Category Archives: Forecasting

Six Questions to Answer When Forecasting

Exciting weather is happening and on the way, so it seems fitting to organize this post around the six questions to answer when forecasting

1. What has happened?

This "winter" has thus far been like 3.5 consecutive Novembers rather than a typical November, December, January, and February sequence.  The average temperature at the Salt Lake City International Airport for 1 December - 17 February was 37.8ºF.  That's just a shade lower than the November mean of about 40ºF.  As everyone knows, we're well below average for snowfall and snowpack, especially in the lowlands.  The photo below was taken this morning looking up City Creek Canyon and indeed it looks more like a scene you might see after a November snowstorm than one would expect in mid February. 

 
This context is important as I suspect most people are entirely unprepared for what is coming.  It will probably seem like the first storm of the year.

2. Why has it happened?

This is a good question and one that I can't answer satisfactorily.  The easy answer is that the warmth and snow drought reflects persistent high pressure and a storm track that has remained predominantly north of northern Utah.  Why that has been the case remains a subject of debate.

3. What is happening?

Wow, what a windy night.  Strong south winds at all elevations.  In the past six hours (ending at 9:20 AM MST), several sites in the northern Wasatch and Bear River Range have gusted over 70 mph and ridgelines in the central Wasatch have seen gusts as high as 71 mph.  I can find many sites in the mid elevations reporting gusts over 50 mph.  Sherwood Hills (5658 ft) near Sardine Canyon guested to 64 mph.   If there was much powder left over from yesterday's feast, I suspect it's been blown to Jackson Hole.  Peak gust at the Salt Lake Airport so far is 43 mph.

With these strong winds, we are seeing some dust.  Concentrations were especially high in the western Salt Lake Valley, which I suspect is due to emissions from the area west of Utah Lake as we have seen in recent events



4. Why is it happening?

The answer here is an approaching frontal trough that at 1500 UTC (0800 MST) was sagging southward into northern Utah.  This has created a strong pressure gradient to its south, with strong gusts at all elevations. 


5. What will happen?
6. Why will it happen?

Loaded questions!  I'll answer them together as it is easier.  As I write this, the surface front just passed Hill Air Force Base.  The HRRR shows it progressing slowly southward today, with frontal passage in the northern Salt Lake Valley around 2000 UTC (1300 MST).  Thus, expect to see a wind shift early this afternoon, if not sooner in the valley. 

Periods of snow are likely in the northern Wastach today and will develop in the central Wasatch later this afternoon.  It's a bit of an oddball situation as the latest NAM shows the frontal band over far northern Utah through 2100 UTC, but then rather than bringing it through continuously, redevelops it to the south tonight. 



Tomorrow brings the post-frontal crapshoot beneath the upper-level trough where much depends on flow direction, moisture, and instability.  The NAM forecast below isn't too bullish on snow, but the 6 Z GFS is more enthusiastic and keeps us in wrap-around moisture (not shown). 


It's worth a look at the 12-km NAM-derived forecast for Alta.  Measureable precipitation begins around 5 PM and is strongest from about 6 PM to 11 PM with the frontal forcing.  Periods of snow continue through 6 PM tomorrow in the unstable post-frontal period.  Total water equivalent is 0.6" with 10" of snow by 9 AM tomorrow and 12" by 7 PM tomorrow. 


Take a peak also at the temperatures for Mt. Baldy.  Keep in mind these are for 11,000 feet, a bit above the lift-served terrain, but this provides some idea of how cold the airmass will be.  It goes sub zero by noon tomorrow and down to -8ºF by 9 AM Tuesday.  I don't think we've seen air this cold yet this year.

To summarize, this looks to be an all elevation storm and you should be prepared for winter conditions.  We haven't had a stress test like this in some time. Thankfully, it is the President's Day Holiday, which will hopefully help with tomorrow's commute.  For Alta-Collins, I lean toward 7-14" by 9 AM tomorrow.  I have a bit more heartburn for totals after that given the variability I'm seeing in the models.  Some snow is likely, but the range of possibilities is large.  Hug a ski patroller or avalanche forecaster when you see them.  They will have a tough job the next few days. 

Finally, this is a high-impact, rapidly evolving situation.  Keep an eye on official forecasts at http://www.weather.gov/slc/. 

Ah, the Blessed Steenburgh Effect

Sidelined with a broken bone in my hand this week, you reap the benefits.

