Sloppy Snow

The storm has arrived! As most of you are already aware, an atmospheric river is penetrating the Intermountain West & Northern Utah. As Peter’s post was titled, these events are typically characterized by a plethora of moisture (wet), relatively warm temperatures, and windy conditions – All of which are currently being observed.  The upper elevations of the Wasatch have already seen 1-2″ of water. In a colder storm that would equate to 1-2 feet of snow. Right now, though, most locations are only reporting a few inches of snow and that’s above 9000 feet! Below 9000 feet periods of a snow/rain mix can be expected. This is what I like to call sloppy snow! In addition to the sloppy snow, wind gusts over 100 mph have been observed on upper elevation ridge lines!

Alta Collins observation - note the large amount of precipitation with only 2 inches of snow
Alta Collins observation – note the large amount of precipitation with only 2 inches of snow

The good news is that a cold front will push through overnight bringing some colder temperatures and snow levels down to 6000-7000 feet. I think Peter’s call of 2-4 inches of liquid is pretty good but with rain at times and dense snow, I’ll be happy if the upper Cottonwoods come out in the 8-16″ range. Below 8000 feet accumulations will be much lower and only occur following frontal passage. As the front clears the area tomorrow morning, snow should begin to taper off.

Additional refreshers are possible later this weekend into early next week, though, models have been very inconsistent with the placement and timing of these features. I’ll let tomorrow’s forecast get more into the details.

 

Warm, Wet, and Windy

As Lucas mentioned, a storm is bearing down on us that promises a period of warm and wet “atmospheric river” conditions. The action will start early Thursday, with the rain/snow line somewhere around 9,000ft, heavy precipitation rates, and winds gusting to near 100mph on the high peaks (50 mph mid mountain at the resorts). Late Thursday into Friday morning, the snow levels will be dropping and the winds will start backing off as colder air works into the region. I expect snow levels to reach about 6-7000ft by Friday morning and remain somewhere around there through the day. The precip should taper off by late Friday.

GFS model meteogram
The meteogram for today’s 6Z run of the GFS model lays things out nicely. Note the heavy precip beginning Thursday and the snow levels starting high and lowering slowly through the event. weather.utah.edu

It looks like the upper Cottonwoods can expect 1.5-2.5″ of liquid, with places like the northern Wasatch around Ben Lomond Peak and the southern Wasatch around Timpanogos likely to get something more like 2-4″ of liquid. Above 9,500ft this should be all snow, and with an average of 8:1 or 10:1 ratios for the storm, that should equate to 1-2ft of snow for the tops of the upper Cottonwoods resorts and 2-3ft of snow for the top of Snowbasin. However, a very large chunk of the skiable terrain in the Wasatch is below 9,500ft, so expect a rain-slush-snow transition throughout the storm with maybe 6-12″ of new snow at 8,000ft by the time all is said and done.

UW WRF GFS
Precip forecast from the U of Washington’s WRF model. I generally take these amounts as a reasonable, but higher-end scenario. Note the widespread >2.5″ for the Wasatch (sorry for the worst unit scale ever).
ECMWF model
Precip forecast from the more conservative ECMWF model. I usually consider these as extremely likely to verify, but often slightly low. models.weatherbell.com

We briefly clear out and warm up on Saturday, and then the next, colder storm is lined up for Sunday.

Warm and Wet

I don’t know about you, but I had a fantastic weekend at Snowbird – it seemed like it didn’t stop snowing all weekend! Periods of fluffy flakes and periods of graupel made for some fun skiing. We ended up with a good amount of snow water, and more is coming to the Wasatch.

As mentioned in previous posts, we are in for quite the atmospheric river event later this week. So far, we are looking at a strong tropical moisture tap that will feed into northern Utah beginning Wednesday, and warm temperatures will impact the snow levels in an unfavorable way. Let’s take a look at the midday NAM 12 km time height section from weather.utah.edu.

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This time-height looks pretty similar to others from several previous model runs and can give us a good idea of what to expect.

Round 1: Some moisture will move into the area beginning Wednesday morning. Combined with moderate southerly/southwesterly winds, this looks like it will mostly be confined to the mountains until afternoon hours. It’ll be fairly warm, so lower elevation resorts will see rain at the bases – especially as the day progresses. Warming temperatures and a rising freezing level (blue line) into Thursday will mean rain at low elevations. Note that the freezing level isn’t exactly where the snow turns to rain, as snowflakes can fall through warmer layers and survive to accumulate, especially in heavier precipitation. For this first round, don’t expect much for accumulation. I’d guess up to 2 inches of snow at upper elevations/higher parts of resorts and up to an inch of wet snow at lower portions of resorts with some rain mixing in.

