Category Archives: Winter Storms

Passage of a Bowling Ball

Interesting forecast situation through Friday night as a closed upper-level low, which rumbles like a bowling ball across the American southwest, moves along the Utah–Arizona border.


If you look at the 3-h NAM precipitation forecasts above (color fill) you can see that northern Utah misses out on the action initially, but does get some after the low enters Colorado thanks to so-called wrap-around precipitation.  In the NAM forecast above, that precipitation primarily affects northeast Utah, especially the Uinta Mountains.  The Wasatch are at the edge of the action and in fact Alta gets no precipitation from the forecast above (but it is right on the edge of it). 

The strong contrast in precipitation probability from west to east across northern Utah is better illustrated by the probability of more than 0.10 inches of precipitation during the 12-hour period ending at 0600 UTC 21 April (0000 MDT Saturday) based on forecasts by the Short Range Ensemble Forecast System (SREF).  Note the sharp gradient from about Antelope Island to the western Uintas.


Indeed, if we downscale those SREF runs to account for local terrain effects, we see large spread at Alta, with members producing anywhere from 0 to 0.7 inches of water.  This would fall mostly as wet snow above 8000 feet, with snow levels possibly flirting with 8500-9000 feet Friday night should precipitation linger.
My take is that thisI is that we may see a few snow showers Friday and Friday night, but accumulations will likely be 1-4 inches.  A skunking is perhaps more likely than a surprise dump.  The latter would require the wrap around to extend further westward than indicated by most of the models I'm looking at this morning. 

After Friday night, the forecast for the weekend looks warm and pleasant.  If we do get a decent dump Friday or Friday night, it will turn to mank quickly on Saturday if it isn't already mank at sunrise. 

My Cup Runneth Over

It has been a long cool season characterized by frequent doldrums, but today my cup runneth over.

Let's talk first about what happened overnight and some of the complexities of cyclone and frontal evolution over the Intermountain West.

With an approaching trough from the west, one often sees a trough develop over the Intermountain West, typically along an axis running from the high Sierra northeastward across northern Utah.  That was the case last night, which featured a very well developed Intermountain Cyclone, as well as a Pacific cold front that was distinct from this feature and pushed across northern Nevada. 

As can be seen in the HRRR-derived analysis below, the Intermountain cyclone progressed northeastward across northern Utah overnight, with the cold front penetrating into the state and eventually overtaking the concomitant trough. 




This resulted in some strange time series in which the wind shifted to northwesterly before the arrival of the cold front.  For example, at Badger Island along the west shore of the Great Salt Lake, the flow shifted from ENE to NNW around 0900 UTC as the low center tracked to just to it's south. However, this was not the surface cold front, which arrived shortly after 1000 UTC, which snapped the wind sharply to NW and was followed by an increase in wind speeds that eventually reached over 35 mph with gusts to near 50. 


This highlights the importance of using the "forecast funnel" and the need to understand what is happening on the larger scales before trying to understand what is happening locally.  

Another issue this morning is that the precipitation dynamics are not favoring anything that looks like the classical cold-frontal precipitation structures that you see in elementary textbooks and most of which are derived from the Norwegian Cyclone Model, which is nearly 100 years old (cold front at left). 

Source: Bjerknes and Solberg (1922)
The Norwegian Cyclone Model was a great advance, but there can be large deviations from that model as far as precipitation is concerned.  Note today how disjointed the precipitation is from the cold front (1300 UTC/0700 MDT analysis and radar below).  This is because the cold-front is shallow at it's leading edge and the pre-frontal airmass is dry, so that the strong ascent at the leading nose is unable to generate precipitation.  Instead, precipitation lags the front and is a result of several factors, including the presence of deeper pacific moisture and forcing associated with the upper-level trough.  


The radar loop below shows how things have filled in nicely along the northern and central Wasatch Front through 1405 UTC (0805 MDT).  At that time, a sharp contrast in weather existed from northern to southwest Salt Lake County, with the latter still dry due to the position of the precipitation and some shadowing by the Oquirrhs. 