Alta Collins has recorded 0.49" of water and 8" of snow through 7 am this morning (the 18" snow interval depth is spurious in the data below, so it's unclear if we may have ticked up or down from the 8" at 6 am).

While not a big storm it's pushing toward my arbitrary "deep powder" threshold of ten inches.  It's also pushing the upper end of predictions.

The radar imagery shows we'll add more to that total, especially in the next hour or two.


Nice to see it snowing in the lowlands as well.  Campus was covered in a thin blanket of white at sunrise this morning.


Make a few turns for me.

The Dribs and Drabs Will Continue Until Morale Improves


In times like these, one learns to appreciate the smaller dumps in life.  My usual definition of a deep-powder day is a 24-hour snowfall of at least 10 inches, but we've only had one of those at Alta since December 3rd!

On the other hand, recent dribs and drabs have certainly helped the skiing some, even as we continue to lose ground to climatology for snowfall amount and snowpack water equivalent.  The 6" of quick snow Saturday afternoon and 7.5" yesterday did create some smiles.  Maybe 6" is the new deep powder day.

You'll be hearing some talk of a pattern shift probably in the coming days, and indeed there are some changes afoot.  The GFS forecast valid 5 AM MST next Tuesday, for example, has a trough over the northwest U.S. and a ridge over the east, something we haven't seen a lot of this winter.



Similarly, the ECMWF model has a trough in the west (with some differing details) as do most (but not all) GEFS ensemble members.

Penn State E-wall
However, the overall pattern is one that remains high amplitude.  Note, for example, the strong ridging over the eastern Pacific and the north Atlantic in the GFS forecast above.  Given the characteristics of this flow pattern over the eastern Pacific and western North America, I'm still not enthused about this pattern opening up the spigot from now through the President's weekend.

Instead, dribs and drabs are likely.  As shown in the NAEFS plume below for Alta, the next round of dribs and drabs looks to be late Wednesday through Thursday AM.  After that, there's a break and then a great range in the timing of possible dribs and drabs Saturday night through Monday.  As usual, there's a couple of more excited ensemble members, so my usual line of keep expectations low and hope for the best applies. 

There is one non-scientific reason for you to be optimistic.  I took a surprisingly hard fall skate skiing on Saturday and learned yesterday that I fractured a bone in my hand.  They tell me I can continue to ski with a splint, but this is likely to slow me down a bit more than usual.  Thus, there may be a partial Steenburgh Effect that increases the likelihood of a deep powder day, although perhaps not as much as when I'm out of town.  This effect, if it exists, will only last 6 weeks, so be ready.  

Probabilistic Snowfall Forecasting

For decades, snowfall forecasts have typically involved the issuance of a range of accumulation amounts, typically (but not always) based on a factor of two.  For example, 3-6 inches, 4-8 inches, etc.

I have no idea why.  Perhaps it is a convenience thing.  Maybe people like it that way.  I don't know what that range even means.  Does it represent the middle 50% of possible outcomes, with a 25% chance of more and a 25% chance of less?  Does it represent the middle 80%?  Why always use a factor of two?  Sometimes the range needs to be bigger, especially in longer range forecasts. 

And then there is my favorite, "higher amounts in favored locations."  What the hell does that really mean and how do you verify it? 

There was a time when snowfall forecasting was truly guesswork, but things are changing.  Computer models are now capable or will soon be capable of simulating smaller storm details, what meteorologists call "cloud scale."  Ensembles can be used to better estimate the future outcomes.  There remains much work to do, but there is great potential to dramatically improve snowfall forecasting. 

The National Weather Service is now producing experimental probabilistic snowfall forecasts, and they are available at https://www.weather.gov/slc/winter.  They provide much richer information about storm potential than a simple range.  For example, ,aps are provided showing the likelihood of snowfall above several thresholds, an example of which is the probability of 6" of snow or more for the period from 5 AM today to 5 PM Sunday, shown below. 

Source: NWS
They also provide a table with snow amount potentials and probabilities of snow within certain ranges, shown below, as well as above certain thresholds. 

Source: NWS
Readers of this blog are snow lovers.  Start perusing these forecasts and provide feedback through the links on the page. 

Now, to clarify some of my scattered comments to yesterday's post about the situation on Saturday.  Although we have a front pushing through tonight, it is a very slow mover.  As a result, this is not a frontal passage in which we quickly get into deep, unstable, northwesterly flow on Saturday morning. 