Round 2: This will be the slug of moisture associated with what is hyped up as the atmospheric river. While a good portion of moisture is usually intercepted by California’s High Sierra, atmospheric rivers can penetrate into Utah and provide good amounts of moisture if the trajectory isn’t directly intercepting the highest parts of California. Today’s 12Z GFS forecast for Thursday morning is just that, as water vapor transport is strong and on a path from extreme southern California into Utah.  the following shows IVT (Integrated Vapor Transport) 48 hours out from the model run that happened at 12Z this morning.

Integrated Vapor transport for Thursday morning from NOAA's Earth Systems Research Laboratory. (https://www.esrl.noaa.gov/psd/psd2/coastal/satres/data/html/ardt_ivt_gfs.php)
Integrated Vapor transport for Thursday morning from NOAA’s Earth Systems Research Laboratory. (https://www.esrl.noaa.gov/psd/psd2/coastal/satres/data/html/ardt_ivt_gfs.php).

For a lot more atmospheric river guidance, check out this site: http://cw3e.ucsd.edu/iwv-and-ivt-forecasts/

Alright, so continuing with Round 2… Note how the freezing level is ~700 mb for most of the larger green blob in the time-height section. This is roughly mountain top height for  our highest northern Utah peaks. So, even the higher resorts will likely have rain/wet snow at their bases for a good chunk of the event. If you’re planning to ski make sure you have your waterproof gear! Snow amounts are somewhat difficult to forecast for this second round considering how impactful the warm temperatures will be for accumulations. Also, there is still a wide range of possibilities – even for higher elevations. Just know that this we’ll be wet and quite warm later this week. To hit that point home, here is the NAEFS plume forecast for this week for Alta. It sure is wet, but sure has some uncertainty!

NAEFS plumes from weather.utah.edu for Alta. Forecast water amounts on top panel, snow on bottom panel.
NAEFS plumes from weather.utah.edu for Alta. Forecast water amounts on top panel, snow on bottom panel.

Check tomorrow as models hone in on some of the details.

-Lucas

Nice Start to the Week

What a weekend we had! It just would not stop snowing in the mountains. The bulk of the snow fell before Sunday morning, but on and off showers persisted through the entire day today. Alta is reporting 2′ as a storm total and Snowbird ~21″. Most other resorts were in the 12-20″ range. All in all a great storm.

As mentioned yesterday, the first half of the week will be stellar. The second half… not so much. Monday and Tuesday will be partly cloudy and chilly. On Wednesday the wind starts to pick up and there’s a chance of showers in the afternoon. And Thursday is when our next big storm should hit. The weather models are all in agreement that an atmospheric river is going to impact Utah Thursday into Friday. Most are predicting an extremely strong one, but there are some dissenters. Right now many models are predicting around 2″ of precipitation out of this. The problem is that this is an atmospheric river event and it’s late March. Snow levels are going to be crazy high. Don’t be surprised if a good chunk of that 2″ falls as rain even at base level.

GFS Forecast via weather.utah.edu
GFS Forecast via weather.utah.edu

This meteogram from the UofU Weather Center shows the problem well. The middle right panel shows snow ratios, and they’re slush for the time Thursday afternoon when it’s precipitating the hardest (bottom left panel). Also snow levels then are close to 10 thousand feet! Cold air might move in right as the precipitation starts to exit, so we might get some drier stuff on top. But it looks like a slush-fest.

Of course, all of this is 4 days out and is liable to change. There could be more cold air present that keeps snow levels lower, or the moisture might not make it this far inland and we don’t get so much rain. Either way, you should try to get outside during the first half the week. It’s going to be fantastic out.

Hundred Inches

It started snowing early last night in the mountains, and it’s dropped 13″ thus far in the Cottonwoods. For the first time this season, Alta-Collins has reached 100″ of snow depth. Snowbird also is reporting 100″.  It’s nice to finally be in the triple digits. Chances are it will keep snowing in the mountains through the end of the day, and I expect another 3-6″, giving a storm total in the neighborhood of 20″. These spring storms sure can produce.

Sunnyside Webcam via Alta
Sunnyside Webcam via Alta

After this storm winds down, we’ll have a rather quiet week. Showers are possible in the evenings several days, especially Sunday and Tuesday. But by and large it’ll be a great week to get out there. We have all this fresh snow, it should be mostly cloudy for the week, and mountaintop temperatures should stay below freezing at least until Wednesday. Perfect weather for the students on spring break.

Peaking ahead into the longer-term, Friday looks like a good time. Many models are producing around 2″ of water, most of which would fall as snow in the mountains. Right now the GFS weather model is producing an atmospheric river set-up, but a lot of pieces have to fall into place before we can determine what’s actually going to happen. It’ll be something to watch in between trips into the mountains this week.