Similarly, at this time, it was not only dry in Park City, but you could see shadows from the sun!

Source: UDOT
The models are advertising the frontal precipitation to move through today, with the event becoming a more "orographic" one later today, with precipitation generated by forced ascent as the northwesterly flow encounters the Wasatch Range.  Temperatures will also be dropping, so rain will be turning to snow on campus this morning (it's mixing in as I write this at 0830 MDT).


The latest weather summary from the NWS is provided below.  Get updated forecasts at https://www.weather.gov/slc/.

Source: NWS
For the mountains, this is still looking like a pretty good spring storm.  The time-height section for Salt Lake City shows the frontal passage this morning, and then periods of moist, unstable, NW flow through tomorrow morning.  

The latest NWS forecast for the upper Cottonwoods is below and tells the tale fairly well.  

Source: NWS
The post-frontal stuff later today and tonight is always a bit of a crapshoot.  Right now, it looks pretty good and I'd say the odds of deep powder tomorrow are high.  If you are skiing later today be aware that there is a chance of thunderstorms and keep and eye to the sky.  Tomorrow, the quality of skiing may be somewhat dependent on winds as the models are advertising fairly strong 700-mb flow through the period.  

Late Season Pow?

We are now moving into mid April where any possible powder day is a blessing from the heavens.  We have potential starting tomorrow.

The latest NAM shows a classic Utah spring storm scenario with an upper-level wave digging into the western U.S. and closing off over Colorado.  The latest NAM forecast for 0900 UTC (0200 MDT) tonight whows the upper-level trough extending from the BC coast to California (dashed lines, 500-mb height contours black) with a classic Intermountain cyclone centered over northern Utah and the trough extending southwestward to the high Sierra. 


At this time, not much is happening precipitation wise except in far northwest Utah.  Some strong southerly winds are possible, however, during this period, and the NWS has issued a high-wind warning for portions of western Utah.  Info below. 

As the upper-level trough pushes inland and the surface cyclone moves northeastward, we get into northwesterly flow and precipitation is expected to develop along the Wasatch Front and Mountains tomorrow morning.


Right now, it appears the lowlands will make it through the morning commute unscathed, but snow-levels will be dropping and the potential for bench and possibly valley snow exists during the day tomorrow.  The forecast below shows 700-mb temperatures (contours), winds (barbs) and 3-h accumulated precipitation for 2100 UTC (1500 MDT). 


The latest NWS forecast calls for snow down to the valley floors by mid-day Thursday. 


Given that this is a late season storm, the ground is quite warm, and the bulk of the storm for the valleys is expected during the day, accumulations will be largest on cold surfaces on the benches and more limited (or non-existent) over pavement.  Much will depend on elevation and precipitation intensity.  The snippet below fro the 8:47 AM NWS forecast discussion illustrates that this is an evolving forecast situation, with updates expected this afternoon.  Monitor official forecasts. 


For the mountains, if precipitation were to start early tomorrow morning, there could be a brief period of rain up to 7000 feet, but otherwise this looks like an all-snow storm.  The latest numbers from the NAM add up to 0.88" of water and 15" of snow by 0900 MDT Friday morning.  The later stages of the storm look quite cold, with temperatures at 11,000 feet falling into the single digits.


As usual, much will depend on the post-frontal crap shoot.  The time-height section below shows the winds shifting to northwesterly early tomorrow morning and a prolong period with the crest-level (700 mb) flow WNW to NW through about midnight Friday.  There is considerable moisture and mid-level instability during this period.  This is an environment in which significant snows can occur if the flow direction is right and moisture is available. 


If this was early February, I would expect that this would be a right-side up storm, but there could be some wildcards.  Tomorrow, with surface heating, we could see some strong convection and that's even possible overnight Friday.  This might produce periods of graupel, especially tomorrow afternoon and evening.  

I'm fairly optimistic about this storm and am going on personal powder alert tomorrow.  If snowfall rates are high, or we get a lot of graupel, a dusk patrol might be warranted tomorrow afternoon, with Friday morning having major April powder potential as well.  The former depends on timing and precipitation intensity tomorrow, the latter on how things go Thursday night.  Spring powder is all about synoptic possibilities.  Whether or not you get the goods or ski on a chundery mix depends on processes not resolved by today's models.  