This is evident in the NAM time-height section for Alta below.  The front at Alta is a late arriver (light blue line), in this case moving through at or just after 0600 UTC (11 PM MST tonight).  Then, look at the winds behind the front on Saturday (circled).  They are NNW at low levels, but NNE near 700 mb (10,000 ft) and then SSW at 600 mb. 


This reflects the slow movement of the front through the area. 

If we look at the sounding for 1800 UTC (11 AM MST) Saturday morning, we see the low level northerly flow, but note how the winds shift to NNE and then SSW with height.  The temperature and dewpoint traces show a sharp inversion just above 700-mb, or 10,000 feet.  
This is not a recipe for our classic northwesterly instability snow showers over Alta for two reasons.  First, the flow direction isn't right.  Second, the instability is too shallow.  

However, if you look at the sounding for 0000 UTC (5 PM MST) tomorrow afternoon, the low level flow is NNW through a deeper layer, although a capping inversion remains based just below 600 mb.  This is closer to what is needed for the NW instability showers, but the capping inversion height is right on the edge of what I would like to see.  Tough to say if it's high enough that Alta can benefit, or just a bit too low so that the mid and lower canyons and east bench do better.  
And that's just one model run.  There are variations in the timing of these changes, wind directions with height, etc., if one looks at other models.  

All of this illustrates what a complex mess this is for Saturday and why probabilistic forecasting is necessary.  The good news is there's enough going on that in the end, this will be a decent storm for the mountains and even the mountain valleys after snow levels lower today and this evening.  

Frontal Snowfall Event On Tap for Late Tomorrow and Tomorrow Night

After another 10-day or so stretch with limited to no accumulations, our next storm will be served up late tomorrow and tomorrow night. 

Most of the precipitation for the central Wasatch looks to be primarily frontally forced.  The large scale setup is shown below and features an upper level trough that is initially tilted from southwest to northeast (referred to as "positively" tilted by meteorologists) that closes off and becomes more north-south oriented as it moves inland across the western US.  




This has both pluses and minuses for snowfall prospects in the central Wasatch.  The plus is that the front may slow as it drags through northern Utah, extending the period of frontal snowfall, as depicted below in the 1200 UTC NAM forecast.  At 000 UTC 20 January (5 PM MST Friday), the surface front is over Utah County with precipitation over the northern Wasatch.  


Frontal precipitation fills in, however, as the front phases with moisture sneaking around the southern end of the Sierra Nevada over the next 3 hours. 


That precipitation continues for another 3 hours as the front makes slow progress into southern and eastern Utah. 


By 0900 UTC 20 March (2 AM MST Saturday) the main frontal band is just downstream of the central Wasatch, with some post-frontal snow showers persisting. 


The minus for snowfall prospects is with the low closing off, the post-frontal winds shift very quickly to northerly, when we would prefer a period of northwesterly flow for better orographic forcing.  Note in the Salt Lake City time height section below that the post frontal flow is predominantly northerly and deepens gradually from about 0Z Saturday through 6Z Sunday.  



Actual numbers derived from the 12Z NAM show the wet bulb zero dropping during the day Friday (snow level is usually about 1000 ft below this level), with values low enough that most of the precipitation produced during this event should fall as snow in the mountain valleys.  Perhaps Mountain Dell might see a bit of rain to start, but then turn over to snow.  Total water equivalent at Alta is 0.64" through Saturday at 8 AM, with snow densities decreasing during the storm for a right-side up snowfall.  



Looking more broadly at the ensembles shows that the NAM is roughly in the upper half of the SREF plume for Alta.  Through 18Z 20 January (11 AM) the SREF members put out anywhere from 0.3 to 0.8 inches of water, the former being a slightly better than dust on crust event adding up to perhaps 4 inches of snow, the latter representing a lower end deep powder day with perhaps 10-12 inches of snow.  

That spread represents variations in the strength and speed of the front.  Increases in precipitation after 18Z 20 January occur in some model runs that are more bullish on the post-frontal precipitation.  

I continue to keep expectations low and hope for the best.  

Wahoo, a Storm!

We have a bit of moisture coming into Utah for the first part of the work week, which I suspect everyone will be glad to see.

The situation this morning features a complex trough off the Pacific coast with a broad region of moist air with high precipitable water (color contours) affecting the entire California coast.