Another weekend (and spring break!) storm

Sometimes it seems like the best way to get a big storm around here is for one of us to skip town… That certainly seems to be the case this weekend, with a handful of us taking advantage of spring break at the U to explore new places. For those sticking around, this storm should be a treat.

Snowfall will likely hold off until late Saturday morning, with the HRRR producing not much more than light snow showers overnight. For those that enjoy storm skiing, Saturday should get better as the day goes on. The main slug of moisture should come through in the early evening, with an intense period of precipitation that will likely be short-lived. Most models wrap up steady precipitation by Sunday morning around when resorts open. Snow showers are likely to continue on and off Sunday morning, ending in the afternoon.

NAM 12km Time-Height
NAM 12km Time-Height

Models have backed off on the amount of water associated with this system, and we are seeing a consensus around .8” of water for the central to northern Wasatch. With the predominant flow direction out of the SW, I don’t expect impressive enhancement over the Cottonwoods. Big, Little, and the Park City ridgeline should all see totals over 6 inches, with about half as much at the Park City base. Max of around 12” likely at the highest elevations at the head of the cottonwoods. Should some strong convection develop, lucky areas in a bullseye may see an inch or two more.

NAM 12km Meteogram at Alta, UT
NAM 12km Meteogram at Alta, UT

For those always thinking ahead, a few inches is possible into/through Monday.

Enjoy it for me!

Mike

March Showers

Some pretty impressive numbers in terms of the liquid water equivalent (SWE) from this storm – 1.8 inches of water at Alta but only 12 inches of snow. This is an average ratio around 7:1 (snowfall:water)…think Sierra Cement. I wasn’t able to sample the goods personally, but I bet it’s actually skiing quite well being right-side up. The high amount of water also means that any crust underneath should be totally covered.

Short Term

Additional light accumulations this evening/overnight should help keep the snow fresh for tomorrow morning, but I wouldn’t expect too much more (1-3 inches). Snow showers should clear out overnight giving way to slightly warmer and drier conditions by tomorrow.

The next storm should begin moving across our area by Saturday afternoon. The GFS has been trending more towards a splitting system with the brunt of the precipitation to the north and south of Northern Utah. The European does show slightly more precipitation, but for right now I’ll take a stab and go with 3-6 inches Saturday afternoon to Sunday morning. Hopefully the models will shift just a tad, and these totals will be upped a little in tomorrow’s forecast. Regardless, the skiing should be good, and a nice change of pace from the Spring skiing we had earlier this week.

Latest GFS model: Saturday - Sunday storm splits
Latest GFS model: Saturday – Sunday storm splits

Long Term

Conditions should clear out as temperatures gradually warm early next week. The next potential storm appears to be mid-late next week, though models have been struggling with the timing.

Nuking

As I write this around 10pm, it is nuking in the Wasatch! Not by snowfall amount though, but by liquid equivalent. Rain or very dense snow is falling at most sites, meaning that although many sites have already received >1″ of water, snowfall totals are maybe 6″. That’s some serious concrete, but it should cover the tracks well.

Snowbasin
Snowbasin snow stake is lookin good!

Fortunately temperatures are dropping with the passage of a cold front a few hours ago, and the snow density should drop accordingly. Snow levels are also dropping, and should get down to the benches by the morning. Precipitation will continue through the day tomorrow, beginning to taper off toward the evening. That leaves us with quite a while to stack up some more snow, which means we will easily hit the snowfall numbers Taylor forecasted, and should likely go even higher than her 16″. Wooohoo!

Looking out further, it looks like another round of precip Friday night into Saturday. Enjoy!

Known Unknowns: Spring Snow Forecasting In the Wasatch

TL;DR:
This week, we will transition from our warm, spring-like weather to the cold, overcast, late winter feels of Utah. The synoptic pattern is becoming more active again, which will bring several rounds of precipitation to both the valley and mountains. Forecast uncertainties exist, but it will snow, and hopefully a lot.


Forecasting for the next week made me think a lot about the words of Donald Rumsfeld (and Jim Steenburgh, because he loves this quote):
“As we know, there are known knowns; there are things we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the one’s we don’t know we don’t know”

So let’s break this down into what we know and what we don’t know.
Known knowns“:  Several short-wave troughs will impact our area from Wednesday through Sunday. These short-wave troughs will bring colder temperatures and precipitation from the valley floor to the mountain tops.  Despite the colder temperatures, weekday snowfall will likely be confined to the high elevations of the Wasatch due to elevated freezing levels.
Known Unknowns“: Timing of each of the tough passages. We have a general idea of the Wednesday – Thursday storm, but as we progress into the weekend, confidence is much lower.  We also know that forecasting snowfall totals (especially around the bases of the resorts) will be tricky. The rest of the post will be devoted to explaining what we know and discussing what we know we don’t know.  As the week progresses, these known unknowns will become more clear.