A Saturday Soaker That Is Simply INCONCEIVABLE!!!

My meteorological excitement factor is relatively elevated today compared to where it has been much of the winter, mainly because of expectations of a strong frontal system pushing across the western US through tomorrow.

Right now we are getting just a bit of an appetizer.  The analysis below is for 1200 UTC (0600 AM) MDT and shows a weak 700-mb trough approaching Utah (brown dashed line).  With this system is a slug of moisture, valley rain showers, and upper-elevation snow showers.


After the remarkably low dewpoints earlier this week, the humidity just hit me in the face and felt so good when I walked out the door this morning.  The meteogram below shows how dewpoints (blue line) have climbed from the low single digits on Tuesday to 46 this morning.  Good for the skin!


It's warm, with temperatures of 34˚F at the base of Alta (8560 ft) and 36˚F at Snowbasin Middle Bowl (7402 ft).  The 8000 ft snow cam at Snowbasin shows about 1.5" of wet snow this morning.  I suspect it is raining at the base.  It's a garbage bag morning if ever there was one.  


Precipitation will taper off after 1800 UTC (Noon MDT).  Then, overnight, Mother Nature revs up the system for Saturday.  At 0600 UTC (Midnight MDT), a deep, 975 mb low is located off the Pacific Northwest coast.  It's a powerful, well developed system.  Ahead of the cold front, northern California, especially the mountains, get considerable rain.  At this time, the warmth of the storm is remarkable with 700-mb temperatures near +4C.  Perhaps only the upper elevations of Mt. Shasta will be seeing rain snow at this time.  Such a waste of water.   


The cold front then pushes inland and redevelops over Nevada.  At 1800 UTC (noon) Saturday, it's forecast to be located over eastern Nevada.  Note the disortion by the Sierra Nevada, as is common, and the lack of precipitation over southern Nevada and southern Utah.  The action is instead concentrated over far northern Utah where the remnant warm front can do its thing.  


The cold front pushes across northern Utah Saturday afternoon [forecast for 2100 UTC (3 PM MDT) below].  I expect for the valleys, the front will bring a pretty good deluge, with a possibility of thunderstorms and locally heavy rain.  It is, however, a fairly mobile feature and moves quickly across the state with precipitation winding down in the evening in Salt Lake.  


The forecast above is from the 0600 UTC initialized NAM.  Let's look more braodly at the possibilities based on our downscaled precipitation estimates from the SREF.  For the Salt Lake City airport, one sees this morning's precipitation, then the afternoon and overnight break, with a couple of members producing showers here or there.  Precipitation chances increase tomorrow morning and all members produce rain tomorrow afternoon.  Mean accumulations with the front are about a half an inch, which is a pretty good for a fast moving front in this part of the world.  The box-and-whisker plot below shows the period of strongest precipitation from about noon to evening (although timing could shift some between now and then).  
Shifting to the mountains, let's first talk about water totals.  In my view, the the pre-frontal period is perhaps the biggest wildcard.  Much will depend on the moisture plume, flow direction, and intensity of the forcing.  The northern Wasatch are likely to get more from that period than the southern.  Everyone should get a pounding with the front.  At Alta-Collins, the SREF mean puts out a bit over an inch of water.  The range, after deducting today's totals, ranges from about 0.5 to 1.7 inches.  The latter would be a big water total for about a 12-hour period.  I would not be surprised to see an hourly accumulation of more than 0.3 inches of water at Alta-Collins tomorrow afternoon.  

This is, however, going to be a warm storm.  Remarkably warm.  The NAM forecast for 2100 UTC (1500 MDT) tomorrow shows us sitting at +4˚C.  I'm not sure if I've ever seen heavy frontal rain in early April with such a high 700-mb temperature.  Someone should dig through the records as my memory is not as sharp as it used to be.