Observed radar, IR satellite imagery and NAM precipitable water and 700-mb wind barbs valid 1500 UTC 8 Jan
By 0600 UTC 9 January (11 PM MST Tonight), the coplex trough is broken into two pieces, one moving over the Pacific Northwest coast, the other approaching California.  Northern Utah is under the influence of bands of moisture moving into the western interior in the southwest flow.  

NAM 500-mb heights, 700-mb wind, satellite imagery, and 3-h accumulated precipitation valid 0600 UTC 9 Jan
That situation persists overnight and for much of the day tomorrow as the two troughs move inland and the 700-mb flow over northern Utah gradually veers (turns clockwise).

NAM 500-mb heights, 700-mb wind, satellite imagery, and 3-h accumulated precipitation valid 2100 UTC 9 Jan
Then, late Tuesday night and early Wednesday morning, the northern trough and accompanying cold front push into northern Utah and the flow switches to northwesterly.  

NAM 500-mb heights, 700-mb wind, satellite imagery, and 3-h accumulated precipitation valid 1500 UTC 10 Jan
So, this will essentially be a three-part storm.  The first part will feature the southwesterly flow and precipitation in waves for the mountains.  The second part is the frontal passage late Tuesday night or early Wednesday.  The third part is the post-frontal period thereafter.  These stages are evident in the NAM time-height section below (remember time increases to the left in this plot — more on how to interpret these diagrams here).  


The blue line in the plot above is the freezing level, which looks to be fairly high during the southwest flow portion of the event.  The freezing level should not be confused with the snow level as those are two different things.  The freezing level is the latitude at which the temperature is 0ºC, but the snow level is below this level.  How far depends on how temperatures change with height in the atmosphere, the relative humidity, and other factors.  Complicating the snow level forecast for the southwesterly flow period is the cold, dry air currently in place in the valleys.  That could help keep the snow level low initially, but it will eventually warm enough that we will be looking at snow levels between 6500 and 8000 feet until the front approaches Tuesday night.  Snow levels will drop with the frontal passage.  

The 0600 UTC NAM generates 1.1 inches of water equivalent at Alta Collins from 7PM tonight through 9 AM Wednesday morning.  Nearly all of that precipitation falls in the southwesterly flow and front periods, with little post-frontal.  Downscaled SREF plumes for Alta Collins show anywhere from about 0.75" to nearly 3" of water.  There is strong clustering by the model used, with the NMB members producing less precipitation in general than the ARW members, which is typical.  A few members to for some significant post-frontal precipitation.  

Looking at things spatially, the downscaled SREF shows high (>90% probability) of 1" or more of water at most high elevations in the Wasatch Range.  Higher elevations regions that typically do well in southwesterly flow, including Ben Lomond, Snowbasin, and Timpanogos Peak have > 80% chance of 2" of water.   Most of this precipitation falls through Wednesday morning, although there are a few members that get higher precipitation totals due to precipitation Wednesday or Wednesday night.  Thus, if you want odds through Wednesday morning, cut those numbers a bit.  

By and large, this looks like a decent wet storm for the high elevations, with high density snow at times tonight through Tuesday night, and snow levels (and snow densities) dropping late Tuesday night or early Wednesday morning with the frontal passage.  Mid elevations will depend on snow levels.  The base of Snowbasin and Park City for example, could see rain at times during the southwesterly flow period.  

I was going to say "tip of the week is to call in sick Wednesday morning" as the right-side up nature of the snowfall should make for great turns in the upper elevations, but the red light will be on for backcountry skiing on or below steep terrain.  At the resorts, keep your friendly ski patrollers happy as they will have their work cut out for them Wednesday morning.  

A Remarkable Forecast of a Beast of a Storm

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


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


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




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

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

Much Needed Tailwind for Santa This Year

With a full-latitude, high-amplitude ridge parked over and just north of Alaska, Santa will have a much needed tailwind after taking off from the North Pole for Utah.  Just check out the strong northerly jet stream (green contours) that will rocket him southward from the North Pole and across much of western Canada tonight! 


That tailwind is a godsend because the reindeer will need to be fresh when they arrive in northern Utah.  With clouds and precipitation along much of the Wasatch Front and in the Wasatch Mountains, they will have a difficult job, even with Rudolph leading the team.  Mountain snow will reduce visibility and, with temperatures climbing, Santa may encounter a rain/snow mix or even a bit of rain in the lower elevations, depending on the timing of his deliveries.