Short Term Forecast (Tuesday – Friday):

Today (Tuesday) expect increasing cloud cover with temperatures reaching the mid- to low-60’s in the valley.
Tomorrow (Wednesday), strong southerly winds will prevail ahead of the main trough axis, moderating valley temperatures and keeping freezing levels high.  A few isolated convective snow showers are possible at crest level during the day tomorrow. Showers will extend to the valley floor in the late evening and overnight hours as a shortwave embedded within the larger trough axis quickly moves through our area.  Freezing levels are not expected to drop much overnight, resulting in rain at the resort bases and below.

NAM 12km Time-height cross-section - weather.utah.edu
NAM 12km Time-height cross-section – weather.utah.edu

Thursday afternoon into the early evening, a shortwave ejecting from the main trough will move through the area. This will finally bring freezing levels to the valley floor as the flow shifts from southerly to more westerly.  Snow will continue through Friday mid-day with accumulations possible down to the valley floor.
Remember that the timing on the freezing level dropping/shift to NW flow may change in the next day or so.. Hence the known unknowns we touched on at the beginning of the post.  Tomorrow we will have a better handle on when to expect precipitation to start, what kind you will see at what elevation, and a more educated guess of how much.
From what we know now: Given the prolonged exposure to the potent available moisture, as well as the instability at crest level, I will forecast that the Cottonwood resorts will see 8-16″ of new snow by Friday.  Look for locally higher amounts in areas favored in the northwest flow regime.

Screen Shot 2018-03-13 at 11.09.04 AM

Long Term (Through the weekend):
This weekend we will have yet another chance for wintry precip in the mountains. The low spinning off to our west will slowly make its way into our area.  Again, timing is uncertain, but model forecasts like our odds of having a powder day on Sunday.  I wont even take a stab at what these totals could be, but enjoy the NAEFS output of liquid and snow to fuel your Tuesday daydreams.

 

-Taylor

 

Models Predict Weak Chance for Snow Next Week, After a Warm Weekend

This weekend is that time to accept the warmer weather and blue skies that hint at spring; I think there are times when that can be exactly what you need in day’s ski…still, that’s as far as I’m going to go excusing mother earth, who is letting the weather escape us this week. I see some powder in most models forecasts but it’s unclear and looks kind of weak.

Before I explain, I’ll start with a quick summary of the weather this weekend.


This Weekend

If you’re in Northern UT, you’ve noticed that it’s warm and the sky was clear all day. The warmth is here to stay for a few days, but clouds will move in beginning around 7PM and stick around through the night. These clouds are associated with a system that is bringing mostly rain to southwest UT.

We’ve been under a weak ridge that has maintained mild temperatures late last week, and will continue the pattern this next week. The ridge won’t stay without a fight…two fights.

A shortwave trough passes through this weekend and splits its energy to our north and our south. Southern Utah takes the cake with the southern piece of energy, although only high elevations will see any snow. Forcing is weaker to our north, and it is especially weak over the rest of Utah, so neither areas will see any more than clouds tonight.


The Powder

The next chance for powder in the Wasatch occurs around Wednesday this week. It’s a weak system and only a deserves a quick blurb in my opinion.

A shallow upper-level trough comes into view Tuesday morning and passes east-southeast from the northern Pacific coast of the US. Both the American (GFS) and the European (ECMWF) global models have a weak shortwave feature spin off and pass just north of Utah. The gif below shows the GFS’s deterministic (one possibility) solution for this shortwave’s trajectory.

model 3- to 5- day forecast solution (GFS)
GFS models 500 hPa relative vorticity (shaded) gif valid 11AM MT Tuesday – 11PM MT Wednesday. Note the slow-moving long-wave trough in the Pacific, and some positive vorticity features that move across the western US (courtesy tropicaltidbits.com)

Beyond this timeframe, the global model solutions diverge. The Euro deepens the trough, then shuffles the system into the continent and weakens it substantially. The GFS solution starts with a weaker system that quickly moves east as it continues to weaken. These are 7-10 day forecasts, and my point is that models don’t easily make sense of the weather in that timeframe and beyond. I’m interested to see what that trough decides to do as we get closer to next weekend. Stay tuned this week!


In summary, we have warm weather for the next few days, a chance for snow on Wednesday, and an interesting set-up to look out for next weekend.

-marcel