If we look at the numbers from the NAM at Alta, the wet-bulb-zero level tomorrow is over 10000 feet, reaching as high as about 12,000 feet from 11-3 PM.  


If that forecast verifies, we are going to see heavy rain perhaps all the way to 11,000 feet at some point tomorrow afternoon.  

I'd like to see some analysis of the past records to see if my gut reaction to these numbers is accurate, but this forecast makes me feel like Vizzini in the Princess Bride, "INCONCEIVABLE."


The GFS is a bit cooler, but I think we need to expect heavy rainfall tomorrow to elevations at least as high as 10,000 feet.  This is the definition of an outlier event.  Expect the unexpected.

Addendum @ 9:15 AM Friday

I feel the need to add a bit more to this post.  The forecast sounding from the NAM below for 2100 UTC (3 PM MDT) Saturday is simply unbelievable.  Fully saturated through the entire troposphere.  A precipitable water of 25 mm (about an inch).  That's monsoon-like moisture with a midlatitude front the likes of which appear unprecedented.  
Let me show you just how just out unprecedented such moisture would be, the yellow dot indicates where that sounding would sit in the sounding climatology for Salt Lake City.  We have never observed a precipitable water above 0.8" in April.  The forecast is for 1.0"!


It's a good thing this will be a short-lived event.  Even still, the potential for heavy valley rain is real and in the mountains I would expect the unexpected.  This is a Cascadian rain storm coming to Utah.  

Intricacies of Atmospheric Rivers

The phrase atmospheric river was first used by researchers in the early 1990s to describe narrow corridors of strong integrated water vapor transport that are frequently found in the mid and high latitudes.  Atmospheric rivers typically achieve high water vapor contents by extracting water vapor from the tropics and/or local moisture convergence.  They are often found along and ahead of cold fronts where flow convergence can concentrate moisture and the strong pre-frontal low-level jet can produce large values of integrated water vapor transport.

Atmospheric rivers are not new phenomenon.  They have played an important role in our weather and climate for ages.  What is new is the name, recognition of their role by atmospheric scientists, and the development of techniques to identify and predict atmospheric rivers.

Two variables are commonly used to identify atmospheric rivers.  The first is the integrated water vapor (IWV), which is the total mass of water vapor in the atmosphere, typically expressed as the depth the water vapor would take if it were all condensed out as rain.  One advantage of IWV is that it can be inferred using satellite and GPS receivers.  The other is the integrated water vapor transport (IVT), which is the total amount of water vapor moving over a location.  IVT is dependent on both the amount of moisture and the strength of the flow and calculating it requires profiles of both moisture and wind.  For this reason, it is most often calculated using three-dimensional gridded analyses or forecasts.

I look at IWV as a measure of moisture availability for storms and IVT as a measure of moisture delivery to storms.  I generally use IVT because it has a stronger correlation with precipitation over the mountain west than IWV, as can be seen in the plots below.  However, in some lowland areas of the western US and in the High Plains, IWV has a higher correlation.   These are areas where high moisture availability plays an important role in fueling convective storms (note, this is an instantaneous correlation – high IWV is often a result of prior moisture transport).
Source: Rutz et al. (2014)
IVT has a strong correlation with precipitation, but the correlation isn't perfect.  This is because precipitation is dependent on many factors, including a mechanism for lift.  The highest correlations between IVT and precipitation are found in the Pacific coastal ranges, Cascade Mountains, and Sierra Nevada (see left-hand panels above).  These mountains are relatively broad and are frequently experience strong cross-barrier moisture transport accompanying atmospheric rivers, with forced lifting by the mountains generating precipitation. 

The correlation is lower, however, over the western interior.  Within that region, the highest values are found in the Idaho Panhandle, central Idaho Mountains, southwest Utah, Mogollon Rim, Tetons, and San Juan Mountains.  These lower correlations reflect several factors, including the greater diversity of processes generating precipitation over the western interior and contrasts in precipitation efficiency between wide and narrow mountain barriers during some storms.