Rumor has it that Santa is considering hanging around and skiing freshies tomorrow at Alta.  Apparently he was spotted yesterday. 


It's rare for Santa to leave the North Pole at this critical time of year, but given the rarity of powder this year, he told the elves to buckle down and cover for him for a couple of hours as he needed a fix to lift the spirit before Christmas.

Happy Holidays from the Wasatch Weather Weenies!

Water Equivalent Is More Important Than Snowfall Amount

Most ski reports and skiers tend to focus on snowfall amount as the measure of storm size, but for many applications, it is the water equivalent of the snowfall (i.e., how much water it contains) that is the most important measure.  After all, the water equivalent determines how much weight (and stress) you are adding to the snowpack, which is important for avalanches, and how much water you are adding to the snowpack for water resource purposes.  

Really, Utah skiers should also be focusing on water equivalent right now instead of snowfall amount.  We need base, and base is a function of how much water is in the snowpack.  Six inches of 15% water content snow goes a lot farther as six inches of 5% for building up base since it contains three times as much water mass.  

As I write this post at 7:15 AM, I'm encouraged by the current storm so far.  Storm-total water equivalents in the Wasatch so far include 0.9" at Ben Lomond Peak (through 6 am), 0.8" at Ben Lomond Trail (6 am), 0.74" at Snowbasin-Boardwalk (7 am), 0.6" at Lookout Peak (6 am), and 0.38" at Alta-Collins.  Those aren't big numbers, but they are welcome and they will help a great deal.  Plus, as can be inferred from the radar image below, snow showers look to continue.  

Source: NCAR/RAL
Indeed, the latest HRRR keeps us in the moisture plume for most of the morning, as evident in the forecast valid 1800 UTC (11 AM).  


Things should taper off in the afternoon, but let's hope we squeeze as much out of this as possible.  It will not get us near an average snowpack, but it's going to help with the ski conditions quite a bit at the resorts.  It should allow North Fork Park to get open for cross country skiing.  I think we're in some sort of ski purgatory right now for backcountry skiing, with a thin, weak snowpack in some areas and not really enough to ski elsewhere, but this gets us closer.  

The Next Storm Could Surprise

Based solely on the large-scale pattern setting up for tonight and Saturday, one might not be very enthused about our snow chances.  There's a huge, high-amplitude ridge centered over Alaska and a ridge upstream of Utah with a short-wave trough moving across Montana.  Show this and nothing else to 10 meteorologists and ask them what will happen in Utah and they probably would say not much.


However, the Devil is in the details, and those details suggest this storm could surprise.  The pattern is highly unusual because of the size of the ridge over Alaska, and we end up in a situation in which two airmasses of dramatically different origin are coming together, one from the subtropics, the other from the high latitudes.  This leads to a strong mid-level front and plume of moisture that extends inland across Utah.


By itself, that probably wouldn't do much, but we have something going for us, the Wasatch Range.  The GFS forecast above shows that the terrain effects are very important, with strong modulation of precipitation as the moisture plume runs over the various topographic features of Oregon, Idaho, Utah, and Colorado.

So, what do the models spit out?  I'll focus on the period through 0600 UTC 24 December (11 PM MST Saturday), although the bulk of the precipitation is expected to fall tonight and during the first half of the day tomorrow (the end of the precip is a bit ill defined with some models calling for it to last longer than others).

For Alta, the 0Z Euro puts out about 0.4 inches of water, the 6Z GFS is just over an inch, and the 6Z NAM 0.8".  The downscaled SREF plumes below show a pretty wide spread, enough to give serious heartburn to any meteorologist, with a range from 0.1" up to 1.75" by 0600 UTC 24 December.


Before it retires in another week or so, we can also have a look at the NCAR ensemble, with a range from about 0.5 to 1.4 inches through 0000 UTC 24 December (5 PM Saturday).


Ultimately, much will depend on the location of the plume and the evolution of the flow.  The Euro and low-end of the ensembles say keep your expectations tempered, but even 0.4" of water would make this the 2nd biggest event since the 3 December storm that put down a season-saving 1.08" of water at Alta.  It's going to help.  Some of the ensemble members are putting out amounts much higher than anything we've seen in a while.  Those are not high probability possibilities, but they could come up with a lucky role of the dice.  Thus, let's see what happens and hope this storm surprises.

Skinny skiers should be happy as a little will go a long ways at Mountain Dell and North Fork Park and the precipitation should be all snow at both locations.