.

Stubai Gletscher opens for the season this coming Friday, 13 September.  My pass is still good!

Friday, September 6, 2019

The Long and Impactful Life of Dorian

By any measure, Hurricane Dorian has already had a long and impactful life, beginning as a tropical wave of western Africa, developing into a tropical depression at 1500 UTC 24 August (the easternmost triangle below), and then strengthening into a tropical storm and hurricane as it continued its march toward the Bahamas.

Source: FleurDeOdile via Wikipedia Commons
Dorian reached category 5 status in the Bahamas, where it stalled over Grand Bahama and caused incredible damage.  It then curved along the southeast coast of the United States, remaining just offshore of Florida, Georgia, South Carolina and most of North Carolina.  At 0900 EDT, the National Hurricane Center reported that Dorian made landfall at 0835 AM over Cape Hatteras.  As illustrated by the radar image below, Cape Hatteras (HAT) was within the eye.  

Although this will be the only landfall in the U.S., Dorian has a long life ahead and is likely to make at least one more landfall.  In fact, Dorian is going to have some major impacts on the weather not only of eastern North America, but also Europe.  

The latest GFS forecast shows Dorian moving off the coast of the Carolinas, moving northeastward, and then curving through eastern Canada.  In this forecast, the low center makes landfall in Nova Scotia, although there is some uncertainty in the precise track.  Further, note that Dorian strengthens as it moves northward.  

In fact, it is likely that Dorian will be a strong system as it moves through eastern Canada.  How can this be?  Shouldn't a hurricane weaken over the cooler waters?

Perhaps, but hurricanes sometimes experience extratropical transition as they move into the extratropics.  Essentially, the energy source for the cyclone transitions to that typically associated with extratropical frontal cyclones.  In this case, Dorian is benefiting from interaction with a very favorable midlatitude pattern with a broad trough over eastern North America and two embedded short-wave troughs (brown dashed lines) that will contribute to its extratropical transition.  

In the loop below, you can see how each of the short-wave troughs overtakes Dorian and contributes to the upper-level (and synonymous lower-level) development.  

The results of all of these tropical and extratropical shenanigans is that Dorian is likely to a be a beast as it moves through eastern Canada.  The low-level (900 mb) wind and temperature pattern below shows the classic frontal structure of an intense extratropical marine cyclone with a warm core seclusion wrapped by an intense temperature gradient known as a bent-back front rearward of the low center.  

Thus, although Dorian will be weaker than it was in the Bahamas, it is expected to produce tropical storm force and possibly hurricane force winds in portions of eastern Canada.  In fact, Environment Canada has issued rainfall and wind warnings and a hurricane watch for Nova Scotia.  Below are the latest for Halifax.

But it doesn't end there.  As Dorian moves further northward and eastward, it builds a ginormous upper-level ridge over the North Atlantic (see top panel below).  

This ridge progresses eastward and eventually results in a closed low and cold-air surge into the Mediterranean Basin.  

So, the impacts of Dorian are widespread.  Further, although it is weaker, you can see Dorian moving across the North Atlantic and just below Iceland in the bottom image above.  It's fate beyond that is less certain, but perhaps it will find a way to survive into the Arctic?  Sadly, I've run out of time today, but maybe we'll take a peek in a future post.