The mountains of northern and central Utah have lower correlations.  This does not mean that we can't get significant precipitation from atmospheric rivers.  Instead, it reflects the fact that we sometimes get significant precipitation from storms that don't feature large cross-barrier moisture fluxes.  An example are cold, post-cold-frontal storms that generate large amounts of snowfall when the water vapor content of the atmosphere is relatively low (in an absolute sense) and the flow weak.

Moisture transport is very important for storm dynamics, but one needs to be cautious about developing AR-myopia.  As shown above, the correlation between IVT and precipitation is not perfect, and is modest or even low in portions of the interior western U.S.  Consideration of the precipitation generation mechanisms, such as frontal or orographic lift, is also important.

As an example, below is the 0600 UTC initialized NAM IVT forecast valid 0000 UTC 7 April.  Most of the eastern Pacific off the California coast is experiencing atmospheric river conditions (IVT>250 kg/m/s), with the highest values along an axis running from about 35˚N, 130˚W to 40˚N, 125˚W.

Source: CW3E

The NAM forecast 3-h accumulated precipitation at this time is, however, nearly non-existent in that area of high IVT and is instead highest over northern California, especially in the coastal ranges, northern Sierra Nevada, and southern Cascades where there is both warm frontal and orographic forcing generating precipitation.  The AR provides the moisture.  The fronts and mountains provide the dynamics.


I hope to look at what will happen as this system moves inland in a future post.  Forecasts currently suggest something that looks like what you get when a November cyclone meets April.

Wild Weather Ahead for Western U.S.

The models are advertising wild weather ahead as a series of storms develop over the eastern Pacific, affect the Pacific coast later this week, and eventually push inland this weekend.  The 0600 3 April initialized GFS forecast valid 0900 UTC (0300 MDT) Saturday shows a 977 mb low off the Washington coast with a well developed inner core and the attendant occluded and cold fronts moving onto the Pacific Coast.  High integrated water vapor (color contours) exceeding 45 mm, exists ahead of the cold front, consistent with atmospheric river conditions.  What a beauty!


The setup for the wild weather the current pattern over the central North Pacific, which at 1200 UTC this morning features a deep upper-level trough with a strong jet and storm track just north of Hawaii. 


This results in relatively low-latitude storms extracting moisture from the tropics and concentrating it into an atmospheric river that extends from Hawaii to California, as depicted by the integrated water vapor contours from the GFS below. 


Integrated vapor transport diagnostics derived from the GFS by the Center for Western Weather and Water Extremes (CW3E) show the export of moisture from the tropics at 0000 UTC 3 April, the transport of moisture to the Pacific cost by 1200 UTC 5 April, the development of an intense atmospheric river with integrated vapor transport exceeding 1600 kg/m/s at 1200 UTC 6 March just upstream of California, and then strong atmospheric river conditions persisting on the California coast at 0600 UTC 7 April.

 
In the images above, you can see the important role played by two cyclones, the first evident near 48N, 145W at 1200 UTC 5 April.  Strong flow along and ahead of the cold front accompanying this cyclone drives moisture transport to the Pacific coast.  Then, a second cyclone, which forms along the trailing cold front and is centered near 43N, 132W at 0600 UTC 7 April further enhances the moisture transport and drives the atmospheric river into California.  It is this second cyclone that is depicted in the image at the top of this post. 

Plume diagrams of integrated vapor transport from a location near Monterrey, California (provided by CW3E and Jason Cordeira, Plymouth State University) show the GFS (black line) producing atmospheric river conditions (integrated vapor transport ≥ 250 kg/m/s) from approximately 0000 UTC 6 April through 1200 UTC 7 April.  The GEFS ensemble members generally confirm this timing, but the intensity varies, due to variations in the strength and position of the atmospheric river. 


Thumbnail plots valid 0000 UTC 7 April show the variations in strength and position.  Forecast details for California will ultimately depend on the intensity and track of the developing cyclone and cold front, which will ultimately control the position, orientation, and strength of the atmospheric river. 


A downscaled NAEFS forecast plume for precipitation and snowfall at the Central Sierra Snow Lab near Donner Pass shows liquid precipitation totals of about 1.25 to as much as 7.5 inches, with a mean of just over 4 inches.  Most of this falls in a 36-hour period.  Snowfall at this elevation (6800 feet) is, however, low for the amount of water due in large part to the warmth of the pre-cold-front environment containing the atmospheric river, which leads to rain in all but the highest elevations of the Sierra. 
Utah can expect impacts as well.  We have a chance of showers Friday (valley rain, upper-elevation snow), but the bulk of the system moves in for the weekend.  Right now Saturday looks warm (GFS forecast 700-mb temperatures are 2-3.5˚C Friday night through Saturday afternoon), with a cold-frontal passage Saturday night.  Timing, however, varies some amongst models and it is too soon to talk details.  Stay tuned and monitor official forecasts. 

The Devil Is in the Details

Nothing gives me heartburn more than the hyping of winter storms days in advance in Utah.  The devil is in the details and snowfall at any given point in our state is strongly dependent on frontal position, wind direction, and the like. 

With those words of caution, let's take a quick look at forecasts for the storm later this week and this weekend.  As I mentioned yesterday, there will be a period of warm-frontal precipitation in southwesterly large-scale flow Thursday afternoon and evening that should some snow to the mountains.  Then, things get interesting.

The 6Z GFS forecast shows a developing trough and strong cold front over the Great Basin and northwest of Salt Lake City at 1500 UTC (8 AM MST) Friday.  We are in the strong southerly flow ahead of the front, with precipitation primarily lagging it. 


At 700-mb (roughly crest level), strong southwesterly flow reaching over 30 m/s (60 knots) covers much of western Utah. 


If happening in the afternoon, this would be a recipe for very strong southerly flow and wide-spread blowing dust.  At night and in the morning, it's a bit more of a crapshoot what will happen in the valleys as sometimes we are able to maintain a nocturnal stable layer or inversion that helps reduce the strength of the surface winds.  I'm inclined to think in this case we will see that inversion mix out and strong southerly winds will develop Thursday night and give us a good blow Friday morning ahead of the cold front. 

The models differ somewhat on the details of the frontal passage.  The 6Z GFS pushes the front through Salt Lake City around noon Friday, with an active zone of frontal precipitation draped over the region Friday afternoon.  Post-frontal precipitation would fall as snow at all elevations.


The GFS is positively bullish on frontal precipitation blanketing our area for much of Friday afternoon and Friday night.  Note the lack of movement from the forecast above, valid 2 PM MST Friday, to the forecast below, valid 11 PM MST Friday. 


The 12Z NAM has a remarkably different forecast as it stalls the front to our northwest, keeping us in prefrontal southerly flow Friday and even Friday evening.  This is a dramatically different forecast that will surely give forecasters heartburn. 




Eventually the NAM pushes the front through late Saturday and Saturday night.  Thus, we can have confidence we're going to have winter weather, but the timing and amounts vary dramatically between these two model forecasts. 

In situations like this, I tend to lean toward the GFS, at least for the gross large-scale details for the following reason.  The GFS is a global model with a later cutoff for ingesting weather observations because it runs later.  The NAM is a regional model, with an earlier cutoff, and it is driven on it's boundaries by the previous GFS forecast. 

Perhaps another reason to lean toward the GFS is that the Euro also favors a frontal passage on Friday and a solution that is more GFS-like than NAM like, at least as far as large-scale features are concerned.

That being said, in terms of the amount of precipitation produced, the GFS is a bit of an outlier and I consider it's precip amount forecast to be a lower-probability outcome.  NCEP has finally corrected a bug that was providing incorrect hourly precipitation amounts in the station output that we use for our model-derived upper cottonwoods forecast guidance (info here).  Thus, I've started processing that data again and as can be seen below, the total amount of water produced for the period, including the warm-front tomorrow, is 3 inches, with nearly 50 inches of snow due to relatively large snow-to liquid ratios. 


This is substantially higher than the Euro, which is producing a bit over an inch.  It is also higher than every member of our downscaled SREF product (Note that the SREF only goes to 0000 UTC 4 March/5 PM MST Saturday, although this still captures most of the storm). 

For the GFS to verify, we'll probably need everything possible to align just right.  Although not out of the realm of possibility, the odds favor water and snowfall totals in the upper Cottonwoods that are lower than forecast by the GFS. 

The bottom line is that we're still a couple of days out from this one and that much is going to depend on small-scale details including the timing and movement of the cold front.  Monitor official forecasts from the National Weather Service and be prepared for winter weather Friday and over the weekend. 

Winter Storms from Salt Lake City to PyeongChang

Lake-effect snows added a bit of frosting to the President's weekend winter storm last night.  Alta is reporting a 22" storm total, but only 3" overnight.   In the end, I think the storm did a little better than anticipated through yesterday morning and a little worse than anticipated yesterday and last night.  Such is the post-frontal crap shoot. There are reports of more than 20 inches (through yesterday afternoon) in Sandy and Cottonwood Heights, so this was an event in which storm totals did not increase significantly with elevation up the canyon.  

Temperatures this morning are bone chilling in the mountains.  The latest from MesoWest shows predominantly teens in the Salt Lake Valley, which isn't all that bad, but above about 8500 feet, most stations are at or below zero, with a -12ºF at 11,000 feet.  

Source: MesoWest
Ski touring on days like these requires constant movement.  I recall doing a huge day several years ago in conditions like this.  The skiing was outstanding, but I was cold the entire day, despite being heavily layered.  There were no breaks.  Did I mention that the skiing was outstanding?  

Turning out attention westward, the schedule for some weather-sensitive events is being shuffled due to the weather forecast.  The fly in the ointment is a surface trough moving across the Yellow Sea and the Korean Peninsula from 0600-1500 UTC 22 Feb (1500-2100 KST Thursday).  




This trough then intensifies into a cyclone over the East Sea (Sea of Japan) and, along with a secondary trough to the west, drives strong winds across the Korean Peninsula on Friday, as illustrated by the forecast valid 0300 UTC 23 February (0900 KST Friday).  


As currently scheduled, the Women's Downhill is 1100 KST Wednesday (7 PM MST Tonight).  This is well in advance of the trough and the main concern will be the omnipresent gusty winds and perhaps some flat light from cloudiness.  I think they will get it in.  Later Wednesday are the Team Cross Country Sprints, which they should also get in.  The DVR will get a workout tonight for that as the finals aren't until 1900 KST (0300 AM MST Tonight).   

Thursday at 10 AM KST is the Men's Slalom Run 1, 1130 AM is the Women's Alpine Combined Downhill, 1330 is the Men's Slalom Run 2, and 1500 is the Women's Alpine Combined Slalom.   This covers a period from 0100-0600 UTC 22 February and as can be seen in the plots above, those events are scheduled to occur before any precipitation arrives, but also as pre-trough southerly flow is increasing.  Official forecasts for the top of the downhill show 5 m/s (10 knot) winds through 1200 KST, then increasing.  It's going to be close.  


Assuming it does go off, tomorrow night (MST) should be a great viewing for ski fans.  Helping in that quest, there's no figure skating scheduled.  Hooray!

So Much Snow, So Little Water

It was wonderful to see the white stuff return to the valley and the Wasatch Front this weekend.  Everybody got some.  Here are some of the bigger totals as reported to the National Weather Service (water equivalent in parentheses).

Bountiful Bench (through 5 PM Saturday): 13.5" (0.87")
Bountiful (5 PM Saturday): 10.8" (not reported)
Cottonwood Heights (7 AM Sunday): 17.5" (1.64")
Holladay (7 AM Sunday): 15.5" (0.91")
Summit Park (12 PM Sunday): 20" (not reported)
Alta Collins (4 PM Sunday): 23" (1.16")
Canyons 8800 ft (3 PM Sunday): 20" (0.90")

It was a complicated storm featuring a frontal phase Friday night, a period Saturday and Saturday evening with strong enhancement along the benches, and a period Saturday night when the Little Cottonwood magic kicked in. 

There are, however, a couple of key observations to be made.  The first is that the water-equivalents were largest along the east bench of the Salt Lake Valley, not in the highest elevations of the Wasatch Mountains.  Alta-Collins, Snowbird, and Alta-Guard were the three wettest sites, each coming in just over an inch of water.  In contrast, two sites in Cottonwood Heights reported over 1.5 inches of water. 

A big reason for this is the first two storm phases, which featured strong frontal forcing and then a period with weak low-level northwesterly flow in the Salt Lake Valley, but light flow aloft with strongly stable conditions.  One can't always assume the climatological increase of precipitation (water equivalent) with elevation, and Friday night and Saturday provided a good example of that. 

The second key observation is that this storm really didn't add much to the snowpack.  The 23 inches of snow that fell at Alta-Collins sounds like a lot, but it had an average water content of only 5%.  The 10 inches that fell Saturday night had a water content of only 3%. 

So much snow, so little water!

Basically, the storm put smiles on skiers faces, but water managers still have heartburn.  The Snowbird SNOTEL snowpack water equivalent didn't go up as much as one might expect from the precipitation gauge observations (this is not uncommon), but even if one jacks up the snowpack water equivalent to account for this, we're still losing ground to average.  At the end of December, we were 6.1" below average snowpack water equivalent.  Today, even with the weekend snow, we're 9.1" of water below average. 

Source: NWS
This time of year, we should be adding about 1.5 inches of water to the upper-elevation snowpack in Little Cottonwood Canyon every week.  In the first 3 weeks of January, we added about 1 inch per week.   We're still losing ground. 

This trend looks to continue for the next week.  Other than some snow showers today, our next change of a storm is in the Thursday-Friday time range.  The downscaled NAEFS ensemble generates anywhere from about 0.25 to 0.9" of water with that storm at Alta Collins.  Some members produce a bit more after that event, but not much.  Only one NAEFS member reaches the coveted 1.5" water mark for the week.   

The end of January roughly marks the midpoint of the winter snow accumulation season at upper elevations.  At Snowbird, for example, the snowpack water equivalent averages 22.1" on February 1, roughly half the average peak of 44 inches on April 27.  With 9.9" currently on the ground, we look to be about 50% of average at the end of the month, with somewhere between 10 and 11.5 inches of snowpack water equivalent.  

We need a real storm cycle and we need it now.  

A Storm with Big Winners and Big Losers

I can't recall a storm in which the winners and losers were more defined than the one that hit us Monday night through Wednesday.  In part, this reflects our meager snowpack, which makes the vulnerability to warmth and rain even more obvious than when we have a more robust low-elevation snowpack.

The winners were clearly the upper-elevations of central Wasatch, above perhaps 8500 or 9000 feet.  The central Wasatch have a great deal going for it, including elevation, and it paid off in spades this week.  Data from the Snowbird SNOTEL shows a nice uptick in snowpack water equivalent. 

Source: NWS
The storm represented only the fourth since mid November, but it finally brings us to something close to mid December snowpack for this aspect and elevation

The losers?  The storm was pretty much a disaster near or below 7000 feet where almost all the precipitation fell in the form of rain.  I was glad to find late yesterday that Mountain Dell still had enough snow to skate ski, but the situation there and along trails in the Park City area is now critical or beyond critical.  Such a shame. 

The Ben Lomond Trail SNOTEL is at 6000 feet and it illustrates the dire situation down low.  Keep in mind this is an extremely snowy location — typically with a much deeper snowpack than found at comparable or even higher elevations in the central Wasatch or Park City area.  There was a bit of a bump in snowpack water equivalent at this location during the storm, with no net change.  I suspect what happened is that the snowpack soaked up the rain initially, causing an increase, afterwhich it experienced net melting and loss.  The site now sits at about 25% of median. 

Source: NWS
Looking forward, we will see some snow showers at times through Friday.  Then, the zombie apocalypse returns with a high amplitude ridge developing over western North America.  


The positive?  It looks like a beautiful Martin Luther King weekend.  For many out-of-towners, and what should be excellent conditions on the groomers, it will be the stuff that dreams are made of.  I expect there will be big smiles at the Cottonwood Canyon resorts.